Physiol Rev
84: 987–1049, 2004; 10.1152/physrev.00040.2003.
Molecular and Cellular Physiology of Renal
Organic Cation and Anion Transport
STEPHEN H. WRIGHT AND WILLIAM H. DANTZLER
Department of Physiology, College of Medicine, University of Arizona, Tucson, Arizona
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Wright, Stephen H., and William H. Dantzler. Molecular and Cellular Physiology of Renal Organic Cation and
Anion Transport. Physiol Rev 84: 987-1049, 2004; 10.1152/physrev.00040.2003.—Organic cations and anions (OCs and
OAs, respectively) constitute an extraordinarily diverse array of compounds of physiological, pharmacological, and
toxicological importance. Renal secretion of these compounds, which occurs principally along the proximal portion
of the nephron, plays a critical role in regulating their plasma concentrations and in clearing the body of potentially
toxic xenobiotics agents. The transepithelial transport involves separate entry and exit steps at the basolateral and
luminal aspects of renal tubular cells. It is increasingly apparent that basolateral and luminal OC and OA transport
reflects the concerted activity of a suite of separate transport processes arranged in parallel in each pole of proximal
tubule cells. The cloning of multiple members of several distinct transport families, the subsequent characterization
of their activity, and their subcellular localization within distinct regions of the kidney now allows the development
of models describing the molecular basis of the renal secretion of OCs and OAs. This review examines recent work
on this issue, with particular emphasis on attempts to integrate information concerning the activity of cloned
transporters in heterologous expression systems to that observed in studies of physiologically intact renal systems.
I. INTRODUCTION
Renal secretion of organic electrolytes of broadly
diverse chemical structures plays a critical role in limiting
the body’s exposure to toxic compounds of exogenous
and endogenous origin (including a wide array of compounds of clinical importance). Because of the physiological, pharmacological, and toxicological importance of
the renal secretion of “organic cations” (including bases;
collectively, OCs) and “organic anions” (including acids;
collectively, OAs), the underlying processes have received increasing attention over the last 20 years. The
field last received an exhaustive review in 1993 (342), a
time at which no transporters associated with the processes of renal OC and OA secretion had yet been cloned.
Indeed, schematic models of renal OC and OA secretion,
based on the then available functional evidence, typically
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showed a single peritubular “avenue of entry” into cells
for each chemical class of substrate (OC or OA) and one
(or two) luminal “avenue(s) of exit.” These functionally
characterized pathways were often (and continue to be)
referred to as the “classical” pathways for renal secretion
of organic cations and anions. As a consequence, however, of the enormous recent growth in information concerning the molecular identity of transporters that accept
OCs and OAs as substrates, we now know that the classical processes of renal OC and OA secretion are the
products of the concerted activity of an extensive array of
discrete molecular entities. These include, but are not
restricted to, the entire spectrum of members of what is
now referred to as the OCT family of amphiphilic solute
transporters, as well as selected members of the multidrug resistance transporters (MDRs and MRPs) and the
organic anion transporting polypeptide (OATP) transport-
0031-9333/04 $15.00 Copyright © 2004 the American Physiological Society
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I. Introduction
II. Renal Organic Cation Transport
A. Overview of the physiological characteristics of renal OC transport
B. Molecular characteristics of renal OC transporters
C. Physiological integration of renal OC transporters
III. Renal Organic Anion Transport
A. Overview of the physiological characteristics of renal OA transport
B. Molecular characteristics of renal OA transporters
C. Physiological integration of renal OA transporters
IV. Speculations and Conclusions
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cussion of issues that we think require future study. Foremost among these is the need to integrate knowledge
obtained on individual transporters into the cellular
events that result in renal OC or OA secretion. These
secretory processes involve the coordinated function of
many separate processes working in the physiological
context of intact cells and tissues. Finally, we offer some
speculative views on evolutionary aspects of renal OC and
OA transport.
II. RENAL ORGANIC CATION TRANSPORT
A. Overview of the Physiological Characteristics
of Renal OC Transport
The proximal tubule is the primary site of renal OC
secretion, as determined by stop-flow, micropuncture,
and microperfusion studies (342, 355). The proximal tubule is also responsible for the reabsorption of a more
limited number of cationic substrates (342). In general,
substrates for the pathways involved in renal OC transport include a diverse array of primary, secondary, tertiary, or quaternary amines that have a net positive charge
on the amine nitrogen at physiological pH. Although a
number of endogenous OCs have been shown to be actively secreted by the proximal tubule [e.g., N1-methylnicotinamide (NMN), choline, epinephrine, and dopamine;
see Ref. 22], it is generally accepted that the principal
function of this process is clearing the body of xenobiotic
agents (342), including a wide range of alkaloids and
other positively charged, heterocyclic compounds of dietary origin; cationic drugs of therapeutic or recreational
use; or other cationic toxins of environmental origin (e.g.,
nicotine). The secretory process is also a site of clinically
significant interactions between OCs in humans. For example, therapeutic doses of cimetidine retard the renal
elimination of procainamide (408, 409) and nicotine (17).
Until recently, models of the cellular basis of renal
OC secretion typically depicted a single basolateral entry
step and a single luminal exit step, a simple view that
effectively explained existing physiological data. That
view is now known to be an oversimplification of the suite
of cellular events that underlies renal OC transport. In
developing a model for the functional basis of this complexity, it is useful to consider the “type I” and “type II”
classifications for different structural classes of organic
cations developed to describe OC secretion in the liver
(274). In general terms, type I OCs are comparatively
small (generally ⬍400 mol wt) monovalent compounds,
such as tetraethylammonium (TEA), tributylmethylammonium (TBuMA), and procainamide ethobromide. Type
II OCs, which are usually bulkier (generally ⬎500 mol wt)
and frequently polyvalent, include d-tubocurarine, vercuronium, and hexafluorenium.
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ers (41, 361, 426). For the purpose of developing an integrated, contemporary view of the cellular processes associated with renal secretion of OCs and OAs, we will
discuss relevant aspects of the structure, location, and
function of all of these recently described transporters.
However, the principal emphasis of this review will be
on recent information concerning the characteristics
and functional behaviors of the OCT family of transporters (which includes an array of both OC and OA
transporters).
In this review we have attempted to compile the
novel material on the molecular and cellular physiology of
renal OC and OA transport that has accrued since the
1993 review of Pritchard and Miller (342). Readers interested in the material preceding 1993 are directed to that
excellent discussion and to several earlier reviews that
assessed the historical sweep of studies in this area (292,
355, 514, 515). Here, we limit ourselves to an examination
of the several families of transport processes currently
suspected of playing significant roles in the renal handling
of organic cations and anions. Readers interested in more
detail on selected issues should consult excellent recent
reviews focused on specific subjects (38, 41, 89, 218 –221,
227, 343, 426, 465, 496, 501, 547). We also note that,
although there is considerable functional overlap of the
OC and OA transport processes expressed in the kidney
and the liver (and gastrointestinal tract), we have limited
our focus to those processes believed to play major roles
in the transport of these compounds in renal cells. Readers are directed to reviews on organic electrolyte transport expressed in the liver (12, 201, 231, 233, 273, 419, 501)
and other organs (14, 157, 220, 224, 311, 398, 547).
The review is organized in the following fashion. The
molecular and cellular physiology of organic cation secretion is presented first. This reflects the fact that the first of
the associated processes to be cloned was a cation transporter (OCT1). The resulting family of transport processes, which includes, as it turns out, processes involved
with the classical OA secretory pathway, is referred to as
the “organic cation transporter family” (253, 361). For
each class of substrate (OC or OA), we initially discuss
the general physiology of renal secretion, particularly as it
was understood in 1993. This is followed by an extended
discussion of the molecular characteristics of individual
members of the several families of transporters implicated in renal transport of these substrates. Then, in the
light of increasing understanding of the molecular properties of these processes, we consider recent, novel observations on the cellular physiology of renal OC or OA
transport, with a particular effort focused on highlighting
where “molecular” information has helped clarify “cellular” information, and where physiological observations
run counter to expectations arising from the results obtained studying cloned transporters. We close with a dis-
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
Figure 1 shows a model for transcellular secretion of
OCs by the proximal tubule that is consistent with observations obtained in studies with isolated renal plasma
membranes and intact proximal tubules (342, 343) and
that is supported by recent molecular data. OCs enter the
cell from the blood across the peritubular membrane. For
type I OCs, this entry step involves either an electrogenic
uniport (facilitated diffusion), driven by the inside-negative electrical potential difference (PD) (407), or an elec-
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troneutral antiport (exchange) of OCs (82, 407) [it is likely
that these two mechanisms represent alternative modes
of action of the same transporter(s); Ref. 44]. The negative PD is sufficient to account for an accumulation of
OCs within proximal cells to levels 10 –15 times that in the
blood. Studies by Ullrich and colleagues (468, 476) on the
structural specificity of inhibition of peritubular OC transport in microperfused rat proximal tubules in vivo indicate a clear correlation between an increase in substrate
hydrophobicity and an increase in inhibitory effectiveness, although it is also clear that steric factors influence
the interaction of type I OCs with basolateral transporters
(e.g., Refs. 15, 470). The marked hydrophobicity of many
type II OCs probably results in a substantial diffusive flux
across the peritubular membrane, providing an alternative, electrically conductive avenue for entry into proximal cells.
Exit of type I OCs across the luminal membrane
involves a carrier-mediated antiport of OC for H⫹ (Fig. 1),
a process observed in brush-border membrane vesicles
(BBMV) isolated from human, rabbit, rat, dog, chicken,
and snake kidneys (342). The electroneutral antiport of
one OC⫹ for one H⫹ (533) permits OCs to leave the cells
and develop a luminal concentration as large as or larger
(depending on the size of the transluminal H⫹ gradient)
than that in the cytoplasm, resulting in net transepithelial
secretion. OC/H⫹ antiport is the active step in this process
because it depends on the displacement of H⫹ away from
electrochemical equilibrium, a state maintained through
the activity in the luminal membrane of the Na⫹/H⫹ exchanger and, to a lesser extent, a V-type H⫹-ATPase (125).
Luminal OC/H⫹ exchange appears to be the rate-limiting
step in transepithelial OC transport (342, 372). Evidence
on the structural specificity of luminal OC transport indicates that, as with the peritubular process, binding to the
OC/H⫹ exchanger is profoundly influenced by substrate
hydrophobicity (536). The secretory flux of type II OCs
appears to require interaction with the apically located
multidrug resistance transporter (MDR1) (102).
We can summarize the current, overall understanding
of the cellular processes associated with secretion of
organic cations as follows: type I OCs enter proximal cells
across the peritubular membrane via electrogenic facilitated diffusion and leave cells across the luminal membrane via electroneutral exchange for H⫹. Type II OCs
diffuse into proximal cells across the peritubular membrane and are exported into the tubule filtrate via the
primary active MDR1 transporter. Importantly, considerable overlap appears to exist in the selectivity of these
parallel transport pathways (502; see also Ref. 403). It is
also evident, as discussed below, that OC transport across
the basolateral and luminal membranes of renal proximal
tubules involves the parallel activity of several distinct
transport processes.
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FIG. 1. Schematic model of the transport processes associated with
the secretion of organic cations (OCs) by renal proximal tubule cells.
Circles depict carrier-mediated transport processes. Arrows indicate the
direction of net substrate transport. Solid lines depict what are believed
to be principal pathways of OC transport; dotted lines indicate pathways
that are believed to be of secondary importance; the dashed line indicates diffusive movement. The following is a brief description of each of
the numbered processes currently believed to play a role, direct or
indirect, in transepithelial OC secretion. 1: Na⫹-K⫹-ATPase, maintains
the K⫹ gradient associated with the inside-negative membrane potential
and the inwardly directed Na⫹ gradient, both of which represent driving
forces associated with active OC secretion. 2, 3, 4: OCT1, OCT2, and
OCT3, support electrogenic facilitated diffusion associated with basolateral uptake of type I OCs (these processes are also believed to
support electroneutral OC/OC exchange, as indicated by the outwardly
directed arrows). 5: MDR1, supports the ATP-dependent, active luminal
export of type II OCs. 6: NHE3, the Na⫹/H⫹ exchanger that plays a
principal role in sustaining the inwardly directed hydrogen electrochemical gradient that, in turn, supports activity of transport processes 7 and
8. 7: Physiologically characterized type I OC/H⫹ exchanger; as depicted,
process 7 includes at least two physiologically characterized OC/H⫹
exchangers: the broadly specific process that accepts tetraethylammonium (TEA) as a prototypic substrate and the narrowly specific process
that accepts guanidine as a substrate. 8: OCTN1, supports electroneutral
OC/H⫹ exchange but with selectivity properties that make it distinct
from process 7. 9: OCTN2, supports Na⫹-carnitine cotransport as well as
electrogenic uptake of TEA and selected type I substrates. 10: Physiologically characterized electrogenic choline reabsorption pathway.
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STEPHEN H. WRIGHT AND WILLIAM H. DANTZLER
B. Molecular Characteristics of Renal
OC Transporters
1. OCT family of OC transporters
FIG. 2. Sequence alignment of 37
members of the organic cation transporter family showing the degree of conservation of signature sequence motifs
characteristic of the major facilitator superfamily (MFS) and the amphiphilic solute facilitator (ASF) family. The MFS motif (left) is 13 amino acids long, and the
ASF motif (right) is 11 amino acids long,
as indicated by the shaded regions within
the aligned sequences. Open boxes
within the shaded regions indicate sequence deviations from the signature
motifs.
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Several alternative nomenclatures are currently in
use for describing members of the OCT family of transporters, and we will commonly provide two of these
when we introduce each member of the family. The first
is the designation within the solute carrier superfamily
(SLC), as defined by the Human Genome Nomenclature
Committee, in which the OCTs (and the related OATs)
are members of group SLC22A. The second reflects the
classification system developed by Saier for transport
proteins (the Transport Commission; Ref. 361), within
which the organic cation transporters are listed as
members of the major facilitator superfamily (MFS)
and are classified as 2.A.1.19: 2 (electrochemical potential driven porters), A (uniporters, symporters, antiporters), 1 (major facilitator superfamily), 19 (organic
cation transporter family). Schömig et al. (382) suggested the alternative name “amphiphilic solute facilitator” (ASF) family for this group of transporters within
the MFS, focusing on the characteristics of substrate
specificity which link all members of this group. Common structural features of MFS proteins, which are
generally shared by the family 19 OCT transporters,
include 12 putative transmembrane spanning domains
(TMDs) and several highly conserved structural motifs,
including a striking degree of conservation for a 13residue sequence found between TMD2 and TMD3: G[RKPATY]- L -[GAS]-[DN]-[RK]-[FY]-G-R-[RK]-[RKP][LIVGST]-[LIM]. Figure 2 shows the remarkable conservation of this “signature sequence” for 37 of the cation
transporting members of the OCT family.
Schömig et al. (382) compared the sequences of nine
members of the OCT (ASF) family to determine whether
these proteins contained any sequence features that
would distinguish them from other members of the MFS.
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
1
We have elected to identify all transport proteins using capital
letters (e.g., OCT1, OAT2, MRP3) and distinguish particular orthologs,
when appropriate, by preceding the acronym with one or two lowercase
letters indicative of the species (h, human; r, rat; m, mouse; p, pig; rb,
rabbit; fl, flounder; ce, C. elegans; dr, Drosophila).
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intracellular domain between TMD 6 and TMD 7 (Fig. 3).
Conserved motifs within OCT1 across all species include
the following (426): 13 cysteine residues, 25 proline residues, 3 N-linked glycosylation sites2 (N 71, 96, 112), 3
protein kinase C (PKC) consensus sites3 (Ser-285, Ser-291,
and Ser/Thr-327), 3 protein kinase A (PKA) consensus
sites (Thr-235, Thr-296, and Thr-347), 1 protein kinase G
(PKG) consensus site (Thr-347), 2 casein kinase II (CKII)
consensus sites (Ser-333 and Thr-524), and 1 Ca2⫹/calmodulin kinase II (CaMII) consensus site (Thr-347). Of
these sites, 7 cysteine residues, 17 proline residues, 1
N-linked glycosylation site (N 72), 1 PKA site (Thr-347), 1
PKC site (Ser-285), 1 PKG site (Ser-347), and 2 CKII sites
(Ser-333 and Thr-524) are conserved across OCT1, OCT2,
and OCT3.
Immunocytochemical evidence supports this postulated secondary structure. Permeabilized and nonpermeabilized HEK 293 cells that stably expressed rOCT1 (276)
were exposed to antibodies prepared against the postulated long extracellular loop and the intracellular COOH
terminus. Whereas the antibody against the extracellular
loop stained both the permeabilized and the nonpermeabilized cells, the antibody against the COOH terminus
stained only the permeabilized cells. These observations
are consistent with the secondary structure profile presented in Figure 3. The gene for rOCT1 (Roct1) has been
mapped to chromosome 1q11–12 (216), and the gene for
the human ortholog has been mapped to chromosome
6q26 (217).
The human gene for OCT1 consists of seven exons
and six introns (152). Several alternatively spliced variants of OCT1 have been described. The rOCT1A variant
(548) arises from a 104-bp deletion between bp 451 and
556 of rOCT1. Although the resulting protein product
lacks putative TMDs 1 and 2 and the large extracellular
loop that separates those two TMDs, the gene product
supports mediated transport of TEA (548). In the human,
four alternatively spliced isoforms are present in human
glioma cells (152): a long (full-length) form and three
shorter forms. Only the long form (hOCT1G/L554) supports transport when expressed in HEK293 cells. The long
form and one of the shorter forms (hOCT1G/L506) are
present in human liver cDNA (152).
II) Tissue and cellular distribution and localization. OCT1 appears to be expressed in many tissues,
although intriguing species differences have been noted.
OCT1 is expressed in the liver of all species tested [rat,
Northern (142); mouse, cDNA library (132); human,
2
Determined using the NetNGlyc 1.0 Server (http://www.cbs.
dtu.dk/services/NetNGlyc/).
3
Unless otherwise indicated, residue numbers reflect the human
sequence for the protein under discussion. Consensus phosphorylation
sites were predicted using PhosphoBase v.2.0 (http://www.cbs.dtu.dk/
databases/PhosphoBase/).
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They suggested the following as possible signature sequences: STIVTEW[D/N]LVC before TMD2; ELYPT after
TMD10, and LP[D/E]TI after TMD12. Figure 2 extends this
comparison for the first of these motifs to 37 OCT family
members. Although a high degree of homology is evident,
it is also clear that a degree of conserved degeneracy
within the listed motif exists at two residues. Within the
11-residue motif, all the orthologs for OCT1, OCT2, and
OCT3 express phenylalanine (instead of tryptophan expressed in OCTNs) at residue seven (as do both of the
Caenorhabditis elegans members of the OCT family).
Also, whereas orthologs of OCT3, OCTN1, OCTN2, and
several of the anion transporters of the OCT family express threonine at residue 2 of this motif, the OCT1 and
OCT2 orthologs cloned to date all express serine (instead
of threonine). Thus the following “signature motif” for
ASF transporters presents itself: S[T/S]IVTE[W/F][D/
N]LVC.
Selected amino acids are conserved in all, or virtually
all, of the OCT family members cloned to date. These
include 4 cysteines and 10 prolines, which suggest an
importance for these residues in establishing the secondary structure of this group of proteins. A number of
charged amino acids are also conserved, and their potential role in the function of these organic electrolyte transporters is discussed in upcoming sections.
A) OCT1. I) Structure. OCT1 (SLC22A1; 2.A.1.19.1)
was cloned in 1994 by Koepsell and colleagues (142) from
a cDNA library prepared from rat kidney.1 Orthologs were
subsequently cloned from three additional mammalian
species [mouse (132), rabbit (451), and human (549)]
displaying from 95 to 78% identity with rOCT1. In addition, related sequences have also been cloned from C.
elegans (ceOCT1; 28.4% identical; Ref. 537) and Drosophila melanogaster (drOCT; 33.7% identical; Ref. 450). The
mammalian isoforms vary in length from 554 to 556 amino
acids.
By hydropathy analysis, OCT1 orthologs appear to
have 11 or 12 TMDs. It is generally acknowledged that 12
TMDs are likely, in accord with the predicted secondary
structure profiles of most MFS members (334), and the
recent high-resolution analyses of the structures of the
MFS members, LacY (lactose transporter of Escherichia
coli; Ref. 1) and GlpT (glycerol-3-phosphate transporter of
E. coli; Ref. 174). The secondary structure predicted from
a 12 TMD configuration includes intracellular NH2 and
COOH termini, a very long extracellular loop between
TMD1 and TMD2 (which typically includes three to four
N-linked glycosylation sites), and a comparatively long
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FIG. 3. A: postulated secondary structure of the human ortholog of OCT1. Features common to other members of
the OCT transporter family include 12 transmembrane spanning domains (TMDs), a large extracellular loop between
TMDs 1 and 2, and a large cytoplasmic loop between TMDs 6 and 7. Indicated in black are the conserved MFS and ASF
motifs. (Secondary structure determined using TMpred [http://www.ch.embnet.org/software/TMPRED_form.html]; layout prepared using TOPO2 [http://www.sacs.ucsf.edu/TOPO/].) B: unrooted phylogenetic tree of the OCT family. The
scale bar indicates nucleotide substitutions per site (prepared using TreeView 1.6.6).
Northern (129, 549); rabbit, RT-PCR (451)]; however, expression in the kidney shows more variability among
species. In rats, OCT1 is heavily expressed in the kidney
(cortex) (402), with substantial expression in the liver
(142). It is also well expressed in the colon, small intestine
(142), skin, and spleen (402). In rat liver, immunocytochemistry shows that OCT1 expression is restricted to the
basolateral (sinusoidal) aspect of hepatocytes (276). This
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distribution was confirmed in Western blots, which
showed a strong 67-kDa band in isolated sinusoidal membranes and a weak reaction (perhaps due to sinusoidal
contamination) with isolated biliary membranes (276).
Western blots of isolated rat renal membrane vesicles also
indicated that OCT1 is restricted to the basolateral domain of cortical cells (489). In a thorough immunocytochemical assessment of OCT distribution in rat kidney
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RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
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ranging from 38 to 251 ␮M. This transport is sensitive to
the trans-membrane electrical potential. Increasing the
external concentration of K⫹ (which depolarizes the oocyte membrane by ⬃20 –50 mV; Refs. 142, 549) reduces
the rate of TEA uptake into Xenopus oocytes injected
with cRNA for rOCT1 (142) and hOCT1 (549). The degree
of inhibition can be small, as shown by the observation
that in oocytes injected with mOCT1 cRNA, the reduction
of TEA uptake associated with elevation of external K⫹
fails to be statistically significant (132).
Direct evidence that OCT1-mediated transport of
TEA and other type I cations results in an inward current
was obtained in studies using voltage-clamped oocytes
injected with rOCT1 (44). The relationship between inward current and the external concentration of substrate
shows saturation, with the apparent Michaelis constants
varying as a function of the holding potential. In one
study, for example, as the holding potential was changed
from ⫺90 to ⫺10 mV, the apparent Kt (Michaelis constant;
substrate concentration resulting in half-maximal transport) for TEA uptake increased from 14 to 49 ␮M (44).
Interestingly, half-saturation constants derived from saturation of inward currents are typically severalfold lower
than those measured using conventional radiotracer techniques. For example, as noted above, the apparent Kt for
TEA uptake mediated by rOCT1 in oocytes is on the order
of 100 –250 ␮M (61, 142). In addition, the rate of TEA
uptake in oocytes inferred from the inward current supported by rOCT1 is about four- to fivefold greater than the
rate of radiolabeled TEA uptake measured in parallel
experiments (44). This difference may reflect a degree of
substrate-induced ion-leak current through rOCT1 (e.g.,
Ref. 509). It may also reflect the fact that transport of
labeled substrates into oocytes (and other cells) that are
not voltage clamped can be expected to depolarize the
cells, thereby decreasing the driving forces that, in turn,
influence (in this case, decrease) the maximal rate of
transport (44).
Every OCT1 ortholog tested to date, including
mOCT1, supports the saturable electrogenic transport of
TEA and other small, type I OCs (e.g., choline, tetramethylammonium, NMN, dopamine) into oocytes (43, 44, 88,
130, 304). Initial studies of electrical currents suggested
that rOCT1 also supports electrogenic transport of several
large, mono- and divalent type II OCs, including quinine,
quinidine, d-tubocurarine, and pancuronium (44). However, subsequent experiments (304) indicated that the
electrogenic currents produced by exposure of rOCT1expressing oocytes to large type II OCs actually reflect
inhibition by these compounds of the rOCT1-mediated
electrogenic efflux of endogenous type I OCs (e.g., choline) (304). Nevertheless, additional evidence supports
the conclusion that the protonated form of quinidine, at
least, can serve as a transported substrate of rOCT1 (502).
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(195), OCT1 expression was restricted to the basolateral
membrane of proximal tubule cells, with the greatest
expression apparent in early (S1) segments of superficial,
midcortical, and juxtamedullary nephrons. rOCT1 was
also evident in proximal S2 segments, although not to the
same extent as in S1 segments. Expression in late (S3)
proximal tubule, although evident, was markedly less
than that in the early (S1) proximal tubule. At the transition between outer stripe (S3) and inner stripe of the
outer medulla, rOCT1 expression was clearly limited to
the late S3 segment (and not in the thin descending limb
of Henle’s loop). Determination of rOCT1 mRNA in renal
tubules by in situ hybridization revealed a distribution for
the mRNA qualitatively similar to that for the protein
shown by immunocytochemistry. However, the relative
abundance of the mRNA in the different tubule segments
differed from that of the protein. Whereas the highest
protein content apparent was in the S1 segments, the
highest concentration of mRNA appeared to be in the S2
and S3 segments (195). No rOCT1 expression (protein or
mRNA) was evident in cells of glomeruli or distal tubules
(see also Ref. 488).
Tissue distribution of rOCT1 during embryonic development appears to differ from that in adults. During
early development (up to day E16), expression of rOCT1
mRNA is restricted to liver and brain (335). After E16,
rOCT1 mRNA is expressed in the kidney, rising through
birth (335) and thereafter (402). The increase in rat OCT1
mRNA from birth through adulthood (which is paralleled
by increases in mRNA for OCT2 and OCT3; Ref. 402)
coincides with increases in TEA uptake into mouse renal
slices through this developmental period (93).
In rabbits, OCT1 is clearly expressed in the kidney
(and small intestine), but it appears to be more strongly
expressed in the liver (451). In humans, OCT1 expression
appears primarily in the liver (129, 549). In one study
(300), immunocytochemistry, Western analysis, and quantitative RT-PCR failed to demonstrate the expression of
OCT1 in human renal tissue. However, the demonstration
in some other studies [by Northern analysis (549) and by
RT-PCR (129)] of modest expression of OCT1 in human
renal tissue has led to the suggestion that this isoform
may play a “housekeeping role” with respect to OC
transport in human tissues other than the liver, where it
may play a predominant role in secretion of type I OCs
(129, 218).
III) Functional characteristics. OCT1-mediated
transport activity has been functionally expressed in several heterologous expressions systems: Xenopus oocytes
(43, 44, 61, 88, 129, 130, 132, 142, 304, 307, 451, 502, 548,
549), HEK-293 cells (32, 43, 130, 256, 270, 276), HeLa cells
(550 –552), Madin-Darby canine kidney (MDCK) cells
(488, 489), Chinese hamster ovary (CHO) (191), and
COS-7 cells (553). All the orthologs tested support saturable transport of TEA, with apparent Michaelis constants
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from these cells is not influenced at all over this same
range of extracellular pH. These data indicate that mediated exchange of TEA for H⫹ is an unlikely operational
mode for rOCT1 (489).
The other cloned orthologs of OCT1 show a varied,
and variable, response to changes in extracellular pH. At
one extreme, the comparatively distantly related OCT1
ortholog from C. elegans, when expressed in HRPE cells,
shows an almost complete elimination of mediated TEA
transport as pH is decreased from 8.5 to 5.5 (537). Mouse
OCT1 expressed in Xenopus oocytes shows a more modest (⬃40%) decrease in TEA uptake over the range of 8.5
to 6.5 (132), and human OCT1, also expressed in oocytes,
shows little (549) or no (129) response to extracellular
pH. The failure of extracellular pH or transmembrane pH
gradients to systematically influence OCT1 activity, combined with the previously discussed electrogenecity of
OCT1-mediated OC transport, supports the conclusion
that OCT1 operates as an electrogenic uniporter that can
also support electroneutral OC/OC exchange.
IV) Substrate structural specificity. The issue of substrate selectivity of OCT1 has been examined in three
ways: 1) directly, through measurement of transmembrane flux of labeled compounds (or transport-induced
current); 2) indirectly, either through determination of
the extent of inhibition of transport of a model substrate
(e.g., TEA) produced by coexposure to a test agent, or as
noted above, by gauging the stimulatory (or inhibitory)
effect on transport of the model substrate following imposition of a trans-gradient of the test agent; and 3) by
means of introducing mutations into the transporter sequence to gauge the influence of physicochemical alterations in protein structure on interaction with transported
substrates. The latter approach, which has provided valuable insights into the mechanisms of transport protein
activity (190), has been used sparingly, to date, in studies
of OCT activity. Gorboulev et al. (130), reasoning that the
binding of cationic substrates likely involves interaction
with anionic residues on the surface(s) of OCTs, considered the effect on rOCT1 of mutating each of the six
acidic residues found in all orthologs of OCT1, OCT2, and
OCT3 (but not in the OCTNs or in the OATs). They
observed a 15-fold decrease in the Kt for MPP uptake into
oocytes injected with mRNA coding for a mutant rOCT1
in which Asp-475 was converted to Glu-475. In addition,
the IC50 values for inhibition of TEA uptake into oocytes
expressing the mutant transporter, measured for several
n-tetraalkylammonium (n-TAA) compounds, all decreased, but the ratio of the IC50 measured for the wildtype versus mutant transporter increased with alkyl chain
length. They concluded that rOCT1 contains a large cation-binding pocket with several interaction domains that
may be responsible for high-affinity binding of structurally different cations and that Asp-475 is located close to
one of these domains. A study by Chen et al. (60) employ-
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In addition to operating as an electrogenic uniporter,
OCT1 can also mediate OC/OC exchange (44, 88, 304,
550 –552). Preloading Xenopus oocytes with unlabeled
TEA, for example, stimulates the uptake of [3H]MPP by
human, rabbit, mouse, and rat OCT1 (88). The symmetry
of this type of trans-effect is apparent in observations of
accelerated efflux of preloaded [3H]MPP from rOCT1expressing oocytes in the presence of inwardly directed
gradients of unlabeled TEA (44) or MPP (304). Human
OCT1 also supports trans-stimulation of both influx and
efflux (of TEA), but quantitative differences in the extent
of these stimulated fluxes produced by some substrates
(e.g., TBuMA) have led to the suggestion of asymmetrical
binding properties on the extracellular versus intracellular face of the transporter (550).
Trans-inhibition of OCT1-mediated transport has
also been noted (44, 88, 304, 550 –552), usually when cells
are exposed to compounds that are also observed to be
high-affinity cis-inhibitors of OCT1-mediated activity. For
example, although quinine and quinidine are (at best)
transported poorly by rOCT1 (304, 502), these substrates
cis-inhibit MPP uptake and trans-inhibit MPP efflux in
Xenopus oocytes (304). Similar results are seen with the
type II OCs d-tubocuarine and cyanine 863 (304), suggesting that when loaded with bulky, hydrophobic substrates,
OCT1 undergoes the conformational changes associated
with translocation much more slowly (if at all) than the
unloaded transporter. At least two alternative explanations can be offered. First, following the conformational
change(s) associated with translocation, hydrophobic
substrates may dissociate slowly from the binding site,
thereby reducing the “physiological turnover” of the substrate-transporter complex (i.e., the sum total of events
associated with net substrate transport: binding of substrate to the transporter at the cis-aspect of the membrane, “translocation” of the substrate-transporter complex, dissociation of substrate from the transporter at the
trans-face of the membrane, and a second translocation
event that returns the transporter to a cis-facing conformation). Second, hydrophobic cations may diffuse across
the membrane and exert what amounts to a cis-inhibition
of efflux at the cytoplasmic face of the transport protein
(also see Ref. 34).
Because of the importance of proton gradients in
energizing the active flux of OCs across the luminal membrane of proximal tubules, via OC/H⫹ exchange, considerable attention has been paid to the effect of pH on the
activity of all the cloned OC transporters to determine if
one of these could function as such an exchanger. When
rOCT1 is expressed in Xenopus oocytes, TEA uptake is
unchanged over an external pH range of 6.5– 8.5 (142).
Moreover, when rOCT1 is stably expressed in MDCK
cells, TEA uptake across the basolateral membrane
shows a fivefold increase as external pH is increased from
5.4 to 8.4 (489), whereas rOCT1-mediated efflux of TEA
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transport in intact rat (468) and rabbit (135) proximal
tubules and with OC/H⫹ exchange in rabbit renal BBMV
(536), at least for structurally related compounds (e.g.,
Ref. 534).
Three-dimensional variations in substrate structure
produce steric constraints on substrate binding, although
for OCT-mediated transport there has been little attention
paid to this issue. In a study employing a computational
approach to the development of a putative pharmacophore of substrate binding to hOCT1, Bednarczyk et al.
(15) observed a systematic effect of planar hydrophobic
mass on binding to the receptor: 4-phenylpyridinium compounds were more effective inhibitors of TEA transport
than were 3-phenylpyridinium compounds, which were,
in turn, more effective than quinolinium compounds. The
deduced pharmacophore consisted of a single cationic
recognition site and three hydrophobic features. Inhibitors that interacted most effectively with hOCT1 contained all these features, and weaker inhibitors generally
displayed fewer of these sterically defined features. It was
telling, however, that despite a strong correlation between predicted and measured IC50 values (r ⫽ 0.86), a
number of outliers were noted, as in the IC50 values for a
“test set” of eight compounds. Algorithms used to develop
a pharmacophore typically seek a single, best-fit structure
for interaction with a receptor that is assumed to possess
a marked degree of structural specificity. However, OC
transporters do not display the narrow specificity of the
typical receptor, which generally accepts a “best” structure to the exclusion of most others. OC transporters, in
contrast, apparently because of the protective role they
play, must accept a broad array of substrate structures,
including compounds to which the host organism may
never have been exposed (e.g., dietary toxins or synthetic
drugs). Consequently, one might predict a selective advantage arising from a transport process that can interact
effectively with a diverse array of environmental chemicals, thus making it desirable for OC transporters to accept chemical structures that fit a generalized format,
rather than one represented by a classical pharmacophore. QSAR analysis, which depends less on steric elements and more on physicochemical properties of the
members of the training set, resulted in a model of the
basis of substrate binding to hOCT1 (r ⫽ 0.95) that more
adequately described the members of the test set (15).
Before proceeding further with our discussion of
substrate specificity, however, we need to highlight the
following issue. There is sufficient variability in reported
values for the kinetic interaction of substrates and inhibitors of OCT1 (and other cloned transporters) to render
suspect most conclusions concerning the quantitative basis of substrate-transporter interaction. There are at least
three sources of this variability in results that may obscure the true structural selectivity of OCT1-mediated
transport. These include differences in 1) species, 2) ex-
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ing chimeras of rOCT1 and rOCT2 implicated TMDs 2–7
as the locus of differences in the interaction of these
homologs with selected nucleosides. Future studies assessing the influence of other regions of and selected
residues in the sequence of OCT1 (and the other members
of the family of OCT transporters) can be expected to add
to the understanding of the molecular basis of substratetransporter interaction.
The results of studies employing the first two approaches outlined above support the general conclusion,
as discussed below, that OCT1 is a polyspecific carrier
that transports a diverse array of cationic compounds.
However, detailed conclusions of the type required to
advance our understanding of the role this process plays
in, for example, renal excretion of selected cationic drugs,
are frequently complicated by confusion concerning the
extent to which results have been influenced by choice of
“model system,” including both the specific ortholog (e.g.,
human versus rat) and the heterologous cell system in
which the cloned transporter is expressed (e.g., Xenopus
oocyte versus cultured mammalian cell).
Measurements of substrate transport, using either
radiochemical or electrical methods, show that OCT1 accepts a broad array of type I organic cations, i.e., monovalent OCs with a molecular weight typically less than
400. This includes, in addition to what have become prototypical substrates for renal OC transporters, i.e., TEA,
MPP, choline, and NMN, a variety of monoamines (e.g.,
dopamine, serotonin, and epinephrine) and nucleosides
[e.g., 2-deoxytubercidin, cytosine arabinoside, and azidothymidine (AZT)]. Although there are clear similarities in
the structural specificity displayed by the several OCT1
orthologs tested to date, there are also significant species
differences in substrate selectivity. For example, electrophysiological measurements show that rat, mouse, human, and rabbit OCT1 orthologs all support mediated
uptake of the n-TAA compounds, TMA and TEA (88).
However, whereas hOCT1 and rbOCT1 also support uptake of tetrapropylammonium (TPrA) and tetrabutylammonium (TBuA), the two rodent orthologs do not (88).
The selectivity of OCT1-mediated transport has also
been probed in studies measuring the inhibitory effectiveness of a vast array of cationic compounds (44, 142, 256,
318, 489, 549, 553). It is, however, difficult to discern a
pattern in the molecular determinants associated with
interaction of (putative) substrates with OCT1. Nevertheless, studies of the effect of increasing alkyl chain length
on the inhibitory interaction of n-TAA ions with hOCT1
have revealed a strong correlation between increasing
chain length (increasing hydrophobicity) and decreasing
IC50 values for inhibition of TEA transport into HeLa cells
transiently expressing hOCT1 (15, 550). This observation
coincides closely with reports of a strong correlation
between increasing hydrophobicity and increasing inhibitory interaction of test agents with contraluminal OC
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STEPHEN H. WRIGHT AND WILLIAM H. DANTZLER
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pressed transport. Again, the database is small and examination of the kinetic constants for different OCT1 orthologs expressed in different systems reveals sufficient
variability that systematic influences of the expression
system used are not clear (Fig. 4). However, a pair of
studies by Inui and colleagues (318, 489) suggests that
systematic differences between expression systems may
exist, at least with respect to the activity of OCT transporters. These studies examined a battery of compounds
as inhibitors of the rat orthologs of OCT1 and OCT2 when
transiently expressed in Xenopus oocytes (318) or stably
expressed in MDCK cells (489). For every test compound,
the apparent Ki (for inhibition of TEA transport) was
lower in the mammalian cell line than in the oocytes. The
extent of the difference was not, however, “predictable.”
For example, whereas the Ki for TEA self-inhibition decreased about 3-fold when the transporter was expressed
in MDCK cells rather than oocytes, the Ki for cimetidine
inhibition of TEA uptake decreased more than 50-fold.
Indeed, whereas in oocytes the Ki for inhibition of radiolabeled TEA transport by cimetidine was greater than the
Ki for inhibition by unlabeled TEA (329 versus 129 ␮M;
Ref. 130), in MDCK cells it was much less (5.7 versus 38
␮M). Interestingly, the pattern of these differences was
quite similar for rOCT1 and rOCT2. Are such differences
noted for all substrates or for all OCT transporters? There
are not sufficient data to draw a conclusion. Neither is it
known if systematic differences exist between transporters expressed in different mammalian cell lines (e.g.,
epithelial cells versus nonpolarized cells).
FIG. 4. Graphic demonstration of the range of literature values for
kinetic parameters (Kt or IC50) for substrate interaction with OCT1.
Solid symbols represent measurements made using the Xenopus oocyte
expression system; open symbols represent measurements made using
various cultured mammalian cells as the expression system. Symbol
shapes represent different orthologs of OCT1: squares, human; circles,
rat; triangles, mouse; diamonds, rabbit. The variation is not clearly
correlated with differences in either species or expression system.
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perimental technique or methodology, and 3) expression
system. The potential influence of each of these sources
of variability must be considered.
The possible influence of species differences in both
quantitative and qualitative aspects of substrate specificity of OCT1 (and all other transporters, as well) is obvious. There is ample evidence that changes in a single
amino acid residue can exert profound differences in
transporter selectivity (e.g., Ref. 512). As noted earlier,
substitution of a glutamate for an aspartate at amino acid
475 in rOCT1 decreases the Kt for TEA transport by
15-fold (130). The fact that the orthologs for OCT1 cloned
to date generally differ by at least 15% from each other
with respect to amino acid identity (despite conservation
of many specific residues at what are likely to be key sites
for transporter function; Refs. 41, 130, 426), makes it
unlikely that any one of these proteins will be quantitatively identical to any other with respect to functional
characteristics. Dresser et al. (88) examined this directly
by comparing the kinetics of n-TAA interaction with the
human, rabbit, rat, and mouse orthologs of OCT1 (expressed in Xenopus oocytes). Significant differences in
the inhibition of OCT1-mediated MPP uptake were noted
for all the orthologs, with the greatest differences noted
between human OCT1 and the two rodent transporters;
rabbit OCT1 showed intermediate properties. In addition,
whereas both hOCT1 and rbOCT1 supported mediated
transport of TMA, TEA, TPrA and tetrabutylammonium
(TBA), the rodent orthologs transported only TMA and
TEA (although, interestingly, TPrA and TBA were more
potent inhibitors of MPP transport mediated by rOCT1
and mOCT1, than by hOCT1 or rbOCT1). Thus the kinetic
and selectivity characteristics noted in one species cannot be assumed to hold for any other species, a fact that
can have important implications in efforts to extrapolate
from animal transport models to humans.
The degree of functional variation among OCT1 orthologs (i.e., species differences) is difficult to assess
given the extent of variation in the literature regarding the
kinetic characteristics of individual orthologs. As noted
above, this variation may reflect differences in technique
between laboratories. Values for the Kt or inhibition constant (Ki) for MPP interaction with rOCT1 expressed in
oocytes ranges from ⬃3 ␮M (130) to ⬃60 ␮M (318); for
NMN, the values range from ⬃125 ␮M (130) to ⬎2 mM
(318). The basis of such variation is not clear, but it could
reflect differences in the level of expression, methods of
analysis, or other aspects related to the technique of
measuring transport. Unfortunately, there are no clear
“benchmark” values for the kinetic constants of OCT1mediated transport in oocytes (or any other expression
system); the database is not yet large enough.
The other potential source of variability that has not
received sufficient attention is the influence of the expression system on the quantitative characteristics of ex-
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
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regional distribution or functional regulation have been
suggested (486).
II) Tissue and cellular distribution and localization. Northern blots show expression of rOCT2 in the
kidney, and in neuronal tissue, but not in the liver (42,
317); hOCT2 and mOCT2 show a similar tissue distribution (129, 296). This restricted distribution was confirmed
by RT-PCR: rat OCT2 is expressed in rat kidney cortex
and medulla, but not in rat liver (489). RT-PCR of adult
mouse tissue gives a similar distribution (296). Intriguingly, RT-PCR on isolated tubule segments from rat kidney showed expression of OCT2 not only in superficial
and juxtamedullary proximal straight and convoluted tubules, but also in medullary thick ascending limbs, distal
convoluted tubules, and cortical collecting ducts (488).
OCT2 mRNA levels in male rat kidneys are approximately
four times greater than those in female rat kidneys when
determined using branched DNA signal amplification
(402). This correlates with greater TEA transport into
renal slices from male than from female rat kidneys (487).
In rats, in situ hybridization revealed that OCT2 mRNA
is expressed throughout the proximal tubules within
the cortex and outer stripe of the outer medulla, with
the heaviest hybridization in cells of the outer stripe
(143, 195).
Like OCT1 expression, OCT2 expression in the kidney appears to be restricted to the basolateral membrane
of proximal tubule cells. Immunocytochemistry showed
that rOCT2 is restricted to the basolateral membrane of
mid to late proximal tubules (S2 and S3 segments) (195,
415). Similarly, in the human kidney, OCT2 expression is
restricted to the basolateral membrane of proximal tubule
cells (300), although it is not clear if the levels of expression differ along the length of the tubule. Functional
mapping of OCT1 and OCT2 transport activity in single
isolated rabbit renal proximal tubules (RPT) showed that
TEA uptake in the S2 segment is dominated by OCT2mediated transport (553). An early report of OCT2 expression in the apical membrane of human distal tubules (129)
may have reflected complications associated with the
quality of the tissue. Another line of evidence consistent
with the basolateral localization of OCT2 in renal epithelia
arises from observations obtained with a fusion protein
construct consisting of rOCT2 plus green fluorescent protein (GFP) (423). When rOCT2/GFP was transiently transfected into polarized MDCK cells, fluorescence was localized to the basolateral membrane (423). Moreover, transient transfection of rOCT2/GFP into intact, isolated
killifish renal proximal tubules resulted in expression of
GFP fluorescence in the basal and lateral aspects of the
tubule cells (423). It is noteworthy that in the porcine
renal cell line, LLC-PK1, OCT2 appears to be expressed in
the apical membrane. This localization is based on the
comparison of the selectivity characteristics of pOCT2
expressed in oocytes with those observed for apical
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Resolution of the issues discussed above is of paramount importance for at least two reasons. First, if the
study of expressed OCT transporters is to provide data
leading to predictions of, for example, drug interactions,
then an understanding of which expression system(s) and
experimental methodology(ies) produce observations
closest to what actually occurs in humans in vivo is of
obvious relevance. Second, if data obtained on the isolated activity of cloned transporters are to be used as the
basis of quantitative hypotheses for the behavior of these
processes in native renal tubules, knowledge of which
expression system(s) provide “reliable” data is clearly
important.
B) OCT2. I) Structure. OCT2 (SLC22A2; 2.A.1.19.5)
was isolated originally from a rat renal library (317).
However, orthologs have now been cloned from human
(129), mouse (296), pig (141), and rabbit (553). The initial
rOCT2 clone had an open reading frame that coded for a
593-amino acid protein (317). The human, mouse, pig, and
rabbit orthologs, in contrast, are 555, 553, 554, and 554
amino acids in length, respectively. Gründemann et al.
(144) isolated a clone of rOCT2 with a 555-amino acid
open reading frame and suggested that the last 38 amino
acids of the original clone were erroneously included due
to difficulties associated with sequencing a poly(A) region
of the 3’-end of the cDNA. The sequences of the several
OCT2 orthologs are 68 –70% identical to that of hOCT1
and 82–92% identical to one another. Conserved motifs
within OCT2 across all species include 13 cysteine residues, 24 proline residues, 1 N-linked glycosylation site (N
72), 2 PKA consensus sites (Thr-348, Thr-489), 1 PKC
consensus site (Ser-286), 1 PKG consensus site (Thr-348),
and 3 CKII consensus sites (Ser-334, Ser-472, Thr-525).
Human kidney expresses at least one splice variant of
OCT2. Designated hOCT2-A, it is characterized by the
insertion of a 1,169-bp sequence arising from the intron
found between exons 7 and 8 of hOCT2 (486). This insertion adds an in-frame stop codon resulting in a truncated
protein of 483 amino acids that is missing the last three
putative TMDs (i.e., 10, 11, 12). Despite the absence of the
last three TMDs, hOCT2-A retains the capacity to transport TEA and cimetidine, although guanidine transport is
lost. The affinity of hOCT2-A for cationic substrates differs markedly from that of OCT2. In side-by-side experiments of expression in HEK 293 cells, the apparent Kt for
TEA was about sevenfold lower in hOCT2-A (63 versus
431 ␮M). The inhibitory profiles of the truncated and
parent transporters also suggest that, for most compounds (including cimetidine, procainamide, and nicotine), hOCT2-A has a higher affinity for substrate than
does hOCT2. However, for a few inhibitors (e.g., norepinephrine and dopamine), hOCT2 displays a higher
apparent affinity than does hOCT2-A. The physiological
role of two closely related OCT2s expressed in the
kidney is not evident, although possible differences in
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STEPHEN H. WRIGHT AND WILLIAM H. DANTZLER
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whether it faces the outward (extracellular) or inward
(cytoplasmic) face of the membrane. When rOCT2 was
expressed in Xenopus oocytes, the nontransported compounds TBuA and corticosterone blocked both inward
and outward currents generated by TEA and choline, as
measured in whole cells (inward currents) and excised
giant patches (outward currents) (507). However, the apparent affinity of the OCT2 binding site for TBuA was four
times higher when the transporter was oriented to face
the extracellular face of the membrane, whereas the affinity for corticosterone was 20-fold lower. The data suggest that the substrate-binding site of rOCT2 is like a
pocket containing overlapping binding domains for ligands, and these binding domains may undergo separate
structural changes.
OCT2, like OCT1, can support OC/OC exchange. Inwardly directed gradients of unlabeled MPP, TEA, and
amantidine, for example, accelerate efflux of labeled MPP
from hOCT2-expressing oocytes (42).
The influence of proton gradients on OCT2-mediated
transport has received considerable attention because of
early functional evidence that this transporter resides in
the apical, rather than basolateral, membrane of selected
(cultured) cells and, in addition, may act as an OC/H⫹
exchanger (141, 144). The initial studies indicating that
pOCT2 may be located in the apical membrane (mentioned above) used decynium-22 as a means to discriminate between apical and basolateral transport of OCs in
LLC-PK1 cells (141). Decynium-22 inhibits TEA uptake in
oocytes mediated by the pOCT2 ortholog cloned from
LLC-PK1 with a Ki of 5.1 nM. This Ki corresponds closely
to the measured Ki of 6.7 nM for inhibition of apical TEA
transport in confluent monolayers of LLC-PK1 (381) (basolateral TEA uptake in LLC-PK1 cells is not blocked by 30
nM decynium-22; Ref. 381), thus supporting the contention that OCT2 is expressed in the apical membrane of
these cells (141). This conclusion has been supported by
independent observations based on the selectivity profile
of OC transport in LLC-PK1 cells (91).
This functional information on localization has been
evaluated in the light of evidence that 1) LLC-PK1 cells
can support net transepithelial transport of TEA (364) and
cimetidine (16) and 2) apical membrane vesicles isolated
from LLC-PK1 cells can support mediated OC/H⫹ (176).
The apical localization of pOCT2 in LLC-PK1 cells would
be consistent with the hypothesis that OCT2 is an OC/H⫹
exchanger involved in active OC secretion in LLC-PK1
cells. This hypothesis was extended from porcine to rodent kidneys on the basis of the selectivity profiles of the
rat orthologs of OCT1, OCT2, and OCT3 (EMT) that were
interpreted as supporting an apical, rather than basolateral, location of rOCT2 (144). Weakening this last conclusion and calling into question the general interpretation of
studies relying on comparisons of substrate selectivity to
determine subcellular location of individual transport
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versus basolateral OC transport in the LLC-PK1 cells
(91, 141).
III) Functional characteristics. OCT2 has been functionally expressed in Xenopus oocytes (5, 34, 42, 61, 129,
317, 318, 423, 425), HEK-293 cells (42, 143), NIH3T3 cells
(333), MDCK cells (423, 425, 489), COS-7 cells (553), and
CHO cells (13). The characteristics of all the OCT2 orthologs studied to date are qualitatively similar to those of
OCT1. All OCT2 orthologs support mediated transport of
TEA, with Michaelis constants ranging from 20 to 393 ␮M.
Transport is electrogenic, with inwardly directed gradients of substrate resulting in the generation of inward
currents (5, 34, 129, 425). These currents are saturable,
with apparent Michaelis constants that are similar to
those measured using radiolabeled substrates. Isolated
giant patches of rOCT2-expressing Xenopus oocytes have
been used to study the electrogenic operation of this
process in some detail (34). The patch configuration permits measurements of currents that correspond to the
efflux mode of transporter operation. Small, type I substrates, including TEA and choline, produce saturable
efflux currents with Kt values for efflux (160 ␮M and 2
mM for TEA and choline, respectively) that are similar to
those measured for the mediated uptake of these compounds. This result is consistent with the assumptions
that only membrane potential and concentration gradients, i.e., the electrochemical potential, of an organic
cation serve as driving forces for rOCT2-mediated transport and that this transport shows little rectification.
Larger, type II cations, including quinine and TBuA,
typically do not produce OCT2-mediated currents (5, 34),
but they do frequently inhibit OCT2-mediated transport
activity, indicating some type of interaction with OCT2.
The nature of this interaction is, however, difficult to
predict. For example, the weak base quinine exerts a
noncompetitive inhibition of TEA uptake in Xenopus oocytes expressing rOCT2 (5), although the results of inhibition experiments performed at different pH values suggest that this effect reflects nonionic diffusion of the
uncharged substrate into the oocyte and a subsequent
competitive interaction at the cytoplasmic face of the
transporter (5). In contrast, the quaternary ammonium
cation TBuA, which is not transported by rOCT2, is a
competitive inhibitor of TEA transport by rOCT2. This
inhibition presumably reflects TEA’s interaction at the
extracellular face of the transporter. Decynium-22, cyanine-863, and tetrapentylammonium, all of which have
fixed cationic charges and are not transported by rOCT2
(5), are presumably restricted to accessing the extracellular face of the transporter. However, each exerts a
noncompetitive inhibition of transport activity, suggesting
the presence on the transporter of an allosteric site that
may interact with hydrophobic cations.
The OCT2 binding site appears to have different affinities for (at least) selected compounds depending on
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permit any specific conclusions concerning the molecular
determinants involved in the transport activity of OCT2.
Of particular importance to future studies of the
relative role of the various OCTs in the kidney is finding
one or more substrates/inhibitors that can effectively discriminate between the activities of different homologs
when expressed in native tissue (where multiple homologs may be coexpressed in the same cell). The parallel
selectivity of OCT2 and OCT1 for at least some OCs is
evident in the side-by-side comparisons, in two different
expression systems, referred to previously (191; see also
Ref. 488). It is, however, also evident that some substrates
show strikingly different affinities for these two processes. Koepsell and colleagues (5) compared the selectivity of the rat orthologs of OCT1 and OCT2 when expressed in Xenopus oocytes. Whereas several compounds
interacted very similarly with both orthologs (e.g., TEA,
MPP, NMN), other compounds interacted very differently.
Most notably, mepiperphenidol and O-methylisoprenaline
had IC50 values for inhibition of transport by rOCT1 that
were 60 times lower than the values for inhibition of
transport by OCT2. On the other hand, guanidine and
corticosterone were 25–35 times more potent in their
inhibition of rOCT2 than rOCT1. It is also noteworthy
that selected nucleotides, including 2-deoxytubercidine,
2-chlorodeoxyadenosine, and cytosine arabinoside, are
transported by rOCT1 but not by rOCT2 (60 – 62). Similarly, human OCT1 and OCT2 display markedly different
relative affinities for n-TAA compounds (90). The rabbit
orthologs of OCT1 and OCT2 also display markedly different affinities for selected substrates (553). Whereas the
affinities of these transporters for TEA is approximately
the same, the apparent affinity of OCT2 for cimetidine and
the fluorescent cationic substrate [2-(4-nitro-2,1,3-benzoxadiazol-7-yl)aminoethyl]trimethylammonium (NBD-TMA)
is 35–100 times higher than that of OCT1 (when expressed in
COS-7 cells).
Exploiting the marked differences in apparent affinity that different OCT homologs have for selected substrates can provide a means to map the functional distribution of OC transport activity in renal cells. The IC50 of
13 ␮M for cimetidine’s inhibition of peritubular TEA
transport in isolated single S2 segments of rabbit proximal tubules is close to the IC50 of 6 ␮M for cimetidine
inhibition of TEA transport by heterologously expressed
rbOCT2, but differs radically from the IC50 of ⬎500 ␮M for
such inhibition of transport by rbOCT1 similarly expressed (553). Although such comparisons require extrapolation of kinetic values obtained with heterologous expression systems to the quantitative behavior of transporters expressed in native tubules (with the attendant
caveats noted previously), these data argue that TEA
transport in the S2 segment of rabbit proximal tubule is
dominated by activity of OCT2 (see also Ref. 191). Extending the demonstration of functional distribution of
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processes is the clear demonstration by the use of immunocytochemistry and OCT/GFP constructs that rOCT2 is
restricted to the basolateral membrane of native proximal
tubule cells (see above) (195, 415).
In addition, OCT2 fails to show convincingly the
physiological characteristics of the apical OC/H⫹ exchanger: 1) changes in extracellular H⫹ concentration
exert a modest effect on OCT2-mediated transport (rat,
Ref. 317; pig, Ref. 141); 2) trans-gradients of H⫹ do not
stimulate rOCT2-mediated transport (425); and 3) OCT2mediated transport (unlike OC/H⫹ exchange, Ref. 533) is
electrogenic, as assessed by the observation of transportinduced currents in rOCT2-expressing oocytes (5, 34). In
addition, the K⫹-induced collapse of the apical membrane
potential inhibits apical metoprolol uptake in LLC-PK1
cells (91). It has been suggested that the discrepancy
between the behavior of OC/H⫹ exchange as measured in
membrane vesicles and the transport activity of OCT2 in
heterologous expression systems reflects differences in
the experimental procedures involved in measurement of
transport (e.g., different buffer systems; Ref. 144). However, inwardly directed H⫹ gradients do stimulate TEA
efflux and outwardly directed H⫹ gradients do stimulate
TEA uptake across the apical membrane of LLC-PK1 cells
(364, 430). Nevertheless, all the data taken together suggest that, although OCT2 may be expressed in the apical
membrane of some renal cells (or some cultured cells of
renal origin), it is unlikely to work as a secondarily active
secretory process and, instead, in those cells may be
limited to mediating trans-apical fluxes in response to the
prevailing electrochemical gradient of the substrate in
question.
III) Substrate structural specificity. OCT2 is a
polyspecific carrier with selectivity characteristics that
are very similar to those observed for OCT1. All OCT2
orthologs tested to date (rat, human, rabbit, and pig)
support the mediated transport of TEA, with Michaelis
constants ranging from 20 to 400 ␮M, with an average
value of ⬃150 ␮M. The observation that, in addition to
expression in the kidney, hOCT2 is expressed in neuronal
tissues (42) has led to the study of the interaction of
monoamine transmitters with this process. The human
ortholog of OCT2 transports (in order of increasing Kt)
serotonin (80 ␮M), dopamine (390 ␮M), histamine (1.3
mM), and norepinephrine (1.9 mM) (42). Since these comparatively low affinity substrates are all relatively small,
polar molecules, these findings are consistent with the
general view that hydrophobicity is an important criterion
in the binding of substrates and inhibitors to OCT transporters (e.g., Ref. 465). However, as in the case of the
substrate selectivity of OCT1-mediated transport, the database is too small and the variability in apparent binding
and transport constants that exists within and between
different orthologs and expression systems is too large to
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tubules in cortex, and little expression is evident in medullary tissues or in glomeruli (539).
III) Functional characteristics. OCT3 has been functionally expressed in Xenopus oocytes (199, 539, 541),
HeLa cells (199, 541), HRPE cells (539, 541), and ARPE-19
cells (349). The qualitative characteristics of OCT3-mediated transport are similar to those for its homologs, OCT1
and OCT2: rOCT3 supports saturable transport of TEA
and MPP (199) and transport is electrogenic. rOCT3-mediated transport activity shows sensitivity to external pH
similar to that seen in selected studies of OCT1 and OCT2
(199). However, in oocytes expressing rOCT3, clamping
the membrane potential to prevent it from changing with
changes in the H⫹-gradient completely eliminates the effect of external pH on OCT3-mediated transport (pH 6.5–
8.5), thereby supporting the contention that OCT3 is a
potential-driven electrogenic uniporter (199).
IV) Substrate structural specificity. In addition to
transporting TEA and MPP, rOCT3 also transports guanidine (199) and dopamine (541). Although a polyspecific
OC transporter like OCT1 and OCT2, OCT3 displays substantial quantitative differences from them with respect to
substrate specificity. OCT3 has a much lower apparent
affinity for TEA (Kt values of 1– 6 mM, Refs. 199, 539) than
either OCT1 or OCT2 (Kt values of 20 – 400 ␮M). The
profile of substrate specificity matches closely that displayed physiologically by the extraneuronal monoamine
transporter, suggesting that OCT3 is the molecular structure involved in this latter process (541). Side-by-side
comparisons of rOCT2 and rOCT3 show that the IC50
values for inhibition of TEA uptake by dopamine and
norepinephrine (in transfected HRPE cells) are 6- and
25-fold higher in OCT2 than in OCT3, respectively (541).
Also, ␤-estradiol blocks rOCT3 activity with an IC50 of 1.1
␮M, compared with 85 ␮M for OCT2 (541). These observations, along with the differences between selectivities
of OCT1 and OCT2, offer potential strategies for defining
the activity and distribution of the several OCTs expressed in native tissues (5).
D) OCTN1. I) Structure. OCTN1 (SLC22A4; 2.A.1.19.2)
was first isolated, using PCR, from a fetal human liver
cDNA library (445). Rat (538) and mouse (443) orthologs
have also been cloned. OCTN1 is a 551- to 553-amino acid
peptide and shares with the other OCT family members a
predicted secondary structure that includes 12 putative
TMDs. The three orthologs show 30 –31% sequence identity to hOCT1. Conserved motifs within OCTN1 across all
species include the following (426): 7 cysteine residues,
24 proline residues, 3 N-linked glycosylation sites (N 57,
64, 91), 1 PKA consensus site (Ser-472), 4 PKC consensus
sites (Ser-164, Ser-225, Ser-280, and Ser-286), and 1 CKII
consensus site (Thr-514).
II) Tissue and cellular distribution and localization. OCTN1 is widely expressed in human tissues. Northern analysis shows the most marked expression in fetal
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transporter activity to different regions of the nephron
remains a critical aspect of integrating molecular information into the context of cellular and organ-level physiology.
The recent discovery of altered transport function of
hOCT1 and hOCT2 containing single nucleotide polymorphisms present in different ethnic populations (237, 238,
393) has underscored the importance of understanding
structure-activity relationships for these processes. For
example, 28 variable sites in the hOCT2 gene were discovered in a collection of 247 ethnically diverse DNA
samples (Caucasian, African-American, Asian-American,
Mexican-American, and Pacific Islander). Eight of these
polymorphisms caused nonsynonymous amino acid
changes, of which four were present in at least 1% of an
ethnic population. These four displayed altered transporter function as seen by a threefold change in Kt values
for MPP and TBA, changes that could result in differences
in the pharmacokinetics of renal drug excretion between
individuals expressing different variants of hOCT2. However, population-genetic analysis suggests that selection
has acted against amino acid changes to hOCT2, which
may reflect a necessary role of OCT2 in the renal elimination of endogenous amines or xenobiotics (238).
C) OCT3. I) Structure. OCT3 (SLC22A3; 2.A.1.19.6)
was initially cloned from a rat placental cDNA library
(199). However, orthologs have now been cloned from
mouse (521) and human [originally referred to as the
extraneuronal monoamine transporter (EMT) (145)].
OCT3 is a 551-amino acid peptide with 12 putative TMDs.
The three orthologs of OCT3 display 47– 48% sequence
identity with hOCT1 and 48 – 49% sequence identity with
hOCT2. Conserved motifs within OCT3 across all species
include the following (426): 15 cysteine residues, 25 proline residues, 3 N-linked glycosylation sites (N 72, 99,
119), 2 PKA consensus sites (Thr-351, Thr-437), 2 PKC
consensus sites (Ser-291, Thr-297), and 3 CKII consensus
sites (Thr-327, Ser-337, Thr-528).
II) Tissue and cellular distribution and localization. Northern analysis indicates that rOCT3 is expressed
most heavily in the placenta and intestine, but it is also
found in the kidney, heart, brain, and lung (199). Using a
branched DNA signal amplification assay, Slitt et al. (402)
found a very broad distribution of OCT3 mRNA in rat
tissues, with highest expression in blood vessels, skin,
and thymus; intermediate expression in kidney, lung, intestine, spleen, and muscle; and low expression in liver,
bladder, and prostate. In mice, OCT3 is also expressed in
retinal pigment epithelium (349). In humans, Northern
analysis shows OCT3 to be expressed strongly in the liver,
placenta, kidney, and skeletal muscle (539), although by
quantitative RT-PCR, OCT3 expression in human kidney
appears to be ⬍10% that of OCT2 (with OCT1 expression
virtually undetectable; Ref. 300). Within the mouse kidney, OCT3 expression is confined to proximal and distal
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changer mode of activity for OCTN1, it is not clear that
this cloned transporter is the process that dominates
transport activity as measured in isolated renal luminal
membranes. OC/H⫹ exchange activity as functionally expressed in renal cells, or more specifically, mediated exchange of TEA for H⫹, appears to be restricted to the
kidney and the liver (299); it is not found in the placenta
or intestine, although both of these organs do express a
transporter that mediates exchange of guanidine for H⫹
(121, 289). OCTN1, however, is expressed in many tissues,
including the placenta and intestine (538). The comparatively low level of expression of OCTN1 in human kidney
(300) also appears to be inconsistent with the observation
that OC/H⫹ exchange is the dominant mechanism for OC
(i.e., TEA and NMN) flux across isolated human renal
BBMV (322). In addition, the kinetic/selectivity characteristics of OCTN1 (discussed below) also appear to be
inconsistent with this process playing a major role in
luminal OC transport. There are, however, too few data
on the function of OCTN1 to draw a firm conclusion about
its role in renal cells.
IV) Substrate structural specificity. OCTN1 is a
polyspecific OC transporter, as shown by its inhibition by
a diverse set of cationic substrates (538, 543). Comparison
of the specificity of hOCTN1 with the selectivity of OC/H⫹
exchange as expressed in native renal cells should provide insight into the role OCTN1 may play at the luminal
membrane. Although there are few data upon which to
make such a comparison, they render suspect the conclusion that OCTN1 serves as the principal route for OC/H⫹
exchange at the luminal membrane. Whereas the transport activity of the human OCTN1 ortholog, for example,
shows no inhibition when exposed to 5 mM NMN (543),
BBMV isolated from human kidney transport NMN with
an apparent Kt of 0.44 mM (322). Also, although the rat
ortholog of OCTN1 shows a very weak interaction with
MPP (⬍40% inhibition of TEA uptake produced by exposure to 5 mM MPP, Ref. 538), MPP transport across the
luminal membrane of microperfused rat proximal tubules
in vivo has an apparent Michaelis constant of 120 ␮M (83).
It should be emphasized that the two methods that were
used to examine transport properties of native cells typically reveal properties of outer cortical proximal tubule
segments; the transport properties of the pars recta are
typically not evident in studies with isolated cortical
membrane vesicles (unless material from the outer medullary stripe is specifically included) or with rat tubules
microperfused in vivo. Therefore, in view of the evidence
for the differential distribution of renal organic cation
(295) and organic anion transporters along the renal tubules, OCTN1 could be expressed in the luminal membrane of late proximal tubule segments and have a selectivity that differs from that of an OC/H⫹ exchanger found
in early proximal segments.
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liver, kidney, and lung and in adult kidney, trachea, and
bone marrow, with weaker signals evident in adult skeletal muscle, prostate, lung, pancreas, placenta, heart,
uterus, spleen, and spinal cord (445); OCTN1 is not evident in adult human liver by Northern analysis. In one
study, quantitative mRNA analysis of expression in human kidney cortex reported barely detectable levels of
OCTN1 (a few percent of that measured for OCT2, and
much less than that measured for OCTN2) (300). The rat
shows a slightly different distribution. Quantitative mRNA
analysis indicated that the highest expression of rOCTN1
is in the kidney, with comparatively high levels also in
intestine, brain, lung, heart, and skin (402; see also Ref.
538). Western analysis of the distribution of mOCTN1
indicated that the greatest expression is in the kidney,
followed by the heart and liver (538). Within the kidney, in
situ hybridization revealed rOCTN1 expression in cortical
and medullary tissues, with expression most abundant in
the outer stripe of the medulla. Expression occurs in
glomeruli and in proximal and distal tubules, being most
evident in the straight portion of the proximal tubules.
There is currently no direct information concerning the
subcellular distribution of OCTN1. There is, however,
reason to postulate a luminal localization of this process.
OCTN1 and OCTN2 have a very high degree of homology
(76% identity between hOCTN1 and hOCTN2, for example), and OCTN2 definitely appears to be expressed in the
luminal membrane of proximal tubule cells (540). Additionally, OCTN1 and OCTN2 exist within the human genome as a tightly linked gene pair (100). Therefore, it is
reasonable to propose that OCTN1 is also expressed at
the apical pole of renal cells.
III) Functional characteristics. OCTN1 has been
functionally expressed in Xenopus oocytes (543), HEK
293 cells (445, 543), and HRPE cells (538). All orthologs
tested support saturable TEA transport with Michaelis
constants ranging from ⬃200 to 400 ␮M for the human
and mouse orthologs (445, 543) to ⬃1 mM for the rat
ortholog (538). As with many of the homologous OCTs,
increases in the extracellular H⫹ concentration inhibit
OCTN1-mediated transport (445, 538, 543). However,
whereas in the other homologs these effects do not appear to reflect a coupled interaction between OC and H⫹
(i.e., via mediated OC/H⫹ exchange), the current evidence, albeit limited, supports the contention that OCTN1
can function as an OC/H⫹ antiporter. First, unlike the
other homologs, OCTN1-mediated transport does not appear to be electrogenic; changes in membrane potential
brought about by raising extracellular K⫹ concentrations
have no effect on the rate of hOCTN1-mediated TEA
uptake (543). Second, efflux of TEA from hOCTN1-expressing HEK 293 cells is stimulated by inwardly directed
H⫹ gradients (543). Both observations suggest that
OCTN1 operates as an electroneutral OC/H⫹ exchanger.
However, despite the evidence supporting an OC/H⫹ ex-
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(313, 387, 441, 510, 540). Nevertheless, the maximal rate of
uptake of each substrate (measured in the presence of
Na⫹) is similar. For example, in rOCTN2-expressing
HRPE cells, the maximal rate of TEA uptake is 1.5 nmol ·
106 cells⫺1 · 30 min⫺1, whereas the maximal rate of carnitine uptake is 1.1 nmol · 106 cells⫺1 · 30 min⫺1 (540).
Chimeric transporters made from different regions of human and rat OCTN2 cDNAs revealed that the primary
sequence sites responsible for TEA and carnitine transport are spatially separated. A chimera that includes the
5⬘-half of hOCTN2 and the 3-half of rOCTN2 displays
“humanlike” Na⫹-dependent carnitine transport and “ratlike” TEA transport. A reverse chimera showed ratlike
carnitine transport and humanlike TEA transport (387).
Nevertheless, the binding of TEA to the rat and human
orthologs of OCTN2 competitively inhibits Na⫹-dependent carnitine transport (387). Thus, whereas the binding
sites may be physically distinct, binding to one site appears to preclude binding to the second site.
Analysis of mutations in human patients reveals that
primary carnitine deficiency can arise from several qualitatively different types of OCTN2 failure. Expression in
HRPE cells of cDNAs containing the naturally occurring
truncation mutation (Trp132Stop) or the missense mutation P478L fails to result in measurable rates of carnitine
transport (447). In contrast, the natural mutation E452K
increases the concentration of Na⫹ required to half-maximally stimulate carnitine transport (KNa) into CHO cells
from the physiological value of ⬃12 to 187 mM (513).
Substitution of glutamine, aspartate, or alanine for Glu452
causes intermediate increases in the KNa for carnitine
transport.
Both the Na⫹-dependent and Na⫹-independent
modes of OCTN2 operation result in inwardly directed
currents. For the Na⫹-dependent transport of carnitine,
which is a zwitterion and carries no net charge at physiological pH, the inward current presumably reflects cotransport of carnitine with cationic Na⫹ (510). For the
Na⫹-independent transport of TEA, this current presumably reflects the electrogenic uniport of this cationic substrate (510). Both modes of OCTN2 operation also display
a modest sensitivity to extracellular pH, with higher rates
of transport of TEA (Na⫹ independent, Ref. 542) and
carnitine (Na⫹ dependent, Refs. 313, 510) observed at
alkaline pH.
IV) Substrate structural specificity. When operating
as a Na⫹-independent OC uniporter, OCTN2 displays a
polyspecific selectivity that is the usual hallmark of OCT
transporters. For example, rOCTN2-mediated transport of
TEA is inhibited by ⬎60% by 5 mM concentrations of
unlabeled TEA, cimetidine, procainamide, dimethylamiloride, clonidine, tetrahexylammonium, desipramine, and
MPTP (540). Poor inhibitors of rOCTN2-mediated TEA
transport (⬍30% inhibition by 5 mM concentrations)
include guanidine, TMA, amphetamine, methamphet-
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E) OCTN2. I) Structure. OCTN2 (SLC22A5; 2.A.1.19.3)
was first cloned from a cDNA library prepared from human choriocarcinoma placental cells (JAR cells) (542).
Subsequently, orthologs from rat and mouse (540) have
been described. These cDNAs code for a protein of 557
amino acids with 12 putative TMDs. The three orthologs
share 31–33% sequence identity with hOCT1 and ⬃85%
sequence identity with each other. Conserved motifs
within OCTN1 across all species include the following
(426): 7 cysteine residues, 24 proline residues, 3 Nlinked glycosylation sites (N 57, 64, 91), 3 PKA consensus sites (Thr-344, Ser-402, Ser-474), 3 PKC consensus
sites (Ser-164, Ser-225, Ser-280), and 1 CKII consensus
site (Thr-311).
II) Distribution and localization. By Northern blot
analysis hOCTN2 appears to be most heavily expressed in
kidney, heart, placenta, skeletal muscle, and pancreas;
expression is also evident, although much weaker, in
brain, lung, and liver (542). hOCTN2 is also expressed in
several human cell lines, including HeLa, Caco-2, JAR,
BeWo, MCF-7, and HKPT (542). Within the rat kidney, in
situ hybridization revealed expression of OCTN2 predominantly in the cortex with very little expression in the
medulla (540). Within the renal cortex, rOCTN2 appears
to be expressed in both proximal and distal tubules and in
glomeruli (540). To date, there is no direct evidence (e.g.,
immunocytochemical data) concerning the subcellular localization of OCTN2. However, as discussed below,
OCTN2, in addition to being an organic cation transporter,
is a Na⫹-dependent carnitine transporter, and lesions in
OCTN2 have been linked to systemic carnitine deficiency
(SCD) that is associated with failure of tubular reabsorption of carnitine (447). This observation strongly supports
the conclusion that OCTN2 is expressed in the apical
membrane of renal tubule cells. In addition, the tissue-toplasma TEA ratio in kidney tissue from OCTN2-deficient
mice (jvs mice) is increased more than twofold compared
with wild-type, supporting the probable brush-border location of OCTN2 (312).
III) Functional characteristics of OCTN2. OCTN2
has been functionally expressed in HeLa cells (120, 387,
542), HEK 293 cells (313, 441, 443, 444), HRPE cells (540),
and CHO cells (513). In addition to transporting TEA
(with an apparent Michaelis constant of 60 –200 ␮M; Refs.
443, 540), OCTN2 also supports Na⫹-dependent transport
of the zwitterion carnitine (Kt ⫽ 4 –20 ␮M; Refs. 120, 313,
443, 513, 540) and various acylcarnitine derivatives (313).
Indeed, OCTN2 is unique in that no other transporter has
been shown to transport some substrates in a Na⫹-dependent manner and other substrates in a Na⫹-independent
manner (387). Whereas OCTN2-mediated uptake of TEA
is completely independent of the presence of Na⫹ in the
extracellular medium (540), uptake of carnitine (and
closely related structural analogs) is almost completely
eliminated by removal of Na⫹ from the external medium
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carnitine, with the relative rate of carnitine-to-TEA transport being on the order of 750:1 (443). Interestingly,
mOCTN3-mediated carnitine transport is completely independent of the presence of Na⫹ in the medium and is
scarcely affected by changes in the extracellular pH (443).
IV) Substrate structural specificity. mOCTN3 has a
comparatively low affinity for TEA (indeed the rate of
TEA transport is sufficiently low that accurate measurement of the Kt for TEA has proven to be difficult, but it
appears to be on the order of 500 ␮M; Ref. 443). However,
the affinity of mOCTN3 for carnitine and acyl-analogs of
carnitine is higher than that displayed by mOCTN2. For
example, whereas 5 ␮M concentrations of L-carnitine,
acetyl-L-carnitine, and butyryl-L-carnitine inhibit OCTN3mediated uptake of L-[3H]carnitine by ⬎⬎50%, this concentration of these substrates reduces OCTN2-mediated
carnitine transport by ⬍50% (measured in the presence of
Na⫹) (443).
2. MDR family of OC transporters
MDR1 (also called the P-glycoprotein, or P-gp;
ABCB1; 3.A.1.201.1) was first characterized within the
context of its role in the development of cross-resistance
of cancer cells to a structurally diverse range of chemotherapeutic agents. The normal expression of MDR1 in
barrier epithelia, including the intestine, liver, and kidney,
supports the conclusion that it plays a role in limiting
absorption (in the intestine) and facilitating excretion (by
the liver and kidney) of xenobiotic compounds. The following is a brief overview of the molecular, cellular, and
physiological characteristics of MDR1 and the current
understanding of the role it plays in the renal transport of
organic electrolytes. The reader is directed to recent reviews that consider the molecular biology and physiology
of MDR1 (P-gp) in more depth (2, 23, 154).
A) MOLECULAR ASPECTS OF MDR1. MDR1 is a member of
the ATP-binding cassette (ABC) superfamily of transport
proteins (3.A.1.201; Ref. 362). The human ortholog of
MDR1 is a protein of 1,279 amino acids (141 kDa) and is
composed of two homologous halves, each containing six
TMDs and an ATP-binding domain, separated by a “linker”
polypeptide.
The molecular mechanism of MDR1-mediated transport is not clear. The purified protein displays both basal
and drug/substrate-stimulated ATPase activity. In general,
most compounds transported by MDR1 stimulate ATPase
activity of the purified protein. Although MDR1 has been
purified and functionally reconstituted into lipid bilayers
(e.g., from human and hamster; see Ref. 388), efforts to
develop kinetic models describing the interaction of substrates with MDR1 have often come to contradictory conclusions. Nevertheless (as reviewed in Ref. 388), MDR1mediated transport 1) clearly appears to be coupled to
ATP hydrolysis, 2) involves conformational changes of
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amine, dopamine, serotonin, norepinephrine, and thiamine (540). A similar pattern of OC interaction is seen
for hOCTN2 (542).
The transport of OCs such as TEA is inhibited by
carnitine and selected other zwitterions, but such interaction is effectively dependent on the presence of Na⫹ in
the medium. For example, whereas the IC50 for unlabeled
TEA’s inhibition of [14C]TEA transport by rOCTN2 is the
same in the presence or absence of Na⫹ (106 versus 107
␮M; Ref. 540), the IC50 for carnitine’s inhibition of TEA
transport is reduced 50-fold by the presence of Na⫹ (15.5
versus 787 ␮M; Ref. 540).
The interaction of selected ␤-lactam antibiotics with
OCTN2 may provide some insight into the basis for the
interaction of zwitterionic substrates with this unique
transporter. Cephaloridine, cefoselis, and cefepime all
block carnitine transport by the human and rat orthologs
of OCTN2 (120). However, the interaction of cephaloridine is Na⫹ dependent, whereas the interaction of cefoselis and cefepime is Na⫹ independent. Similarly, the inhibition of TEA transport by cephaloridine is Na⫹ dependent, whereas inhibition of TEA transport by the cefoselis
and cefepime is Na⫹ independent. The structure of cefoselis and cefipime includes an amino group and a modified
imino group as substituents in the cepham nucleus, and it
has been suggested that these groups may occupy the
Na⫹-binding site, occupancy of which is typically required
for transport of zwitterions. Consequently, the requirement for Na⫹ may be eliminated for the interaction of
these selected zwitterions with OCTN2 (120). In addition,
whereas OCTN2 has an affinity for cefipime and cefoselis
several orders of magnitude lower than for cephaloridine
in the presence of Na⫹, it displays a similar affinity for all
three compounds in the absence of Na⫹. This might be
explained if the binding of cephaloridine to OCTN2 leaves
the Na⫹ binding site accessible to Na⫹, whereas the binding of the other two compounds to OCTN2 makes the Na⫹
binding site inaccessible to Na⫹. (120). The interaction of
␤-lactam antibiotics with OCTN2 may explain the carnitine deficiency that is associated with therapeutic use of
cephaloridine (120).
F) OCTN3. I) Structure. OCTN3 has been cloned from
mouse embryo mRNA using RT-PCR (443). The cDNA for
mOCTN3 codes for a 564-amino acid peptide that is 34%
identical to hOCT1, 66% identical to hOCTN1, and 69%
identical to mOCTN1.
II) Tissue and cellular distribution and localization. By both RT-PCR and Western blot analysis,
mOCTN3 is strongly expressed in adult testis and weakly
expressed in kidney, with virtually no expression in other
tissues (443).
III) Functional characteristics. mOCTN3 has been
functionally expressed in HEK 293 cells (443), in which it
transports both TEA and carnitine. Uptake of TEA is,
however, very modest compared with that measured for
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cesses acknowledged as being largely specific for type I
OCs. However, several studies have reported enhanced
transepithelial cimetidine secretion across cultured cell
monolayers that express MDR1 [e.g., Caco-2 (72), LLCPK1 (92), MDCK (332)]. Indeed, such observations have
been used as the basis for other studies using cimetidine
as a tool for probing MDR1 distribution (156). However,
other reports seem to contradict this conclusion. For
example, in one study, although verapamil, vincristine,
and quinidine enhanced cellular accumulation of doxorubicin by LLC-PK1 (presumably by blocking pMDR1-mediated doxorubicin export), cimetidine had no effect (85).
In another, cimetidine had no effect on rhodamine-123
(Rho) accumulation in doxorubicin-resistant colon carcinoma cells and failed to reverse resistance of these cells
to anticancer hMDR1 substrates (223). In another, Rho
clearance by the liver was blocked by the rMDR1 inhibitors cyclosporin A and colchicine, but not by cimetidine
(3). Similar contradictory observations have been made
concerning the possible interaction of MPP⫹ with human
and mouse MDR1 (e.g., Refs. 24, 411).
Although the basis for these contradictory observations is not clear, it could be influenced by the cultured
cell lines used for such studies. Caco-2 cells, which clearly
do express hMDR1 in the apical membrane (24), also
appear to express a number of other OC-selective processes (24, 47, 291). This expression could complicate
interpretation of inhibition studies. LLC-PK1 cells have
been used in some studies because they apparently express (pig) MDR1 (e.g., Refs. 85, 242) and in others because they apparently either do not express MDR1 (e.g.,
Ref. 75) or express it at comparatively low levels (e.g.,
Refs. 405, 446). Different behaviors have been noted for
OC transport in LLC-PK1 cells before, with some studies
reporting the presence of OC/H⫹ exchange in the apical
membrane (176), including cimetidine/H⫹ exchange (16),
whereas others found no evidence for such a process
(92). Again, the basis for such differences is not clear, but
it could reflect differences in culture methods, cell passage number, or other selection criteria. Nevertheless,
Smit et al. (405) used LLC-PK1 cells to provide strong
evidence that MDR1 can accept at least some type I OCs
as substrates. Using several clonal lines of LLC-PK1 cells
that stably expressed transfected human or mouse MDR1,
they measured the influence of MDR1 expression on the
transepithelial flux of selected type II and type I OCs.
There was a modest net secretory flux in the wild-type
(nontransfected) cells of two type II substrates, vinblastine and vercuronium, and two type I substrates, TBuMA
and azidoprocainamide (APM). Expression of MDR1 in
the apical membrane of the transfected cells significantly
increased transepithelial flux of all the substrates, supporting the conclusion that MDR1 can support transport
of at least some type I OCs. A similar study employing
MDCK cells showed that cells that stably express human
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MDR1/substrate complexes, and 3) is capable of directly
generating substantial substrate concentration gradients
arising from the active efflux of substrates from cells (or
into inside-out membrane vesicles). Current theories for
the mechanism of MDR1 activity include variations of the
traditional “pump model” in which the conformation of
the transport protein alternates between forms that allow
access to one or more binding sites on one side of the
membrane or the other. An alternative hypothesis has
MDR1 displaying a “flippase” mode of action in which
substrate accesses the transporter from within the lipid
bilayer, with subsequent conformational changes of the
protein shifting the substrate to the outer leaflet of the
bilayer, thereby creating a steady-state concentration gradient across the thickness of the membrane (155).
We will limit the present discussion of MDR1 to three
subjects. The first two of these, i.e., the location within
the kidney of MDR1 expression and its substrate specificity, are addressed in this section. The third subject, i.e.,
the physiological role of MDR1 in mediating OC secretion,
is dealt with in section IIC.
B) LOCATION OF MDR1 EXPRESSION IN RENAL TISSUES. By
immunocytochemistry, MDR1 is expressed in the apical
membrane of proximal tubule cells in human (455) and
mouse kidneys (101). MDR1 is also expressed, albeit at
apparently lower levels, in the mesangium, thick ascending limb of Henle’s loop, and collecting tubule of the
normal human kidney (103).
C) SPECIFICITY OF MDR1. MDR1 supports ATP-dependent
export of a structurally diverse range of comparatively
bulky, hydrophobic cationic substrates that, in general,
fall within the type II OC classification. These “traditional”
substrates include the vinca alkaloids (e.g., vinblastine,
vincristine), cyclosporine, anthracyclines (e.g., daunorubicin, doxorubicin), and verapamil. In addition, MDR1
mediates the transport of a number of relatively hydrophobic compounds that are either uncharged or are neutral at physiological pH, including digoxin, colchicine,
propafenone, and selected corticosteroids. Many of these
compounds also inhibit the renal secretion of smaller type
I OCs (464, 465). Consequently, the extent to which the
transporters generally viewed as being specific for type I
OCs (e.g., the OCTs previously discussed) accept these
bulky type II compounds as substrates is not clear. Indeed, there is evidence in intact tubules that at least some
of the traditional MDR1 substrates [e.g., rhodamine-123 in
the rat (3, 259); daunomycin in killifish (277)] can also be
transported by OCTs.
There is even greater confusion concerning the extent to which MDR1 can accept type I OCs as substrates.
The contradictory evidence concerning the interaction of
cimetidine with MDR1 will serve as a case in point. Cimetidine (mol wt 252) is actively secreted by renal proximal tubules (267, 269) and is transported by both the
basolateral (267, 269) and apical (266) transport pro-
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
MDR1 can support enhanced secretion of cimetidine
(332), thus providing the least ambiguous evidence to
date for the contention that cimetidine, in addition to
TBuMA and APM, is an MDR1 substrate.
C. Physiological Integration of Renal
OC Transporters
1. Cellular organization and mechanisms of renal OC
transport
Physiological characteristics of OC transport in renal
proximal tubules have been studied with intact tubules
Physiol Rev • VOL
and isolated membrane vesicles from a wide variety of
species, including representative mammalian (rat, rabbit,
dog), avian (chicken), reptilian (snake), and teleost
(flounder) species. The physiological profile of OC transport observed in these studies suggests that the cellular
mechanism of renal OC secretion is phenotypically conserved over many phyla. The secretion of type I OCs
involves (indeed, may be dominated by) electrogenic,
facilitated diffusion at the basolateral membrane, and
electroneutral OC/H⫹ exchange at the apical (luminal)
membrane. Secretion of type II OCs, on the other hand,
appears to involve a largely diffusive flux across the basolateral membrane, followed by P-glycoprotein-mediated
export at the luminal membrane.
A) BASOLATERAL/CONTRALUMINAL TRANSPORT MECHANISM.
Studies with isolated renal basolateral membranes and
intact renal tubules have clearly implicated electrogenic,
facilitated diffusion as a principal element in the renal
secretion of organic cations. Uptake of NMN, TEA, and
other prototypic type I OCs into isolated mammalian renal
basolateral membrane vesicles (BLMV) (211, 294, 407,
429, 533) consistently reveals that transport is saturable
and stimulated by intravesicular electrical negativity. Importantly, an inside negative membrane potential has
been shown to be sufficient to support the transient uphill
accumulation of TEA in isolated renal BLMV (294, 407).
The characteristics of basolateral OC uptake in intact
tubules are also consistent with an electrogenic mode of
transport. In in vivo micropuncture of rat proximal tubules, depolarization of the basolateral membrane potential significantly inhibits NMN uptake (468). Similarly,
depolarization of the basolateral membrane inhibits uptake of TEA across the basolateral membrane of isolated
single snake and flounder proximal tubules (205, 207,
406). Furthermore, addition of TEA to the medium bathing isolated snake (205) or flounder (406) renal proximal
tubules causes a depolarization of the basolateral membrane, supporting the contention that basolateral OC
transport includes electrogenic facilitated diffusion. Interestingly, efflux of TEA from preloaded snake proximal
tubules is not sensitive to depolarization of the membrane
potential, leading to the suggestion that basolateral OC
efflux may involve a process distinct from the pathway(s)
that dominates influx (205, 206). It should be noted that
the electrogenicity of OCT1-, OCT2-, and OCT3-mediated
transport in heterologous expression systems, discussed
previously, is typically used as support for the probable
basolateral location of these processes.
OC transport across the basolateral membrane can
also operate in an OC/OC exchange mode. For example,
outwardly directed gradients of the OCs TEA and mepiperphenidol (Darstine) support the transient, uphill accumulation of labeled TEA in voltage-clamped rabbit renal
BLMV (294). Also, in isolated single rabbit proximal tubules, an inwardly directed gradient of TMA, TEA, or
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Although great strides have been made in understanding the molecular and cellular physiology of renal
OC transport through studies employing cloned transport
proteins in heterologous expression systems, the observations so obtained must be interpreted with the caveat
that they indicate how a process works outside its normal
physiological milieu. In the light of the growing list of
processes known to contribute to renal transport of OCs,
we believe that determination of the net result of the
concerted activity of suites of these processes is essential
if we are to understand renal secretion within the context
of physiologically intact systems. In this section, we examine the results of studies employing “native” transport
systems. These include studies of transport by renal tubules in vivo, isolated tubules and cells, isolated renal
membrane preparations, and cultured cell systems.
It is convenient to organize the great diversity of such
studies into groups that focus on particular areas of general emphasis. These include 1) the functional organization of renal OC transporters and 2) the structural specificity of renal OC transport. The first category can be
subdivided further into studies primarily focused on understanding mechanisms and regulation of renal OC
transport in general and studies primarily focused on
understanding mechanisms of transport of a particular
compound of major physiological, pharmacological, or
toxicological interest. The second category includes efforts to establish both the molecular determinants associated with binding to and translocation by renal OC
transporters and the presence of multiple processes operating in series in the basolateral or apical poles of renal
cells. When possible, we will attempt to correlate observations made with intact or native systems with evidence
obtained using cloned transporters in an initial effort to
establish the molecular basis of the integrated processes
of renal OC transport. As noted earlier, the emphasis here
is on studies conducted since 1993; reference to earlier
studies is largely restricted to those that form a necessary
backdrop to more recent work. The reader is directed to
previous reviews that effectively summarize the earlier
literature (76, 342, 352, 355).
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threefold increase in the apparent Kt for amantidine transport and a two- to threefold reduction in maximum flux
(Jmax) as well (106). This is a much more pronounced
effect than that noted for contraluminal NMN transport in
rat proximal tubules in vivo noted previously (468). The
effect of bicarbonate on amantidine transport is also
present in suspensions of rat distal tubules (106), which is
one of the few observations of OC transport in a nephron
segment other than the proximal tubule (105, 106).
Significantly, the bicarbonate-sensitive component of
amantidine uptake appears to involve a process distinct
from the well-characterized transporter for TEA, NMN,
and other OCs. rOCT1 and rOCT2 both support amantidine transport that is blockable by TEA (126). However,
whereas amantidine is a potent inhibitor of TEA uptake
into suspensions of both proximal and distal rat renal
tubules, TEA has no effect on amantidine uptake in these
preparations (127). Bicarbonate-sensitive amantidine is
not influenced by treating tubules with acetazolamide,
suggesting that the intracellular bicarbonate pool exerts
little or no effect on the rate of basolateral amantidine
transport (106). Unlike basolateral transport of TEA,
NMN, and other OCs, bicarbonate-sensitive amantidine
uptake is not responsive to changes in the extracellular
K⫹ concentration and, therefore, does not appear to be
electrogenic (105). The amantidine transport pathway
shows a marked stereoselective interaction with quinine
and quinidine (105). The bicarbonate-sensitive component of amantidine transport in proximal tubules is
blocked with threefold greater potency by quinine than by
its diastereomer, quinidine. In distal tubules, however,
amantidine transport is blocked equally well by quinine
and quinidine. Lactate inhibits the bicarbonate-sensitive
component of amantidine transport in both proximal and
distal tubules (104). Acute increases in blood bicarbonate
concentration are correlated with a decrease in renal
amantidine clearance (128), suggesting that the effect of
bicarbonate on renal OC clearance is influenced by the
effects of bicarbonate on a battery of bicarbonate-sensitive cellular events. These observations, combined with
those discussed in an upcoming section, support the conclusion that there are multiple, mechanistically distinct
OC pathways in the basolateral membrane of renal proximal (and distal) tubules.
Basolateral OC transport is also influenced to variable degrees by the extracellular concentrations of other
inorganic ions. Reducing the extracellular Ca2⫹ concentration from 2.5 to 0.25 mM has no effect on the rate of
amantidine uptake in rat renal proximal tubules (105).
Similarly, removal of extracellular Ca2⫹ has no effect on
the initial rate of TEA uptake by isolated snake renal
proximal tubules, but it does reduce the 1-h, steady-state
accumulation of TEA and increases TEA efflux (206).
Exposure to Ba2⫹ inhibits TEA uptake into isolated snake
proximal tubules, but this appears to be secondary to
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choline (82, 135) significantly stimulates efflux of labeled
TEA from preloaded tubules. Operationally, this mode of
activity is similar to that observed for the mediated exchange of OCs supported by OCT1 and OCT2 (34, 44).
B) INFLUENCE OF INORGANIC IONS ON BASOLATERAL OC TRANS⫹
PORT. Na
does not exert a direct effect on basolateral
transport of OCs in proximal cells. Uptake of TEA into rat
renal BLMV is not influenced by the removal of Na⫹ (429),
and imposition of either an outwardly or inwardly directed Na⫹ gradient has no effect on TEA transport in
rabbit renal BLMV (294). In microperfused rat renal proximal tubules in vivo, removal of Na⫹ causes a modest
inhibition of NMN uptake (468). However, the degree of
inhibition observed is similar to that noted when extracellular K⫹ is elevated or bicarbonate is removed, so this
effect has been ascribed to the associated depolarization
of the membrane potential caused by these three manipulations (468).
Although H⫹ concentrations and trans-membrane
⫹
H gradients have been found to influence basolateral OC
transport in some experiments, these effects do not appear to indicate a direct coupling of H⫹ in the transport
process. Hydrogen ion gradients typically have little or no
effect on OC transport in isolated renal BLMV (168, 429,
533), and when such effects have been observed, they
generally have been found to reflect the indirect effect of
H⫹ gradients on membrane potential (407). In this regard,
the effect of H⫹ gradients on OC transport in BLMV
appears to parallel the pH sensitivity of TEA transport
into oocytes expressing rOCT3 that disappears when the
oocyte membrane potential is clamped (199).
In intact tubules, changes in ambient pH do influence
basolateral OC transport in some cases (207), although
whether this reflects a direct effect on basolateral transporters, or indirect effects arising from changes in transmembrane potentials or other factors that influence transport, is not clear. In studies on isolated snake renal tubules, alterations in extracellular and intracellular pH
provided no evidence for trans-stimulation of either TEA
uptake or efflux (207). However, in these tubules there
appeared to be an optimal intracellular H⫹ concentration
for TEA uptake corresponding to that found at the physiological intracellular pH of 7.1 and an optimal extracellular H⫹ concentration for TEA efflux corresponding to
that found at the physiological extracellular pH of 7.4
(207). Although the mechanism involved in this relationship is unknown, these data support the concept noted
above that OC efflux may occur by a pathway distinct
from the pathway(s) involved in uptake in these tubules.
Basolateral transport of selected OCs can be markedly influenced by extracellular bicarbonate concentration. In suspensions of isolated rat proximal tubules, replacement of bicarbonate with phosphate, Tris-HEPES, or
acetate buffers inhibits basolateral uptake of the organic
base amantidine. The inhibition reflects both a two- to
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
2. Specificity and multiplicity of basolateral OC
transport
A) SPECIFICITY OF BASOLATERAL OC TRANSPORTERS. In light of
the role played by the kidney in mediating excretion of
cationic drugs, substantial effort has been applied to understanding the molecular determinants that define interaction of substrates with OC transporters. Ullrich and
co-workers (410, 468 – 470, 474 – 476) have made an extensive examination of the factors that influence selectivity
of renal organic cation transport. Applying the in vivo
stopped-flow capillary microperfusion technique to the
rat kidney, they characterized the contraluminal transport
of NMN and TEA, concluding that these two substrates
shared what could be adequately described as a single
common transport pathway (475). In the light of immunocytochemical data on the distribution of OCTs in the
basolateral membrane of rat proximal tubule (195), it is
likely that Ullrich’s observations reflected the combined
influence of both OCT1 and OCT2. It is, in hindsight,
difficult to determine the extent to which the selectivity
profile for basolateral OC transport evident from Ullrich’s
studies represents the specific characteristics of either of
these transporters, or (more likely) reflects the combined
influence of both of them. In Ullrich’s studies, selectivity
of basolateral OC transport was tested by assessing the
apparent Ki for inhibition of basolateral NMN transport
with substances having a diverse array of chemical structures, including, primary, secondary, tertiary, mono- and
bisquaternary amines (468), anilines, phenylalkylamines
(catecholamines), pyridines, quinolines, acridines (475, 476),
imadazoles, guanidines, hydrazines, piperidines, piperazines,
azepines (475), dipeptides, cephalosporins, quinolone-carboxylate gyrase inhibitors, steroid hormones, cyclophosphamides (474), sulfamoyl-, sulfonyl-urea-, thiazide- and diphenylamine-carboxylate analogs (467). The inhibitory profiles
Physiol Rev • VOL
led Ullrich to develop five rules for interaction of substrates
with the basolateral NMN/TEA (i.e., OC) transport system
(465). The first two rules, summarized here, are the most
general.
1) Interaction increases with increasing substrate
hydrophobicity.
2) Interaction increases with increasing substrate
pKa (i.e., increasing “basicity”).
The last three rules, as described below, reflect the
apparent basis for exceptions to the first two rules.
3) Compounds with the amino group arising directly
from a benzene ring (e.g., amino benzenes and aminonapthelenes) interact poorly with the basolateral OC
transport process.
4) Selected, nonionizable substrates (e.g., selected
steroid hormones, Ref. 474) can interact with basolateral
OC transport. Also, basolateral OC transport does not
appear to sense the degree of ionization of a substrate
(469).
5) Electrophilic (e.g., Cl⫺, Br⫺, or NO2) groups produce varied effects, which may reflect their influence on
pKa values and direct interactions with the transport process (475).
Each of these rules deserves discussion.
The influence of hydrophobicity on the interaction of
substrates with organic cation transporters has received
considerable attention. Early studies on substrate selectivity, involving renal OC clearance (131) and transport in
renal slices (353), revealed a correlation between inhibition of transport and increasing hydrophobicity of test
inhibitors. Ullrich’s work (468, 476) confirmed and extended this general observation, and it has been further
supported by additional in vivo work with rat kidneys (28)
and by in vitro studies employing isolated single rabbit
proximal tubules (135). Importantly, the underlying molecular processes believed to be the basis of the tubular
transport examined in Ullrich’s studies, i.e., OCT1 and
OCT2, have both been shown to display a qualitatively
similar profile of interaction with substrates of increasing
hydrophobicity (90, 550; W. Barendt, unpublished observations). It should, however, be emphasized that studies
of the interaction of test agents with OC transporters have
largely been limited to an assessment of the effectiveness
of the test compounds as inhibitors of OC transport.
Generally, it is not known if the agents are themselves
transported, nor is it known if the inhibition is, in fact,
competitive. Ullrich speculated that high-affinity inhibitors (i.e., those with very low Ki values) may bind to the
transporter but are translocated through the membrane
slowly, if at all (468). Consistent with this suggestion,
Groves et al. (135) found that n-TAA compounds of comparatively low affinity (TMA and TEA, with IC50 values of
1.3 and 0.08 mM, respectively) trans-stimulated efflux of
radiolabeled TEA from preloaded rabbit proximal tubules, whereas high affinity n-TAAs (TBA and TPeA, with
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depolarization of the membrane potential (205, 206).
However, exposure to Ba2⫹ also inhibits TEA efflux, even
in the face of the concomitant depolarization of the basolateral membrane potential (206). These data suggest
that TEA flux across the basolateral membrane in snake
renal tubules, as noted above, is kinetically and, perhaps,
mechanistically asymmetrical (206). Removal of Cl⫺ from
the medium bathing isolated rabbit proximal tubules (S2
segment) virtually eliminates TEA accumulation (163),
with approximately half activity noted at an external concentration of between 30 and 70 mM Cl⫺. Bromide can
replace chloride, but iodide, sulfate, and nitrate cannot
(163). The presence of TEA in the extracellular solution
has no effect on the basolateral flux of Cl⫺, arguing
against the existence of Cl⫺-OC cotransport in the basolateral membrane (163). The mechanistic basis of these
inorganic ion effects on basolateral OC transport is not
known.
1007
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STEPHEN H. WRIGHT AND WILLIAM H. DANTZLER
Physiol Rev • VOL
no effect on the rate of basolateral uptake of radiolabeled
cimetidine. They concluded that basolateral OC transporters do not “sense” the degree of substrate ionization. This
is the basis of rule 4 above.
However, these data run counter to the more recent
observation that the charge state of substrates does exert
a marked influence on binding to the human ortholog of
OCT2 (stably expressed in CHO cells; Ref. 13). Whereas a
change in external pH (from 7 to 8) had no effect on the
interaction of quaternary (fixed charge) ammonium compounds (i.e., TEA and MPP) with hOCT2, an increase in
external pH markedly diminished the interaction of weak
bases (cimetidine, trimethoprim, and 4-phenylpyridine)
with this transporter. Increases in external pH also decreased the rate of hOCT2-mediated uptake of labeled
cimetidine. In addition, whereas the interaction of these
charged compounds with hOCT2 was competitive, inhibition produced by the neutral steroid corticosterone was
noncompetitive and, therefore, may not have reflected an
interaction at the transport binding site. Consequently,
the very effective inhibitory interaction of several nonionizable steroid hormones with basolateral OC transport in
rat proximal tubules, including corticosterone and testosterone, should not be used as evidence that noncharged
substrates can bind effectively to the transport site of
basolateral OCTs (474).
A number of organic electrolytes (anionic, cationic,
or zwitterionic) have been found to interact (to some
degree) with both OC transporters and transporters that
typically interact with organic anions. For example, the
prototypic inhibitor of renal OA transport, probenecid,
also inhibits, albeit with comparatively low affinity, renal
OC transporters (e.g., Ref. 269). Ullrich et al. (474, 475)
compared the interaction of a number of compounds with
rat renal basolateral OC and OA transport, and substrates
that interacted with both processes were referred to as
“bisubstrates.” Three groups of bisubstrates were noted:
1) zwitterions (containing both potentially anionic and
cationic groups), including hydrophobic dipeptides,
amino cephalosporins and gyrase inhibitors; 2) substances with a potential cationic N-group and one or more
electronegative (electron-accepting) or hydrogen bonding
groups (e.g., Cl⫺, Br⫺, or NO2 groups); and 3) substances
with one or more partially charged or hydrogen-bond
forming groups in a suitable spatial arrangement (this
latter group includes a number of noncharged steroids).
Typically, bisubstrates that have a comparatively high
affinity for one pathway have a comparably low affinity
for the other. In other words, compounds with a combination of steric and physico/chemical characteristics that
render them high-affinity substrates of, for example, basolateral OC transport, generally interact with renal OA
transport with low affinity. There are, however, a number
of compounds, including the azepines (e.g., chlorimiramine, chloropromazine, and flurazepam) and many of the
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IC50 values of 4 and 0.8 ␮M) trans-inhibited efflux (135).
The interaction of TPeA with basolateral OC transport
appears to have both high- and low-affinity components,
with the high-affinity component (Ki of ⬍1 ␮M) being
competitive (140). Interestingly, basolateral TEA transport in isolated rabbit proximal tubules continues to be
reduced for 10 min following only a brief (⬍10 s) exposure of the basolateral membrane to TPeA (140). These
data, which further support Ullrich’s speculation (468),
suggest that high-affinity, hydrophobic cations may dissociate slowly from their binding site(s), thereby reducing
physiological turnover of OC transporters, a concept discussed previously in the context of trans-inhibition noted
in studies with cloned transport proteins.
As for basicity, a strong correlation has been observed between the inhibitory interaction of weak bases
with basolateral OC transport and their pKa values, the
latter being defined as the strength of the partial ionic
charge of the basic nitrogen atom(s) within a substrate
molecule (28, 476). Of the two physical parameters, hydrophobicity appears to exert more influence than pKa in
determining interaction with basolateral OC transporters.
Pyridine, for example, is both a very weak base (pKa of
5.2) and only moderately hydrophobic (calculated log
octanol:water partition coefficient [log P] of 0.65). Thus it
is not surprising that it shows no significant inhibitory
interaction (Ki ⬎ 7 mM) with basolateral OC transport
(476). 2-Benzylpyridine is also a very weak base (pKa of
5.1). However, the addition of the methyl-benzyl moiety
increases its apparent log P to 2.66. The substantial increase in apparent affinity of basolateral OC transport for
2-benzylpyridine (Ki of 2.3 mM), compared with the failure of pyridine to interact with the transport pathway,
suggests that binding to basolateral OC transporters involves an interplay between hydrophobicity of a substrate
and its partial cationic charge, with hydrophobicity (apparently) playing the more critical role in stabilizing substrate-transporter interaction. Presence of an amino
group as a partial charge centered directly on an unsaturated ring, however, is sufficient to eliminate interaction
with OC transporters, despite large log P values (e.g.,
aminonapthalenes; Ref. 476). This is the basis of rule 3
given above.
Related to the issue of the influence of pKa/ionic
charge on binding of substrate to OC transporters is the
question of whether the substrate must be positively
charged to “be recognized” by or bind to the transporter.
Ullrich and Rumrich (469) raised this issue in a study that
examined the influence of pH on the inhibitory interaction
of several weak bases having pKa values of ⬃7 with
basolateral OC transport in rat proximal tubules. Significantly, they found that varying extracellular pH from 6 to
8 had no effect on the degree of inhibition of basolateral
NMN transport exerted by these weak bases, including
cimetidine (pKa of 6.98). Moreover, extracellular pH had
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
Physiol Rev • VOL
amantidine blocks TEA transport into isolated rat proximal tubules but TEA does not block amantidine transport
(126, 127) is strong evidence that at least two parallel OC
transport processes must be coexpressed in these tubules. In addition, there is strong evidence, both physiological and immunocytochemical, for the potential presence of at least three OCT homologs (OCT1, OCT2, and
OCT3) in the basolateral membrane of renal proximal
tubule cells (e.g., Refs. 195, 415). Thus it is likely that the
basolateral OC uptake measured in native renal tissues
(including intact proximal tubules and isolated BLMV) is
influenced, if not dominated, by the activity of some combination of these cloned transport proteins. So, to what
extent can we compare the quantitative and qualitative
characteristics of the cloned transporters (i.e., their kinetic and selectivity characteristics) to those measured in
renal tubules? Can conclusions be drawn concerning the
functional distribution and integrated activity of the several OC transporters that are expressed in the kidney
based on transport behavior observed in intact tubules?
We deal first with the issue of evidence that multiple OC
transporters must be expressed in the basolateral membrane of native tissues/cells.
C) MULTIPLE TRANSPORTERS IN NATIVE RENAL CELLS. The
profile of inhibitor interactions with the transport of prototypic OC substrates (e.g., TEA, NMN, MPP) in intact rat
and rabbit proximal tubules is consistent with the presence of a single pathway for small, monovalent (type I)
OCs (135, 468), although immunocytochemical data support the expression, at least in the rat, of significant levels
of at least two similar transporters, i.e., OCT1 and OCT2
(195, 415). The results of studies with OCT1 and OCT2
knockout mice provide the most direct evidence that both
of these processes play a role in OC clearance in rodents.
Interestingly, OCT1⫺/⫺ mice show an increase (rather
than a decrease) in renal TEA clearance. But this paradoxical effect presumably reflects the ability of OCT2 in
the kidney to deal with the increase in plasma TEA concentration resulting from the marked decrease in hepatic
uptake and clearance of TEA in OCT1⫺/⫺ mice (184).
Significantly, deficiency in OCT2 by itself has little effect
on renal TEA clearance, indicating the presence of sufficient OCT1 in the kidney to handle the load (185). However, simultaneous elimination of both OCT1 and OCT2
virtually eliminates renal TEA clearance (185), providing
the most direct evidence for the parallel importance of
both OCT1 and OCT2 in the rodent kidney. This latter
observation suggests that OCT3 expression in the kidney
is not likely to play a quantitatively significant role in TEA
(or other type I OC) secretion. Thus it is not surprising
that elimination of OCT3 has no effect on the renal accumulation of MPP⫹ (554).
Studies employing the cultured cell line LLC-PK1 are
generally interpreted as being consistent with the presence in these cells of a single pathway for basolateral
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quinolines, which have moderately high affinities for both
OC and OA transport at the basolateral membrane (475).
Finally, a few molecules, including a number of the nonionic steroid hormones (e.g., corticosterone, aldosterone,
and testosterone), display comparatively high affinity for
both OC and OA transporters (474). As noted previously,
in at least some cases, the inhibitory interactions these
compounds exert with OC and/or OA transport are not
competitive (13). In particular, extremely hydrophobic
compounds might be expected to interact with hydrophobic regions of transport proteins distinct from the transport receptor.
Steric (spatial) parameters appear to play a comparatively minor role in influencing affinity of basolateral OC
transport for type I substrates. This has been examined
most extensively in studies comparing the relative interaction of a number of chiral cations with OC transport in
rat and human renal tissue (173, 410, 527). Comparisons
of the relative affinity for basolateral OC transport in rat
tubules of 10 pairs of cationic enantiomers/diastereomers,
including (R)-(⫹)- and (S)-(⫺)pindolol, quinine/quinidine,
and several analogs of ephedrine, revealed differences no
greater than a factor of three (410). This is the same
degree of difference reported for quinidine and quinine as
inhibitors of the amantidine-selective OC transport pathway in rat proximal (105) and distal (526) tubules. Studies
of the renal clearance of stereoisomers by human kidney
also show the influence of structure on rates of clearance,
but these effects are generally very modest (⬍2-fold; Refs.
410, 463, 527). In addition, owing to the number of events
associated with the process of renal clearance, it is not
certain where the influence of substrate structure is occurring, i.e., at the basolateral membrane, the luminal
membrane, and/or at one or more intracellular events
(e.g., sequestration). Evaluation of three-dimensional
models of the interaction of ephedrine enantiomers/diastereomers with basolateral OC transport in the rat suggests that the spatial orientation of the amino group and
the OH group is important. The importance of the threedimensional location of OH groups on the interaction of
morphine analogs with the basolateral OC transporter in
rat kidney (470) further supports the conclusion that
steric interactions can influence (usually modestly) the
binding of substrates to basolateral OC transporters. We
discussed previously the systematic influence of selected
steric parameters on the binding of substrates to hOCT1
(15), results that underscore the fact that three-dimensional structure, and general hydrophobicity, both influence the interaction of substrates with basolateral OC
transporters.
B) MULTIPLICITY OF BASOLATERAL OC TRANSPORTERS. Although the observations of Ullrich et al. (468) are consistent with the operation of a single mediated pathway for
OCs in the basolateral membrane, other observations are
not. For example, the observation mentioned above that
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Physiol Rev • VOL
ments of the inhibitory interactions of guanidine and corticosterone with basolateral OC transport (474, 475). Corticosterone’s apparent Ki in intact rat tubules is 200 ␮M
(474). When this value is compared with the IC50 values of
150 and 4 ␮M for rOCT1 and rOCT2, respectively, as
expressed in oocytes (5), we could conclude that OC
transport in the rat tubule is dominated by OCT1. Similarly, the interaction of guanidine with basolateral OC
transport (Apparent Ki of 1 mM; Ref. 475), is more consistent with an interaction dominated by OCT1 (Kt for
guanidine transport of 1.6 mM when expressed in oocytes;
Ref. 5) than with OCT2 (Kt of 170 ␮M when expressed in
oocytes; Ref. 5).
The microperfusion method is limited to tubule segments at the surface of the kidney and, consequently, is
effectively restricted to early (S1) and mid (S2) segments
of proximal tubule. By immunocytochemistry, these regions of the proximal tubule are expected to contain both
OCT1 (prevalent in the S1 segment) and OCT2 (prevalent
in the S2 segment). Thus the apparent absence of transport activity dominated by or, at least, influenced by
activity of OCT2 in the in vivo studies is, perhaps, surprising. But, how comparable are kinetic values obtained with
native cell/tissue preparations to those obtained with
cloned transporters in expression systems? We emphasized earlier the caution that should be employed when
comparing kinetic parameters for cloned transporters obtained with different expression systems. In this regard, it
is relevant to compare results obtained with cloned transporters to those obtained in native tissues in which we
can be reasonably confident that transport activity is
strongly influenced, if not completely dominated, by activity of that protein. Figure 5 compares apparent Ki
FIG. 5. Comparison of kinetic parameters for inhibition of basolateral organic cation transport measured in intact rat proximal tubules
(x-axis) to those measured in studies of the rat ortholog of OCT1
(y-axis). Intact tubule data are from studies by Ullrich et al. (466). OCT1
data include Kt, Ki, and IC50 values measured in studies employing a
variety of expression systems. The solid line indicates the line of unity
for this comparison.
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uptake of type I OCs (111, 115, 268, 364, 428, 430, 458).
The snake proximal tubule shows a different pattern;
whereas TEA transport displays the routine characteristics of OC transport described above, NMN transport
appears to be dominated by a distinct set of pathways that
are Na⫹ dependent and that result in net reabsorption
(rather than secretion). Another example of distinct basolateral pathways for type I OCs is the suite of processes
responsible for amantidine versus TEA transport in rat
proximal tubules, described previously (127). There is
also evidence for a basolateral transport process (in rabbit proximal tubules; Ref. 137) for small, divalent OCs
(e.g., paraquat) that is distinct from the process(es) that
accept monovalent OCs (54, 55, 250).
D) COMPARISON OF CLONED TRANSPORTERS TO TRANSPORT ACTIVITY IN NATIVE RENAL CELLS. Although phenotypic evidence
mostly suggests the presence of a single transport process
for type I OCs in the basolateral membrane of proximal
cells, there is ample evidence to suggest that, in at least
some species (e.g., the rat), multiple OCT processes may
be expressed. Functional data with intact or native (i.e.,
vesicular) experimental systems have generally not lent
themselves to conclusions concerning the relative contributions to cellular OC transport of the different OCTs
expressed in the kidney. Whereas the kinetics of transport
of prototypic OC substrates, including TEA, NMN, and
MPP, show no evidence of being influenced by interaction
with more than one mediated pathway (468, 475), such
evidence is only expected if parallel pathways display
markedly different affinities for these substrates (306).
Indeed, in practice, apparent affinities of parallel pathways for a common substrate must differ by a factor of
5–10 or more if kinetic evidence for the presence of
separate process is to be evident. The apparent affinities
for TEA, NMN, and MPP, though rather different from one
another, are very similar in OCT1 and OCT2. Thus it is
unlikely that the parallel expression of these processes in
renal tubules would be evident from the kinetic profile of
substrate transport.
Inhibitors that selectively block individual OC transporters are useful in assessing the extent of coexpression
of multiple OCTs that display similar affinities for prototypic substrates. Several substrates that discriminate between rat OCT1 and OCT2 expressed in oocytes have
been identified (5). These include mepiperphenidol and
O-methylisoprenaline (O-methylisopreteranol), which selectively interact with rOCT1 over rOCT2, and guanidine
and corticosterone, which selectively interact with rOCT2
over rOCT1. These compounds may permit the functional
assessment of coexpression of rOCTs in native renal cells.
The largest database currently available for comparing the characteristics of transport in native tubules with
those of cloned transporters is that provided by the observations of Ullrich and colleagues (83) using in vivo
microperfusion techniques. Their work included measure-
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
3. Regulation of basolateral OC transport
Comparatively little attention has been paid to the
regulation of peritubular OC transport. OC transport appears to be influenced by acute activation of protein
kinases, although the profile of such responses is not yet
clear. Activation of PKA or PKC [via treatment with either
forskolin or 1,2-dioctanoylglycerol (DOG), respectively]
resulted in a marked stimulation in rOCT1-mediated uptake of the cationic dye, 4-(4-(dimethylamino)styryl)-Nmethylpyridinium (ASP⫹) into HEK293 cells stably exPhysiol Rev • VOL
pressing this transporter (270). These effects were
blocked by coexposure to inhibitors of PKA (KT5720) or
PKC (tamoxifen). The increase in uptake was associated
with an increase in the apparent affinity of rOCT1 for
TEA, TPA, and quinine, and with an increase in phosphorylation of rOCT1 (270). Interestingly, kinase activation
produced different effects in two renal cell lines that
phenotypically display OC transport. Whereas activation
of PKC also increased ASP⫹ uptake into IHKE-1 cells, it
decreased uptake into LLC-PK1 cells (164). Different regulatory profiles in these cells were also noted following
activation of PKA and PKG; ASP⫹ uptake was stimulated
in IHKE-1 cells, but remained unchanged in LLC-PK1 cells
(164). The authors noted that these differences could be
caused by differences in protein trafficking or expression
of different isoforms in the different cell lines. Changes in
the trafficking of organic anion transporters at the level of
the plasma membrane in response to activation of PKC
has been shown to influence OA uptake (150, 331) and
may represent a means of influencing the level of OC
transport as well.
In one study with isolated, nonperfused S2 segments of rabbit renal proximal tubule, activation of
PKC (via exposure to phorbol ester) produced a
marked stimulation of TEA uptake (162). The effect
was time and dose dependent and was blocked by
coexposure to the nonspecific PKC-inhibitor staurosporine. In contrast, activation of PKC, as well as PKA
and PKG, in single, isolated human proximal tubules
resulted in an inhibition of ASP⫹ uptake (337). The
authors noted that the selectivity profile of ASP⫹ uptake into isolated human tubules suggested activity of
hOCT2, rather than hOCT1, an observation consistent
with immunocytochemical and mRNA expression data
discussed previously (300). Thus the difference in the
response to activation of PKC in human and rabbit
tubules could reflect differences in the isoforms expressed in these species. However, OCT2 appears to be
the only homolog expressed at appreciable levels in the
human kidney (300), and OC transport in the S2 segment of rabbit proximal tubule is dominated by expression of OCT2 as well (553). Both the human and rabbit
orthologs share consensus sites for PKC-mediated
phosphorylation, including one (Ser-285/286) that is
shared by all five of the cloned OCT2 orthologs (426,
553).
Activation of G protein-coupled receptors by carbachol results in a decrease in hOCT2-mediated transport
activity when expressed in HEK293 cells (51). This effect
does not appear to involve activation of PKC because the
permeable diacylglyceral analog DOG has no effect on
hOCT2 activity. [Interestingly, this contrasts with the observation, noted above, that activation of PKC in human
tubules causes an apparent downregulation of OCT2 activity (337), suggesting the influence of different regula-
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values determined in intact rat proximal tubules with Kt,
Ki, or IC50 values determined for the same compounds
used as substrates or inhibitors of transport mediated by
rOCT1 in heterologous expression systems. Included in
the comparison are results obtained using both the oocyte
and mammalian cultured cell expression systems. It is
evident that there is not a strong correlation between
kinetic observations made in intact tubules with those
made with cells expressing rOCT1.
Despite the absence of such a correlation, OC transport in the studies with intact rat tubules is probably
strongly influenced, if not dominated, by rOCT1 activity.
Why, then, are apparent Ki values measured in intact
tubules routinely much higher than those for the cloned
transporter measured in various expression systems? At
least two possibilities suggest themselves. First, the techniques associated with use of the different experimental
systems are very different (e.g., microperfusion versus
uptake in single oocytes or cultured cells) and could lend
themselves to introduction of analytical artifacts. Second,
and almost certainly a contributor to the apparent discrepancies highlighted in Figure 5, the physical, chemical,
and regulatory environments of the native tubule, including the complicating influence of extracellular binding of
substrates to blood proteins, differ from that of each of
the various expression systems used to characterize
cloned transporters.
Indeed, the strength of expression systems, i.e., the
ability to physiologically isolate a process from the complexity of its native environment, is also the basis of an
important “weakness” of such studies. Whereas observations of transport activity in an expression system provide
information on how a cloned transporter can act, they do
not necessarily provide precise information on how the
transporter does act within the physiological context of
the native cell or tissue. Nevertheless, as noted previously, kinetic parameters obtained using multiple expression systems have been used as the basis for quantitative
tests of the functional distribution of OCT1 and OCT2 in
rabbit proximal tubules (553), leading to the conclusion
that OCT1 activity predominates in the S1 segment and
that OCT2 activity predominates in the S2 and S3 segments (553; K. Evans, unpublished observations).
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4. Apical/brush border/luminal OC transport:
mechanism
The luminal step in renal secretion of type I OCs
appears to be dominated by carrier-mediated exchange of
extracellular H⫹ for intracellular OC. This scheme was
first put forward by Holohan and Ross in 1981 (168) and
has not changed appreciably since then (342). The activity
of OC/H⫹ exchangers has been identified in isolated luminal membrane vesicles from every mammalian species
examined, including dog, rat, rabbit, and human (e.g.,
Refs. 168, 322, 429, 533), and in all nonmammalian species
examined, including birds (504, 505) and snakes (81). The
exchange of small, monovalent (type I) OCs for H⫹ is
electroneutral (429, 533). Because intracellular H⫹ is kept
far from electrochemical equilibrium (through the activity
of luminal Na/H⫹ exchange and the V-type H⫹-ATPase),
the downhill influx of H⫹ serves as the driving force for
active transepithelial OC secretion. At least two separate
OC/H⫹ exchangers have been identified in isolated membrane preparations. Of the two, the process that accepts
TEA and MPP is generally accepted as being the major
route of luminal type I OC efflux (342). The second process displays much narrower substrate specificity, with
guanidine being the prototypic substrate (290) (although
studies with LLC-PK1 cells have implicated a guanidinesensitive transporter in the apical flux of quinolone derivatives; Ref. 366). In addition to these two OC/H⫹ exchangers, the presence of at least three other OC transporters is
generally accepted. The first is an electrogenic facilitated
diffusion process with a comparatively narrow selectivity
for choline and structurally related analogs (471, 535).
This process has been suggested to play a role in the
routine reabsorption of choline from the tubular filtrate
(80, 342, 535). The second is the Na⫹-dependent carnitine
cotransporter, which has proven to be the cloned transporter OCTN2 (314, 350, 413, 540). Although OCTN2 plays
a clear role in the physiological reabsorption of carnitine
(441), it can also operate as an OC transporter, suggesting
a possible role in the reabsorption of selected OCs (312,
387, 540). OCTN2 also supports mediated carnitine/TEA
exchange (312), and mice lacking functional OCTN2 show
depressed rates of TEA clearance (312), suggesting a
physiological role for OCTN2 in net tubular OC secretion
as well. The third luminal transport process involved in
renal OC secretion is the multidrug resistance transporter
MDR or P-glycoprotein, which is postulated to play a
significant role in the renal secretion of a wide array of
type II OCs (377).
To date, efforts to clone a transporter with characteristics paralleling those of luminal OC/H⫹ exchange
have been unsuccessful. As discussed earlier, OCTN1
does display pH sensitivity consistent with OC/H⫹ exchange activity (543). However, the tissue distribution
and kinetic profile of OCTN1 are quite different from the
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tory profiles in different tissues or cell types.] Instead,
carbachol-induced activation of G protein receptors appears to involve phosphatidylinositol 3-kinase. Additional
evidence suggests that OCT1 (rat) and OCT2 (human) are
also downregulated by cGMP (378) and by activation of
PKA (51). Activation of calmodulin-dependent kinase, instead, stimulates hOCT2-mediated transport (51). Future
studies of the acute effect of kinase activation on both OC
transport activity mediated by cloned OCTs and OC transport by native renal cell systems will be required to determine the basis, and the physiological role, of acute
OCT regulation.
There is strong evidence that expression of OCT
activity in the kidney is under the control of steroid
hormones. Early studies of OC accumulation in renal
slices showed a marked gender difference, with TEA
uptake in male rats being greater than that in females
(27). This has been confirmed in more recent studies (487,
490). These latter studies compared levels of OCT mRNA
in male and female rat kidneys, in an effort to determine
the basis for the sex difference in renal OC transport.
Whereas no sex-based differences were observed in the
renal expression of either OCT1 (402, 487) or OCT3 (402,
487) mRNA, the OCT2 mRNA level was approximately
three- to fourfold higher in kidneys from male rats than in
kidneys from female rats (402, 487). Treatment of male
and female rats with testosterone significantly increased
expression of OCT2 mRNA (490). This was correlated
with an increase in OCT2 protein expression and increased rate of TEA uptake into renal slices from both
sexes (490). Neither male nor female rats showed a significant decrease in OCT2 mRNA expression following
treatment with estradiol (490). However, treatment of
male rats with estradiol resulted in a decrease in OCT2
protein expression, and this was correlated with a decrease in TEA uptake in renal slices from treated males
(but not females) (490). Gonadectomized adult male rats
showed a marked decrease in expression of OCT2 mRNA;
this treatment had no effect on expression of OCT2
mRNA in females (402). As noted previously, TEA uptake
is substantially greater in renal slices from male rat kidney (487), and this is correlated with greater expression
of OCT2 mRNA (487, 490), and greater expression of
OCT2 protein (487) in male rat kidney. Consistent with
this observation is the increase of both TEA transport and
OCT2 expression in MDCK cells following treatment with
testosterone (392). Treatment of MDCK cells with dexamethasone and corticosterone significantly increases
TEA transport and OCT2 expression as well (392). In line
with the above observations, the reduction in renal secretion of cimetidine by male rats that is associated with
chronic renal failure (following 5/6 nephrectomy) is correlated with a reduction of both plasma testosterone and
renal expression of OCT2 (183).
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
5. Specificity of luminal OC transport
In the absence of cloned candidates for the role of
the luminal OC/H⫹ exchanger, the emphasis in recent
years has been on the use of native cell and membrane
preparations to extend the understanding of the luminal
efflux step in renal secretion of type I OCs. These studies,
in turn, have tended to be dominated by efforts to understand the molecular determinants associated with substrate binding and transport by the luminal OC/H⫹ exchanger, using isolated BBMV as well as intact renal
tubules.
The physicochemical parameters that influence interaction of OCs with the luminal OC/H⫹ exchanger are
similar to those discussed previously for basolateral OC
transport. Ullrich et al. (83) compared the interactions of
a large number of OCs with luminal MPP transport (presumably reflecting interaction with the luminal OC/H⫹
exchanger) with those observed in their previous work on
the basolateral NMN transporter (reflecting activity of one
or more basolateral OC transporters). Again, the most
general observation was the positive correlation between
increasing substrate affinity and increasing substrate hydrophobicity, confirming and extending an earlier assessPhysiol Rev • VOL
ment made for TEA transport in isolated rabbit renal
BBMV (536). Ullrich and co-workers (83) also noted three
quantitative differences in the interaction of OCs with the
two sets of transport processes. First, the luminal transporter interacts more readily with large OCs than does the
basolateral OC transporter. Second, there is a more
marked relationship between substrate hydrophobicity
and apparent affinity for luminal transport than for basolateral transport. Third, there is a more marked relationship between substrate pKa and apparent affinity for luminal transport than for basolateral transport. The consequence of these relationships is that, for a common set
of substrates, there is a broader range of affinities observed for luminal OC transport than for basolateral OC
transport.
A principal characteristic of renal OC transporters,
and one shared by the luminal OC/H⫹ exchanger, is their
ability to accept an extremely diverse array of chemical
structures as substrates. This characteristic implies that
steric factors play a relatively modest role in stabilizing
the binding of substrate on (within) the receptor region of
the OC/H⫹ exchanger. This issue was examined in a study
with rabbit renal BBMV that compared the relative interaction of a set of pyridinium/quinolinium analogs with the
OC/H⫹ exchanger (534). These compounds all had a cationic nitronium moiety at the 1-position of a six-member
aromatic ring and varied sterically in the position of a
planar (a flat aromatic ring) hydrophobic mass. Intriguingly, the position of the planar hydrophobic mass had no
effect on the IC50 values displayed by these compounds
against the transport of TEA. Instead, affinity of the transporter for planar hydrophobic OCs is predicted by their
relative hydrophobicity. This information was used to
postulate the presence of a 9 Å ⫻ 12 Å planar receptor
surface with which OCs interact to stabilize binding to the
OC/H⫹ exchanger (534).
The influence of steric factors on luminal OC transport has also been assessed by determining the relative
inhibitory effect of stereoisomers on activity of the
OC/H⫹ exchanger. In vivo micropuncture of rat renal
proximal tubules found that a wide array of enantiomers/
diastereomers (including several ephedrine derivatives,
deprenyl, transcycloporomine, disopyramide, verapamil,
pindolol, and quinine/quinidine) have IC50 values for luminal OC transport that differ by less than twofold (410).
Consistent with this observation is the finding that in rat
renal BBMV the IC50 values for several stereoisomers
(verapamil, hydroxychloroquine, and quinine/quinidine)
differ by less than twofold (134). Similar results have been
reported for rabbit renal BBMV and isomers of ephedrine,
nicotine, pindolol, and quinidine/quinidine (321, 534). In
contrast, however, are results obtained with dog renal
BBMV and cultured OK cells. In the former, IC50 values
for inhibition of apical TEA transport differed by 3-fold
for quinine and quinidine and by ⬎10-fold for S-(⫺)- and
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physiological profile of OC/H⫹ exchange activity observed in most experimental systems. Indeed, mRNA for
OCTN1 was effectively absent from samples of human
renal mRNA studied using quantitative real-time PCR
(300). Thus OCTN1 is an unlikely candidate for the principal OC/H⫹ exchanger implicated in renal OC secretion.
Several studies have attempted to characterize the biochemical profile of the luminal OC/H⫹ exchanger. Treatment of isolated membrane vesicles or the apical membrane of cultured OK and LLC-PK1 cells with various
group-selective modifying reagents implicates one or
more histidine (197), lysine (319), carboxyl (208), and
sulfhydryl (197) groups as being important in the activity
of luminal OC/H⫹ exchange. Cationic photoaffinity labels
have also been used in efforts to identify protein associated with luminal OC/H⫹ exchange. Treating rat renal
BBMV with an azido group-containing analog of cimetidine irreversibly reduces TEA transport, and transport
activity is protected by coexposure with TEA (210). This
radiolabeled photoaffinity probe labeled a 36-kDa protein,
which was decreased by coexpsoure during labeling with
TEA, NMN, and several other OCs, thereby implicating
the 36-kDa protein in OC/H⫹ exchange. A similar approach has been used to label a 41-kDa protein in dog
renal BBMV with radiolabeled azidopine (124, 169). Both
these proteins are much smaller than the 61- to 63-kDa
size of all current cation transporters in the OCT family of
proteins. These data suggest that a separate family of
transport proteins may be responsible for luminal OC
transport.
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STEPHEN H. WRIGHT AND WILLIAM H. DANTZLER
6. Role of MDR1 in renal OC secretion
As discussed earlier, MDR1 supports active export of
type II and at least some type I OCs. Expression of MDR1
in the apical membrane of proximal tubule cells suggests
that it is likely to play a role in the secretion of at least
some OCs. Indeed, there is clear evidence that MDR1
dominates the secretion of daunomycin (277) and fluorescent analogs of cyclosporin (383) and rapamycin (282) by
intact, isolated renal proximal tubules. There are, however, contradictory indications of the quantitative role
played by MDR1 in renal secretion of some OCs. In one
study (259), renal clearance of Rho by the intact isolated
rat kidney was blocked by MDR1 inhibitors known to also
inhibit type I transporters (quinidine and verapamil), but
not by inhibitors that do not influence type I transporters
Physiol Rev • VOL
(cyclosporine A and digoxin). Rho clearance was also
blocked by cimetidine, a known substrate for type I OC
transporters, but one for which interaction with MDR1 is
less conclusive. The authors concluded that, in the intact
kidney, Rho secretion is dominated by type I transporters,
rather than by MDR1. Net secretion of Rho across LLCPK1 cells also appears to involve type I transporters,
rather than MDR1 (493a). The results of these studies are,
however, difficult to reconcile with measured rates of
Rho efflux in single isolated perfused proximal tubules
from wild-type mice and mice failing to express either of
the murine MDR1 isoforms (mMDR1a or mMDR1b)
(MDR1a/1b ⫺/⫺ mice) (460). The half-time for Rho efflux
from tubule cells was increased from the control value of
34 to 407 s in the MDR1a/b ⫺/⫺ tubules. This latter value
correlated closely with the 434 s t1/2 measured in control
tubules when verapamil was added to the perfusate. Importantly, addition of TEA to the perfusate had no effect
on efflux of Rho from control tubules, suggesting that, in
the native proximal tubule, type I transporters play, at
best, a modest role in mediating Rho secretion. However,
it is increasingly apparent that Rho can interact with
multiple OC transport pathways, making it difficult to
draw firm conclusions about the quantitative role of distinct, parallel transport processes.
As noted previously, digoxin is a substrate for MDR1
expressed in cultured cells (179, 316, 448) and, importantly, digoxin clearance by rat kidney is blocked by
traditional MDR1 inhibitors, including cyclosporin, quinidine, and verapamil (87, 170, 316), but not by the type I OC
TEA (170). These data suggest that, in the normal kidney,
digoxin clearance is dominated by its interaction with
MDR1. Conclusions drawn from studies employing MDR1
knockout mice are, however, more equivocal. Although
MDR1a/1b ⫺/⫺ mice show a profound reduction in hepatic
and intestinal clearance of digoxin, renal clearance of
digoxin is actually enhanced (198, 404) (although these
mice also show an increase in renal digoxin content; Ref.
460). Importantly, the MDR1a/1b ⫺/⫺ animals also show
an increase in renal clearance of the type I MDR1 substrates, TBuMA and APM (404), which are likely substrates for both MDR1 and type I OC secretory pathways
(405). Taken together, the enhanced renal secretion of
selected OCs in MDR1a/1b ⫺/⫺ animals may reflect (over)expression of additional mechanisms for renal OC secretion (198). Interestingly, there appears to be no upregulation of either OCT2 or OCT3 in OCT1 ⫺/⫺ mice (184).
MDR1 activity in renal proximal tubules appears to
be regulated acutely. Activation of PKC, either through
exposure to phorbol esters (286) or via an endothelin
receptor-mediated response (261), reduces luminal secretion of fluorescent cyclosporin A. This effect is blocked by
inhibition of PKC (286).
In summary, it is likely that MDR1 plays a quantitatively significant role in the secretion of type II OCs and
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R-(⫹)-verapamil (320). These results, which may reflect
species differences in the selectivity of the apical OC/H⫹
exchanger, suggest that relatively modest differences in
the structure of the transport protein may lead to substantial differences in the influence of steric factors on substrate binding.
There is comparatively little information on the relationship between binding of substrate to a luminal OC
transporter and subsequent turnover of the substratetransporter complex. Ullrich et al. (475) observed in their
studies using in situ microperfusion methods in the rat
kidney that initial rates of OC transport increased with
decreasing Kt (or Ki), though, importantly, they noted that
this general trend reflected transport from “low substrate
concentrations.” Potentially pertinent to this observation,
there is an inverse relationship between affinity of the
OC/H⫹ exchanger for substrate and actual turnover of the
substrate:exchanger complex, i.e., substrates for which
the OC/H⫹ exchanger has a high affinity have comparatively low Jmax values (maximal rates of transport). It is,
however, instructive to consider the ratio of Jmax to Kt as
a means for comparing the relative rates of carrier-mediated solute flux from the low concentrations (i.e., ⬍⬍Kt)
to which transporters are most likely to be exposed. The
traditional units of Jmax are moles per unit time per unit
area, and those for Kt are moles per unit volume. The ratio
of Jmax to Kt, therefore, has units of velocity (e.g., cm/s)
and, so, is a measure of the “carrier-mediated permeability” of the membrane when rates of transport are nearly
first-order functions of substrate concentrations. Viewed
in this context, “high affinity” substrates are predicted to
cross the membrane more efficiently (i.e., higher rates at
low concentrations) than “low affinity” substrates, consistent with Ullrich’s observation. Importantly, as also suggested by Ullrich (475), compounds for which OC transporters display an extremely high affinity (Kt or Ki ⬍⬍1
␮M) may dissociate from the transporter sufficiently
slowly to be transported poorly, if at all.
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
some hydrophobic neutral substrates. It is not clear, however, if MDR1 plays a significant role in the renal excretion of type I OCs, including the vast array of cationic
drugs known to interact with the several OCTs described
previously.
7. Intracellular OC sequestration
Physiol Rev • VOL
III. RENAL ORGANIC ANION TRANSPORT
A. Overview of the Physiological Characteristics
of Renal OA Transport
As with OCs, the proximal tubule is the primary site
of renal OA secretion, as determined by studies employing stop flow, micropuncture, and microperfusion (see
Refs. 292, 342). Substrates for the pathways involved in
renal OA transport include a diverse array of weak acids
that have a net negative charge on carboxylate or sulfonyl
residues at physiological pH. Although a number of endogenous OAs have been shown to be actively secreted by
the proximal tubule (e.g., 5⬘-hydroxyindoleacetic acid,
riboflavin), it is generally accepted that the principal function of this process is clearing the body of xenobiotic
agents (292, 342), including many of the products of phase
I and phase II hepatic biotransformation, as well as anionic drugs of therapeutic or recreational use (41, 420).
As discussed in section II, dealing with renal transport of OCs, until recently, models of renal OA secretion
typically depicted a single basolateral entry step and a
single luminal exit step, a simple view that adequately
explained existing physiological data. As in the case of the
renal handling of OCs, that view is now known to oversimplify a process that entails a suite of cellular events
that underlies renal OA transport. We previously made
use of the type I and type II classifications for OCs to
assist in the discussion of the multiple transporters with
distinct selectivities for different structural classes of substrate. Although a parallel classification of OAs has not
been formally recognized, it is useful to acknowledge that
OAs can be separated into different structural divisions,
based largely on molecular weight, net charge, and hydrophobicity. Therefore, we will define type I OAs as being
comparatively small (generally ⬍400 mol wt) and typically monovalent compounds, such as p-aminohippurate
(PAH), probenecid, and fluorescein. Type II OAs are bulkier (generally ⬎500 mol wt) and frequently polyvalent and
include, for example, calcein and estradiol-17␤-D-glucoronide (E17␤-D). As we shall see, OA transporters can be
distinguished, at least in part, on the basis of the substrates with which they most efficiently interact.
1. Peritubular OA transport
In the 1980s and 1990s, Ullrich and colleagues (465,
477) characterized what has frequently been referred to as
the “classical” OA secretory pathway, i.e., the one associated with the active secretion of a wide variety of xenobiotic compounds exemplified by the type I OA PAH. The
peritubular entry of a type I OA is generally recognized as
the active step in the transepithelial secretion of these
compounds. In this regard, steady-state secretion of PAH
has been modeled as reflecting the concerted behavior of
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Transepithelial OC secretion across renal tubules involves three compartments: peritubular, cellular, and luminal. Substantial evidence suggests that a fourth compartment, i.e., an intracellular “vesicular” compartment,
could also influence the flux dynamics of OCs. The potential influence of intracellular vesicular sequestration on
reducing the toxic effect of chemotherapeutic agents has
received considerable attention (e.g., Refs. 258, 347, 348).
Endosomes isolated from rat renal cortex show an ATPdependent accumulation of TEA that is linked to an outwardly directed H⫹ gradient generated by the V-type H⫹ATPase (344). The carrier-mediated TEA transport was
suggested to reflect the presence in the endosomal membrane of the OC/H⫹ exchanger, possibly arising from
cycling of luminal membrane through the early endosomal compartment (344). Accumulation of OCs within an
intracellular vesicular compartment could play at least
two roles. First, sequestration could spare sensitive cytoplasmic constituents from exposure to potentially toxic
OCs. This would only be effective if the intravesicular
compartment were “emptied” (to the outside of the cell)
rapidly or often enough that steady-state is not achieved.
However, the parameters of endosomal turnover in some
cells, at least, suggest that this could occur and, theoretically, reduce the maximal intracellular concentration
(348). Second, sequestration of OCs could play a role in
transcellular OC transport by a mechanism similar to that
proposed for the demonstrated transcellular movement of
sequestered organic anions in renal epithelia (see below)
(284). Indeed, the transcellular flux of the fluorescent OC
quinacrine across choroid plexus includes a component
that resides within a vesicular compartment that can be
visualized as it releases its contents following fusion with
the basolateral membrane (287). The two postulated roles
for OC sequestration are not mutually exclusive; sequestration of OCs within a population of vesicles that moves
across the cells could increase the rate of active secretion
(by increasing intracellular OC content) while simultaneously sparing sensitive cytoplasmic constituents from
exposure to potentially toxic levels of selected compounds.
It should be noted that ATP-dependent TEA transport has also been observed in preparations of renal
BBMV (265). Whereas it is unlikely that the characteristics of transport observed in the cortical endosomal preparation represent contamination with brush-border membranes (344), the reverse is certainly possible.
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STEPHEN H. WRIGHT AND WILLIAM H. DANTZLER
three basolateral transport processes arranged in parallel.
Figure 6 shows a model for transcellular secretion of type
I OAs by the proximal tubule that reflects an attempt to
integrate observations obtained in studies with isolated
renal plasma membranes and intact proximal tubules
(342, 343) with those supported by recent molecular data.
Type I OAs are taken up into the cell via mediated exchange with intracellular ␣-ketoglutarate (␣-KG) (342).
The outwardly directed ␣-KG gradient is maintained, at
least in part, through the activity of a basolateral Na⫹dicarboxylate (NaDC) cotransporter. The Na⫹ gradient
that supports activity of the NaDC cotransporter is maintained, in turn, through activity of the Na⫹-K⫹-ATPase.
Molecular candidates for each of the OA transport processes have been cloned, including three OA-selective
members of the “OCT” family of transport proteins
(OAT1, OAT2, and OAT3) that are expressed in the basolateral membrane of renal proximal tubule cells, at least
two of which (OAT1 and OAT3) serve as OA/DC exchangers with functional characteristics that correlate closely
with those observed in physiological studies of OA secretion in intact renal systems (386, 421, 427) and an NaDC
cotransporter (NaDC-3; Refs. 63, 200, 511) expressed in
the basolateral membrane of proximal tubules (153).
FIG. 6. Schematic model of the transport processes associated with
the secretion of organic anions (OAs) by renal proximal tubule cells.
Circles depict carrier-mediated transport processes. Arrows indicate the
direction of net substrate transport during active secretion. Solid lines
depict what are believed to be principal pathways of OA transport;
dotted lines indicate pathways that are believed to be of secondary
importance; the dashed line indicates diffusive movement. The following
are brief descriptions of each of the numbered processes currently
believed to play a role, direct or indirect, in transepithelial OA secretion.
1: Na⫹-K⫹-ATPase, maintains the K⫹ gradient associated with insidenegative membrane potential and the inwardly directed Na⫹ gradient,
each of which represents a driving force associated with active basolateral organic anion uptake. 2: NaDC-3, the sodium-dicarboxylate cotransporter that assists in the maintenance of an outwardly directed gradient
of ␣KG. 3, 4, 5: OAT1, OAT2, and OAT3. OAT1 and OAT3 have been
shown to support secondary active uptake of type I organic anions via
carrier-mediated exchange of intracellular ␣-ketoglutarate (␣-KG) for an
extracellular organic anion (the function and subcellular location of
OAT2 remain unclear). 6: MRP2, supports the ATP-dependent active
luminal export of type II OAs. 7: URAT1, supports reabsorption of urate
through mediated exchange with intracellular type I OAs (including
p-aminohippurate). 8: OAT4, suspected of mediating efflux of selected
type I OAs. 9: Oatp1, OAT-K1/K2, suggested to play a role in luminal
influx of selected type I and type II OAs. 10: OATV1, NPT1, suggested to
play a role in luminal efflux of selected type I OAs. 11: Physiologically
characterized OA/OA exchanger.
Physiol Rev • VOL
In contrast to the current consensus on the mechanism
of peritubular OA transport, comparatively little is known
about luminal OA flux (Fig. 6). Transport into the lumen can
be inhibited (59), implicating a mediated mode of efflux.
Luminal secretion of the type I OA fluorescein has been
shown to be saturable and described by Michaelis-Menten
kinetics (397). There is, however, little agreement on the
mechanism for efflux of PAH and other “classical” OAs, with
evidence in some species (dog and rat) consistent with
carrier-mediated anion exchange, and in other species (pig
and rabbit), consistent with electrogenic facilitated diffusion
(342). In human and bovine luminal membranes, the presence of demonstrable OA/␣-KG exchange activity (357, 380)
further complicates efforts to develop an integrated view of
how peritubular and luminal events work in concert to
produce transepithelial OA secretion. Recent evidence has
implicated the multidrug resistance-associated transport
protein MRP2 in the active, ATP-dependent export of some
OAs, including PAH (201). Also, the type I Na⫹-phosphate
cotransporter, NaPi-1 or NPT1 (45, 462), has been shown to
support the electrogenic transport of selected organic anions, and a homolog of NPT1 (i.e., OATV1; Ref. 188) has
been cloned that displays a number of the properties characteristic of a postulated electrogenic OA pathway. Immunocytochemical data have placed OATV1, as well as OAT4
and several members of the distinct “organic anion transporter family” of proteins (rOATP1, OAT-K1 and OAT-K2) in
the luminal membrane of proximal cells, where they have
been suggested to play roles in mediating OA transport. It is,
however, not clear if their mode of activity will support net
import or export of anionic substrates.
B. Molecular Characteristics of Renal
OA Transporters
Renal OA transport involves the concerted activity of
a suite of organic electrolyte transport processes belonging to at least five different families of transport protein.
The “active step” in the classical OA secretory pathway,
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2. Luminal OA transport
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
1. OCT family of OA transporters
OAT1 was the first of the transport proteins cloned
from the kidney to express the “physiological fingerprint”
indicative of a role in the active secretion of OAs by renal
proximal tubules, i.e., transport of PAH driven by an
oppositely oriented gradient of ␣-KG. The primary sequence of this protein contains sufficient homology
(⬎30%) to, and similarity of predicted secondary structure with, OCT1 to support its classification within the
OCT family of the MFS (2.A.1.19). This is also true of the
primary sequence of the several homologs of OAT1
(OAT2– 4 and URAT1) subsequently cloned. All four OAT
homologs share the MFS and ASF structural motifs described previously (Fig. 2).
A) OAT1. I) Structure. OAT1 (SLC22A6; 2.A.1.19.4)
was cloned from rat kidney and functionally identified as
an organic anion transporter in 1997 by two independent
groups (OAT1, Ref. 386; ROAT1, Ref. 427). It proved to be
the rat ortholog of a protein first cloned from mouse
kidney, originally named NKT, for which no function was
identified (246). In addition to rat and mouse OAT1, orthologs have now been cloned from human (172, 248, 346,
351), rabbit (8), pig (149), flounder (525), and C. elegans
(123). The mammalian orthologs display ⬃80% sequence
identity with one another. The flounder and C. elegans
OAT1s, although sharing marked functional similarities
with the mammalian orthologs, are more distantly related,
showing 48 and 19% similarity to human OAT1, respectively (Fig. 3). The cDNAs for the four cloned mammalian
OAT1s have open reading frames that code for proteins
that range from 545 to 551 amino acids in length (ceOAT1
is 526 amino acids in length; Ref. 123). Splice variants
have been cloned in both the human (9, 172) and pig
(149). In the human, four splice variants were identified,
OAT1–1, OAT1–2, OAT1–3, and OAT1– 4 (9, 172). OAT1–1
and OAT1–2 show comparable rates of PAH transport
Physiol Rev • VOL
(172). OAT1–1 is a 563-amino acid protein, and OAT1–2
has a 13-amino acid deletion within the putative intracellular COOH terminus following TMD 12 (W523-R535).
These two variants arose as a consequence of the use of
alternative 5⬘-splice sites in exon 9 (175). A 132-bp deletion (Q455-S498), which results in loss of TMD 11 and half
of TMD 12, produces the splice variants OAT1–3 (which
also lacks the 13 amino acids in the COOH terminus) and
OAT1– 4 (9), neither of which transports PAH (A. Bahn,
personal communication). The 550-amino acid peptide
hOAT1–2 was independently cloned by four groups and is
commonly referred to as the human ortholog of OAT1
(172, 248, 346, 351). The hOAT1 gene is localized to chromosome 11 (11q13.1-g13.2) and spans 8.2 kb with 10
exons divided by 9 introns (9; see also Refs. 175, 346).
Conserved motifs within OAT1 include 12 cysteine
residues, 25 proline residues, 4 N-linked glycosylation
sites (N 39, 56, 92, 97), 3 PKA consensus sites (Ser-195,
Thr-456, and Ser-469), and 3 CKII consensus sites (Ser325, Thr-515, and Ser-543).
II) Tissue and cellular distribution and localization. Northern blots indicated that the mRNA coding for
OAT1 in human, rat, and mouse is almost exclusively
expressed in the kidney, although long exposures typically reveal weak expression in the brain and skeletal
muscle (69, 172, 246, 248, 346, 386). Quantitative measurements of mRNA in rat tissues using branched-DNA signal
amplification confirmed the observation that rOAT1 expression is largely limited to the kidney (35). Expression
in rat kidney increases shortly after birth, and this is
correlated with a threefold increase in PAH accumulation
in renal slices from adult rat kidneys, compared with
uptake in slices from neonatal rat kidneys (305). Quantitative real-time RT-PCR revealed that OAT1 expression in
human renal cortex is greater than five times that of either
OAT2 or OAT4; expression of hOAT3 mRNA, however, is
about three times that of hOAT1 (300). Immunocytochemical studies indicated that OAT1 expression in the kidney
is restricted to the basolateral membrane of proximal
tubule cells in the human (172, 300) and rat (457), and
GFP-rOAT1 fusion proteins are expressed in the basolateral membrane of cultured cells and native proximal tubule cells (422, 545). Initial efforts to resolve the distribution of OAT1 along the length of the rat proximal tubule
concluded that expression was restricted to the basolateral membrane of cells in the S2 segment (457). Subsequent studies, however, revealed positive, albeit weaker,
signals in the basolateral membrane within the S1 and S3
segments as well (222). This distribution of OAT1 protein
is consistent with previous physiological data showing
highest PAH secretion in the S2 segment of rabbit renal
proximal tubules (390, 391, 528).
III) Functional characteristics. OAT1 has been functionally expressed in Xenopus oocytes (4, 40, 69, 172, 177,
181, 338, 346, 386, 427, 459, 491– 493, 508, 523) and in HeLa
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i.e., peritubular uptake of anionic substrates, appears to
involve at least two members of the OCT family of transporters: OAT1 and OAT3. Their activity is functionally
linked to that of the several NaDC transporter homologs
that are expressed in renal proximal tubules, and the
reader is directed to several excellent reviews on the
molecular physiology of the NaDCs (326 –328). Two additional OCT family members have been identified (OATs 2
and 4), although their roles in renal OA transport remain
speculative. The physiological basis of the luminal step in
renal OA secretion, as noted above, is still unclear. Nevertheless, there are multiple candidates for transporters
that could play a role in exporting selected OAs into the
tubular filtrate. These processes include homologs 2 and 4
of the multidrug resistance associated protein, MRP2 and
MRP4, as well as members of the OCT, OAT, and NPT
families of membrane transporter.
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Physiol Rev • VOL
This inhibition was reduced or eliminated by exposure to
the PKC inhibitor staurosporine. Exposure of mOAT1expressing LLC-PK1 cells to the phosphatase inhibitor
okadaic acid also resulted in a downregulation of PAH
uptake and was associated with an increase in phosphorylation of OAT1 (545). Interestingly, however, the inhibition of mOAT1 activity resulting from activation of PKC
was not correlated with an increase in phosphorylation of
OAT1 protein, but from a decrease in the Jmax for PAH
transport (with no effect on Kt) (545). These observations
led to the suggestion that activation of PKC may increase
internalization of membrane transporters, or inhibit recruitment of preformed transporters from a cytoplasmic
pool (545). In fact, the decrease in hOAT1-mediated PAH
transport that follows activation of PKC (in Xenopus oocytes) is correlated with an internalization of the transport protein (524). Interestingly, although consensus motifs for phosphorylation mediated by PKC are conserved
in OAT transporters (41, 426), elimination of these sites in
hOAT1 had no effect on the decrease in transport activity
that followed activation of PKC (524). Consensus motifs
for phosphorylation mediated by PKA and CKII are also
conserved in OAT transporters (41, 426), but whether any
of these sites are involved in the regulation of OAT1 is not
known.
Sex steroids also influence the level of expression of
OAT1 mRNA. Although Northern blot analysis indicates
no apparent difference in the level of OAT1 mRNA in male
and female (Wistar) rat kidneys (214, 230), quantitative
analysis of OAT1 mRNA levels using branched-DNA signal amplification reveals a trend toward higher OAT1
levels in the kidneys of male than of female (SpragueDawley) rats (35). This is consistent with Western blot
analyses that found ⬃50% more OAT1 protein in basolateral membranes from kidneys of male than of female
(Wistar) rats (48). Treating female rats with testosterone
results in a significant increase in the OAT1 mRNA in their
kidneys, to a level equal to that in male rat kidneys (230).
Transcriptional regulation of OAT1 (or other OATs)
has received little attention to date. However, examination of phylogenetic footprints (evolutionarily conserved
regions within noncoding sequences) of related OATs has
revealed a number of conserved transcription factor binding sites that are suggested to be potential sites of regulation (100). These include PAX1, PBX, WT1, and HNF1.
Additionally, OAT1 and OAT3, and other members of the
OCT transporter family, typically occur within the human
and mouse genomes as tightly linked pairs of closely
related paralogs (e.g., OAT1 and OAT3; OAT4 and URAT1;
OCT1 and OCT2; OCTN1 and OCTN2). This pairing
correlates well with the observation that pair members
generally have similar profiles of cellular and tissue
distribution.
V) Substrate structural specificity. OAT1 interacts
with a diverse structural array of OAs, supporting the
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(123, 177, 248), CHO (68, 138, 159), LLC-PK1 (545), Cos-7
(138, 235), mouse S2 (186, 209, 432, 434, 435), and mouse
S3 cells (181, 437). OAT1-mediated uptake of PAH is
saturable, with apparent Michaelis constants ranging
from 5–20 ␮M for human OAT1, to 15–70 ␮M for rat OAT1,
to 0.43 mM for C. elegans OAT1. The activity of OAT1 is
influenced by ␣-KG, glutarate, and selected other divalent
anions. Cis-concentrations of these compounds inhibit
uptake of PAH and other anions (68, 69, 172, 235, 346, 386,
427, 491), whereas elevation of the trans-concentration of
these compounds (for example, by preloading OAT1-expressing cells with glutarate) stimulates uptake of PAH
and other OAs (4, 6, 69, 123, 248, 386, 427, 459, 493, 508).
These functional characteristics of OAT1-mediated transport in heterologous expression systems are sufficiently
similar to those observed for basolateral uptake of PAH
and other OAs in native renal tubules to support the
conclusion that OAT1 is a principal contributor to the
classical renal OA secretory pathway.
The stoichiometry of PAH/␣-KG exchange mediated
by hOAT1 is 1:1, as determined by static-head experiments employing membrane vesicles isolated from
hOAT1-expressing MDCK cells (6). The exchange of a
monovalent substrate (e.g., PAH) for a divalent substrate
(e.g., ␣-KG) suggests that the translocation process may
be electrogenic. In fact, evidence on the electrogenicity of
OA/DC exchange in renal basolateral membrane vesicles
has been equivocal, with some studies exhibiting an effect
of an imposed diffusion potential on the rate of exchange
(252), whereas others failed to obtain such effects (340).
PAH uptake into oocytes that are expressing rOAT1 is not
influenced by increases in extracellular K⫹, suggesting
that the rate of exchange is not sensitive to membrane
potential (427). However, uptake of PAH (and other selected monovalent OAs) into oocytes expressing rOAT1
(6) or flOAT1 (37, 40) results in an inward current. Extracellular concentrations of glutarate inhibit these currents,
and the application of glutarate by itself causes no currents, consistent with the electroneutral exchange of divalent anions (40). The PAH-induced current and the
current calculated from the flOAT1-mediated uptake of
[3H]PAH are of similar magnitude. These observations
support the contention that OAT1 typically supports uptake of monovalent OAs through the mediated exchange
for intracellular divalent OAs (physiologically, ␣-KG) and
that this process is energetically supported by the insidenegative membrane potential that drives the net efflux of
negative charge.
IV) Regulation of OAT1. The regulatory influence of
PKC activation has been examined for several OAT1 orthologs. In all cases examined [hOAT1 expressed in HeLa
cells (248); rOAT1 expressed in oocytes (491); mOAT1
expressed in LLC-PK1 cells (545)], activation of PKC
through exposure to phorbol ester resulted in a time- and
dose-dependent inhibition (⬃30 –70%) of PAH transport.
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
Physiol Rev • VOL
transporters (130). It is tempting to speculate that comparatively modest changes in the primary sequence of an
archetypal member of the OCT family resulted in the
dramatic differences in specificity (e.g., for anions versus
cations) and underlying energetic mechanisms found today across this large family of transport proteins.
B) OAT2– 4/URAT1. These homologs are 535 to 565
amino acids in length and have 6 –12 predicted TMDs
(depending on the algorithms upon which the prediction
is based). Given the strong homology among the OCT
transporters, it is generally assumed that, like OAT1,
OATs 2– 4 and URAT1 probably share the 12 TMD structure associated with most members of the MFS transporter families. In addition, OATs 2 through 4 and URAT1
share the sequence motifs common to MFS and ASF
transporters, as discussed previously.
I) OAT2 (SLC22A7). This was cloned initially from a
rat liver library and named NLT (novel liver transporter;
Refs. 385, 399). Orthologs from the human (98) and mouse
(215) have subsequently been cloned. There is a marked
gender-based difference in the level and tissue distribution of OAT2 expression. The mRNA for OAT2 is predominantly expressed in the liver of male rats, with very
modest expression in the kidney (35, 214). In contrast, the
level of OAT2 mRNA in female rat kidneys is substantially
larger than that in female liver (the latter value being
comparable to that in male rat liver) (35, 214, 230). Expression of OAT2 mRNA is significantly increased in kidneys from castrated male rats, and treating castrated rats
with testosterone reverses this increase (230). The sex/
tissue profile is very different in the mouse. OAT2 expression in the male mouse is virtually restricted to the kidney, whereas in the female, there is substantial expression
of OAT2 mRNA in both kidney and liver (215).
Just as there are marked species differences in the
organ distribution of OAT2, there are also marked species
differences in the subcellular localization of OAT2 within
the kidney. In the rat kidney, OAT2 appears to be localized to the apical membrane of medullary thick ascending
limb and the cortical and medullary collecting ducts
(222). However, in the human kidney, OAT2 appears to be
restricted to the basolateral membrane of the proximal
tubule (98).
OAT2 has been functionally expressed in Xenopus
oocytes (215, 385), LLC-PK1 cells (298), and mouse S2
cells (98, 99, 203, 209). OAT2 transports a variety of
substrates, including PAH, methotrexate, ochratoxin A,
PGE2, PGF2␣, salicylate, ␣-KG, and glutarate (98, 215, 298,
385). The affinity for PAH, however, is very low (compared to OAT1 and, as we shall see, OAT3), with an IC50
for inhibition of salicylate transport of ⬎⬎1 mM (298).
The affinity of OAT2 for PGE2, on the other hand, is
comparatively high, with Kt values for transport being 5
nM, 713 nM, and 39 ␮M for mOAT2 (215), hOAT2 (209),
and rOAT2 (298), respectively. OAT2 shows little or no
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frequent use of the descriptor “polyspecific” or “multispecific” for this process. For the most part, evidence for
“interaction” has been limited to demonstration that a test
agent inhibits OAT1-mediated transport. With the use of
this criterion, OAT1 has been shown to interact with
variety of endogenous compounds, including cGMP (386),
prostaglandin E2 (386), urate (172, 175, 386, 427), and
␣-KG (172, 235, 248, 386, 427, 491). Inhibitory criteria have
been used to implicate OAT1 in the transport of an extremely diverse array of drugs, toxins, and other xenobiotic compounds. In many cases, OAT1 has actually been
shown to mediate transport of these compounds. These
studies include the intriguing observation that OAT1 can
support the transport of the nonionized form of the weak
base cimetidine (37). However, there is evidence that at
least some OAs that are potent inhibitors of OAT1 activity
are not transported themselves. For example, whereas the
antidiabetic agent glibenclamide is a potent competitive
inhibitor of rOAT1 activity (Ki of 1.6 ␮M), there is no
increase in glibenclamide uptake into rOAT1-expressing
oocytes compared with water-injected controls (492).
Also, furosemide, although a potent inhibitor of PAH
transport mediated by the flounder ortholog of OAT1,
does not evoke an inward current, suggesting that it is
transported slowly, if at all, by flOAT1 (40). Indeed, differences in OAT1 substrate specificity observed between
species, combined with the presence of multiple OAT
transporter isoforms (as discussed in an upcoming section), have led to suggestions that the renal secretion of a
diverse array of anionic compounds may be the result of
several different transporters operating in parallel. These
processes, including OAT1, may individually have narrower selectivity than implied in studies of inhibition of
PAH transport in intact renal preparations (248, 465).
Site-directed mutagenesis of the flounder OAT1 ortholog identified two cationic residues that may play a
role in the binding of anionic substrates to OAT1. Wolff et
al. (523) reasoned that the homology that exists between
members of the OCT transporter family would be reflected in conservation of sites involved in the basic elements of substrate recognition. If the binding of cationic
substrates to OCTs involves interaction with one or more
anionic residues (as suggested by the study of Gorboulev
et al., Ref. 130), then OATs might be expected to display
conservation of cationic residues at homologous sites.
Three sites fulfilling these criteria were identified: H34,
K394, and R478. These were exchanged individually for
the corresponding amino acids present in OCTs. Each of
these mutations retained PAH transport, albeit at rates
reduced compared with the wild type. However, mutations K394A and R478D resulted in complete insensitivity
of PAH transport to the presence of glutarate and loss of
the PAH/glutarate exchange. Interestingly, R478 in flOAT1
is homologous to the D475 site in OCT1–3 that appears to
play a role in the binding of cationic substrates to these
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Physiol Rev • VOL
basolateral aspect of the medullary and cortical thick
ascending limbs, the connecting tubules, and the cortical
and medullary collecting duct (222).
OAT3 has been functionally characterized in Xenopus oocytes (52, 86, 112, 232), mouse S2 cells (187, 209,
436), and LLC-PK1 cells (416). Transported substrates
include estrone sulfate, indoxyl sulfate, PAH, ochratoxin
A, PGE2, benzylpenicillin, cephaloridine, E17␤-D, and
glutarate (86, 112, 187, 209, 232, 416). Although it is evident that the selectivity of OAT3 overlaps that of OAT1
(and OAT2), affinities for several substrates appear to
permit discrimination between OAT3 and these other
transporters. Estrone sulfate (ES) has been frequently
used as a test substrate in studies of OAT3 activity. OAT3
displays a moderately high affinity for ES (Kt of 3–35 ␮M;
Refs. 52, 112, 232, 436), whereas OAT1 interacts little with
ES (424). Salicylate is transported by mouse and rat OAT2
(215, 298, 385), but not by rat and human OAT3 (52, 232).
Other candidate substrates that have the potential for
permitting discrimination between homologous anion
transporters include citrinin (186) and taurocholate (424).
Interestingly, OAT3 transports the weak base cimetidine (pKa 6.9) (52, 112). Indeed, the affinity of human and
rat OAT3 for cimetidine (Kt of 40 – 60 ␮M; Refs. 52, 112) is
comparable to that for PAH (60 –90 ␮M; Refs. 52, 232).
Whereas the hydrophilic quaternary ammonium compound TEA has little effect on rOAT3 activity (52, 112),
the hydrophobic cation quinidine has generally been
found to exert a more marked inhibitory effect on OAT3
activity (52, 112, 232).
Giacomini and colleagues (112) examined the influence of two cationic amino acid residues within the primary structure of rOAT3 on the interaction of substrates
with this transporter. Their results supported the following conclusions. First, substrate recognition by the organic anion and organic cation transporters of the OCT
family of proteins resides in the COOH-terminal half of
the transporters. Second, multiple domains within the
COOH-terminal half of the protein differentially contribute to the interaction of rOAT3 with hydrophilic versus
hydrophobic organic anions. Third, two conserved
amino acids within this region, arginine-454 and lysine370, play important roles in substrate-transporter interaction. Fourth, changing these amino acids (R454D and
K370A) shifts the specificity of the transporter from
preferential transport of the organic anion PAH to the
organic cation MPP.
In a separate study, Giacomini and colleagues (113)
identified several aromatic amino acid residues in TMD 7
of rOAT3 that exerted a specific effect on the affinity of
the transporter for the hydrophilic substrates PAH and
cimetidine, but had little effect on interaction with the
hydrophobic substrate ES. They concluded that rOAT3
does not have a single binding site but, rather, has multiple binding domains that can participate in binding and
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interaction with tetraethylammonium, cholate, taurocholate, and estrone sulfate (385). Significantly, efforts to
show trans-stimulation of OAT2 activity with gradients of
glutarate have failed to show an effect, leading to speculation that, unlike OAT1 (and OAT3), OAT2 does not
operate as an OA exchanger (385). Consequently, the
energetic basis of OAT2-mediated OA transport is not
clear. However, if OAT2 were to operate as an electrogenic uniporter, its expression in the basolateral membrane of renal proximal tubules (in the human, at least;
Ref. 98) makes it unlikely that OAT2 plays a major role in
the secretory flux of OAs. Expression of OAT2 in the
apical membrane of selected regions of the nephron, as
appears to be the case in rat kidneys (222), would, in
contrast, permit an electrogenic mode of operation to
support a role in secretion of anionic substrates.
II) OAT3 (SLC22A8; 2.A.1.19.7). The cDNA for
OAT3, cloned from rat kidney, encodes for a protein 536
amino acids long (232). It has subsequently been cloned
from mouse (424), rabbit (X. Zhang and A. Bahn, unpublished observations), and human (52) kidney. Two OAT3
orthologs have been cloned from human kidney (52, 346).
The second of these, hOAT3, codes for a 542-amino acid
protein that supports transport of a wide range of OAs,
including PAH, estrone sulfate, and methotrexate (52). In
contrast, the first reported clone of human OAT3 (346;
referred to here as hOAT3*) encodes for a 568-amino acid
protein. This longer cDNA, however, does not appear to
support transport. The difference between these hOAT3
clones lies in four specific regions of the sequence, including the presence of 25 extra amino acids inserted in the
sequence in the near portion of the COOH terminus (52).
These sequence differences may alter the conformation of
the protein in a manner that precludes transport function.
It remains to be determined if hOAT3 and hOAT3* represent examples of alternative splicing of the same gene or
are encoded by different genes.
Northern-blot analysis indicated that rOAT3 is most
heavily expressed in kidney, liver, and brain (214, 232).
Quantitative analysis of rOAT3 mRNA expression using
branched-DNA signal amplification showed the highest
levels of rOAT3 expression in rat kidney, with no difference in renal expression between male and female rats
(35). The distribution of OAT3 in humans differs from that
in rats, with Northern blots showing expression to be
virtually restricted to the kidney (52, 346). Interestingly,
quantitative analysis of the mRNA levels of OATs 1, 2, 3,
and 4 in the human kidney revealed OAT3 expression to
be the highest, about 3-fold that of OAT1 and ⬎10-fold
higher than either OAT2 or OAT4 (300). Within human
and rat kidney, OAT3 is expressed in the basolateral
membrane of S1, S2, and S3 segments of the proximal
tubule (52, 222, 300). In the human kidney, expression
levels in the proximal tubule appear to run S1 ⬎ S2 ⫽ S3
(52). In the rat kidney, OAT3 is also expressed at the
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
Physiol Rev • VOL
blot analysis indicated that OAT4 is most heavily expressed in the kidney but is also expressed in the placenta
(53). Within the human kidney, however, OAT4 mRNA
expression is only 5–10% of OAT1 and OAT3 mRNA expression and is comparable to OAT2 expression (300). In
contrast to OATs 1, 2 and 3, which are typically restricted
to the basolateral aspect of the proximal tubule, OAT4
protein appears to be localized to the luminal membrane
of human proximal tubule cells (7). Activity of hOAT4 has
been studied in Xenopus oocytes (53) and in mouse S2
cells (7, 98, 209). Human OAT4 displays a wide range of
apparent affinities for the substrates transported. For example, the Kt values for ES, dehydroepiandrosterone,
PGE2, and PGF2␣ are all ⱕ1 ␮M (53, 209); the Kt for
ochratoxin A is ⬃20 ␮M (7); and for PAH, although it is
transported, the Kt is ⬎⬎1 mM (53). Potent inhibitors of
OAT4-mediated ES transport include sulfobromophthalein and probenecid, whereas taurocholate (a substrate
for mOAT3), glutarate (a trans-exchanging substrate for
OAT1 and OCT3), and the cationic TEA have little or no
interaction with hOAT4 (53).
The energetic basis of OAT4-mediated transport is
not clear. Mouse S2 cells that express hOAT4 can accumulate ochratoxin A (OTA) to levels well above that in the
surrounding solution, although, in the absence of information on intracellular binding of OTA, it is not clear if
this represents net flux against an electrochemical gradient (7). Transport mediated by hOAT4 is Na⫹ independent, and trans-concentrations of unlabeled ES do not
stimulate efflux of radiolabeled ES from hOAT4-expressing oocytes (53). The presence of OAT4 on the luminal
membrane lends support to a role for this process in
transepithelial secretion of OAs, or luminal reabsorption
of selected substrates, or both. If OAT4 is an electrogenic
process, as existing evidence implies, then a potential role
in supporting luminal efflux of anionic substrates is evident. However, the rather narrow selectivity of OAT4,
including its poor interaction with PAH, makes it unlikely
that OAT4 is the principal transporter associated with the
classical renal secretory process. However, the fact that
OAT4 can transport substrates like OTA, suggests that it
can serve as an avenue of luminal entry of selected anionic toxins, thus making OAT4 potentially important in
influencing the nephrotoxicity of selected compounds (7).
IV) URAT1 (SLC22A12; 2.A.1.19.11). URAT1 was
cloned from human kidney and is 52% identical with
hOAT4 (97). In the human, URAT1 mRNA appears to be
restricted to the kidney, and immunocytochemistry indicates that expression of URAT1 is limited to the apical
membrane of proximal tubules cells. When expressed in
Xenopus oocytes, hURAT1 supports urate transport, and
it has been suggested that URAT1 represents the principal
pathway in the human kidney for urate reabsorption (97).
URAT1-mediated urate transport is neither electrogenic
nor dependent on Na⫹. Instead, URAT1 supports an elec-
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translocation of different substrates depending on their
structural and physical properties. Interestingly, the several residues that correlated with the ability of rOAT3 to
interact with both PAH and cimetidine are all conserved
in the rabbit ortholog of OAT3 (which transports cimetidine and ES but does not interact well with PAH; Zhang
and Bahn, unpublished observations), indicating that
these residues alone are not sufficient to account for
selectivity of the OAT3 binding site(s). Nevertheless, the
results of these studies suggest how substrate/charge selectivity of the homologous OCT transporter family may
have evolved by mutations of a very few amino acids.
The mechanism of OAT3-mediated OA transport has
been the source of some confusion. Because inwardly
directed gradients of unlabeled PAH and ES did not transstimulate efflux of labeled ES from rOAT3-expressing
oocytes in initial studies, the investigators concluded that,
in contrast to OAT1, OAT3 does not operate as an anion
exchanger (52, 232). However, its location in the basolateral membrane of proximal tubule cells and its overlapping specificity with OAT1 substrates (e.g., PAH) that are
actively secreted by the proximal tubule make an electrogenic mode of action unlikely. Additionally, renal slices
from kidneys of OAT3-knockout mice show a ⬎60% reduction in uptake of PAH, as well as profound decreases
in uptake of ES and taurocholate (424). Thus it was not
surprising that a more focused examination of the mechanisms of OAT3 activity revealed that it can operate as an
OA/dicarboxylate exchanger. When rOAT3-expressing oocytes were preloaded with glutarate, the rate of ES and
PAH uptake was markedly stimulated (421). Moreover,
oocytes that coexpressed rOAT3 and rbNaDC1 showed
enhanced ES uptake in the presence of glutarate, and this
effect was blocked by the addition of Li⫹ or methylsuccinate (421), known inhibitors of NaDC-1 activity. The
human and rabbit orthologs of OAT3 also support exchange of glutarate for ES (and urate as well; Ref. 10 and
Zhang and Bahn, unpublished observations). Collectively,
these observations implicate OAT3 as a principal contributor, along with OAT1, in the classical renal OA secretory
process.
The parallels between OAT3 and OAT1 include the
regulatory response to PKC activation. As seen with
OAT1, treatment of OAT3-expressing oocytes with phorbol ester resulted in a time- and dose-dependent downregulation of transport activity (436). rOAT3-mediated ES
transport was decreased by ⬃40% by pretreating oocytes
for 10 min with 10⫺7 M phorbol 12-myristate 13-acetate
(PMA), and this effect was blocked by coexposure of
oocytes to PMA plus the PKC inhibitor chelerythrine chloride. The kinetic basis of this downregulation of rOAT3
activity was a decrease in Jmax, with no change in Kt, just
as in the response of mOAT1 to exposure to PMA (436).
III) OAT4 (SLC22A9; 2.A.1.19.10). OAT4 was
cloned from a human renal cDNA library (53). Northern-
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STEPHEN H. WRIGHT AND WILLIAM H. DANTZLER
2. OATP family of OA transporters
The family of transport proteins usually referred to as
organic anion transporting polypeptides (OATPs) is classified as 2.A.60 [the organic anion transporter (OAT) family] within the Transport Commission Protein Database
(361), and also as the solute carrier gene family Slc21a/
SLC21A. Members of the OATP family generally share a
common postulated secondary structure that includes 12
TMDs and several long extracellular and intracellular
loops (147), although there is no primary sequence homology with members of the OCT family. The research on
this group of transport proteins has focused on their
well-documented role in mediating the Na⫹-independent
transport of bile salts in the liver, and the reader is directed to recent reviews that deal extensively with this
subject (147, 148, 272, 456, 501). Several members of the
OATP family are, however, expressed in the kidney and
have been suggested to play a role in the secretion/reabsorption of selected anionic substrates.
A) OATP1 (SLC21A1; 2.A.60.1.1). Originally cloned from
rat liver (180), OATP1 consists of 670 amino acids with a
calculated molecular mass of 74 kDa. Expressed primarily
in the liver and kidney, it is also expressed in brain, lung,
skeletal muscle, and proximal colon (180, 244). In rat
liver, expression is restricted to the basolateral (sinusoidal) membrane of hepatocytes (18). However, in rat kidney, OATP1 expression is restricted to the apical membrane of cells in the S3 segment of the proximal tubule
(18). The differences in subcellular localization of
rOATP1 in liver and renal cells may reflect differences in
the biosynthetic processing of this protein in these two
tissues (18). When SDS-PAGE gels were run under standard reducing conditions, immunoblots of liver cell membranes using a polyclonal rOATP1 antibody revealed a
single 80-kDa band. Membranes from rat kidney run under the same conditions revealed two bands, 33 and 37
kDa. Immunoblots of renal membranes separated under
Physiol Rev • VOL
nonreducing conditions (to maintain disulfide bonds) revealed a single 80-kDa rOATP1 band (18).
The regulatory response of rOATP1 to sex steroids
also differs markedly in liver and renal tissues. Whereas
there is no difference in expression of OATP1 mRNA in
male and female rat liver, expression in male kidneys is
⬃20 times greater than that in female kidneys (244).
Administering testosterone to female rats increases, and
administering estradiol to male rats decreases, renal
OATP1 mRNA expression (247). These treatments have
no effect on OATP1 expression in rat liver (247).
The range of rOATP1 substrates is extremely broad
and includes anionic, cationic, and uncharged substrates
(94, 180, 193, 271, 500). Of relevance to the issues raised
here, OATP1 substrates include taurocholate, ES, and
OTA (193). Of equal relevance is the observation that
PAH, ␣-KG, and fluorescein have little or no interaction
with rOATP1 (296). rOATP1 supports mediated exchange
of anionic substrates for HCO⫺
3 (367) and glutathione
(GSH) (243). Whereas the prevailing electrochemical gradient for HCO⫺
3 across hepatic cell membranes would not
typically support net uptake of anionic substrates from
the blood, the exchange of extracellular substrates for
intracellular GSH is consistent with the postulated role of
rOATP1 in the liver, i.e., uptake of a broad array of
endogenous and exogenous anions that are then secreted
(via separate mechanisms) across the canalicular membrane into the bile (243). Proximal tubule cells contain a
relatively large (5 mM) concentration of GSH (236) that
could serve to support uphill accumulation of anions,
consistent with a role for rOATP1 in reabsorption of
selected substrates from the tubular filtrate. It has also
been suggested that rOATP1 could mediate the electrogenic secretion of E17␤-D, a physiological metabolite of
estradiol (194), and there is evidence that rOATP1 can
support an electrogenic mode of activity (288).
The several homologs of rOATP1 identified in human
kidney are not orthologous to those identified in the rat
(442). On the basis of criteria drawn from differences in
sequence homology and substrate specificity, it has been
concluded that the human orthologs for rOATP1 and several other rat OATPs have not been identified (293). The
current list of human homologs includes two, OATP-D
and OATP-E, which are expressed in the kidney (442).
Both proteins support transport of ES and PGE2 (442).
However, their mechanism of transport and subcellular
distribution within the kidney are not known.
B) OAT-K1/OAT-K2. In addition to rOATP1, three other
members of the organic anion transporting polypeptide
family, i.e., Oatp5, OAT-K1, and OAT-K2, are highly expressed in rat kidney. Expression in mouse of OATP5
(SLC21A13) appears to be effectively restricted to the
kidney (67), but it has not been functionally characterized. OAT-K1 (SLC21A4; 2.A.60.1.4) was cloned from rat
kidney and is 72% identical to rOATP1 (363). OAT-K2
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troneutral urate transport that is trans-stimulated by gradients of Cl⫺ (and other halides, including Br⫺ and I⫺)
and by gradients of lactate, nicotinate, and by the uricosuric drug pyrazinecarboxylate. Three different mutations
of URAT1 found in patients with idiopathic renal hypouricemia were shown to express proteins that did not support urate transport, consistent with a central role of
URAT1 in renal urate reabsorption (97).
RST (renal-specific transporter; Ref. 297) was isolated from mouse kidney. Although no function has been
reported for this protein, it was originally suspected to be
an organic cation transporter, based on its 30% homology
with rOCT1 (297). However, its sequence similarity with
hURAT1 (74% identical; see also Fig. 2) suggests it may be
the mouse ortholog of URAT1.
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
Physiol Rev • VOL
MTX and folate, but not taurocholate (264). However,
when expressed in MDCK cells, where biosynthetic processing results in apical localization of the transporter
(263) (see above), rOAT-K1 supports the transport of
taurocholate and a wide variety of other anionic substrates as well (438). It was suggested that the membrane
sorting mechanism and the proteolytic processing and/or
excision of rOAT-K1 found in renal tubular cells are retained in MDCK cells, but not in LLC-PK1 cells and Xenopus oocytes (438). These observations underscore the
caveats we introduced previously concerning the care
that must be exercised when interpreting quantitative or
qualitative aspects of cloned transporter function as determined in heterologous expression systems.
3. NPT family of Pi/OA transporters
NPT1 (SLC17A1; 2.A.1.14.6), or NaPi-I, is in one of the
three current families of Na⫹-phosphate cotransporters. It is
most heavily expressed in the luminal membrane of renal
proximal tubules and is also expressed in liver and brain
(301, 302). Originally cloned from rabbit kidney (519), and
subsequently cloned from rat, mouse, and human tissues
(301), NaPi-I is a 465-amino acid protein with 6– 8 TMDs.
Although it supports Na⫹-PO⫺
3 cotransport when expressed
in Xenopus oocytes (519), it is not believed to play a major
role in renal phosphate handling (518). In addition to supporting Na⫹-PO⫺
3 cotransport, however, rbNPT1 also supports an anionic conductance and the mediated uptake of
several organic anions, including penicillin G and probenecid when expressed in oocytes (45). Interestingly, the human ortholog of NPT1 (but not rbNPT1) accepts PAH as a
transported substrate (45, 462).
OATV1 (voltage-driven organic anion transporter 1;
2.A.1.14.15) is a protein cloned from pig renal cortex that
shares 60 – 65% sequence identity with human, rat, rabbit,
and mouse NPT1 (188). OATV1 supports voltage-dependent
transport of PAH. Although the Kt for PAH uptake into
oocytes that express pOATV1 is comparatively high (4.4
mM), it interacts with a structurally diverse array of anionic
compounds (including probenecid, furosemide, ES, and penicillin G). Immunocytochemical localization placed OATV1
in the apical membrane of pig proximal tubules. The physiological role of OATV1 (and NPT1) is not clear, but its
characteristics indicate that it could be an element in the
conductive transport of organic anions observed in the luminal membrane of some species (360, 462).
4. MRP family of OA transporters
The multidrug resistance-associated proteins, or
MRPs, are members of the ABCC subfamily of the ABC
transporter superfamily (conjugate transporter-2 or CT2;
3.A.1.208). Nine MRPs have been identified, of which six
have been functionally characterized. The MRPs are large
proteins (1,300 –1,500 amino acids) compared, for exam-
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(SLC21A4) was also cloned from rat kidney (262). It is
91% identical in sequence to OAT-K1, with the principal
structural difference being an NH2-terminal truncation of
OAT-K1 that eliminates the first four (of 12) postulated
TMDs. The mRNA for both transporters is expressed in
the straight portion of proximal tubule in rat kidneys (262,
263). The mRNA for OAT-K2 is also heavily expressed in
the proximal convoluted tubule (262). Western blots of
isolated rat renal BBMV and BLMV revealed that OAT-K1
and OAT-K2 expression is limited to the apical pole of
proximal tubule cells (262, 263). It is of interest that,
whereas expression of both transporters is restricted to
the apical pole of stably transfected MDCK cells (262,
264), the initial functional characterization of rOAT-K1 in
LLC-PK1 cells indicated that the transporter was restricted to the basolateral membrane (363). The differences in tracking of the cloned transport protein in different cells types has been suggested to be correlated
with differences in substrate specificity found to occur
when rOAT-K1 is expressed in different heterologous expression systems, as discussed below.
rOAT-K1 and rOAT-K2 are multispecific transporters
with broadly overlapping selectivities for anionic substrates. Substrates transported by both processes (as expressed in MDCK cells) include ES, E17␤-D, thyroxine,
taurocholate, OTA, methotrexate (MTX), and folate (262,
264, 438). PAH exerts a modest but significant inhibition
of rOAT-K2-mediated taurocholate transport (262), but
whether PAH is transported by this process at appreciable
rates is not known. Considerable attention has been given
to the interaction of MTX with rOAT-K1 and rOAT-K2.
Trans-gradients of MTX stimulate uptake of folic acid into
MDCK cells expressing rOAT-K1 or rOAT-K2 (438), and
inwardly directed folic acid gradients stimulate the efflux
of MTX from cells expressing rOAT-K1 (440). This latter
observation was the basis of studies examining the role of
rOAT-K1 and rOAT-K2 in mediating “folinic acid rescue”
therapy as a means of reducing MTX-induced nephrotoxicity (439). OAT-K1 and OAT-K2 mRNA levels were significantly reduced in 5/6 nephrectomized rats, suggesting
that the progressive renal failure associated with this
treatment is accompanied by decreased expression of
both of these transporters (SGLT1, SGLT2, and MRP2
levels were not affected by the treatment) (439). The
changes in expression of the two OAT-K homologs were
correlated with a marked decrease in renal clearance of
MTX, and the addition of folinic acid resulted in an increase in MTX clearance in sham-operated rats, but not in
the OAT-K-deficient 5/6 nephrectomized rats (439).
It is noteworthy that in the initial characterization of
rOAT-K1 (363), where functional expression of transport
activity in stably transfected LLC-PK1 cells was restricted
to the basolateral membrane (see above), MTX transport
was supported but taurocholate transport was not. Similarly, rOAT-K1 expressed in Xenopus oocytes transports
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STEPHEN H. WRIGHT AND WILLIAM H. DANTZLER
Physiol Rev • VOL
protecting these cells from the toxic effects of endogenous and exogenous compounds (241).
B) MRP2 (ABCC2; 3.A.1.208.2). Long before MRP2 was
cloned, there was physiological evidence for the presence
of an ATP-dependent transporter in the canilicular (apical) membrane of hepatocytes (323). A protein capable of
supporting this process, originally cloned and characterized as the canilicular multispecific organic anion transporter (cMOAT; Ref. 449), is now referred to as MRP2
(ABCC2). Orthologs of MRP2 have been cloned from a
number of mammals, including the human (449), rat (33),
mouse (116), and rabbit (495, 499). Of particular relevance to the present report, MRP2 has been shown to be
expressed in the apical membrane of both human (373)
and rat (374) renal proximal tubule cells [in addition to
the apical membrane of liver (33) and intestinal (494)
cells]. In addition, hMRP2 is localized to the apical pole of
MDCK cells where it supports active secretion of a wide
array of type II OAs (73, 74, 109, 365).
Like MRP1, MRP2 supports the ATP-dependent export of a structurally diverse array of GSH conjugates
(202). There is also strong evidence supporting the contention that MRP2 mediates the ATP-dependent cotransport of GSH with a number of nonconjugated substrates
(see Refs. 11, 46, 108, 239, 308, 497). Interestingly, MRP2
has been shown to support ATP-dependent transport of
the type I OA PAH (human, Ref. 239; rabbit, Ref. 497).
MRP2’s location on the apical membrane and its secretory
mode of action raises the question as to what fraction of
the luminal efflux of PAH (and other type I OAs) could be
supported by MRP2. We think it unlikely, given current
evidence, that MRP2 represents a major avenue for type I
OA secretion. This issue is discussed in more detail in
section IIIC.
C) MRP3 (ABCC3; 3.A.1.208.9). Although MRP3 is expressed in the kidney, its principal site of expression in
the rat (66) and human (225) is the intestine. In the human
kidney, MRP3 is expressed in the basolateral membranes
of distal convoluted tubule cells, but not in proximal
convoluted tubule cells (376). Whereas MRP1 and MRP2
display a broad selectivity for anionic conjugates, MRP3
shows a marked preference for glucuronate conjugates as
substrates, including E17␤-D (158). The physiological
function of MRP3 remains to be established.
D) MRP4 (ABCC4; 3.A.1.208.7). MRP4 and MRP5 are the
“short” MRPs, lacking TMDs 1–5. Interestingly, MRP4
(ABCC4) is expressed in the apical membrane of human
renal proximal tubule cells (498), which immediately implicates it in the secretion of selected OAs. Although
MRP4 transports a broad range of anionic conjugates,
including GSH, glucuronate, and phosphate conjugates
(64, 498), the quantitative aspects of MRP4 selectivity
differ rather markedly from those of MRP1, MRP2, and
MRP3 (241). hMRP4 transports methotrexate and has
been suggested to play a role in mediating efflux of folate
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ple, with the OCTs and OATs (⬃550 amino acids). MRPs
1, 2, 3, 6, and 7 have a predicted secondary structure that
contains 17 TMDs. MRPs 4 and 5 are smaller owing to the
absence of TMDs 1–5. The MRPs that have been functionally characterized share the ability to support primary
active (ATP-dependent) export of a broad array of anionic
substrates from cells. The interested reader is directed to
recent reviews that discuss in detail the molecular and
functional aspects of this group of transporters (25, 26,
166, 224, 241). We will limit our discussion to observations that pertain to the potential role of MRPs in the renal
handling of organic anions.
A) MRP1 (ABCC1; 3.A.1.208.1). First cloned from multidrug resistant human lung cancer cells in 1992 (71), MRP1
(ABCC1) is broadly expressed in many cells types (66,
225), with the highest expression generally found in the
basolateral membrane of epithelial cells (241). When expressed in the renal cell line LLC-PK1, hMRP1 is localized
to the basolateral membrane (110). Studies of the distribution of MRP1 in mouse kidney (i.e., mMRP1) (522)
showed greatest expression in the loop of Henle and
collecting duct. Interestingly, there was no evidence of
mMRP1 expression in proximal convoluted tubules.
hMRP1, like MRPs 2, 3, and 4, supports the ATPdependent transport of a diverse array of amphiphilic type
II organic anions. In particular, hMRP1 generally displays
a high affinity (Kt generally ⬍1 ␮M) for glutathione, glucuronide, and sulfate conjugates, including those of many
toxic xenobiotics (241). Nonconjugated hMRP1 substrates include selected nonconjugated type I OAs, such
as fluorescein (418) and PAH (201), glutathione disulfide
(GSSG; Ref. 240), and vinca alkaloids (178). Transport of
nonconjugated substrates is typically sensitive to the intracellular presence of physiological concentrations of
GSH, leading to suggestions of modes of activity that
include cotransport of substrates with GSH and/or an
allosteric GSH activator site on the transport protein
(241). It has been suggested that hMRP1 has a multipartite
binding pocket that permits interaction with and transport of conjugated substrates (in particular, GSH conjugates) and interaction with and transport of selected nonconjugated substrates when the appropriate binding
pocket is occupied by GSH (241). The issue of whether
hMRP1 supports the transport of unconjugated GSH, in
the absence of other substrates, is less clear. Evidence
supporting this contention includes the observations that
hMRP1-expressing cells show enhanced release of GSH to
the surrounding medium (25, 241) and that inside-out
membrane vesicles isolated from hMRP1 expressing cells
accumulate GSH, albeit at low rates and with low affinity,
in an ATP-dependent fashion (345). Although the apparent affinity for GSH is quite low, the relatively high concentration of GSH in renal cells suggests that hMRP1 may
play a role in maintaining the normal redox status of the
renal tubular cells in which it is expressed, in addition to
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
C. Physiological Integration of Renal
OA Transporters
Here we discuss the results of studies that have
examined the renal transport of organic anions by “native” systems, including renal tubules in vivo, isolated
tubules and cells, isolated renal membrane preparations,
and cultured cell systems. As with the previous discussion
of renal OC transport, we have organized this section into
categories that focus on particular areas of general emphasis. Areas that received particular attention in recent
years include 1) function of renal OA transporters and 2)
regulation of renal OA transport. The first category can be
subdivided further into studies primarily focused on issues that pertain to the mechanism(s) of renal OA transport, the specificity of renal OA transporters, the physiological organization of renal OA transporters, or selected
aspects of transport of a particular compound of major
physiological, pharmacological, or toxicological interest.
The second category includes efforts to assess the extent
to which activation of signal transduction pathways results in modulation of renal OA transport. We will close
with a presentation of the evidence documenting the carrier-mediated sequestration of OAs in renal proximal tubule cells and the possible physiological role(s) intracellular sequestration can play in the renal handling of OAs.
When possible, we will attempt to correlate observations
made with intact or native systems with evidence obtained using cloned transporters in an initial effort to
establish the molecular basis of the integrated processes
of renal OA transport. Again, the emphasis here is on
studies conducted since 1993; reference to earlier studies
is largely restricted to those that form a necessary backdrop to more recent work. The reader is directed to
Physiol Rev • VOL
previous reviews that effectively summarize the earlier
literature (76, 292, 339, 342, 352, 355).
1. Mechanisms, selectivity, and functional
organization of renal OA transport
Physiological characteristics of OA transport in renal
proximal tubules have been studied with intact animals,
intact tubules, and isolated membrane vesicles from a
variety of species, including representative mammalian
(e.g., rat, mouse, rabbit), reptilian (snake), and teleost
(flounder, killifish) species. The physiological profile of
OA transport observed in these studies suggests that the
mechanism of peritubular (basolateral) OA secretion is
phenotypically conserved over many phyla and involves
(indeed, may be dominated by) carrier-mediated exchange of intracellular ␣-KG for an extracellular substrate
anion. The secretory flux of type I OAs across the luminal
(apical, brush border) membrane appears to involve a
collection of processes that are more mechanistically diverse and that show more species diversity than transport
into the cells across the peritubular membrane. The secretion of the bulkier type II OAs presumably involves
diffusion across the basolateral membrane, followed by
MRP2- or MRP4-mediated export at the luminal membrane.
A) BASOLATERAL/PERITUBULAR TRANSPORT. I) Mechanisms.
Following the demonstration in 1987 (339, 389) that PAH
transport in isolated renal BLMV can be driven by oppositely oriented gradients of selected dicarboxylates, considerable attention was directed toward defining the physiological characteristics of “OA/DC exchange.” Of particular importance were the initial studies showing that
manipulation of intracellular dicarboxylate concentrations resulted in changes in basolateral (peritubular) PAH
transport, i.e., demonstration that conditions capable of
driving OA transport in isolated membrane vesicles are
also operative in intact native renal cells, as well. Pritchard (341) showed that the rate of PAH uptake into rat
renal slices is directly correlated with the intracellular
concentration of ␣-KG when the latter is manipulated by
preincubating slices in increasing concentrations of ␣-KG.
In a separate study, elevation of intracellular ␣-KG concentrations using the transaminase inhibitor aminooxyacetate increased uptake of fluorescein in rat renal slices
(309). In both of these studies, the stimulatory influence
of exogenous ␣-KG was linked to the activity of Na-␣-KG
cotransport in the peritubular membrane of renal cells
through the demonstration that addition of lithium to
experimental solutions blunted or eliminated the stimulation of OA uptake caused by preincubating tissue with
␣-KG, an effect associated with lithium’s inhibition of
basolateral Na-␣-KG cotransport (95, 234). Similarly, the
inhibitory effect of valproic acid on OA uptake into iso-
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derivatives (65). hMRP4 also supports export of a variety
of cyclic nucleotides (64, 498), and it has been suggested
that it may play a role in the release of cAMP and cGMP
into the urine, thereby supporting their role as paracrine
modulators of renal fluid homeostasis (498).
E) MRP5 (ABCC5). MRP5 supports the transport of a
wide array of anionic conjugates, including cAMP and
cGMP (182). The distribution of MRP5 in the kidney is,
however, not yet established.
F) MRP6 (ABCC6; 3.A.1.208.10). This member of the family is most heavily expressed in liver and kidney (226).
Within human kidney, MRP6 is localized to the basolateral
membrane of proximal tubule cells (375). Transport function has been difficult to establish for MRP6, although at
least one compound, the endothelin receptor antagonist
BQ-123, is clearly transported in an ATP-dependent fashion by MRP6 (and MRP2) (251). Although the physiological function of MRP6 is not clear, it has been suggested
that it may play a housekeeping role (251).
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STEPHEN H. WRIGHT AND WILLIAM H. DANTZLER
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The stimulation of OA transport produced by exposure to ␣-KG in the aforementioned studies involved activity of a basolateral NaDC cotransporter (presumably
NaDC-3) (153, 200). However, proximal tubule cells also
express a robust NaDC cotransporter in the luminal membrane (NaDC-1) (324) for reabsorption of dicarboxylates,
including ␣-KG, from the tubular filtrate (327). Addition of
100 ␮M ␣-KG to the perfusate results in a twofold increase
in steady-state secretion of PAH by single perfused S2
segments of rabbit renal proximal tubule (79). This stimulating effect was blocked by the addition of lithium to
the perfusate. Whereas this effect initially was only noted
in tubules incubated in a bicarbonate-free buffer (i.e., a
condition in which intracellular ␣-KG production is low),
a subsequent study of the differential effect of bath versus
luminal exposure to ␣-KG implicated both luminal and
peritubular NaDC cotransport in the stimulation of OA
secretion under physiological conditions. Steady-state fluorescein secretion by single S2 segments of rabbit renal
proximal tubule was increased by ⬃15% by the presence
of either 10 ␮M ␣-KG in the bathing medium or 50 ␮M
␣-KG in the luminal perfusate, and these effects were
additive in the presence of both bath and luminal ␣-KG
(395). Taken together, these observations suggest that the
combined influence of luminal and peritubular NaDC cotransport activity (including uptake of exogenous ␣-KG
and the recycling of originally intracellular ␣-KG) is responsible for supporting between 30 and 60% of the
steady-state rate of transepithelial OA secretion.
B) SPECIFICITY AND MULTIPLICITY OF BASOLATERAL OA TRANSPORT. I) Specificity of basolateral OA transporters. Ullrich
and colleagues (464, 465) described the presence of three
principal pathways for the basolateral entrance of anionic
substrates in renal proximal tubule cells: 1) a PAH/dicarboxylate exchange system, 2) a NaDC cotransport system, and 3) a sulfate/anion exchange system (464, 465).
The presence of these three processes was inferred based
on patterns of inhibition of basolateral uptake of either
PAH, succinate, or sulfate, produced by a chemically and
structurally diverse array of substrates during in vivo et
situ microperfusions in the rat kidney (refer to Ref. 465).
The last of these three processes mediates the efflux of
SO2⫺
3 from proximal tubule cells in exchange for the entry
of bicarbonate, hydroxyl, thiosulfate, and sulfate ions, but
not chloride, phosphate, lactate, or PAH. Its narrow selectivity makes it a minor contributor to the renal secretion of OAs, although it may contribute to the secretion of
selected sulfamoyl compounds (473, 482). The candidate
transport protein for this process in the rat, SAT-1 (sulfate-anion transporter; SLC26A1; 2.A.53), has been cloned
and characterized (255), and the interested reader is directed to a recent review of the molecular and cellular
characteristics of renal sulfate transport (254).
The second of these processes, i.e., basolateral NaDC
cotransport, also probably plays only a minor role in
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lated rat renal proximal tubules appears to reflect, in large
part, reduction of cellular ␣-KG (546).
Experiments with isolated single perfused and nonperfused tubules (56, 57) provided further support for the
developing contention that ␣-KG is the primary intracellular counterion used by intact proximal tubules to drive
the active accumulation of extracellular OA. Preloading
isolated single S2 segments of rabbit proximal tubules
with ␣-KG increased peritubular uptake of PAH (57) by
three- to sixfold. Importantly, preloading tubules with
␣-KG also resulted in a three- to sixfold increase in the
rate of transepithelial secretion of PAH (56, 57) by single
perfused proximal tubules. This observation is consistent
with basolateral entry’s being rate-limiting in active transepithelial secretion of OAs.
Although elevation of intracellular ␣-KG (as well as
glutarate, suberate and adipate; Ref. 417) clearly results in
stimulation of basolateral OA/DC exchange activity, the
physiological contribution to basolateral OA transport of
exogenous ␣-KG was not evident from the initial studies
that typically used large dicarboxylate concentrations,
nonphysiological buffers, and extended preincubations to
show that such coupling can occur. Mitochondrial metabolism produces endogenous ␣-KG (400), and the routine
observation of active renal accumulation of PAH (and
other organic anions) in the absence of exogenous ␣-KG
(e.g., Ref. 212) clearly shows that metabolic sources of
␣-KG are sufficient to support active OA uptake. Welborn
et al. (517) used an optical method to monitor peritubular
fluorescein transport in real-time to determine the extent
to which uptake of exogenous ␣-KG results in an increase
in basolateral OA transport. Acute exposure of single S2
segments of rabbit renal proximal tubules to 10 ␮M ␣-KG
in the bathing medium (a representative plasma concentration), in the presence of physiological bicarbonate concentrations, increased peritubular fluorescein transport
by ⬃75% within 7–10 s, an effect that was completely
blocked by the presence of lithium. The stimulatory effect
of extracellular ␣-KG is even larger when applied in a
HEPES-based, bicarbonate-free bathing medium (57,
517), presumably reflecting the decrease in intracellular
metabolic production of ␣-KG that occurs upon removal
of bicarbonate (401). Real-time measurements of fluorescein transport also revealed the extent to which reaccumulation of endogenous ␣-KG (i.e., that which is lost to
the peritubular extracellular space following exchange
for a transported OA substrate) supports renal OA transport (517). In the absence of added ␣-KG in the bathing
medium, blocking basolateral NaDC cotransport resulted
in an immediate and reversible 25% reduction in the initial
rate of fluorescein uptake. The authors concluded that the
combined influence of accumulating exogenous extracellular (i.e., plasma) ␣-KG and recycling ␣-KG lost from
proximal cells following OA/DC exchange is responsible
for supporting about 60% of peritubular OA transport.
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
Physiol Rev • VOL
dicarboxylate transport. Neither the luminal nor the basolateral NaDC cotransporters interact effectively with
monocarboxylates, and the affinity of both is largely influenced by the distance between the two carboxylate
residues (465). Both processes display comparatively high
affinities for succinate and glutarate (with carboxylate
residues separated by 2 or 3 carbons, respectively), but
they have virtually no affinity for oxalate and malonate
(carboxyl residues separated by 0 or 1 carbon, respectively). Longer chain dicarboxylates (e.g., adipate and
pimelate) show a systematic decrease in their inhibitory
interactions with both processes (479). However, the basolateral NaDC cotransporter is more tolerant of substitutions (⫽O, -OH, -SH, or -CH3) on internal carbons than
is the luminal transporter (465). Of particular interest is
the observation that, whereas 2,3-dimethylsuccinate is a
potent inhibitor of basolateral NaDC cotransport, it has
comparatively little interaction with the luminal transporter (531). It is significant, therefore, that succinate and
dimethylsuccinate appear to be equally effective at interacting with rNaDC-3 (200). The ability of rNaDC-3 to
interact with disubstituted dicarboxylates is the probable
basis for the recent observation that the heavy metal
chelator 2,3-dimercaptosuccinic acid (DMSA) is transported by the flounder ortholog of NaDC-3 (39). This
finding suggests that this transporter may play a clinically
significant role in clearing renal cells of heavy metals.
III) PAH/dicarboxylate exchange selectivity. Ullrich
and co-workers (470, 473– 475, 478 – 485) used the in vivo
stopped-flow capillary microperfusion technique in the
rat kidney to examine the selectivity of basolateral PAH
transport. Factors that influence selectivity of PAH/dicarboxylate exchange were deduced from the patterns of
inhibition of basolateral PAH transport produced by a
diverse array of chemical structures, including monocarboxylates; aliphatic dicarboxylates; mono- and polysubstituted benzene analogs; phenolphthaleins; sulfonphthaleins; fluoresceins; amino acids; di- and oligopeptides;
methyl-, acetyl-, and berizoylderivatives; glutathione and
cysteine conjugates; cyclic nucleotides; eicosanoids; corticosteroids; ␤-lactam antibiotics; sulfamoyl compounds
and sulfonylurea derivatives; oxazaphosphorines; analgesics (including salicylates); aniline derivatives; weak organic acid derivatives; mercaptocompounds and their Hgcomplexes; and several other chemicals and drugs which
act as bisubstrates [i.e., they interact with both the basolateral PAH- and NMN-transporter(s)]. These studies were
extended by observations from other groups that used a
variety of intact renal systems to study the interaction of
selected groups of compounds with basolateral renal OA
transport, including anionic pesticides (84), herbicides
(506), and nucleoside derivatives (151, 279). The inhibitory profiles indicated that interaction with the basolateral PAH transporter is primarily influenced by 1) the
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directly mediating renal OA secretion. However, as indicated previously, basolateral NaDC cotransport plays a
quantitatively important role in supporting active OA secretion by maintaining the large, outwardly directed ␣-KG
gradient that, in turn, supports the first of the three processes listed above, PAH/dicarboxylate exchange. Interested readers are directed to several recent reviews on
the cellular and molecular physiology of NaDC cotransport (326 –328). Here, we provide an overview of those
kinetic and selectivity characteristics of the basolateral
NaDC cotransport process and its suspected molecular
transporter, NaDC-3, that are of particular relevance to
renal OA secretion.
II) NaDC cotransport selectivity. It is pertinent to
compare the kinetic and selectivity characteristics of the
basolateral NaDC cotransporter to the functionally similar process found in the luminal membrane. Kinetically,
the two processes are clearly distinguishable: the basolateral transporter has a markedly higher affinity for its
prototypical substrates (succinate and ␣-KG) than does
the luminal transporter. In the rabbit, for example, the
apparent Kt for succinate transport in renal BLMV is 10
␮M, compared with 0.6 mM in luminal BBMV (532); and in
the rat, the apparent Kt for ␣-KG transport in renal BLMV
is 15 ␮M, versus 158 ␮M in BBMV (95). The physiological
significance of high-affinity basolateral NaDC cotransport
becomes evident when it is considered that the circulating
concentration of ␣-KG is on the order of 10 –20 ␮M (354),
and exposure of intact proximal tubules to ␣-KG concentration as low as 10 ␮M is sufficient to increase basolateral OA uptake by ⬎50% (517). The NaDC cotransporter
NaDC-1 (SLC13A2; 2.A.47.1.1) has been cloned in human
(325), rabbit (324), rat (2937), and mouse (414) and localized to the luminal membrane of proximal tubule cells by
immunocytochemistry (330, 384). The similarity of its Kt
for succinate (0.4 mM; Ref. 324) to that observed in luminal BBMV supports the contention that NaDC-1 is the
NaDC cotransporter, the function of which has been characterized in isolated BBMV and intact, microperfused tubules (117, 466). NaDC-3 (SLC13A3; 2.A.47.1.4) has been
cloned in human (511), rat (63, 200), mouse (329), and
flounder (412) and has also been shown by immunocytochemistry to be in the basolateral membrane of proximal
tubule cells (153). In addition, the similarity between
the affinity of NaDC-3 for succinate (Kt of 2 to 20 ␮M;
Refs. 63, 511) and the affinity of BLMV for contraluminal succinate transport (Kt of 10 –90 ␮M; Refs. 479, 532)
supports the contention that NaDC-3 is the molecular
candidate most likely to be the basolateral NaDC cotransporter (200).
The selectivities of basolateral and luminal NaDC
cotransport show marked similarities and a few significant differences. Indeed, the differences in selectivity of
these two processes lend further support to the contention that NaDC-3 plays a dominant role in basolateral
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STEPHEN H. WRIGHT AND WILLIAM H. DANTZLER
Physiol Rev • VOL
FIG. 7. Comparison of kinetic parameters for inhibition of basolateral organic anion transport measured in intact rat proximal tubules
(x-axis) to those measured in studies of the rat ortholog of OAT1
(y-axis). Intact tubule data are from studies by Ullrich et al. (466). OAT1
data include Kt, Ki, and IC50 values measured in studies employing a
variety of expression systems. The solid line indicates the line of unity
for this comparison.
OA transport in rat renal tubules could reflect interaction
with OAT1 and/or OAT3, inhibition by PAH of transport in
rabbit tubules implies an interaction with OAT1. Thus the
ability of PAH to effectively block secretion of, for example, riboflavin (544), the angiotensin II receptor antagonist losartan (96), and the artificial sweetening agent
stevioside (189) and its metabolite, steviol (58), is strongly
suggestive that secretion of these compounds in the rabbit is dominated by an interaction with OAT1 and not
OAT3.
OAT1 and OAT3 are coexpressed in rabbit RPT (249).
OAT3 is expressed in (at least) the S1 and S2 segments of
rabbit RPT. ES is a relatively specific, high-affinity (Kt of
4.5 ␮M) substrate for OAT3 in the rabbit (Zhang and Bahn,
personal communication). Thus the observation that single S2 segments of rabbit RPT display high-affinity ES
transport (Kt of 3 ␮M) that is only weakly inhibited by
PAH (249) probably reflects the functional expression of
OAT3 in S2 cells. Interestingly, whereas the Jmax for ES
transport in single S2 segments is only ⬃10% that for PAH,
the Jmax values for ES and PAH transport are virtually
identical in isolated rabbit cortical RPT, a preparation
enriched in both S1 and S2 segments (249). Consequently,
whereas OAT1 may be expressed at higher levels in S2
cells, OAT3 expression may be predominant in S1 cells.
The profile of basolateral transport of the mycotoxin
OTA into rabbit RPT is consistent with the contention of
differential expression of OAT1 and OAT3 in different
regions of RPT. OTA is accumulated by isolated S2 segments of rabbit RPT (516), suspensions of cortical RPT
(136), and primary cultured cells derived from rabbit
cortical RPT (139). In a study with isolated S2 segments of
rabbit RPT, 1 mM PAH blocked ⬃90% of total OTA accu-
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number and location of anionic charges or partial charges
and 2) the size and placement of hydrophobic regions.
Ullrich (465) developed a model of the chemical and
spatial features of a PAH transporter binding site. The
general requirements for interaction with the putative
binding site are a single (or partial) anionic charge and a
hydrophobic domain with a length of at least 4 Å. A
second full or partial anionic charge located 6 –7 Å from
the first is also acceptable, thereby accounting for the
interaction of dicarboxylates such as ␣-KG with the transporter. The affinity increases with increasing hydrophobicity, up to a hydrophobic domain length of 8 –10 Å
(located slightly off-axis to that of the two anionic
charges).
IV) Multiplicity of basolateral OA transporters. Ullrich’s model should be viewed in the context of current
knowledge that there are multiple OA transporters coexpressed in the basolateral membrane of the proximal
tubule. Furthermore, there appear to be marked species
differences in the relative distribution of different OAT
proteins within renal tubules, complicating the development of models that attempt to explain or predict the
basis of the selectivity of renal OA secretion. For example, the human kidney expresses OAT1, OAT2, and OAT3
in the basolateral membrane of renal proximal tubule
(RPT) cells, although OAT2 expression appears to be
comparatively low, and OAT1 appears to be more broadly
distributed than OAT3 along the length of the RPT (300).
In contrast, in rat kidney OAT1 and OAT3 are restricted to
the basolateral membrane of the RPT and OAT2 is limited
to the apical membrane of the thick ascending limb and
collecting duct (222). In addition, within the rat RPT,
OAT1 expression is largely restricted to the S2 segment,
whereas OAT3 appears to be expressed in the basolateral
membrane of S1, S2, and S3 segments (222). Finally,
marked differences between kinetic values obtained with
intact tubules and those obtained with cloned transporters in heterologous expression systems (Fig. 7) further
complicate development of models of the mechanistic
basis of tubular OA transport.
The rabbit kidney offers an opportunity to compare
functional expression of different OAT transporters in
different regions of the proximal tubule. PAH secretion is
4- to 10-fold higher in the S2 segment than in the S1 or S3
segments (390, 391, 528), which is qualitatively consistent
with the distribution of OAT1 protein in the rat noted
previously (222). Unlike the rat and human, rabbit OAT3
interacts very weakly with PAH, and transport of PAH
appears to be effectively restricted to OAT1 in the rabbit
kidney (249). Consequently, the profile of PAH secretion
along the length of the rabbit proximal tubule probably
reflects the distribution of OAT1. The apparent specificity
of PAH for OAT1 in rabbit kidney clarifies interpretation
of studies that have used PAH as an inhibitor of secretion
of other anionic substrates. Whereas inhibition by PAH of
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
Physiol Rev • VOL
ing the secretory flux of selected organic anions. Following is a discussion of recent studies employing isolated
membrane vesicles and intact renal tubules that address
issues of mechanism, selectivity, and multiplicity of luminal OA efflux.
B) MECHANISM OF LUMINAL PAH (I.E., TYPE I) OA TRANSPORT. In
isolated renal tubules, the apparent permeability of the
luminal membrane to OAs is several times greater than
the apparent passive permeability of the basolateral membrane (76, 461). This favors movement into the tubule
lumen of OAs that have entered the tubule cells at the
basolateral side. Although this movement from the cells
into the lumen could involve simple passive diffusion, the
apparent permeability of the luminal membrane is much
greater than would be expected for simple passive diffusion of a charged molecule across a biological membrane.
Moreover, transport can be inhibited by probenecid and
other inhibitors in isolated tubules and BBMV from a
number of species (21, 77, 78, 211). Unidirectional luminal
efflux of fluorescein has been measured in isolated single
perfused rabbit tubules (397) and was found to be saturable
with a Kt of ⬃560 ␮M and a Jmax of 635 fmol · min⫺1 · mm⫺1.
Comparison of these values with those measured for net
transepithelial fluorescein secretion in the same system
(maximal rate of secretion of ⬃280 fmol · min⫺1 · mm⫺1,
half-saturated at ⬃4 ␮M; Ref. 395) reveals the luminal efflux
step to be of both lower affinity and higher capacity than
the basolateral influx step, consistent with the view that
the active basolateral uptake step is rate limiting during
transepithelial secretion. The comparatively low affinity
of the luminal efflux process(es) for what is usually considered to be a “high-affinity” substrate (i.e., fluorescein)
also lends credibility to the potential role in secretion of
several cloned transporters, including MRP2 and OATV1,
that have been shown to transport type I OAs but to have
a low (millimolar) affinity for these substrates (e.g., Refs.
188, 239).
The energetic basis of the luminal OA efflux step is
not clear. Although an OA exchanger that can be shared
by PAH, urate, Cl⫺, and OH⫺ has been physiologically
characterized in the luminal membrane of some mammalian species (e.g., dog, rat; Refs. 146, 192, 211), changes in
luminal pH do not appear to influence efflux of PAH from
proximal cells in intact rat tubules (472). Also, replacement of Cl⫺ in the luminal fluid does not eliminate PAH
secretion by isolated perfused rabbit proximal tubules
(59). Isolated BBMV from human (358) and bovine (379,
380) kidney display mediated PAH/␣-KG exchange with
characteristics similar to those observed in BLMV. However, studies employing the doubly perfused rat renal
proximal tubule method (472) failed to demonstrate a
trans-stimulation of luminal PAH efflux in the presence of
inwardly directed gradients of either PAH or ␣-KG. Interestingly, the presence in the luminal fluid of substrates for
which the luminal pathway was suspected to have a high
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mulation, suggesting that, in the S2 segment, OTA transport is dominated by its interaction with OAT1. However,
in the studies with cortical tubule suspensions and primary cultured cells, the block of OTA transport produced
by 2.5 mM PAH was not complete (⬃40%), although other
anionic compounds, including probenecid and octanoic
acid, blocked OTA uptake by 80 –90%. These observations
suggest that OTA uptake into these preparations involved
interaction with OAT1 and (at least) one more process.
Thus the differential distribution of multiple OAT transporters with overlapping, but nevertheless distinct specificities, must be considered when modeling the renal
secretion of anionic compounds.
OAT1 and OAT3 share the same energetic mechanism of transport (i.e., mediated exchange of OAs for
intracellular dicarboxylates; Refs. 386, 421, 427) and, as
noted above, are coexpressed in at least some renal proximal cells (249, 300). Moreover, although OAT1 and OAT3
clearly have overlapping substrate specificities (e.g., human, rat and mouse OAT3 all transport PAH; Refs. 52, 232,
424), they can also discriminate between selected substrates (whereas, for example, mOAT3 and rbOAT3 transport ES, mOAT1 and rbOAT1 do not; Ref. 424). Ullrich’s
model for the molecular basis of selectivity by the PAH/
dicarboxylate exchanger must, therefore, be viewed as
representing “average” physical and structural criteria
that influence the physiological secretion of organic anions by a suite of processes working in parallel. The
ability to predict the extent to which any one of these
processes influences total secretion of a given compound
is critically important to efforts to predict and thereby
prevent unwanted drug interactions at the level of OAT
transporters (e.g., Refs. 203, 204, 433). Accurate predictions will, however, require an understanding of both the
degree of interaction of specific substrates with each
member of the entire suite of transporters expressed in
the RPT and the profile of distribution within the tubule of
each of the transporters that interact significantly with the
selected agents. Therefore, additional studies that focus
on the selectivity characteristics of the individual transporters, in conjunction with their functional distribution
along the length of nephron, will be required before a
physiological model of the structural basis of renal OA
secretion can be developed.
V) Apical/brush border/luminal OA transport. Although mediated entry of type I OAs across the basolateral membrane appears to be largely restricted to two or
three processes, each of which (i.e., OAT1, OAT2 and
OAT3) has been cloned and characterized, luminal OA
efflux, and the processes underlying it, is less well understood. Evidence based on immunocytochemical and
Western blot data discussed previously indicates that
OAT4, OAT-K1, OAT-K2, OATP1, MRP2, NaPi-1, and
OATV1 are expressed in the luminal membrane of renal
proximal tubules. Thus each could play a role in mediat-
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tial role of this process in renal OA transport is discussed
at greater length below.
There is physiological evidence that luminal efflux of
at least some OAs can involve the parallel activity of
multiple transport processes. The fluorescent sulfonated
anion Lucifer yellow (LY) is actively secreted by flounder
proximal tubules (260). Whereas basolateral LY uptake in
flounder RPT appears to be limited to an interaction with
an OA/DC exchanger (probably including flOAT1), secretion of LY across the luminal membrane is partially
blocked by leukotriene C4 (LTC4), a stereotypic inhibitor
of MRP2, suggesting that ⬃50% of luminal LY efflux involves MRP2. LY is a divalent anion of molecular weight
443 and, so, is on the cusp of the loose definition of type
I versus type II OAs employed here. Nevertheless, luminal
secretion of fluorescein was not affected by LTC4, consistent with the failure of fluorescein to inhibit rbMRP2mediated transport (495). This further supports the contention that multiple processes play a role in luminal OA
efflux.
PAH has also been shown to be a substrate for MRP2
(human, Ref. 239; rabbit, Ref. 497). The apparent affinity
of MRP2 for PAH is comparatively low: the Kt values for
ATP-dependent PAH transport are 0.9 and 1.1 mM for the
human and rabbit orthologs of MRP2, respectively. These
values contrast sharply with the Kt values of 1.5 ␮M for
vinblastine transport by rbMRP2 (495) and 7.2 ␮M for
E17␤-D transport by hMRP2 (73). However, as noted
above, given that the apparent Kt for luminal fluorescein
efflux in intact rabbit tubules is ⬃560 ␮M (397), the
comparatively low affinity of MRP2 for PAH should not be
interpreted as being indicative of a quantitatively insignificant role for this process in luminal secretion of PAH.
Indeed, for hMRP2 there is less than a 10-fold difference
in the Jmax/Kt ratio for PAH and the prototypic MRP2
substrate E17␤-D (239), suggesting that PAH is at least a
modestly effective substrate for MRP2.
The unanswered question is, “What percentage of
total luminal efflux of PAH (or any other type I OA) is
carried by each of the growing list of candidate transporters?” On the basis of reported interactions with PAH
(albeit, in some cases, weak), this list now includes MRP2,
OATV1/NaPi-1, OAT4, and OAT-K2. Unfortunately, with
the exception of the study on LY secretion in flounder
tubules mentioned previously, no studies have determined the effect on total tubular substrate secretion of
selectively blocking one or more of these potential pathways. Such studies will be difficult to design and interpret
because few compounds are known to selectively interact
with a restricted population of suspected luminal transport processes. Nevertheless, the issue under question,
i.e., mapping the contribution to luminal OA efflux of
different transport proteins, is important enough to the
overall understanding of renal OA secretion to warrant
the identification (e.g., using heterologous expression sys-
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affinity, including a variety of uricosuric drugs (e.g., losartan), generally produced a trans-inhibition of luminal
PAH efflux (472). In those mammalian species in which a
luminal OA exchanger exists, it appears poised to reabsorb OAs (e.g., urate), rather than to secrete OAs (e.g.,
PAH) (356).
Although luminal OA flux may, in some species, involve anion exchange, it may also involve a potentialdriven conductive pathway. Indeed, given the presence of
active accumulation of PAH (and other type I OAs) at the
basolateral membrane via OA/DC exchange, the simplest
luminal transport mechanism consistent with net secretion is an electrogenic PAH transporter or “channel.”
Studies with pig (228, 520), rabbit (213, 257), and rat (171,
315) BBMV have described an inhibitable conductive
pathway for PAH (and urate). In pig BBMV, for example,
mediated PAH and urate efflux clearly involve a saturable,
low-affinity (half-saturated at 12 mM) electrogenic process that is distinct from the conductive pathway for Cl⫺
in the same membrane (228). The cloning of an electrogenic PAH transporter from pig kidney that is expressed
in the apical membrane of proximal tubule cells (OATV1;
Ref. 188) offers a novel new molecular candidate for this
process. Conductive pathways may exist in parallel with
exchange pathways, as suggested by studies with isolated
rat renal BBMV (315). However, experiments with intact
rat (472) and flounder (283) proximal tubules failed to
show any change in luminal OA efflux following depolarization of the luminal membrane (via elevation of extracellular [K⫹]). It has been suggested that species that
display net urate secretion (e.g., rabbit, pig) may depend
on potential-driven, conductive pathways to support luminal OA efflux, whereas species that exhibit net urate
absorption (rat, dog, human) may typically have both
voltage-driven and exchanger/mediated modes of luminal
PAH flux (356).
C) SPECIFICITY AND MULTIPLICITY OF LUMINAL OA TRANSPORTER
PATHWAYS. The molecular identity of the luminal pathway
for PAH efflux is not clear. As noted above, there is
evidence, in different species, supporting the presence of
both conductive processes and those that involve mediated PAH/anion exchange. OAT-K1 (363), OAT-K2 (262),
and OATP1 (416) display little or no interaction with PAH
and other type I OAs and so are unlikely to be major
elements within the classical OA secretory pathway.
OAT4 does support PAH transport, albeit with comparatively low affinity (7). Although the human ortholog of
NPT1 (NaPi-1) does accept PAH as a substrate (462),
rabbit NaPi-1 does not (45), making it unlikely that NaPi-1
is the principal luminal element in the OA secretory pathway. The NPT1 homolog from pig kidney, OATV1 (188),
supports electrogenic PAH transport and is localized
within proximal cells to play a potential role in transepithelial OA secretion. Human and rabbit MRP2 both support the ATP-dependent transport of PAH, and the poten-
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
tems) of discriminatory inhibitors for all the candidate
transporters, and then applying this information in studies
(e.g., using in vitro tubular perfusion or in vivo microperfusion methods) comparing basolateral uptake and transepithelial secretion of a range of type I OA substrates.
2. Regulation of tubular OA transport
Physiol Rev • VOL
The extent to which PKC-mediated regulation of type
I OA transport involves direct phosphorylation of the
transporter(s) is not known. In a study employing LLCPK1 cells that stably expressed mOAT1 (545), exposure to
okadaic acid (to inhibit phosphatase activity) resulted in
both a downregulation of PAH uptake and an increase in
phosphorylation of mOAT1. However, the quantitatively
similar decrease in transport activity that followed exposure to phorbol ester was not associated with a change in
mOAT1 phosphorylation. The decrease of mOAT1-mediated PAH reflected a decrease in the Jmax for transport
and not a change in Kt (545), leading to the suggestion
that the regulation of mOAT1 activity induced by activation of PKC may involve changes in the trafficking of
transport protein, an observation confirmed in studies of
the PKC-mediated downregulation of hOAT1 activity expressed in Xenopus oocytes (524).
Other kinases have also been implicated in the regulation of renal transport of type I OAs. Inhibition of CKII
activity has been shown to decrease PAH uptake into
isolated rabbit RPT (118). Activation of PKA with forskolin has also been shown to inhibit PAH uptake in isolated
rabbit RPT (133). This latter observation is, however, at
odds with the recent observation that basolateral PAH
uptake into OK cells is stimulated by exposure to forskolin (371). The basis for this apparent discrepancy is not
clear, but it may reflect the different concentrations of
forskolin employed in these two studies (100 versus 5 ␮M,
respectively). Indeed, the stimulatory influence of PKA
activation on basolateral OA transport is consistent with
the observed stimulation of PAH transport in OK cells
(369, 370) and intact rabbit RPT (368) by exposure to
epidermal growth factor (EGF). Furthermore, inhibition
of the mitogen-activated protein kinase (MAPK) pathway
(through exposure to PD98059) inhibits basolateral PAH
uptake in intact rabbit RPT (119). These data support a
cascade of events that includes EGF receptor activation
of the MAPK, MEK (mitogen-activated extracellular signal-regulated kinase kinase) and ERK1/2 (extracellular
signal-regulated kinase isoforms 1 and 2), and the subsequent activation of phospholipase A2 and an increase in
cytoplasmic arachidonic acid (AA). Cyclooxygenase-1
(COX-1) mediates metabolism of AA to prostaglandin E2
(PGE2) that then activates adenylate cyclase. The resulting rise in cAMP leads to activation of PKA and finally to
the previously noted increase in basolateral OA transport.
The activity of basolateral OA/DC exchange is functionally linked to the activity of NaDC cotransport (340,
389, 517). Consequently, regulation of NaDC cotransport
offers an alternative means to influence basolateral uptake of OAs. Flounder NaDC-3 activity (expressed in Xenopus oocytes) responds to activation of PKC with a
marked downregulation of transport that occurs in parallel with the endocytosis of NaDC-3 protein (150). As noted
earlier for PKC-induced downregulation of OAT1 activity,
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Organic anion transport in the kidney is influenced
by a wide array of regulatory processes (19, 453). Regulation of PAH transport has generally been the focus of
study, and the potential influence of multiple transporters
that accept PAH as a substrate in both basolateral and
luminal membranes complicates the interpretation of
much of the work to date. Nevertheless, it is evident that
renal secretion of organic anions can be dynamically
modulated. The following is a discussion of observations
that pertain to mechanisms of regulation of renal transport of type I and type II OAs that appear to involve rapid,
kinase-mediated events, and long-term, steroid hormonemediated events.
A) KINASE-MEDIATED MODULATION OF RENAL BASOLATERAL OA
TRANSPORT. Activation of PKC influences transport of type
I OAs in intact renal tubules (122, 161, 165, 278, 396) and
in cultured OK cells (303, 369, 431). Transport has been
influenced by both direct activation of PKC using phorbol
esters or diacylglycerol analogs (122, 161, 278, 369, 396,
431), and by indirect, receptor-mediated activation (122,
303, 396). In all these cases, the effects of PKC activation
were reversed or blunted by preexposure of cells to PKC
inhibitors. Although in one study activation of PKC
caused a stimulation of PAH uptake into intact rabbit RPT
(161), the profile observed in all other studies has involved a dose-dependent decrease in basolateral OA uptake. For example, exposing isolated single nonperfused
S2 segments of rabbit RPT to bradykinin or phenylephrine
caused a dose-dependent decrease (25– 40%) in basolateral uptake (122) or trans-tubular secretion (396) of fluorescein. This effect occurred within 10 min of exposure,
was maximal within 60 min, and was blocked by preexposure to the PKC inhibitor bisindolylmaleimide I (122).
Depending on the substrate involved, the molecular
basis of the regulation of basolateral type I OA transport
in native tubules or cultured cells could reflect modulation of either OAT1 or OAT3 (or both). Activation of PKC
with phorbol esters has been shown to decrease activity
of both OAT1 [human, expressed in HeLa cells (248); rat,
Xenopus oocytes (491); mouse, LLC-PK1 cells (545)] and
OAT3 [rat, Xenopus oocytes (436)]. The decrease in transport noted in rabbit tubules (122, 396), for example, reflected changes in measured rates of accumulation of
fluorescein, which in the rabbit is a substrate for both
OAT1 (8) and OAT3 (Zhang and Bahn, personal communication). Consequently, how much these effects reflect
modulation of one or both of these processes is not clear.
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STEPHEN H. WRIGHT AND WILLIAM H. DANTZLER
Physiol Rev • VOL
step (122) without the need to invoke a PKC-mediated
change in the luminal exit step.
Transepithelial PAH secretion across confluent
monolayers of OK cells shows a different profile in which
activation of PKC results in a downregulation of both
basolateral PAH entry and luminal exit (369). Similarly,
inhibition of MEK resulted in a downregulation of both
the basolateral and luminal transport steps associated
with PAH secretion in OK cells (369). It is not clear if the
apparent difference in apical response to PKC activation
in cultured OK cells versus intact rabbit and flounder
tubules reflects species differences in luminal exit mechanism(s), differences in substrate studied [i.e., luminal FL
exit may involve pathway(s) different from those for
PAH], or simply the technical difficulty of clearly assessing functional activity of luminal efflux processes (particularly in intact tubules).
MRP2, which mediates luminal secretion of type II
OAs (260) and may play a role in the luminal efflux of at
least some type I OAs (239, 497), is also regulated by PKC.
Activation of PKC in human hepatoblastoma (HepG2)
cells results in a downregulation of MRP2-mediated efflux
of fluorescent type II substrates by a process that involves
internalization of transport protein from canalicular membrane (229). Miller and colleagues (261, 280, 310, 452)
have also linked activation of PKC in intact teleost renal
proximal tubules to the rapid downregulation of MRP2mediated efflux of the type II OA fluorescein-methotrexate. Their studies documented a cascade of events involving endothelin (ET), nitric oxide (NO), and PKC (see Refs.
280, 310). The scheme involves 1) release of ET from
proximal tubule cells, 2) the autocrine/paracrine activation of ETB receptors on proximal tubule cells, 3) activation of NO synthase and production of NO, and 4) NOdependent activation of PKC. Activation of PKC then
downregulates MRP2, although it is not clear if PKC
changes MRP2 trafficking in the kidney, as seen in the
liver (229). ET is frequently released in response to renal
injury, including nephrotoxic insults (70, 160). The shortterm response of this regulatory scheme, including as it
does the PKC-mediated downregulation of basolateral OA
accumulation, may play a protective role for renal cells.
However, the parallel decrease in MRP2-mediated luminal
efflux may also place renal cells at increased risk of the
effects of toxic anions that are no longer exported from
the cells across the luminal membrane (280, 454).
NaDC-1 responds to physiological regulation in a
manner similar to that noted earlier for NaDC-3. Activation of PKC caused a 95% decrease in Na⫹-dependent
succinate uptake into oocytes expressing NaDC-1, and
this effect was blocked by preexposure to the PKC inhibitor staurosporine (331). Mutations of two consensus PKC
sites within the NaDC-1 sequence did not prevent the
PKC-mediated downregulation of succinate transport. Oocytes treated with PMA displayed a 30% decrease in
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mutation of the five consensus PKC phosphorylation sites
of flNaDC3 had no effect on the downregulation of transport activity that followed exposure of oocytes to phorbol
ester. The extent to which this regulatory response to
PKC activation occurs in intact tubules is not clear in light
of the observation that exposure of isolated rabbit S2
segments to phorbol ester appears to have no effect on
the rate of Na⫹-dependent glutarate transport (359). In
addition, exposure to PD98059 (MAPK inhibitor),
genistein (tyrosine kinase inhibitor), wortmannin (phosphatidylinositol 3-kinase inhibitor), or KN93 (CaMII inhibitor) has no effect on basolateral glutarate uptake in intact
tubules (118, 119).
B) KINASE-MEDIATED MODULATION OF RENAL LUMINAL OA
TRANSPORT. Twenty-five years ago, E. Wright likened the
existing knowledge of transport in the basolateral aspect
of the intestinal epithelium to the previous decade’s
knowledge of “the dark side of the moon” (530). The
substantial difference in understanding of transport processes of the luminal versus basolateral side of intestinal
cells reflected, in large part, the ease of experimental
access to the former, compared with the latter. An inverse
relationship exists with respect to the study of renal
transporters; whereas the basolateral (peritubular) side of
renal tubules is easily accessed, the luminal aspect is the
“dark side” of the renal epithelium. Generally speaking,
experimental access to the luminal membrane of living
renal cells is limited to technically challenging studies
that employ microperfusion methods. Even then, assessment of the contribution of luminal processes to transepithelial secretion is typically complicated by the necessity
that substrates cross the basolateral membrane before
they can interact with luminal membrane transporters.
Although such complications have plagued all aspects of
the study of secretory processes, they have had a particular impact on the study of the regulation of luminal
transport processes simply because of the difficulty of
testing the influence of signal cascade activation on the
initial rate of transport across the luminal membrane.
Two studies have examined the influence of activation of PKC on the steady-state transepithelial flux of the
type I OA fluorescein in intact renal proximal tubules.
Treating isolated flounder proximal tubules with phorbol
ester (PMA) reduces intracellular accumulation of fluorescein but has no effect on the lumen-to-cell ratio of
fluorescein distribution across the apical membrane
(278). Whereas the former effect is consistent with the
well-documented downregulation of basolateral OA/dicarboxylate exchange, the latter observation suggests
that activation of PKC has no effect on luminal fluorescein
efflux. Similarly, the decrease in transepithelial fluorescein secretion measured in isolated perfused rabbit proximal tubules following activation of PKC (396) can be
explained by the downregulation of the basolateral entry
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
Physiol Rev • VOL
transport could result in sex-based differences in rates of
drug excretion or exposures of renal tissues to anionic
drugs and nephrotoxicants.
OATP1 expression in the kidney, where it appears to
be restricted to the luminal membrane of the S3 segment
of renal proximal tubules (18), is under the control of sex
steroids. In Northern blots of renal total mRNA, male rats
display much higher levels of OATP1 mRNA than do
female rats (247). A similar profile of OATP1 mRNA expression was observed in a study employing the
branched-DNA signal amplification method (244). Administering testosterone to female rats increases, and administering estradiol to male rats decreases, renal expression
of OATP1 mRNA (247). In contrast to the marked sex
steroid sensitivity of renal OATP1 expression, sex steroids do not influence expression of OATP1 in rat liver
(18). It has been suggested that increased expression of
OATP1 in the apical membrane of proximal tubule cells in
male rats facilitates clearance of glucuronidated steroids,
including conjugated estradiol, thereby assisting in the
maintenance of “maleness.”
3. Intracellular OA sequestration
In addition to the basolateral entry and luminal exit
steps that are requisite elements of net transepithelial OA
transport, secreted substrates must traverse the cell cytoplasm. Although this process is frequently tacitly assumed to involve simple, passive diffusion through an
aqueous cytoplasm, a number of observations indicate
that secreted OAs are not uniformly distributed within the
intracellular compartment. Binding to cytoplasmic constituents has been known for sometime to contribute to
intracellular accumulation of selected OAs, including phenol red, PAH, and probenecid (e.g., Refs. 20, 107, 167; see
also Ref. 342). However, in addition to passive binding of
OAs, several observations support the contention that at
least some OAs can be actively sequestered within intracellular vesicles. Optical methods were used to demonstrate directly the “punctate” sequestration of FL within
intact flounder proximal tubules, cultured OK cells, and
crab urinary bladder (281, 285). When FL was directly
injected into the cytoplasm of OK cells, its accumulation
within these punctate compartments was energy dependent and blocked by coinjection of PAH. Treating tubules
with the microtubule inhibitor nocodazole significantly
reduced transepithelial secretion of fluorescein into the
lumen of isolated flounder proximal tubules (284). Studies
of the effect of nocodazole on fluorescein distribution in
crab urinary bladder (281) offered some insight into the
possible mechanistic basis of the reduction in secretion
observed in flounder tubules. Accumulated fluorescein
within control bladder cells was found in two discontinuous compartments: one that was diffuse and evenly distributed through the cytoplasm and one that was punc-
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NaDC-1 protein in the plasma membrane, suggesting that
the downregulation of transport activity involves (but
may not be restricted to) internalization of transport protein. Downregulation of NaDC-1 by activation of PKC may
also play a small role in the downregulation of basolateral
OA uptake in perfused tubules observed with PKC activation because ␣-KG transport by NaDC-1 apparently
accounts for a small portion of the outwardly directed
␣-KG gradient at the basolateral membrane (395, 396).
Activation of PKA appears to have no effect on NaDC-1
activity (331).
C) STEROID-MEDIATED MODULATION OF RENAL OA TRANSPORT.
Tubular transport of PAH is influenced by glucocorticoids
(30), 1,25-dihydroxycholecalciferol (31), and sex steroids
(29, 48 –50). PAH accumulation in human renal slices is
increased by exposure to dexamethasone (114). Treatment of rats (particularly 5- to 10-day-old rats with immature kidney function) with the glucocorticoids prednisolone or dexamethasone results in increased renal excretion of PAH and increased PAH accumulation in renal
cortical slices (30). In contrast, treatment with mineralocorticoids (desoxycorticosterone, aldosterone) had no effect (30). Synergistic interactions were also observed following treatment with multiple hormones. Calcitriol
(1,25-dihydroxycholecalciferol), which by itself reduces
PAH accumulation in renal cortical slices in rats of
various age groups, can prevent the increase in renal
PAH transport that follows treatment with triiodothyronine (31).
Sex steroids have a marked influence on renal PAH
transport. Cortical slices from adult male rat kidneys
display higher tissue-to-medium ratios of PAH accumulation than do slices from female rat kidney (29), and this
difference is correlated with a more than twofold higher
expression of OAT1 protein (by Western analysis) in
BLMV from male than from female rat kidneys (48).
Whereas castration of male rats results in a significant
decrease in renal accumulation of PAH, ovarectomy has
no effect on PAH transport in female rat kidneys (29).
This relative refractoriness of sex steroid regulation of
PAH transport in female rat kidneys is also observed
when treating rats with sex steroids; whereas PAH accumulation in male rat kidneys increases significantly following treatment of the animals with testosterone, it does
not change in female rat kidneys with the same treatment.
Generally, the higher rates of PAH transport in kidneys of male rats are correlated with higher levels of
expression of mRNA for OAT1 and OAT3 in renal tissue
from male rats (compared with female rats) (35, 36, 196,
230). The most extreme sex difference in expression,
however, is the difference in mRNA for OAT2, which is
four- to eightfold higher in female rat kidney (35, 36, 196,
230). The functional consequences of these differences in
OAT expression to actual rates of renal OA uptake or
secretion are not clear, but sexual dimorphism of OA
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STEPHEN H. WRIGHT AND WILLIAM H. DANTZLER
IV. SPECULATIONS AND CONCLUSIONS
Our understanding of the renal handling of organic
cations and anions has been profoundly influenced by the
introduction of molecular methods for the study of transport proteins. Since the cloning of OCT1 in 1994, at least
12 distinct members of the OCT family of organic electrolyte transporters (alternatively, amphiphilic solute facilitators) have been identified, and selected orthologs
cloned in some nine species, including six mammalian
species. In addition, functional evidence has been obtained implicating members of at least five other transport
protein families in the secretion of selected OCs and OAs.
Studies employing these cloned transporters and associated molecular tools have established that the “classical” pathways for renal secretion of organic cations and
anions consist of multiple processes arranged in parallel
and in series, processes that involve different energetic
mechanisms with overlapping yet distinct substrate specificities. Indeed, that very complexity of organization with
multiple redundancies speaks to the physiological importance of efficient xenobiotic excretion.
It now seems reasonable to speculate on “why” renal
OC and OA secretion is organized the way it appears to
be. Extant transport processes probably arose, in evolutionary terms, in response to the extraordinarily diverse
array of secondary plant products to which heterotrophs
are routinely exposed (now including man-made “variants” of these plant products). The chemical/structural
diversity of these agents was the probable “selective pressure” that led to the retention within evolving genomes of
multiple transport processes that represented an increasPhysiol Rev • VOL
ing number of “variations” on a general theme. The central element of this theme may have been a binding site
that 1) interacted with molecules that shared a limited set
of physical determinants, i.e., a degree of hydrophobicity
plus a cationic or anionic charge center, and 2) imposed
a limited (but finite) set of steric constraints on binding to
a structurally diverse population of substrates. Inclusion
within genomes of multiple transport homologs with
overlapping yet distinct selectivity characteristics would
then have increased the structural range of xenobiotic
agents that could be effectively handled by animals that
were, in turn, exposed to an ever-increasing array of
plant-derived products.
The requirement for transport processes that could
interact efficiently with a structurally diverse array of
substrates may also have been the pressure that led to
reliance on exchange processes, rather than cotransport
processes, as the principal strategy for moving (type I)
organic anions and cations against their electrochemical
gradients. The binding of a given substrate to a cotransporter apparently occurs when a conformational change
in the cotransporter, consequent to its binding of one or
more activator ions (typically Na⫹), increases its affinity
for the substrate in question (245, 275, 336, 529). The
magnitude of such conformational changes is not clear,
but it seems reasonable to infer that a structural change
that, for example, increases the affinity of SGLT1 for
D-glucose but not for L-glucose is comparatively subtle. It
seems likely that the conformational changes required by
this cotransporter paradigm are ill suited for transport
processes that must be capable of the multiselectivity
shown by OCTs and OATs. However, future studies on the
molecular nature of substrate binding and translocation
will be required before firm conclusions can be drawn
about this issue.
The advent of molecular approaches to the study of
renal OC and OA transport should result in continuing
advances in understanding of both the molecular and
systems physiology of these processes. As noted above,
the ability to examine the activity of single transport
proteins should lead to a clearer understanding of the
structural basis of the multiselectivity of renal OC and OA
transporters, which has been a hallmark of their secretory
activity. Application of molecular tools is also enhancing
studies of the mechanistic basis of the physiological and
developmental regulation of OA and OC transport. At the
cellular and organ level, the challenge is now to integrate
the information being obtained from the study of individual molecular processes into the physiological context of
intact renal tissues. Particular areas of emphasis will need
to involve establishing the functional distribution of these
distinct processes and their profiles of physiological and
developmental regulation. Given the importance of these
processes for human health, we believe that other focal
points for future studies should include the influence of
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tate. Accumulation in both compartments was blocked by
addition of PAH or glutarate to the bath, implicating entry
into bladder cells via OA/dicarboxylate exchange. Moreover, whereas treating the cells with nocodazole had no
effect on the level of accumulation in the diffuse distributed compartment, it significantly reduced accumulation
in the punctate compartment. Confocal microscopy revealed that the individual sites of punctate accumulation
displayed a net basolateral-to-apical movement of ⬃0.8
␮m/min. Nocodazole nearly abolished this movement and
significantly reduced net transepithelial secretion of fluorescein across crab bladder (281). These data suggest that
intravesicular sequestration can play a quantitatively significant role in the transepithelial flux of selected OAs in
(at least) some epithelial systems. It should be stressed,
however, that active sequestration of OAs has not been
shown to play a role in the secretion of OAs by physiologically intact mammalian renal tubules. Consequently,
the potential role of intracellular compartmentalization to
the processes of secretion and detoxification of OAs by
the kidney remains to be established.
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
sex differences on expression of renal OC and OA transporters and the consequences of polymorphisms of OC
and OA transporters on the pharmacokinetics and toxicokinetics of renal secretion.
NOTE ADDED IN PROOF:
We gratefully acknowledge support from National Institutes of Health Grants DK-58251, DK-56224, ES-04940, ES-06694,
and HL-07249 and from National Science Foundation Grant
IBN-9814448.
Address for reprint requests and other correspondence:
S. H. Wright, Dept. of Physiology, College of Medicine, Univ. of
Arizona, Tucson, AZ 85724 (E-mail: [email protected]).
REFERENCES
1. Abramson J, Smirnova I, Kasho V, Verner G, Kaback HR, and
Iwata S. Structure and mechanism of the lactose permease of
Escherichia coli. Science 301: 610 – 615, 2003.
2. Ambudkar SV, Dey S, Hrycyna CA, Ramachandra M, Pastan I,
and Gottesman MM. Biochemical, cellular, and pharmacological
aspects of the multidrug transporter. Annu Rev Pharmacol Toxicol
39: 361–398, 1999.
3. Ando H, Nishio Y, Ito K, Nakao A, Wang L, Zhao YL, Kitaichi
K, Takagi K, and Hasegawa T. Effect of endotoxin on P-glycoprotein-mediated biliary and renal excretion of rhodamine-123 in
rats. Antimicrob Agents Chemother 45: 3462–3467, 2001.
4. Apiwattanakul N, Sekine T, Chairoungdua A, Kanai Y, Nakajima N, Sophasan S, and Endou H. Transport properties of
nonsteroidal anti-inflammatory drugs by organic anion transporter
1 expressed in Xenopus laevis oocytes. Mol Pharmacol 55: 847–
854, 1999.
5. Arndt P, Volk C, Gorboulev V, Budiman T, Popp C, UlzheimerTeuber I, Akhoundova A, Koppatz S, Bamberg E, Nagel G, and
Koepsell H. Interaction of cations, anions, and weak base quinine
with rat renal cation transporter rOCT2 compared with rOCT1.
Am J Physiol Renal Physiol 281: F454 –F468, 2001.
6. Aslamkhan A, Han YH, Walden R, Sweet DH, and Pritchard
JB. Stoichiometry of organic anion/dicarboxylate exchange in
membrane vesicles from rat renal cortex and hOAT1-expressing
cells. Am J Physiol Renal Physiol 285: F775–F783, 2003.
7. Babu E, Takeda M, Narikawa S, Kobayashi Y, Enomoto A,
Tojo A, Cha SH, Sekine T, Sakthisekaran D, and Endou H.
Role of human organic anion transporter 4 in the transport of
ochratoxin A. Biochim Biophys Acta 1590: 64 –75, 2002.
Physiol Rev • VOL
8. Bahn A, Knabe M, Hagos Y, Rodiger M, Godehardt S, GraberNeufeld DS, Evans KK, Burckhardt G, and Wright SH. Interaction of the metal chelator 2,3-dimercapto-1-propanesulfonate
with the rabbit multispecific organic anion transporter 1 (rbOAT1).
Mol Pharmacol 62: 1128 –1136, 2002.
9. Bahn A, Prawitt D, Buttler D, Reid G, Enklaar T, Wolff NA,
Ebbinghaus C, Hillemann A, Schulten HJ, Gunawan B, Fuzesi
L, Zabel B, and Burckhardt G. Genomic structure and in vivo
expression of the human organic anion transporter 1 (hOAT1)
gene. Biochem Biophys Res Commun 275: 623– 630, 2000.
10. Bakhiya A, Bahn A, Burckhardt G, and Wolff N. Human organic
anion transporter 3 (hOAT3) can operate as an exchanger and
mediate secretory urate flux. Cell Physiol Biochem 13: 249 –256,
2003.
11. Bakos E, Evers R, Sinko E, Varadi A, Borst P, and Sarkadi B.
Interactions of the human multidrug resistance proteins MRP1 and
MRP2 with organic anions. Mol Pharmacol 57: 760 –768, 2000.
12. Ballatori N and Rebbeor JF. Roles of MRP2 and oatp1 in hepatocellular export of reduced glutathione. Semin Liver Dis 18:
377–387, 1998.
13. Barendt WM and Wright SH. The human organic cation transporter (hOCT2) recognizes the degree of substrate ionization.
J Biol Chem 277: 22491–22496, 2002.
14. Barthe L, Woodley J, and Houin G. Gastrointestinal absorption
of drugs: methods and studies. Fundam Clin Pharmacol 13: 154 –
168, 1999.
15. Bednarczyk D, Ekins S, Wikel JH, and Wright SH. Influence of
molecular structure on substrate binding to the human organic
cation transporter, hOCT1. Mol Pharm 63: 489 – 498, 2003.
16. Bendayan R, Lo B, and Silverman M. Characterization of cimetidine transport in LLC-PK1 cells. J Am Soc Nephrol 5: 75– 84, 1994.
17. Bendayan R, Sullivan JT, Shaw C, Frecker RC, and Sellers
EM. Effect of cimetidine and ranitidine on the hepatic and renal
elimination of nicotine in humans. Eur J Clin Pharmacol 38:
165–169, 1990.
18. Bergwerk AJ, Shi X, Ford AC, Kanai N, Jacquemin E, Burk
RD, Bai S, Novikoff PM, Stieger B, Meier PJ, Schuster VL, and
Wolkoff AW. Immunologic distribution of an organic anion transport protein in rat liver and kidney. Am J Physiol Gastrointest
Liver Physiol 271: G231–G238, 1996.
19. Berkhin EB and Humphreys MH. Regulation of renal tubular
secretion of organic compounds. Kidney Int 59: 17–30, 2001.
20. Berndt WO. Probenecid binding by renal cortical slices and homogenates. Proc Soc Exp Biol Med 126: 123–126, 1967.
21. Berner W and Kinne R. Transport of p-aminohippurate acid by
plasma membrane vesicles isolated from rat kidney cortex.
Pflügers Arch 361: 269 –277, 1976.
22. Besseghir K, Pearce LB, and Rennick B. Renal tubular transport
and metabolism of organic cations by the rabbit. Am J Physiol
Renal Fluid Electrolyte Physiol 241: F308 –F314, 1981.
23. Blackmore CG, McNaughton PA, and van Veen HW. Multidrug
transporters in prokaryotic and eukaryotic cells: physiological
functions and transport mechanisms. Mol Membr Biol 18: 97–103,
2001.
24. Bleasby K, Chauhan S, and Brown CD. Characterization of
MPP⫹ secretion across human intestinal Caco-2 cell monolayers:
role of P-glycoprotein and a novel Na⫹-dependent organic cation
transport mechanism. Br J Pharmacol 129: 619 – 625, 2000.
25. Borst P, Evers R, Kool M, and Wijnholds J. The multidrug
resistance protein family. Biochim Biophys Acta 1461: 347–357,
1999.
26. Borst P, Evers R, Kool M, and Wijnholds J. A family of drug
transporters: the multidrug resistance-associated proteins. J Natl
Cancer Inst 92: 1295–1302, 2000.
27. Bowman HM and Hook JB. Sex differences in organic ion transport by rat kidney. Proc Soc Exp Biol Med 141: 258 –262, 1972.
28. Brändle E, Fritzsch G, and Greven J. Affinity of different local
anesthetic drugs and catecholamines for the contraluminal transport system for organic cations in proximal tubules of rat kidneys.
J Pharmacol Exp Ther 260: 734 –741, 1992.
29. Bräunlich H, Meinig T, and Grosch U. Postnatal development of
sex differences in renal tubular transport of p-aminohippurate
(PAH) in rats. Exp Toxicol Pathol 45: 309 –313, 1993.
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
The reader is directed to several recently published studies
that expand and/or clarify topics discussed in this review. 1)
Human OAT4 was shown to support mediated exchange of
selected OAs, including PAH and estrone-3-sulfate for dicarboxylates (i.e., glutarate) (96a). 2) RST was shown to support
exchange of urate for Cl⫺ or glutarate, supporting the contention that RST is the mouse ortholog of URAT1 (171a). 3) Phylogenetic analysis of members of the SLC22 family suggests that
organic anion and cation transporter-like genes already existed
before the divergence of vertebrates and invertebrates. The
authors suggest that subsequently, within the vertebrate lineage,
this gene family expanded through independent tandem duplications in each of the OAT, OCT, and OCTN subfamilies, producing multiple tandem gene pairs. The argument is developed
that this reflects a requirement for redundancy or broader substrate specificity in vertebrates (compared with invertebrates),
due to their greater physiological complexity and thus potentially broader exposure to organic ions (100a).
1035
1036
STEPHEN H. WRIGHT AND WILLIAM H. DANTZLER
Physiol Rev • VOL
49. Cerrutti JA, Quaglia NB, Brandoni A, and Torres AM. Effects
of gender on the pharmacokinetics of drugs secreted by the renal
organic anions transport systems in the rat. Pharmacol Res 45:
107–112, 2002.
50. Cerrutti JA, Quaglia NB, and Torres AM. Characterization of
the mechanisms involved in the gender differences in p-aminohippurate renal elimination in rats. Can J Physiol Pharmacol 79:
805– 813, 2001.
51. Cetinkaya I, Ciarimboli G, Yalcinkaya G, Mehrens T, Velic A,
Hirsch JR, Gorboulev V, Koepsell H, and Schlatter E. Regulation of human organic cation transporter hOCT2 by PKA, PI3K, and
calmodulin-dependent kinases. Am J Physiol Renal Physiol 284:
F293–F302, 2003.
52. Cha SH, Sekine T, Fukushima JI, Kanai Y, Kobayashi Y, Goya
T, and Endou H. Identification and characterization of human
organic anion transporter 3 expressing predominantly in the kidney. Mol Pharmacol 59: 1277–1286, 2001.
53. Cha SH, Sekine T, Kusuhara H, Yu E, Kim JY, Kim DK, Sugiyama Y, Kanai Y, and Endou H. Molecular cloning and characterization of multispecific organic anion transporter 4 expressed in
the placenta. J Biol Chem 275: 4507– 4512, 2000.
54. Chan BS, Lazzaro VA, Seale JP, and Duggin GG. Characterisation and uptake of paraquat by rat renal proximal tubular cells in
primary culture. Hum Exp Toxicol 15: 949 –956, 1996.
55. Chan BS, Seale JP, and Duggin GG. The mechanism of excretion
of paraquat in rats. Toxicol Lett 90: 1–9, 1997.
56. Chatsudthipong V and Dantzler WH. PAH-␣KG countertransport stimulates PAH uptake and net secretion in isolated snake
renal tubules. Am J Physiol Renal Fluid Electrolyte Physiol 261:
F858 –F867, 1991.
57. Chatsudthipong V and Dantzler WH. PAH/␣-KG countertransport stimulates PAH uptake and net secretion in isolated rabbit
renal tubules. Am J Physiol Renal Fluid Electrolyte Physiol 263:
F384 –F391, 1992.
58. Chatsudthipong V and Jutabha P. Effect of steviol on paraaminohippurate transport by isolated perfused rabbit renal proximal tubule. J Pharmacol Exp Ther 298: 1120 –1127, 2001.
59. Chatsudthipong V, Jutabha P, Evans KK, and Dantzler WH.
Effects of inhibitors and substitutes for chloride in lumen on
p-aminohippurate transport by isolated perfused rabbit renal proximal tubules. J Pharmacol Exp Ther 288: 993–1001, 1999.
60. Chen R, Jonker JW, and Nelson JA. Renal organic cation and
nucleoside transport. Biochem Pharmacol 64: 185–190, 2002.
61. Chen R and Nelson JA. Role of organic cation transporters in the
renal secretion of nucleosides. Biochem Pharmacol 60: 215–219,
2000.
62. Chen R, Pan BF, Sakurai M, and Nelson JA. A nucleosidesensitive organic cation transporter in opossum kidney cells. Am J
Physiol Renal Physiol 276: F323–F328, 1999.
63. Chen X, Tsukaguchi H, Chen XZ, Berger UV, and Hediger MA.
Molecular and functional analysis of SDCT2, a novel rat sodiumdependent dicarboxylate transporter. J Clin Invest 103: 1159 –1168,
1999.
64. Chen ZS, Lee K, and Kruh GD. Transport of cyclic nucleotides
and estradiol 17-beta-D-glucuronide by multidrug resistance protein
4. Resistance to 6-mercaptopurine and 6-thioguanine. J Biol Chem
276: 33747–33754, 2001.
65. Chen ZS, Lee K, Walther S, Raftogianis RB, Kuwano M, Zeng
H, and Kruh GD. Analysis of methotrexate and folate transport by
multidrug resistance protein 4 (ABCC4): MRP4 is a component of
the methotrexate efflux system. Cancer Res 62: 3144 –3150, 2002.
66. Cherrington NJ, Hartley DP, Li N, Johnson DR, and Klaassen
CD. Organ distribution of multidrug resistance proteins 1, 2, and 3
(Mrp1, 2, and 3) mRNA and hepatic induction of Mrp3 by constitutive androstane receptor activators in rats. J Pharmacol Exp
Ther 300: 97–104, 2002.
67. Choudhuri S, Ogura K, and Klaassen CD. Cloning, expression,
and ontogeny of mouse organic anion-transporting polypeptide-5, a
kidney-specific organic anion transporter. Biochem Biophys Res
Commun 280: 92–98, 2001.
68. Cihlar T and Ho ES. Fluorescence-based assay for the interaction
of small molecules with the human renal organic anion transporter
1. Anal Biochem 283: 49 –55, 2000.
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
30. Bräunlich H, Rassbach H, and Vogelsang S. Stimulation of renal
tubular transport of p-aminohippurate in rats of different ages by
treatment with adrenocortical steroids. Dev Pharmacol Ther 19:
1–5, 1992.
31. Bräunlich H and Schmidt M. Effects of calcitriol on expression of
triiodothyronine-stimulated renal tubular transport of p-aminohippurate in rats of different ages. Biol Neonate 66: 352–358, 1994.
32. Breidert T, Spitzenberger F, Gründemann D, and Schomig E.
Catecholamine transport by the organic cation transporter type 1
(OCT1). Br J Pharmacol 125: 218 –224, 1998.
33. Buchler M, König J, Brom M, Kartenbeck J, Spring H, Horie
T, and Keppler D. cDNA cloning of the hepatocyte canalicular
isoform of the multidrug resistance protein, cMrp, reveals a novel
conjugate export pump deficient in hyperbilirubinemic mutant rats.
J Biol Chem 271: 15091–15098, 1996.
34. Budiman T, Bamberg E, Koepsell H, and Nagel G. Mechanism
of electrogenic cation transport by the cloned organic cation transporter 2 from rat. J Biol Chem 275: 29413–29420, 2000.
35. Buist SC, Cherrington NJ, Choudhuri S, Hartley DP, and
Klaassen CD. Gender-specific and developmental influences on
the expression of rat organic anion transporters. J Pharmacol Exp
Ther 301: 145–151, 2002.
36. Buist SC, Cherrington NJ, and Klaassen CD. Endocrine regulation of rat organic anion transporters. Drug Metab Dispos 31:
559 –564, 2003.
37. Burckhardt BC, Brai S, Wallis S, Krick W, Wolff NA, and
Burckhardt G. Transport of cimetidine by flounder and human
renal organic anion transporter 1. Am J Physiol Renal Physiol 284:
F503–F509, 2003.
38. Burckhardt BC and Burckhardt G. Transport of organic anions
across the basolateral membrane of proximal tubule cells. Rev
Physiol Biochem Pharmacol 146: 95–158, 2003.
39. Burckhardt BC, Drinkuth B, Menzel C, Konig A, Steffgen J,
Wright SH, and Burckhardt G. The renal Na⫹-dependent dicarboxylate transporter, NaDC-3, translocates dimethyl- and disulfhydryl-compounds and contributes to renal heavy metal detoxification. J Am Soc Nephrol 13: 2628 –2638, 2002.
40. Burckhardt BC, Wolff NA, and Burckhardt G. Electrophysiologic characterization of an organic anion transporter cloned from
winter flounder kidney (fROAT). J Am Soc Nephrol 11: 9 –17, 2000.
41. Burckhardt G and Wolff NA. Structure of renal organic anion and
cation transporters. Am J Physiol Renal Physiol 278: F853–F866,
2000.
42. Busch AE, Karbach U, Miska D, Gorboulev V, Akhoundova A,
Volk C, Arndt P, Ulzheimer JC, Sonders MS, Baumann C,
Waldegger S, Lang F, and Koepsell H. Human neurons express
the polyspecific cation transporter hOCT2, which translocates
monoamine neurotransmitters, amantadine, and memantine. Mol
Pharmacol 54: 342–352, 1998.
43. Busch AE, Quester S, Ulzheimer JC, Gorboulev V, Akhoundova A, Waldegger S, Lang F, and Koepsell H. Monoamine
neurotransmitter transport mediated by the polyspecific cation
transporter rOCT1. FEBS Lett 395: 153–156, 1996.
44. Busch AE, Quester S, Ulzheimer JC, Waldegger S, Gorboulev
V, Arndt P, Lang F, and Koepsell H. Electrogenic properties and
substrate specificity of the polyspecific rat cation transporter
rOCT1. J Biol Chem 271: 32599 –32604, 1996.
45. Busch AE, Schuster A, Waldegger S, Wagner CA, Zempel G,
Broer S, Biber J, Murer H, and Lang F. Expression of a renal
type I sodium/phosphate transporter (NaPi-1) induces a conductance in Xenopus oocytes permeable for organic and inorganic
anions. Proc Natl Acad Sci USA 93: 5347–5351, 1996.
46. Cantz T, Nies AT, Brom M, Hofmann AF, and Keppler D.
MRP2, a human conjugate export pump, is present and transports
fluo 3 into apical vacuoles of Hep G2 cells. Am J Physiol Gastrointest Liver Physiol 278: G522–G531, 2000.
47. Cavet ME, West M, and Simmons NL. Fluoroquinolone (ciprofloxacin) secretion by human intestinal epithelial (Caco-2) cells.
Br J Pharmacol 121: 1567–1578, 1997.
48. Cerrutti JA, Brandoni A, Quaglia NB, and Torres AM. Sex
differences in p-aminohippuric acid transport in rat kidney: role of
membrane fluidity and expression of OAT1. Mol Cell Biochem 233:
175–179, 2002.
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
Physiol Rev • VOL
89. Dresser MJ, Leabman MK, and Giacomini KM. Transporters
involved in the elimination of drugs in the kidney: organic anion
transporters and organic cation transporters. J Pharm Sci 90:
397– 421, 2001.
90. Dresser MJ, Xiao G, Leabman MK, Gray AT, and Giacomini
KM. Interactions of n-tetraalkylammonium compounds and biguanides with a human renal organic cation transporter (hOCT2).
Pharm Res 19: 1244 –1247, 2002.
91. Dudley AJ, Bleasby K, and Brown CD. The organic cation transporter OCT2 mediates the uptake of beta-adrenoceptor antagonists
across the apical membrane of renal LLC-PK(1) cell monolayers.
Br J Pharmacol 131: 71–79, 2000.
92. Dudley AJ and Brown CDA. Mediation of cimetidine secretion by
P-glycoprotein and a novel H⫹-coupled mechanism in cultured
renal epithelial monolayers of LLC-PK1 cells. Br J Pharmacol 117:
1139 –1144, 1996.
93. Dutt A, Priebe TS, Teeter LD, Kuo MT, and Nelson JA. Postnatal development of organic cation transport and MDR gene expression in mouse kidney. J Pharmacol Exp Ther 261: 1222–1230,
1992.
94. Eckhardt U, Schroeder A, Stieger B, Hochli M, Landmann L,
Tynes R, Meier PJ, and Hagenbuch B. Polyspecific substrate
uptake by the hepatic organic anion transporter oatp1 in stably
transfected CHO cells. Am J Physiol Gastrointest Liver Physiol
276: G1037–G1042, 1999.
95. Edwards RM, Stack E, and Trizna W. alpha-Ketoglutarate transport in rat renal brush-border and basolateral membrane vesicles.
J Pharmacol Exp Ther 281: 1059 –1064, 1997.
96. Edwards RM, Stack EJ, and Trizna W. Transport of [3H]losartan
across isolated perfused rabbit proximal tubule. J Pharmacol Exp
Ther 290: 38 – 42, 1999.
96a.Ekaratanawong S, Anzai N, Jutabha P, Miyazaki H, Noshiro
R, Takeda M, Kanai Y, Sophasan S, and Endou H. Human
organic anion transporter 4 is a renal apical organic anion/dicarboxylate exchanger in the proximal tubules. J Pharmacol Sci 94:
297–304, 2004.
97. Enomoto A, Kimura H, Chairoungdua A, Shigeta Y, Jutabha P,
Cha SH, Hosoyamada M, Takeda M, Sekine T, Igarashi T,
Matsuo H, Kikuchi Y, Oda T, Ichida K, Hosoya T, Shimokata
K, Niwa T, Kanai Y, and Endou H. Molecular identification of a
renal urate anion exchanger that regulates blood urate levels.
Nature 417: 447– 452, 2002.
98. Enomoto A, Takeda M, Shimoda M, Narikawa S, Kobayashi Y,
Kobayashi Y, Yamamoto T, Sekine T, Cha SH, Niwa T, and
Endou H. Interaction of human organic anion transporters 2 and 4
with organic anion transport inhibitors. J Pharmacol Exp Ther 301:
797– 802, 2002.
99. Enomoto A, Takeda M, Taki K, Takayama F, Noshiro R, Niwa
T, and Endou H. Interactions of human organic anion as well as
cation transporters with indoxyl sulfate. Eur J Pharmacol 466:
13–20, 2003.
100. Eraly SA, Hamilton BA, and Nigam SK. Organic anion and
cation transporters occur in pairs of similar and similarly expressed genes. Biochem Biophys Res Commun 300: 333–342, 2003.
100a.Eraly SA, Monte JC, and Nigam SK. Novel slc22 transporter
homologs in fly, worm, and human clarify the phylogeny of organic
anion and cation transporters. Physiol Genomics 2004. Mar 30
[Epub ahead of print]
101. Ernest S and Bello-Reuss E. Expression and function of Pglycoprotein in a mouse kidney cell line. Am J Physiol Cell Physiol
269: C323–C333, 1995.
102. Ernest S and Bello-Reuss E. P-glycoprotein functions and substrates: possible roles of MDR1 gene in the kidney. Kidney Int
Suppl 65: S11–S17, 1998.
103. Ernest S, Rajaraman S, Megyesi J, and Bello-Reuss EN. Expression of MDR1 (multidrug resistance) gene and its protein in
normal human kidney. Nephron 77: 284 –289, 1997.
104. Escobar MR, Goralski K, and Sitar DS. L(⫹)- and D(⫺)-lactate
modulate rat renal tubular accumulation of amantadine in the
presence and absence of bicarbonate. J Pharmacol Exp Ther 275:
1317–1323, 1995.
105. Escobar MR and Sitar DS. Site-selective effect of bicarbonate on
amantadine renal transport: quinine-sensitive in proximal vs. qui-
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
69. Cihlar T, Lin DC, Pritchard JB, Fuller MD, Mendel DB, and
Sweet DH. The antiviral nucleotide analogs cidofovir and adefovir
are novel substrates for human and rat renal organic anion transporter 1. Mol Pharmacol 56: 570 –580, 1999.
70. Clavell AL and Burnett JC Jr. Physiologic and pathophysiologic
roles of endothelin in the kidney. Curr Opin Nephrol Hypertens 3:
66 –72, 1994.
71. Cole SP, Bhardwaj G, Gerlach JH, Mackie JE, Grant CE,
Almquist KC, Stewart AJ, Kurz EU, Duncan AM, and Deeley
RG. Overexpression of a transporter gene in a multidrug-resistant
human lung cancer cell line. Science 258: 1650 –1654, 1992.
72. Collett A, Higgs NB, Sims E, Rowland M, and Warhurst G.
Modulation of the permeability of H2 receptor antagonists cimetidine and ranitidine by P-glycoprotein in rat intestine and the human
colonic cell line Caco-2. J Pharmacol Exp Ther 288: 171–178, 1999.
73. Cui Y, Buchholz JK, Spring H, Leier I, and Keppler D. Drug
resistance and ATP-dependent conjugate transport mediated by the
apical multidrug resistance protein, MRP2, permanently expressed
in human and canine cells. Mol Pharmacol 55: 929 –937, 1999.
74. Cui Y, König J, and Keppler D. Vectorial transport by doubletransfected cells expressing the human uptake transporter
SLC21A8 and the apical export pump ABCC2. Mol Pharmacol 60:
934 –943, 2001.
75. Cvetkovic M, Leake B, Fromm MF, Wilkinson GR, and Kim
RB. OATP and P-glycoprotein transporters mediate the cellular
uptake and excretion of fexofenadine. Drug Metab Dispos 27:
866 – 871, 1999.
76. Dantzler WH. Organic acid (or anion) and organic base (or cation)
transport by renal tubules of nonmammalian vertebrates. J Exp
Zool 249: 247–257, 1989.
77. Dantzler WH and Bentley SK. Effects of inhibitors in lumen on
PAH and urate transport by isolated renal tubules. Am J Physiol
Renal Fluid Electrolyte Physiol 236: F379 –F386, 1979.
78. Dantzler WH and Bentley SK. Bath and lumen effects of SITS on
PAH transport by isolated perfused renal tubules. Am J Physiol
Renal Fluid Electrolyte Physiol 238: F16 –F25, 1980.
79. Dantzler WH and Evans KK. Effect of ␣-KG in lumen on PAH
transport by isolated perfused rabbit renal proximal tubules. Am J
Physiol Renal Fluid Electrolyte Physiol 271: F521–F526, 1996.
80. Dantzler WH, Evans KK, and Wright SH. Basolateral choline
transport in isolated rabbit renal proximal tubule. Pflügers Arch
436: 899 –905, 1998.
81. Dantzler WH, Wright SH, and Brokl O. Tetraethylammonium
transport by snake renal brush-border membrane vesicles. Pflügers
Arch 418: 325–332, 1991.
82. Dantzler WH, Wright SH, Chatsudthipong V, and Brokl O.
Basolateral tetraethylammonium transport in intact tubules: specificity and trans-stimulation. Am J Physiol Renal Fluid Electrolyte
Physiol 261: F386 –F392, 1991.
83. David C, Rumrich G, and Ullrich KJ. Luminal transport system
for H⫹/organic cations in the rat proximal tubule. Kinetics, dependence on pH; specificity compared with the contraluminal organic
cation-transport system. Pflügers Arch 430: 477– 492, 1995.
84. Dawson MA and Renfro JL. Interaction of structurally similar
pesticides with organic anion transport by primary cultures of
winter flounder renal proximal tubule. J Pharmacol Exp Ther 266:
673– 677, 1993.
85. Decorti G, Peloso I, Favarin D, Klugmann FB, Candussio L,
Crivellato E, Mallardi F, and Baldini L. Handling of doxorubicin
by the LLC-PK1 kidney epithelial cell line. J Pharmacol Exp Ther
286: 525–530, 1998.
86. Deguchi T, Ohtsuki S, Otagiri M, Takanaga H, Asaba H, Mori
S, and Terasaki T. Major role of organic anion transporter 3 in the
transport of indoxyl sulfate in the kidney. Kidney Int 61: 1760 –
1768, 2002.
87. De Lannoy IAM, Koren G, Klein J, Charuk J, and Silverman
M. Cyclosporin and quinidine inhibition or renal digoxin excretion:
evidence for luminal secretion of digoxin. Am J Physiol Renal
Fluid Electrolyte Physiol 263: F613–F622, 1992.
88. Dresser MJ, Gray AT, and Giacomini KM. Kinetic and selectivity differences between rodent, rabbit, and human organic cation
transporters (OCT1). J Pharmacol Exp Ther 292: 1146 –1152, 2000.
1037
1038
106.
107.
108.
109.
111.
112.
113.
114.
115.
116.
117.
118.
119.
120.
121.
122.
123.
124.
nine-sensitive sites in distal tubules. J Pharmacol Exp Ther 273:
72–79, 1995.
Escobar MR, Wong LTY, and Sitar DS. Bicarbonate-dependent
amantadine transport by rat renal cortical proximal and distal
tubules. J Pharmacol Exp Ther 270: 979 –986, 1994.
Eveloff J, Morishige WK, and Hong SK. The binding of phenol
red to rabbit renal cortex. Biochim Biophys Acta 448: 167–180,
1976.
Evers R, Kool M, Smith AJ, van Deemter L, de Haas M, and
Borst P. Inhibitory effect of the reversal agents V-104, GF120918
and Pluronic L61 on MDR1 Pgp-. Br J Cancer 83: 366 –374, 2000.
Evers R, Kool M, van Deemter L, Janssen H, Calafat J, Oomen
LC, Paulusma CC, Oude Elferink RP, Baas F, Schinkel AH,
and Borst P. Drug export activity of the human canalicular multispecific organic anion transporter in polarized kidney MDCK cells
expressing cMOAT (MRP2) cDNA. J Clin Invest 101: 1310 –1319,
1998.
Evers R, Zaman GJ, van Deemter L, Jansen H, Calafat J,
Oomen LC, Oude Elferink RP, Borst P, and Schinkel AH.
Basolateral localization and export activity of the human multidrug
resistance-associated protein in polarized pig kidney cells. J Clin
Invest 97: 1211–1218, 1996.
Fauth C, Rossier B, and Roch-Ramel F. Transport of tetraethylammonium by a kidney epithelial cell line (LLC-PK/1X). Am J
Physiol Renal Fluid Electrolyte Physiol 254: F351–F357, 1988.
Feng B, Dresser MJ, Shu Y, Johns SJ, and Giacomini KM.
Arginine 454 and lysine 370 are essential for the anion specificity of
the organic anion transporter, rOAT3. Biochemistry 40: 5511–5520,
2001.
Feng B, Shu Y, and Giacomini KM. Role of aromatic transmembrane residues of the organic anion transporter, rOAT3, in substrate recognition. Biochemistry 41: 8941– 8947, 2002.
Fleck C, Bachner B, Gockeritz S, Karge E, Strohm U, and
Schubert J. Ex vivo stimulation of renal tubular p-aminohippurate
transport by dexamethasone and triiodothyronine in human renal
cell carcinoma. Urol Res 28: 383–390, 2000.
Fouda AK, Fauth C, and Roch-Ramel F. Transport of organic
cations by kidney epithelial cell line (LLC-PK1). J Pharmacol Exp
Ther 252: 286 –292, 1990.
Fritz F, Chen J, Hayes P, and Sirotnak FM. Molecular cloning
of the murine cMOAT ATPase. Biochim Biophys Acta 1492: 531–
536, 2000.
Fritzsch G, Haase W, Rumrich G, Fasold H, and Ullrich KJ. A
stopped flow capillary perfusion method to evaluate contraluminal
transport parameters of methylsuccinate from interstitium into
renal proximal tubular cells. Pflügers Arch 400: 250 –256, 1984.
Gabriels G, Kramer C, Stark U, and Greven J. Role of the
calcium/calmodulin-dependent protein kinase II in the regulation
of the renal basolateral PAH and dicarboxylate transporters. Fundam Clin Pharmacol 13: 59 – 66, 1999.
Gabriels G, Werners A, Mauss S, and Greven J. Evidence for
differential regulation of renal proximal tubular p-aminohippurate
and sodium-dependent dicarboxylate transport. J Pharmacol Exp
Ther 290: 710 –715, 1999.
Ganapathy ME, Huang W, Rajan DP, Carter AL, Sugawara M,
Iseki K, Leibach FH, and Ganapathy V. Beta-lactam antibiotics
as substrates for OCTN2, an organic cation/carnitine transporter.
J Biol Chem 275: 1699 –1707, 2000.
Ganapathy V, Ganapathy ME, Nair CN, Mahesh VB, and
Leibach FH. Evidence for an organic cation-proton antiport system in brush-border membranes isolated from the human term
placenta. J Biol Chem 263: 4561– 4568, 1988.
Gekle M, Mildenberger S, Sauvant C, Bednarczyk D, Wright
SH, and Dantzler WH. Inhibition of initial transport rate of basolateral organic anion carrier in renal PT by BK and phenylephrine.
Am J Physiol Renal Physiol 277: F251–F256, 1999.
George RL, Wu X, Huang W, Fei YJ, Leibach FH, and Ganapathy V. Molecular cloning and functional characterization of a
polyspecific organic anion transporter from Caenorhabditis elegans. J Pharmacol Exp Ther 291: 596 – 603, 1999.
Gilsdorf JS, Rebbeor JF, and Holohan PD. Evidence that the
organic cation/H⫹ exchanger in the brush border membrane of dog
Physiol Rev • VOL
125.
126.
127.
128.
129.
130.
131.
132.
133.
134.
135.
136.
137.
138.
139.
140.
141.
142.
143.
144.
kidney is a 41-kDa protein. J Pharmacol Exp Ther 280: 1043–1050,
1997.
Gluck S and Nelson R. The role of the V-ATPase in renal epithelial
H⫹ transport. J Exp Biol 172: 205–218, 1992.
Goralski KB, Lou G, Prowse MT, Gorboulev V, Volk C, Koepsell H, and Sitar DS. The cation transporters rOCT1 and rOCT2
interact with bicarbonate but play only a minor role for amantadine
uptake into rat renal proximal tubules. J Pharmacol Exp Ther 303:
959 –968, 2002.
Goralski KB and Sitar DS. Tetraethylammonium and amantadine
identify distinct organic cation transporters in rat renal cortical
proximal and distal tubules. J Pharmacol Exp Ther 290: 295–302,
1999.
Goralski KB, Smyth DD, and Sitar DS. In vivo analysis of
amantadine renal clearance in the uninephrectomized rat: functional significance of in vitro bicarbonate-dependent amantadine
renal tubule transport. J Pharmacol Exp Ther 290: 496 –504, 1999.
Gorboulev V, Ulzheimer JC, Akhoundova A, Ulzheimer-Teuber I, Karbach U, Quester S, Baumann C, Lang F, Busch AE,
and Koepsell H. Cloning and characterization of two human
polyspecific organic cation transporters. DNA Cell Biol 16: 871–
881, 1997.
Gorboulev V, Volk C, Arndt P, Akhoundova A, and Koepsell H.
Selectivity of the polyspecific cation transporter rOCT1 is changed
by mutation of aspartate 475 to glutamate. Mol Pharmacol 56:
1254 –1261, 1999.
Green RE, Ricker WE, Attwood WL, Koh YS, and Peters L.
Studies of the renal tubular transport characteristics of N1-methylnicotinamide and tetraalkylammonium compounds in the avian
kidney. J Pharmacol Exp Ther 126: 195–201, 1959.
Green RM, Lo K, Sterritt C, and Beier DR. Cloning and functional expression of a mouse liver organic cation transporter.
Hepatology 29: 1556 –1562, 1999.
Greven J and Gabriels G. Role of the protein kinases A and C and
of the calcium/calmodulin-dependent protein kinase II in the regulation of the renal basolateral PAH and dicarboxylate transporters. Nephrol Dial Transplant 14 Suppl 4: 10 –11, 1999.
Gross AS and Somogyi AA. Interaction of the stereoisomers of
basic drugs with the uptake of tetraethylammonium by rat renal
brush-border membrane vesicles. J Pharmacol Exp Ther 268:
1073–1080, 1994.
Groves CE, Evans K, Dantzler WH, and Wright SH. Peritubular
organic cation transport in isolated rabbit proximal tubules. Am J
Physiol Renal Fluid Electrolyte Physiol 266: F450 –F458, 1994.
Groves CE, Morales M, and Wright SH. Peritubular transport of
ochratoxin A in rabbit renal proximal tubules. J Pharmacol Exp
Ther 284: 943–948, 1998.
Groves CE, Morales MN, Gandolfi AJ, Dantzler WH, and
Wright SH. Peritubular paraquat transport in isolated renal proximal tubules. J Pharmacol Exp Ther 275: 926 –932, 1995.
Groves CE, Munoz L, Bahn A, Burckhardt G, and Wright SH.
Interaction of cysteine conjugates with human and rabbit organic
anion transporter 1. J Pharmacol Exp Ther 304: 560 –566, 2003.
Groves CE, Nowak G, and Morales M. Ochratoxin A secretion in
primary cultures of rabbit renal proximal tubule cells. J Am Soc
Nephrol 10: 13–20, 1999.
Groves CE and Wright SH. Tetrapentylammonium (TPeA):
slowly dissociating inhibitor of the renal peritubular organic cation
transporter. Biochim Biophys Acta 1234: 37– 42, 1995.
Gründemann D, Babin-Ebell J, Martel F, Örding N, Schmidt
A, and Schömig E. Primary structure and functional expression of
the apical organic cation transporter from kidney epithelial LLCPK1 cells. J Biol Chem 272: 10408 –10413, 1997.
Gründemann D, Gorboulev V, Gambaryan S, Veyhl M, and
Koepsell H. Drug excretion mediated by a new prototype of
polyspecific transporter. Nature 372: 549 –552, 1994.
Gründemann D, Koster S, Kiefer N, Breidert T, Engelhardt M,
Spitzenberger F, Obermuller N, and Schomig E. Transport of
monoamine transmitters by the organic cation transporter type 2,
OCT2. J Biol Chem 273: 30915–30920, 1998.
Gründemann D, Liebich G, Kiefer N, Koster AS, and Schömig
E. Selective substrates for non-neuronal monoamine transporters.
Mol Pharmacol 56: 1–10, 1999.
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
110.
STEPHEN H. WRIGHT AND WILLIAM H. DANTZLER
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
Physiol Rev • VOL
166. Holland IB and Blight MA. ABC-ATPases, adaptable energy generators fuelling transmembrane movement of a variety of molecules in organisms from bacteria to humans. J Mol Biol 293: 381–
399, 1999.
167. Holohan PD, Pessah NI, and Ross CR. Binding of N1-methylnicotinamide and p-aminohippuric acid to a particulate fraction from
dog kidney. J Pharmacol Exp Ther 195: 22–33, 1975.
168. Holohan PD and Ross CR. Mechanisms of organic cation transport in kidney plasma membrane vesicles. 2. ⌬pH studies. J Pharmacol Exp Ther 216: 294 –298, 1981.
169. Holohan PD, White KE, Sokol PP, and Rebbeor J. Photoaffinity
labeling of the organic cation/H⫹ exchanger in renal brush border
membrane vesicles. J Biol Chem 267: 13513–13519, 1992.
170. Hori R, Okamura N, Aiba T, and Tanigawara Y. Role of Pglycoprotein in renal tubular secretion of digoxin in the isolated
perfused rat kidney. J Pharmacol Exp Ther 266: 1620 –1625, 1993.
171. Hori R, Takano M, Okano T, Kitazawa S, and Inui K. Mechanisms of p-aminohippurate transport by brush-border and basolateral membrane vesicles isolated from rat kidney cortex. Biochim
Biophys Acta 692: 97–100, 1982.
171a.Hosoyamada M, Ichida K, Enomoto A, Hosoya T, and Endou
H. Function and localization of urate transporter 1 in mouse kidney. J Am Soc Nephrol 15: 261–268, 2004.
172. Hosoyamada M, Sekine T, Kanai Y, and Endou H. Molecular
cloning and functional expression of a multispecific organic anion
transporter from human kidney. Am J Physiol Renal Physiol 276:
F122–F128, 1999.
173. Hsyu PH and Giacomini KM. Stereoselective renal clearance of
pindolol in humans. J Clin Invest 76: 1720 –1726, 1985.
174. Huang Y, Lemieux MJ, Song J, Auer M, and Wang DN. Structure and mechanism of the glycerol-3-phosphate transporter from
Escherichia coli. Science 301: 616 – 620, 2003.
175. Ichida K, Hosoyamada M, Kimura H, Takeda M, Utsunomiya
Y, Hosoya T, and Endou H. Urate transport via human PAH
transporter hOAT1 and its gene structure. Kidney Int 63: 143–155,
2003.
176. Inui KI, Saito H, and Hori R. H⫹-gradient-dependent active transport of tetraethylammonium cation in apical-membrane vesicles
isolated from kidney epithelial cell line LLC-PK1. Biochem J 227:
199 –203, 1985.
177. Islinger F, Gekle M, and Wright SH. Interaction of 2,3-dimercapto-1-propane sulfonate with the human organic anion transporter hOAT1. J Pharmacol Exp Ther 299: 741–747, 2001.
178. Ito K, Olsen SL, Qiu W, Deeley RG, and Cole SP. Mutation of a
single conserved tryptophan in multidrug resistance protein 1
(MRP1/ABCC1) results in loss of drug resistance and selective loss
of organic anion transport. J Biol Chem 276: 15616 –15624, 2001.
179. Ito S, Woodland C, Sarkadi B, Hockmann G, Walker SE, and
Koren G. Modeling of P-glycoprotein-involved epithelial drug
transport in MDCK cells. Am J Physiol Renal Physiol 277: F84 –
F96, 1999.
180. Jacquemin E, Hagenbuch B, Stieger B, Wolkoff AW, and
Meier PJ. Expression cloning of a rat liver Na⫹-independent organic anion transporter. Proc Natl Acad Sci USA 91: 133–137, 1994.
181. Jariyawat S, Sekine T, Takeda M, Apiwattanakul N, Kanai Y,
Sophasan S, and Endou H. The interaction and transport of
beta-lactam antibiotics with the cloned rat renal organic anion
transporter 1. J Pharmacol Exp Ther 290: 672– 677, 1999.
182. Jedlitschky G, Burchell B, and Keppler D. The multidrug resistance protein 5 functions as an ATP-dependent export pump for
cyclic nucleotides. J Biol Chem 275: 30069 –30074, 2000.
183. Ji L, Masuda S, Saito H, and Inui K. Down-regulation of rat
organic cation transporter rOCT2 by 5/6 nephrectomy. Kidney Int
62: 514 –524, 2002.
184. Jonker JW, Wagenaar E, Mol CA, Buitelaar M, Koepsell H,
Smit JW, and Schinkel AH. Reduced hepatic uptake and intestinal excretion of organic cations in mice with a targeted disruption
of the organic cation transporter 1 (Oct1 [Slc22a1]) gene. Mol Cell
Biol 21: 5471–5477, 2001.
185. Jonker JW, Wagenaar E, Van Eijl S, and Schinkel AH. Deficiency in the organic cation transporters 1 and 2 (Oct1/Oct2
[Slc22a1/Slc22a2]) in mice abolishes renal secretion of organic
cations. Mol Cell Biol 23: 7902–7908, 2003.
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
145. Gründemann D and Schomig E. Gene structures of the human
non-neuronal monoamine transporters EMT and OCT2. Hum Genet
106: 627– 635, 2000.
146. Guggino SE, Martin GJ, and Aronson PS. Specificity and modes
of the anion exchanger in dog renal microvillus membranes. Am J
Physiol Renal Fluid Electrolyte Physiol 244: F612–F621, 1983.
147. Hagenbuch B. Molecular properties of hepatic uptake systems for
bile acids and organic anions. J Membr Biol 160: 1– 8, 1997.
148. Hagenbuch B and Meier PJ. The superfamily of organic anion
transporting polypeptides. Biochim Biophys Acta 1609: 1–18, 2003.
149. Hagos Y, Bahn A, Asif AR, Krick W, Sendler M, and Burckhardt G. Cloning of the pig renal organic anion transporter 1
(pOAT1). Biochimie 84: 1219 –1222, 2002.
150. Hagos Y, Burckhardt BC, Larsen A, Mathys C, Gronow T,
Bahn A, Wolff NA, Burckhardt G, and Steffgen J. Regulation of
the sodium dicarboxylate cotransporter, NaDC-3, from winter
flounder kidney by protein kinase C. Am J Physiol Renal Physiol
286: F86 –F93, 2004.
151. Hasegawa M, Kusuhara H, Endou H, and Sugiyama Y. Contribution of organic anion transporters to the renal uptake of anionic
compounds and nucleoside derivatives in rat. J Pharmacol Exp
Ther 305: 1087–1097, 2003.
152. Hayer M, Bonisch H, and Bruss M. Molecular cloning, functional
characterization and genomic organization of four alternatively
spliced isoforms of the human organic cation transporter 1
(hOCT1/SLC22A1). Ann Hum Genet 63: 473– 482, 1999.
153. Hentschel H, Burckhardt BC, Scholermann B, Kuhne L,
Burckhardt G, and Steffgen J. Basolateral localization of flounder Na⫹-dicarboxylate cotransporter (fNaDC-3) in the kidney of
Pleuronectes americanus. Pflügers Arch 446: 578 –584, 2003.
154. Higgins CF, Callaghan R, Linton KJ, Rosenberg MF, and Ford
RC. Structure of the multidrug resistance P-glycoprotein. Semin
Cancer Biol 8: 135–142, 1997.
155. Higgins CF and Gottesman MM. Is the multidrug transporter a
flippase? Trends Biochem Sci 17: 19 –21, 1992.
156. Hilgendorf C, Spahn-Langguth H, Regardh CG, Lipka E, Amidon GL, and Langguth P. Caco-2 versus Caco-2/HT29-MTX cocultured cell lines: permeabilities via diffusion, inside- and outsidedirected carrier-mediated transport. J Pharm Sci 89: 63–75, 2000.
157. Hipfner DR, Deeley RG, and Cole SP. Structural, mechanistic
and clinical aspects of MRP1. Biochim Biophys Acta 1461: 359 –
376, 1999.
158. Hirohashi T, Suzuki H, and Sugiyama Y. Characterization of the
transport properties of cloned rat multidrug resistance-associated
protein 3 (MRP3). J Biol Chem 274: 15181–15185, 1999.
159. Ho ES, Lin DC, Mendel DB, and Cihlar T. Cytotoxicity of
antiviral nucleotides adefovir and cidofovir is induced by the expression of human renal organic anion transporter 1. J Am Soc
Nephrol 11: 383–393, 2000.
160. Hocher B, Thone-Reineke C, Bauer C, Raschack M, and Neumayer HH. The paracrine endothelin system: pathophysiology and
implications in clinical medicine. Eur J Clin Chem Clin Biochem
35: 175–189, 1997.
161. Hohage H, Löhr M, Querl U, and Greven J. The renal basolateral
transport system for organic anions: properties of the regulation
mechanism. J Pharmacol Exp Ther 269: 659 – 664, 1994.
162. Hohage H, Mörth DM, Querl IU, and Greven J. Regulation by
protein kinase C of the contraluminal transport system for organic
cations in rabbit kidney S2 proximal tubules. J Pharmacol Exp
Ther 268: 897–901, 1994.
163. Hohage H, Querl IU, Morth DM, and Greven J. The basolateral
organic cation transport system of rabbit kidney proximal tubules.
Influence of anorganic anions. J Pharmacol Exp Ther 279: 1086 –
1091, 1996.
164. Hohage H, Stachon A, Feidt C, Hirsch JR, and Schlatter E.
Regulation of organic cation transport in IHKE-1 and LLC-PK1
cells. Fluorometric studies with 4-(4-dimethylaminostyryl)-N-methylpyridinium. J Pharmacol Exp Ther 286: 305–310, 1998.
165. Hohage H, Steinmetz M, Mergelsberg U, Lohr M, and Greven
J. Regulation of the renal basolateral transport system for organic
anions by calcium channel blockers. Pharmacol Res 41: 25–30,
2000.
1039
1040
STEPHEN H. WRIGHT AND WILLIAM H. DANTZLER
Physiol Rev • VOL
205.
206.
207.
208.
209.
210.
211.
212.
213.
214.
215.
216.
217.
218.
219.
220.
221.
222.
223.
224.
225.
porters with nonsteroidal anti-inflammatory drugs. J Pharmacol
Exp Ther 303: 534 –539, 2002.
Kim YK and Dantzler WH. Relation of membrane potential to
basolateral TEA transport in isolated snake proximal renal tubules.
Am J Physiol Regul Integr Comp Physiol 268: R1539 –R1545, 1995.
Kim YK and Dantzler WH. Ca2⫹ and Ba2⫹ effects on basolateral
tetraethylammonium transport in isolated snake renal proximal
tubules. Pflügers Arch 435: 28 –33, 1997.
Kim YK and Dantzler WH. Effects of pH on basolateral tetraethylammonium transport in snake renal proximal tubules. Am J
Physiol Regul Integr Comp Physiol 272: R955–R961, 1997.
Kim YK, Kim TI, Jung DK, Jung JS, and Lee SH. Inhibition of
H⫹/organic cation antiport by carboxyl reagents in rabbit renal
brush-border membrane vesicles. J Pharmacol Exp Ther 266: 500 –
505, 1993.
Kimura H, Takeda M, Narikawa S, Enomoto A, Ichida K, and
Endou H. Human organic anion transporters and human organic
cation transporters mediate renal transport of prostaglandins.
J Pharmacol Exp Ther 301: 293–298, 2002.
Kimura M, Nabekura T, Katsura T, Takano M, and Hori R.
Identification of organic cation transporter in rat renal brush-border membrane by photoaffinity labeling. Biol Pharm Bull 18: 388 –
395, 1995.
Kinsella JL, Holohan PD, Pessah NI, and Ross CR. Transport of
organic ions in renal cortical luminal and antiluminal membrane
vesicles. J Pharmacol Exp Ther 209: 443– 450, 1979.
Kippen I and Klinenberg JR. Effects of renal fuels on uptake of
PAH and uric acid by separated renal tubules of the rabbit. Am J
Physiol Renal Fluid Electrolyte Physiol 235: F137–F141, 1978.
Knorr BA, Beck JC, and Abramson RG. Classical and channellike urate transporters in rabbit renal brush border membranes.
Kidney Int 45: 727–736, 1994.
Kobayashi Y, Hirokawa N, Ohshiro N, Sekine T, Sasaki T,
Tokuyama S, Endou H, and Yamamoto T. Differential gene
expression of organic anion transporters in male and female rats.
Biochem Biophys Res Commun 290: 482– 487, 2002.
Kobayashi Y, Ohshiro N, Shibusawa A, Sasaki T, Tokuyama S,
Sekine T, Endou H, and Yamamoto T. Isolation, characterization
and differential gene expression of multispecific organic anion
transporter 2 in mice. Mol Pharmacol 62: 7–14, 2002.
Koehler MR, Gorboulev V, Koepsell H, Steinlein C, and
Schmid M. Roct1, a rat polyspecific transporter gene for the excretion of cationic drugs, maps to chromosome 1q11–12. Mamm
Genome 7: 247–248, 1996.
Koehler MR, Wissinger B, Gorboulev V, Koepsell H, and
Schmid M. The two human organic cation transporter genes
SLC22A1 and SLC22A2 are located on chromosome 6q26. Cytogenet Cell Genet 79: 198 –200, 1997.
Koepsell H. Organic cation transporters in intestine, kidney, liver,
and brain. Annu Rev Physiol 60: 243–266, 1998.
Koepsell H and Endou H. The SLC22 drug transporter family.
Pflügers Arch 447: 666 – 676, 2003.
Koepsell H, Gorboulev V, and Arndt P. Molecular pharmacology
of organic cation transporters in kidney. J Membr Biol 167: 103–
117, 1999.
Koepsell H, Schmitt BM, and Gorboulev V. Organic cation
transporters. Rev Physiol Biochem Pharmacol 150: 36 –90, 2003.
Kojima R, Sekine T, Kawachi M, Cha SH, Suzuki Y, and Endou
H. Immunolocalization of multispecific organic anion transporters,
OAT1, OAT2, and OAT3, in rat kidney. J Am Soc Nephrol 13:
848 – 857, 2002.
Komatsubara M, Nagata T, Miyauchi S, and Kamo N. Cimetidine enhances the antiproliferative effect of 5-fluorouracil on colon
carcinoma SW620. Anticancer Res 19: 1153–1157, 1999.
König J, Nies AT, Cui Y, Leier I, and Keppler D. Conjugate
export pumps of the multidrug resistance protein (MRP) family:
localization, substrate specificity, and MRP2-mediated drug resistance. Biochim Biophys Acta 1461: 377–394, 1999.
Kool M, de Haas M, Scheffer GL, Scheper RJ, van Eijk MJ,
Juijn JA, Baas F, and Borst P. Analysis of expression of cMOAT
(MRP2), MRP3, MRP4, and MRP5, homologues of the multidrug
resistance-associated protein gene (MRP1), in human cancer cell
lines. Cancer Res 57: 3537–3547, 1997.
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
186. Jung KY, Takeda M, Kim DK, Tojo A, Narikawa S, Yoo BS,
Hosoyamada M, Cha SH, Sekine T, and Endou H. Characterization of ochratoxin A transport by human organic anion transporters. Life Sci 69: 2123–2135, 2001.
187. Jung KY, Takeda M, Shimoda M, Narikawa S, Tojo A, Kim K,
Chairoungdua A, Choi BK, Kusuhara H, Sugiyama Y, Sekine
T, and Endou H. Involvement of rat organic anion transporter 3
(rOAT3) in cephaloridine-induced nephrotoxicity: compared with
rOAT1. Life Sci 70: 1861–1874, 2002.
188. Jutabha P, Kanai Y, Hosoyamada M, Chairoungdua A, Kim
DK, Iribe Y, Babu E, Kim JY, Anzai N, Chatsudthipong V, and
Endou H. Identification of a novel voltage-driven organic anion
transporter present at apical membrane of renal proximal tubule.
J Biol Chem 278: 27930 –27938, 2003.
189. Jutabha P, Toskulkao C, and Chatsudthipong V. Effect of
stevioside on PAH transport by isolated perfused rabbit renal proximal tubule. Can J Physiol Pharmacol 78: 737–744, 2000.
190. Kaback HR, Sahin-Toth M, and Weinglass AB. The kamikaze
approach to membrane transport. Nat Rev Mol Cell Biol 2: 610 – 620,
2001.
191. Kaewmokul S, Chatsudthipong V, Evans KK, Dantzler WH,
and Wright SH. Functional mapping of rbOCT1 and rbOCT2 activity in the S2 segment of rabbit proximal tubule. Am J Physiol
Renal Physiol 285: F1149 –F1159, 2003.
192. Kahn AM, Branham S, and Weinman EJ. Mechanisms of urate
and p-aminohippurate transport in rat renal microvillus membrane
vesicles. Am J Physiol Renal Fluid Electrolyte Physiol 245: F151–
F158, 1983.
193. Kanai N, Lu R, Bao Y, Wolkoff AW, and Schuster VL. Transient
expression of oatp organic anion transporter in mammalian cells:
identification of candidate substrates. Am J Physiol Renal Fluid
Electrolyte Physiol 270: F319 –F325, 1996.
194. Kanai N, Lu R, Bao Y, Wolkoff AW, Vore M, and Schuster VL.
Estradiol 17 beta-D-glucuronide is a high-affinity substrate for oatp
organic anion transporter. Am J Physiol Renal Fluid Electrolyte
Physiol 270: F326 –F331, 1996.
195. Karbach U, Kricke J, Meyer-Wentrup F, Gorboulev V, Volk C,
Loffing-Cueni D, Kaissling B, Bachmann S, and Koepsell H.
Localization of organic cation transporters OCT1 and OCT2 in rat
kidney. Am J Physiol Renal Physiol 279: F679 –F687, 2000.
196. Kato Y, Kuge K, Kusuhara H, Meier PJ, and Sugiyama Y.
Gender difference in the urinary excretion of organic anions in rats.
J Pharmacol Exp Ther 302: 483– 489, 2002.
197. Katsura T, Takano M, Tomita Y, Yasuhara M, Inui KI, and
Hori R. Characteristics of organic cation transporter in rat renal
basolateral membrane. Biochim Biophys Acta 1146: 197–202, 1993.
198. Kawahara M, Sakata A, Miyashita T, Tamai I, and Tsuji A.
Physiologically based pharmacokinetics of digoxin in mdr1a
knockout mice. J Pharm Sci 88: 1281–1287, 1999.
199. Kekuda R, Prasad PD, Wu X, Wang H, Fei YJ, Leibach FH, and
Ganapathy V. Cloning and functional characterization of a potential-sensitive, polyspecific organic cation transporter (OCT3) most
abundantly expressed in placenta. J Biol Chem 273: 15971–15979,
1998.
200. Kekuda R, Wang H, Huang W, Pajor AM, Leibach FH, Devoe
LD, Prasad PD, and Ganapathy V. Primary structure and functional characteristics of a mammalian sodium-coupled high affinity
dicarboxylate transporter. J Biol Chem 274: 3422–3429, 1999.
201. Keppler D, König J, Schaub T, and Leier I. Molecular basis of
ATP-dependent transport of anionic conjugates in kidney and liver.
Nova Acta Leopoldina 78: 213–221, 1998.
202. Keppler D, Leier I, Jedlitschky G, and König J. ATP-dependent
transport of glutathione S-conjugates by the multidrug resistance
protein MRP1 and its apical isoform MRP2. Chemico-Biol Interact
111–112: 153–161, 1998.
203. Khamdang S, Takeda M, Babu E, Noshiro R, Onozato ML,
Tojo A, Enomoto A, Huang XL, Narikawa S, Anzai N, Piyachaturawat P, and Endou H. Interaction of human and rat organic anion transporter 2 with various cephalosporin antibiotics.
Eur J Pharmacol 465: 1–7, 2003.
204. Khamdang S, Takeda M, Noshiro R, Narikawa S, Enomoto A,
Anzai N, Piyachaturawat P, and Endou H. Interactions of human organic anion transporters and human organic cation trans-
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
Physiol Rev • VOL
246. Lopez-Nieto CE, You G, Bush KT, Barros EJ, Beier DR, and
Nigam SK. Molecular cloning and characterization of NKT, a gene
product related to the organic cation transporter family that is
almost exclusively expressed in the kidney. J Biol Chem 272:
6471– 6478, 1997.
247. Lu R. Regulation of renal oatp mRNA expression by testosterone.
Am J Physiol Renal Fluid Electrolyte Physiol 270: F332–F337,
1996.
248. Lu R, Chan BS, and Schuster VL. Cloning of the human kidney
PAH transporter: narrow substrate specificity and regulation by
protein kinase C. Am J Physiol Renal Physiol 276: F295–F303,
1999.
249. Lungkaphin A, Chatsudthipong V, Evans KK, Groves CE,
Wright SH, and Dantzler WH. Interaction of the metal chelator,
DMPS, with OAT1 and OAT3 in intact isolated rabbit renal proximal tubules. Am J Physiol Renal Physiol 286: F68 –F76, 2004.
250. Machaalani R, Lazzaro V, and Duggin GG. The characterisation
and uptake of paraquat in cultured baboon kidney proximal tubule
cells (bPTC). Hum Exp Toxicol 20: 90 –99, 2001.
251. Madon J, Hagenbuch B, Landmann L, Meier PJ, and Stieger B.
Transport function and hepatocellular localization of mrp6 in rat
liver. Mol Pharmacol 57: 634 – 641, 2000.
252. Makhuli MJ, Polkowski CA, and Grassl SM. Organic anion
transport in rabbit renal basolateral membrane vesicles. J Pharmacol Exp Ther 273: 146 –153, 1995.
253. Marger MD and Saier MH Jr. A major superfamily of transmembrane facilitators that catalyse uniport, symport and antiport.
Trends Biochem Sci 18: 13–20, 1993.
254. Markovich D. Physiological roles and regulation of mammalian
sulfate transporters. Physiol Rev 81: 1499 –1533, 2001.
255. Markovich D, Bissig M, Sorribas V, Hagenbuch B, Meier PJ,
and Murer H. Expression of rat renal sulfate transport systems in
Xenopus laevis oocytes. Functional characterization and molecular
identification. J Biol Chem 269: 3022–3026, 1994.
256. Martel F, Vetter T, Russ H, Gründemann D, Azevedo I, Koepsell H, and Schömig E. Transport of small organic cations in
the rat liver. The role of the organic cation transporter OCT1.
Naunyn-Schmeidebergs Arch Pharmacol 354: 320 –326, 1996.
257. Martinez F, Manganel M, Montrose-Rafizadeh C, Werner D,
and Roch-Ramel F. Transport of urate and p-aminohippurate in
rabbit renal brush-border membranes. Am J Physiol Renal Fluid
Electrolyte Physiol 258: F1145–F1153, 1990.
258. Martinez-Zaguilan R, Raghunand N, Lynch RM, Bellamy W,
Martinez GM, Rojas B, Smith D, Dalton WS, and Gillies RJ. pH
and drug resistance. I. Functional expression of plasmalemmal
V-type H⫹-ATPase in drug-resistant human breast carcinoma cell
lines. Biochem Pharmacol 57: 1037–1046, 1999.
259. Masereeuw R, Moons MM, and Russel FG. Rhodamine 123
accumulates extensively in the isolated perfused rat kidney and is
secreted by the organic cation system. Eur J Pharmacol 321:
315–323, 1997.
260. Masereeuw R, Moons MM, Toomey BH, Russel FG, and Miller
DS. Active Lucifer yellow secretion in renal proximal tubule: evidence for organic anion transport system crossover. J Pharmacol
Exp Ther 289: 1104 –1111, 1999.
261. Masereeuw R, Terlouw SA, van Aubel RA, Russel FG, and
Miller DS. Endothelin B receptor-mediated regulation of ATPdriven drug secretion in renal proximal tubule. Mol Pharmacol 57:
59 – 67, 2000.
262. Masuda S, Ibaramoto K, Takeuchi A, Saito H, Hashimoto Y,
and Inui KI. Cloning and functional characterization of a new
multispecific organic anion transporter, OAT-K2, in rat kidney. Mol
Pharmacol 55: 743–752, 1999.
263. Masuda S, Saito H, Nonoguchi H, Tomita K, and Inui KI.
mRNA distribution and membrane localization of the OAT-K1 organic anion transporter in rat renal tubules. FEBS Lett 407: 127–
131, 1997.
264. Masuda S, Takeuchi A, Saito H, Hashimoto Y, and Inui K.
Functional analysis of rat renal organic anion transporter OAT-K1:
bidirectional methotrexate transport in apical membrane. FEBS
Lett 459: 128 –132, 1999.
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
226. Kool M, van der LM, de Haas M, Baas F, and Borst P. Expression of human MRP6, a homologue of the multidrug resistance
protein gene MRP1, in tissues and cancer cells. Cancer Res 59:
175–182, 1999.
227. Kosoglou T and Vlasses PH. Drug interactions involving renal
transport mechanisms: an overview. DICP 23: 116 –122, 1989.
228. Krick W, Wolff NA, and Burckhardt G. Voltage-driven p-aminohippurate, chloride, and urate transport in porcine renal brushborder membrane vesicles. Pflügers Arch 441: 125–132, 2000.
229. Kubitz R, Huth C, Schmitt M, Horbach A, Kullak-Ublick G,
and Haussinger D. Protein kinase C-dependent distribution of the
multidrug resistance protein 2 from the canalicular to the basolateral membrane in human HepG2 cells. Hepatology 34: 340 –350,
2001.
230. Kudo N, Katakura M, Sato Y, and Kawashima Y. Sex hormoneregulated renal transport of perfluorooctanoic acid. Chem Biol
Interact 139: 301–316, 2002.
231. Kullak-Ublick GA. Regulation of organic anion and drug transporters of the sinusoidal membrane. J Hepatol 31: 563–573, 1999.
232. Kusuhara H, Sekine T, Utsunomiya-Tate N, Tsuda M, Kojima
R, Cha SH, Sugiyama Y, Kanai Y, and Endou H. Molecular
cloning and characterization of a new multispecific organic anion
transporter from rat brain. J Biol Chem 274: 13675–13680, 1999.
233. Kusuhara H, Suzuki H, and Sugiyama Y. The role of P-glycoprotein and canalicular multispecific organic anion transporter in
the hepatobiliary excretion of drugs. J Pharm Sci 87: 1025–1040,
1998.
234. Kutzer M, Meer S, Haller S, and Greven J. Basolateral glutarate
transport by isolated S2 segments of rabbit kidney proximal tubules. J Pharmacol Exp Ther 277: 316 –320, 1996.
235. Kuze K, Graves P, Leahy A, Wilson P, Stuhlmann H, and You
G. Heterologous expression and functional characterization of a
mouse renal organic anion transporter in mammalian cells. J Biol
Chem 274: 1519 –1524, 1999.
236. Lash LH. Glutathione status and other antioxidant defense mechanisms. In: Comprehensive Toxicology. Renal Toxicology, edited
by R. S. Goldstein. New York: Pergamon, 1997, vol. 7, p. 403– 428.
237. Leabman MK, Huang CC, DeYoung J, Carlson EJ, Taylor TR,
De La CM, Johns SJ, Stryke D, Kawamoto M, Urban TJ,
Kroetz DL, Ferrin TE, Clark AG, Risch N, Herskowitz I, Giacomini KM, and Pharmacogenetics of Membrane Transporters Investigators. Natural variation in human membrane transporter genes reveals evolutionary and functional constraints. Proc
Natl Acad Sci USA 100: 5896 –5901, 2003.
238. Leabman MK, Huang CC, Kawamoto M, Johns SJ, Stryke D,
Ferrin TE, DeYoung J, Taylor T, Clark AG, Herskowitz I, and
Giacomini KM. Polymorphisms in a human kidney xenobiotic
transporter, OCT2, exhibit altered function. Pharmacogenetics 12:
395– 405, 2002.
239. Leier I, Hummel-Eisenbeiss J, Cui Y, and Keppler D. ATPdependent para-aminohippurate transport by apical multidrug resistance protein MRP2. Kidney Int 57: 1636 –1642, 2000.
240. Leier I, Jedlitschky G, Buchholz U, Center M, Cole SP, Deeley
RG, and Keppler D. ATP-dependent glutathione disulphide transport mediated by the MRP gene-encoded conjugate export pump.
Biochem J 314: 433– 437, 1996.
241. Leslie EM, Deeley RG, and Cole SP. Toxicological relevance of
the multidrug resistance protein 1, MRP1 (ABCC1) and related
transporters. Toxicology 167: 3–23, 2001.
242. Leung S and Bendayan R. Role of P-glycoprotein in the renal
transport of dideoxynucleoside analog drugs. Can J Physiol Pharmacol 77: 625– 630, 1999.
243. Li L, Lee TK, Meier PJ, and Ballatori N. Identification of glutathione as a driving force and leukotriene C4 as a substrate for
oatp1, the hepatic sinusoidal organic solute transporter. J Biol
Chem 273: 16184 –16191, 1998.
244. Li N, Hartley DP, Cherrington NJ, and Klaassen CD. Tissue
expression, ontogeny, and inducibility of rat organic anion transporting polypeptide 4. J Pharmacol Exp Ther 301: 551–560, 2002.
245. Loo DD, Hirayama BA, Gallardo EM, Lam JT, Turk E, and
Wright EM. Conformational changes couple Na⫹ and glucose
transport. Proc Natl Acad Sci USA 95: 7789 –7794, 1998.
1041
1042
STEPHEN H. WRIGHT AND WILLIAM H. DANTZLER
Physiol Rev • VOL
287. Miller DS, Villalobos AR, and Pritchard JB. Organic cation
transport in rat choroid plexus cells studied by fluorescence microscopy. Am J Physiol Cell Physiol 276: C955–C968, 1999.
288. Mittur A, Wolkoff AW, and Kaplowitz N. The thiol sensitivity of
glutathione transport in sidedness-sorted basolateral liver plasma
membrane and in Oatp1-expressing HeLa cell membrane. Mol
Pharmacol 61: 425– 435, 2002.
289. Miyamoto Y, Ganapathy V, and Leibach FH. Transport of guanidine in rabbit intestinal brush-border membrane vesicles. Am J
Physiol Gastrointest Liver Physiol 255: G85–G92, 1988.
290. Miyamoto Y, Tiruppathi C, Ganapathy V, and Leibach FH.
Multiple transport systems for organic cations in renal brush-border membrane vesicles. Am J Physiol Renal Fluid Electrolyte
Physiol 256: F540 –F548, 1989.
291. Mizuuchi H, Katsura T, Saito H, Hashimoto Y, and Inui KI.
Transport characteristics of diphenhydramine in human intestinal
epithelial Caco-2 cells: contribution of pH-dependent transport
system. J Pharmacol Exp Ther 290: 388 –392, 1999.
292. Moller JV and Sheikh MI. Renal organic anion transport system:
pharmacological, physiological, and biochemical aspects. Pharmacol Rev 34: 315–358, 1982.
293. Montfoort JEV, Stieger B, Meijer DKF, Weinmann HJ, Meier
PJ, and Fattinger KE. Hepatic uptake of the MRI-contrast agent
gadoxetate by the organic anion transporting polypeptide Oatp1.
J Pharmacol Exp Ther 1999.
294. Montrose-Rafizadeh C, Mingard F, Murer H, and Roch-Ramel
F. Carrier-mediated transport of tetraethylammonium across rabbit renal basolateral membrane. Am J Physiol Renal Fluid Electrolyte Physiol 257: F243–F251, 1989.
295. Montrose-Rafizadeh C, Roch-Ramel F, and Schäli C. Axial
heterogeneity of organic cation transport along the rabbit renal
proximal tubule: studies with brush-border membrane vesicles.
Biochim Biophys Acta 904: 175–177, 1987.
296. Mooslehner KA and Allen ND. Cloning of the mouse organic
cation transporter 2 gene, Slc22a2, from an enhancer-trap transgene integration locus. Mamm Genome 10: 218 –224, 1999.
297. Mori K, Ogawa Y, Ebihara K, Aoki T, Tamura N, Sugawara A,
Kuwahara T, Ozaki S, Mukoyama M, Tashiro K, Tanaka I, and
Nakao K. Kidney-specific expression of a novel mouse organic
cation transporter-like protein. FEBS Lett 417: 371–374, 1997.
298. Morita N, Kusuhara H, Sekine T, Endou H, and Sugiyama Y.
Functional characterization of rat organic anion transporter 2 in
LLC-PK1 cells. J Pharmacol Exp Ther 298: 1179 –1184, 2001.
299. Moseley RH, Jarose SM, and Permoad P. Organic cation transport by rat liver plasma membrane vesicles: studies with tetraethylammonium. Am J Physiol Gastrointest Liver Physiol 263: G775–
G785, 1992.
300. Motohashi H, Sakurai Y, Saito H, Masuda S, Urakami Y, Goto
M, Fukatsu A, Ogawa O, and Inui KI. Gene expression levels and
immunolocalization of organic anion transporters in the human
kidney. J Am Soc Nephrol 13: 866 – 874, 2002.
301. Murer H, Hernando N, Forster I, and Biber J. Proximal tubular
phosphate reabsorption: molecular mechanisms. Physiol Rev 80:
1373–1409, 2000.
302. Murer H, Hernando N, Forster L, and Biber J. Molecular mechanisms in proximal tubular and small intestinal phosphate reabsorption. Mol Membr Biol 18: 3–11, 2001.
303. Nagai J, Yano I, Hashimoto Y, Takano M, and Inui KI. Inhibition of PAH transport by parathyroid hormone in OK cells: involvement of protein kinase C pathway. Am J Physiol Renal Physiol 273:
F674 –F679, 1997.
304. Nagel G, Volk C, Friedrich T, Ulzheimer JC, Bamberg E, and
Koepsell H. A reevaluation of substrate specificity of the rat cation
transporter rOCT1. J Biol Chem 272: 31953–31956, 1997.
305. Nakajima N, Sekine T, Cha SH, Tojo A, Hosoyamada M, Kanai
Y, Yan K, Awa S, and Endou H. Developmental changes in
multispecific organic anion transporter 1 expression in the rat
kidney. Kidney Int 57: 1608 –1616, 2000.
306. Neame KD and Richards TG. Elementary Kinetics of Membrane
Carrier Transport. New York: Wiley, 1972, p. 1–120.
307. Nelson JA, Dutt A, Allen LH, and Wright DA. Functional expression of the renal organic cation transporter and P-glycoprotein
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
265. McKinney TD and Hosford MA. ATP-stimulated tetraethylammonium transport by rabbit renal brush border membrane vesicles.
J Biol Chem 268: 6886 – 6895, 1993.
266. McKinney TD and Kunnemann ME. Cimetidine transport in
rabbit renal cortical brush-border membrane vesicles. Am J
Physiol Renal Fluid Electrolyte Physiol 252: F525–F535, 1987.
267. McKinney TD, Myers P, and Speeg KV Jr. Cimetidine secretion
by rabbit renal tubules in vitro. Am J Physiol Renal Fluid Electrolyte Physiol 241: F69 –F76, 1981.
268. McKinney TD, Scheller MB, Hosford M, Lesniak ME, and
Haseley TS. Basolateral transport of tetraethylammonium by a
clone of LLC-PK1 cells. J Am Soc Nephrol 2: 1507–1515, 1992.
269. McKinney TD and Speeg KV Jr. Cimetidine and procainamide
secretion by proximal tubules in vitro. Am J Physiol Renal Fluid
Electrolyte Physiol 242: F672–F680, 1982.
270. Mehrens T, Lelleck S, Cetinkaya I, Knollmann M, Hohage H,
Gorboulev V, Boknik P, Koepsell H, and Schlatter E. The
affinity of the organic cation transporter rOCT1 is increased by
protein kinase C-dependent phosphorylation. J Am Soc Nephrol 11:
1216 –1224, 2000.
271. Meier PJ, Eckhardt U, Schroeder A, Hagenbuch B, and
Stieger B. Substrate specificity of sinusoidal bile acid and organic
anion uptake systems in rat and human liver. Hepatology 26: 1667–
1677, 1997.
272. Meier PJ and Stieger B. Bile salt transporters. Annu Rev Physiol
64: 635– 661, 2002.
273. Meijer DK, Smit JW, Hooiveld GJ, van Montfoort JE, Jansen
PL, and Muller M. The molecular basis for hepatobiliary transport
of organic cations and organic anions. Pharm Biotechnol 12: 89 –
157, 1999.
274. Meijer DKF, Mol WEM, Müller M, and Kurz G. Carrier-mediated transport in the hepatic distribution and elimination of drugs,
with special reference to the category of organic cations. J Pharmacokin Biopharmaceut 18: 35–70, 1990.
275. Meinild AK, Hirayama BA, Wright EM, and Loo DD. Fluorescence studies of ligand-induced conformational changes of the
Na⫹/glucose cotransporter. Biochemistry 41: 1250 –1258, 2002.
276. Meyer-Wentrup F, Karbach U, Gorboulev V, Arndt P, and
Koepsell H. Membrane localization of the electrogenic cation
transporter rOCT1 in rat liver. Biochem Biophys Res Commun 248:
673– 678, 1998.
277. Miller DS. Daunomycin secretion by killifish renal proximal tubules. Am J Physiol Regul Integr Comp Physiol 269: R370 –R379,
1995.
278. Miller DS. Protein kinase C regulation of organic anion transport
in renal proximal tubule. Am J Physiol Renal Physiol 274: F156 –
F164, 1998.
279. Miller DS. Nucleoside phosphonate interactions with multiple
organic anion transporters in renal proximal tubule. J Pharmacol
Exp Ther 299: 567–574, 2001.
280. Miller DS. Xenobiotic export pumps, endothelin signaling, and
tubular nephrotoxicants—a case of molecular hijacking. J Biochem
Mol Toxicol 16: 121–127, 2002.
281. Miller DS, Barnes DM, and Pritchard JB. Confocal microscopic
analysis of fluorescein compartmentation within crab urinary bladder cells. Am J Physiol Regul Integr Comp Physiol 267: R16 –R25,
1994.
282. Miller DS, Fricker G, and Drewe J. P-glycoprotein-mediated
transport of a fluorescent rapamycin derivative in renal proximal
tubule. J Pharmacol Exp Ther 282: 440 – 444, 1997.
283. Miller DS, Letcher S, and Barnes DM. Fluorescence imaging
study of organic anion transport from proximal tubule cell to
lumen. Am J Physiol Renal Fluid Electrolyte Physiol 271: F508 –
F520, 1996.
284. Miller DS and Pritchard JB. Nocodazole inhibition of organic
anion secretion in teleost renal proximal tubules. Am J Physiol
Regul Integr Comp Physiol 267: R695–R704, 1994.
285. Miller DS, Stewart DE, and Pritchard JB. Intracellular compartmentation of organic anions within renal cells. Am J Physiol Regul
Integr Comp Physiol 264: R882–R890, 1993.
286. Miller DS, Sussman CR, and Renfro JL. Protein kinase C regulation of P-glycoprotein-mediated xenobiotic secretion in renal
proximal tubule. Am J Physiol Renal Physiol 275: F785–F795, 1998.
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
308.
309.
310.
311.
312.
314.
315.
316.
317.
318.
319.
320.
321.
322.
323.
324.
325.
326.
327.
328.
329.
Physiol Rev • VOL
330. Pajor AM and Sun N. Characterization of the rabbit renal Na⫹dicarboxylate cotransporter using antifusion protein antibodies.
Am J Physiol Cell Physiol 271: C1808 –C1816, 1996.
331. Pajor AM and Sun N. Protein kinase C-mediated regulation of the
renal Na⫹/dicarboxylate cotransporter, NaDC-1. Biochim Biophys
Acta 1420: 223–230, 1999.
332. Pan BF, Dutt A, and Nelson JA. Enhanced transepithelial flux of
cimetidine by Madin-Darby canine kidney cells overexpressing human P-glycoprotein. J Pharmacol Exp Ther 270: 1–7, 1994.
333. Pan BF, Sweet DH, Pritchard JB, Chen R, and Nelson JA. A
transfected cell model for the renal toxin transporter, rOCT2. Toxicol Sci 47: 181–186, 1999.
334. Pao SS, Paulsen IT, and Saier MH Jr. Major facilitator superfamily. Microbiol Mol Biol Rev 62: 1–34, 1998.
335. Pavlova A, Sakurai H, Leclercq B, Beier DR, Yu AS, and Nigam
SK. Developmentally regulated expression of organic ion transporters NKT (OAT1), OCT1, NLT (OAT2), and Roct. Am J Physiol
Renal Physiol 278: F635–F643, 2000.
336. Peerce BE and Wright EM. Sodium-induced conformational
changes in the glucose transporter of intestinal brush borders.
J Biol Chem 259: 14105–14112, 1984.
337. Pietig G, Mehrens T, Hirsch JR, Cetinkaya I, Piechota H, and
Schlatter E. Properties and regulation of organic cation transport
in freshly isolated human proximal tubules. J Biol Chem 276:
33741–33746, 2001.
338. Pombrio JM, Giangreco A, Li L, Wempe MF, Anders MW,
Sweet DH, Pritchard JB, and Ballatori N. Mercapturic acids
(N-acetylcysteine S-conjugates) as endogenous substrates for the
renal organic anion transporter-1. Mol Pharmacol 60: 1091–1099,
2001.
339. Pritchard JB. Luminal and peritubular steps in the renal transport
of p-aminohippurate. Biochim Biophys Acta 906: 295–308, 1987.
340. Pritchard JB. Coupled transport of p-aminohippurate by rat kidney basolateral membrane vesicles. Am J Physiol Renal Fluid
Electrolyte Physiol 255: F597–F604, 1988.
341. Pritchard JB. Intracellular ␣-ketoglutarate controls the efficacy of
renal organic anion transport. J Pharmacol Exp Ther 274: 1278 –
1284, 1995.
342. Pritchard JB and Miller DS. Mechanisms mediating renal secretion of organic anions and cations. Physiol Rev 73: 765–796, 1993.
343. Pritchard JB and Miller DS. Renal secretion of organic anions
and cations. Kidney Int 49: 1649 –1654, 1996.
344. Pritchard JB, Sykes DB, Walden R, and Miller DS. ATP-dependent transport of tetraethylammonium by endosomes isolated from
rat renal cortex. Am J Physiol Renal Fluid Electrolyte Physiol 266:
F966 –F976, 1994.
345. Qian YM, Song WC, Cui H, Cole SP, and Deeley RG. Glutathione stimulates sulfated estrogen transport by multidrug resistance
protein 1. J Biol Chem 276: 6404 – 6411, 2001.
346. Race JE, Grassl SM, Williams WJ, and Holtzman EJ. Molecular
cloning and characterization of two novel human renal organic
anion transporters (hOAT1 and hOAT3). Biochem Biophys Res
Commun 255: 508 –514, 1999.
347. Raghunand N and Gillies RJ. pH and drug resistance in tumors.
Drug Resist Update 3: 39 – 47, 2000.
348. Raghunand N, Martinez-Zaguilan R, Wright SH, and Gillies
RJ. pH and drug resistance. II. Turnover of acidic vesicles and
resistance to weakly basic chemotherapeutic drugs. Biochem
Pharmacol 57: 1047–1058, 1999.
349. Rajan PD, Kekuda R, Chancy CD, Huang W, Ganapathy V, and
Smith SB. Expression of the extraneuronal monoamine transporter in RPE and neural retina. Current Eye Res 20: 195–204, 2000.
350. Rebouche CJ and Mack DL. Sodium gradient-stimulated transport of L-carnitine into renal brush border membrane vesicles:
kinetics, specificity, and regulation by dietary carnitine. Arch Biochem Biophys 235: 393– 402, 1984.
351. Reid G, Wolff NA, Dautzenberg FM, and Burckhardt G. Cloning of a human renal p-aminohippurate transporter, hROAT1. Kidney Blood Press Res 21: 233–237, 1998.
352. Rennick B. Renal tubule transport of organic cations. Am J
Physiol Renal Fluid Electrolyte Physiol 240: F83–F89, 1981.
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
313.
in Xenopus laevis oocytes. Cancer Chemother Pharmacol 37: 187–
189, 1995.
Nies AT, Cantz T, Brom M, Leier I, and Keppler D. Expression
of the apical conjugate export pump, Mrp2, in the polarized hepatoma cell line, WIF-B. Hepatology 28: 1332–1340, 1998.
Nikiforov AA. Stimulation by aminooxyacetate of fluorescein uptake in rat renal tubules in vitro: role of intracellular ␣-ketoglutarate. J Pharmacol Exp Ther 274: 1204 –1208, 1995.
Notenboom S, Miller DS, Smits P, Russel FG, and Masereeuw
R. Role of NO in endothelin-regulated drug transport in the renal
proximal tubule. Am J Physiol Renal Physiol 282: F458 –F464,
2002.
Oh DM, Han HK, and Amidon GL. Drug transport and targeting.
Intestinal transport. Pharm Biotechnol 12: 59 – 88, 1999.
Ohashi R, Tamai I, Nezu JJ, Nikaido H, Hashimoto N, Oku A,
Sai Y, Shimane M, and Tsuji A. Molecular and physiological
evidence for multifunctionality of carnitine/organic cation transporter OCTN2. Mol Pharmacol 59: 358 –366, 2001.
Ohashi R, Tamai I, Yabuuchi H, Nezu JI, Oku A, Sai Y, Shimane M, and Tsuji A. Na⫹-dependent carnitine transport by
organic cation transporter (OCTN2): its pharmacological and toxicological relevance. J Pharmacol Exp Ther 291: 778 –784, 1999.
Ohnishi S, Saito H, Fukada A, and Inui K. Distinct transport
activity of tetraethylammonium from L-carnitine in rat renal brushborder membranes. Biochim Biophys Acta 1609: 218 –224, 2003.
Ohoka K, Takano M, Okano T, Maeda S, Inui K, and Hori R.
p-Aminohippurate transport in rat renal brush-border membranes:
a potential-sensitive transport system and an anion exchanger. Biol
Pharm Bull 16: 395– 401, 1993.
Okamura N, Hirai M, Tanigawara Y, Tanaka K, Yasuhara M,
Ueda K, Komano T, and Hori R. Digoxin-cyclosporin A interaction: modulation of the multidrug transporter P-glycoprotein in the
kidney. J Pharmacol Exp Ther 266: 1614 –1619, 1993.
Okuda M, Saito H, Urakami Y, Takano M, and Inui KI. cDNA
cloning and functional expression of a novel rat kidney organic
cation transporter, OCT2. Biochem Biophys Res Commun 224:
500 –507, 1996.
Okuda M, Urakami Y, Saito H, and Inui K. Molecular mechanisms of organic cation transport in OCT2-expressing Xenopus
oocytes. Biochim Biophys Acta 1417: 224 –231, 1999.
Ott RJ, Chan JK, Ramanathan VK, Hui AC, and Giacomini
KM. Effect of phenylisothiocyanate on organic cation transport in
opossum kidney cells. J Pharmacol Exp Ther 269: 204 –208, 1994.
Ott RJ and Giacomini KM. Stereoselective interactions of organic cations with the organic cation transporter in OK cells.
Pharm Res 10: 1169 –1173, 1993.
Ott RJ, Hui AC, Hsyu PH, and Giacomini KM. Interactions of
quinidine and quinine and (⫹)- and (⫺)-pindolol with the organic
cation/proton antiporter in renal brush membrane vesicles. Biochem Pharmacol 41: 142–145, 1991.
Ott RJ, Hui AC, Yuan G, and Giacomini KM. Organic cation
transport in human renal brush-border membrane vesicles. Am J
Physiol Renal Fluid Electrolyte Physiol 261: F443–F451, 1991.
Oude Elferink RP, Meijer DK, Kuipers F, Jansen PL, Groen
AK, and Groothuis GM. Hepatobiliary secretion of organic compounds: molecular mechanisms of membrane transport. Biochim
Biophys Acta 1241: 215–268, 1995.
Pajor AM. Sequence and functional characterization of a renal
sodium/dicarboxylate cotransporter. J Biol Chem 270: 5779 –5785,
1995.
Pajor AM. Molecular cloning and functional expression of a sodium-dicarboxylate cotransporter from human kidney. Am J
Physiol Renal Fluid Electrolyte Physiol 270: F642–F648, 1996.
Pajor AM. Citrate transport by the kidney and intestine. Semin
Nephrol 19: 195–200, 1999.
Pajor AM. Sodium-coupled transporters for Krebs cycle intermediates. Annu Rev Physiol 61: 663– 682, 1999.
Pajor AM. Molecular properties of sodium/dicarboxylate cotransporters. J Membr Biol 175: 1– 8, 2000.
Pajor AM, Gangula R, and Yao X. Cloning and functional characterization of a high-affinity Na⫹/dicarboxylate cotransporter
from mouse brain. Am J Physiol Cell Physiol 280: C1215–C1223,
2001.
1043
1044
STEPHEN H. WRIGHT AND WILLIAM H. DANTZLER
Physiol Rev • VOL
372. Schäli C, Schild L, Overney J, and Roch-Ramel F. Secretion of
tetraethylammonium by proximal tubules of rabbit kidneys. Am J
Physiol Renal Fluid Electrolyte Physiol 245: F238 –F246, 1983.
373. Schaub TP, Kartenbeck J, König J, Spring H, Dorsam J,
Staehler G, Storkel S, Thon WF, and Keppler D. Expression of
the MRP2 gene-encoded conjugate export pump in human kidney
proximal tubules and in renal cell carcinoma. J Am Soc Nephrol 10:
1159 –1169, 1999.
374. Schaub TP, Kartenbeck J, König J, Vogel O, Witzgall R, Kriz
W, and Keppler D. Expression of the conjugate export pump
encoded by the mrp2 gene in the apical membrane of kidney
proximal tubules. J Am Soc Nephrol 8: 1213–1221, 1997.
375. Scheffer GL, Hu X, Pijnenborg AC, Wijnholds J, Bergen AA,
and Scheper RJ. MRP6 (ABCC6) detection in normal human
tissues and tumors. Lab Invest 82: 515–518, 2002.
376. Scheffer GL, Kool M, de Haas M, de Vree JM, Pijnenborg AC,
Bosman DK, Elferink RP, d. van V, Borst P, and Scheper RJ.
Tissue distribution and induction of human multidrug resistant
protein 3. Lab Invest 82: 193–201, 2002.
377. Schinkel AH. The physiological function of drug-transporting Pglycoproteins. Semin Cancer Biol 8: 161–170, 1997.
378. Schlatter E, Monnich V, Cetinkaya I, Mehrens T, Ciarimboli
G, Hirsch JR, Popp C, and Koepsell H. The organic cation
transporters rOCT1 and hOCT2 are inhibited by cGMP. J Membr
Biol 189: 237–244, 2002.
379. Schmitt C and Burckhardt G. Modulation by anions of p-aminohippurate transport in bovine renal basolateral membrane vesicles.
Pflügers Arch 425: 241–247, 1993.
380. Schmitt C and Burckhardt G. p-Aminohippurate/2-oxoglutarate
exchange in bovine renal brush-border and basolateral membrane
vesicles. Pflügers Arch 423: 280 –290, 1993.
381. Schömig E, Babin-Ebell J, and Russ H. 1,1⬘-Diethyl-2,2⬘-cyanine
(decynium22) potently inhibits the renal transport of organic cations. Arch Pharm 347: 379 –383, 1993.
382. Schomig E, Spitzenberger F, Engelhardt M, Martel F, Ording
N, and Gründemann D. Molecular cloning and characterization of
two novel transport proteins from rat kidney. FEBS Lett 425:
79 – 86, 1998.
383. Schramm U, Fricker G, Wenger R, and Miller DS. P-glycoprotein-mediated secretion of a fluorescent cyclosporin analogue by
teleost renal proximal tubules. Am J Physiol Renal Fluid Electrolyte Physiol 268: F46 –F52, 1995.
384. Sekine T, Cha SH, Hosoyamada M, Kanai Y, Watanabe N,
Furuta Y, Fukuda K, Igarashi T, and Endou H. Cloning, functional characterization, and localization of a rat renal Na⫹-dicarboxylate transporter. Am J Physiol Renal Physiol 275: F298 –F305,
1998.
385. Sekine T, Cha SH, Tsuda M, Apiwattanakul N, Nakajima N,
Kanai Y, and Endou H. Identification of multispecific organic
anion transporter 2 expressed predominantly in the liver. FEBS
Lett 429: 179 –182, 1998.
386. Sekine T, Watanabe N, Hosoyamada M, Kanaim Y, and Endou
H. Expression cloning and characterization of a novel multispecific
organic anion transporter. J Biol Chem 272: 18526 –18529, 1997.
387. Seth P, Wu X, Huang W, Leibach FH, and Ganapathy V. Mutations in novel organic cation transporter (OCTN2), an organic
cation/carnitine transporter, with differential effects on the organic
cation transport function and the carnitine transport function.
J Biol Chem 274: 33388 –33392, 1999.
388. Sharom FJ. The P-glycoprotein efflux pump: how does it transport
drugs? J Membr Biol 160: 161–175, 1997.
389. Shimada H, Moewes B, and Burckhardt G. Indirect coupling to
Na⫹ of p-aminohippuric acid uptake into rat renal basolateral
membrane vesicles. Am J Physiol Renal Fluid Electrolyte Physiol
253: F795–F801, 1987.
390. Shimomura A, Chonko AM, and Grantham JJ. Basis for heterogeneity of para-aminohippurate secretion in rabbit proximal
tubules. Am J Physiol Renal Fluid Electrolyte Physiol 240: F430 –
F436, 1981.
391. Shpun S, Evans K, and Dantzler WH. Interaction of ␣-KG with
basolateral organic anion transporter in isolated rabbit renal S3
proximal tubules. Am J Physiol Renal Fluid Electrolyte Physiol
268: F1109 –F1116, 1995.
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
353. Reynard AM. The reversible and irreversible inhibition of N1methylnicotinamide uptake into rat kidney cortex slices. J Pharmacol Exp Ther 163: 461– 467, 1968.
354. Rocchiccioli F, Leroux JP, and Cartier PH. Microdetermination
of 2-ketoglutaric acid in plasma and cerebrospinal fluid by capillary
gas chromatography mass spectroscopy: application in pediatrics.
Biomed Mass Spectrom 24: 28, 1984.
355. Roch-Ramel F, Besseghir K, and Murer H. Renal excretion and
tubular transport of organic anions and cations. In: Handbook of
Physiology. Renal Physiology. Bethesda, MD: Am Physiol Soc,
1992, sect. 8, vol. II, p. 2189 –2262.
356. Roch-Ramel F and Diezi J. Renal transport of organic ions and
uric acid. In: Diseases of the Kidney, edited by R. W. Schrier and
C. W. Gottschalk. Boston, MA: Little, Brown, 1997, p. 231–249.
357. Roch-Ramel F, Guisan B, Jaeger P, and Diezi J. Transport of
urate and other organic anions by anion exchange in human renal
brush-border membrane vesicles. Cell Physiol Biochem 6: 60 –71,
1996.
358. Roch-Ramel F, Guisan B, and Schild L. Indirect coupling of
urate and p-aminohippurate transport to sodium in human brushborder membrane vesicles. Am J Physiol Renal Fluid Electrolyte
Physiol 270: F61–F68, 1996.
359. Rover N, Kramer C, Stark U, Gabriels G, and Greven J. Basolateral transport of glutarate in proximal S2 segments of rabbit
kidney: kinetics of the uptake process and effect of activators of
protein kinase A and C. Pflügers Arch 436: 423– 428, 1998.
360. Russel FG, Masereeuw R, and van Aubel RA. Molecular aspects
of renal anionic drug transport. Annu Rev Physiol 64: 563–594,
2002.
361. Saier MH Jr. A functional-phylogenetic classification system for
transmembrane solute transporters. Microbiol Mol Biol Rev 64:
354 – 411, 2000.
362. Saier MH Jr and Paulsen IT. Phylogeny of multidrug transporters. Semin Cell Dev Biol 12: 205–213, 2001.
363. Saito H, Masuda S, and Inui KI. Cloning and functional characterization of a novel rat organic anion transporter mediating basolateral uptake of methotrexate in the kidney. J Biol Chem 271:
20719 –20725, 1996.
364. Saito H, Yamamoto M, Inui KI, and Hori R. Transcellular transport of organic cations across monolayers of kidney epithelial cell
line LLC-PK1. Am J Physiol Cell Physiol 262: C59 –C66, 1992.
365. Sasaki M, Suzuki H, Ito K, Abe T, and Sugiyama Y. Transcellular transport of organic anions across a double-transfected Madin-Darby canine kidney II cell monolayer expressing both human
organic anion-transporting polypeptide (OATP2/SLC21A6) and
Multidrug resistance-associated protein 2 (MRP2/ABCC2). J Biol
Chem 277: 6497– 6503, 2002.
366. Sasaya M, Hatakeyama Y, Saitoh H, and Takada M. Stereoselective permeation of new fluorinated quinolone derivatives across
LLC-PK1 cell monolayers. Biol Pharm Bull 22: 707–712, 1999.
367. Satlin LM, Amin V, and Wolkoff AW. Organic anion transporting
polypeptide mediates organic anion/HCO⫺
3 exchange. J Biol Chem
272: 26340 –26345, 1997.
368. Sauvant C, Hesse D, Holzinger H, Evans KK, Dantzler WH,
and Gekle M. Action of EGF and PGE2 on basolateral organic
anion uptake in isolated proximal renal tubules (S2) and human
OAT1 expressed in immortalized human kidney epithelial (IHKE)
cells. Am J Physiol Renal Physiol 286: F774 –F783, 2004.
369. Sauvant C, Holzinger H, and Gekle M. Modulation of the basolateral and apical step of transepithelial organic anion secretion in
proximal tubular opossum kidney cells. Acute effects of epidermal
growth factor and mitogen-activated protein kinase. J Biol Chem
276: 14695–14703, 2001.
370. Sauvant C, Holzinger H, and Gekle M. Short-term regulation of
basolateral organic anion uptake in proximal tubular OK cells: EGF
acts via MAPK, PLA(2), and COX1. J Am Soc Nephrol 13: 1981–
1991, 2002.
371. Sauvant C, Holzinger H, and Gekle M. Short-term regulation of
basolateral organic anion uptake in proximal tubular OK cells:
protaglandin E2 acts via EP4-receptor and activation of PKA. J Am
Soc Nephrol 14: 3017–3026, 2003.
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
Physiol Rev • VOL
412. Steffgen J, Burckhardt BC, Langenberg C, Kuhne L, Muller
GA, Burckhardt G, and Wolff NA. Expression cloning and characterization of a novel sodium-dicarboxylate cotransporter from
winter flounder kidney. J Biol Chem 274: 20191–20196, 1999.
413. Stieger B, O’Neill B, and Krahenbuhl S. Characterization of
L-carnitine transport by rat kidney brush-border-membrane vesicles. Biochem J 309: 643– 647, 1995.
414. Strausberg RL, Feingold EA, Grouse LH, Derge JG, Klausner
RD, Collins FS, Wagner L, Shenmen CM, Schuler GD, Altschul
SF, Zeeberg B, Buetow KH, Schaefer CF, Bhat NK, Hopkins
RF, Jordan H, Moore T, Max SI, Wang J, Hsieh F, Diatchenko
L, Marusina K, Farmer AA, Rubin GM, Hong L, Stapleton M,
Soares MB, Bonaldo MF, Casavant TL, Scheetz TE, Brownstein MJ, Usdin TB, Toshiyuki S, Carninci P, Prange C, Raha
SS, Loquellano NA, Peters GJ, Abramson RD, Mullahy SJ,
Bosak SA, McEwan PJ, McKernan KJ, Malek JA, Gunaratne
PH, Richards S, Worley KC, Hale S, Garcia AM, Gay LJ, Hulyk
SW, Villalon DK, Muzny DM, Sodergren EJ, Lu X, Gibbs RA,
Fahey J, Helton E, Ketteman M, Madan A, Rodrigues S,
Sanchez A, Whiting M, Madan A, Young AC, Shevchenko Y,
Bouffard GG, Blakesley RW, Touchman JW, Green ED, Dickson MC, Rodriguez AC, Grimwood J, Schmutz J, Myers RM,
Butterfield YS, Krzywinski MI, Skalska U, Smailus DE,
Schnerch A, Schein JE, Jones SJ, and Marra MA. Generation
and initial analysis of more than 15,000 full-length human and
mouse cDNA sequences. Proc Natl Acad Sci USA 99: 16899 –16903,
2002.
415. Sugawara-Yokoo M, Urakami Y, Koyama H, Fujikura K, Masuda S, Saito H, Naruse T, Inui K, and Takata K. Differential
localization of organic cation transporters rOCT1 and rOCT2 in the
basolateral membrane of rat kidney proximal tubules. Histochem
Cell Biol 114: 175–180, 2000.
416. Sugiyama D, Kusuhara H, Shitara Y, Abe T, Meier PJ, Sekine
T, Endou H, Suzuki H, and Sugiyama Y. Characterization of the
efflux transport of 17beta-estradiol-D-17beta-glucuronide from the
brain across the blood-brain barrier. J Pharmacol Exp Ther 298:
316 –322, 2001.
417. Sullivan LP and Grantham JJ. Specificity of basolateral organic
anion exchanger in proximal tubule for cellular and extracellular
solutes. J Am Soc Nephrol 2: 1192–1200, 1992.
418. Sun H, Johnson DR, Finch RA, Sartorelli AC, Miller DW, and
Elmquist WF. Transport of fluorescein in MDCKII-MRP1 transfected cells and mrp1- knockout mice. Biochem Biophys Res Commun 284: 863– 869, 2001.
419. Suzuki H and Sugiyama Y. Transporters for bile acids and organic anions. Pharm Biotechnol 12: 387– 439, 1999.
420. Sweet DH, Bush KT, and Nigam SK. The organic anion transporter family: from physiology to ontogeny and the clinic. Am J
Physiol Renal Physiol 281: F197–F205, 2001.
421. Sweet DH, Chan LM, Walden R, Yang XP, Miller DS, and
Pritchard JB. Organic anion transporter 3 [Slc22a8] is a dicarboxylate exchanger indirectly coupled to the Na⫹ gradient. Am J
Physiol Renal Physiol 284: F763–F769, 2003.
422. Sweet DH, Miller DS, and Pritchard JB. Localization of an
organic anion transporter-GFP fusion construct (rROAT1-GFP) in
intact proximal tubules. Am J Physiol Renal Physiol 276: F864 –
F873, 1999.
423. Sweet DH, Miller DS, and Pritchard JB. Basolateral localization
of organic cation transporter 2 in intact renal proximal tubules.
Am J Physiol Renal Physiol 279: F826 –F834, 2000.
424. Sweet DH, Miller DS, Pritchard JB, Fujiwara Y, Beier DR, and
Nigam SK. Impaired organic anion transport in kidney and choroid
plexus of organic anion transporter 3 (Oat3 [Slc22a8]) knockout
mice. J Biol Chem 277: 26934 –26943, 2002.
425. Sweet DH and Pritchard JB. rOCT2 is a basolateral potentialdriven carrier, not an organic cation/proton exchanger. Am J
Physiol Renal Physiol 277: F890 –F898, 1999.
426. Sweet DH and Pritchard JB. The molecular biology of renal
organic anion and organic cation transporters. Cell Biochem Biophys 31: 89 –118, 1999.
427. Sweet DH, Wolff NA, and Pritchard JB. Expression cloning and
characterization of ROAT1: the renal basolateral organic anion
transporter in rat kidney. J Biol Chem 272: 30088 –30095, 1997.
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
392. Shu Y, Bello CL, Mangravite LM, Feng B, and Giacomini KM.
Functional characteristics and steroid hormone-mediated regulation of an organic cation transporter in Madin-Darby canine kidney
cells. J Pharmacol Exp Ther 299: 392–398, 2001.
393. Shu Y, Leabman MK, Feng B, Mangravite LM, Huang CC,
Stryke D, Kawamoto M, Johns SJ, DeYoung J, Carlson E,
Ferrin TE, Herskowitz I, Giacomini KM, and the Pharmacogenetics of Membrane Transporters Investigators. Evolutionary conservation predicts function of variants of the human organic
cation transporter, OCT1. Proc Natl Acad Sci USA 100: 5902–5907,
2003.
395. Shuprisha A, Lynch RM, Wright SH, and Dantzler WH. Realtime assessment of ␣-ketoglutarate effect on organic anion secretion in perfused rabbit proximal tubules. Am J Physiol Renal
Physiol 277: F513–F523, 1999.
396. Shuprisha A, Lynch RM, Wright SH, and Dantzler WH. PKC
regulation of organic anion secretion in perfused S2 segments of
rabbit proximal tubules. Am J Physiol Renal Physiol 278: F104 –
F109, 2000.
397. Shuprisha A, Wright SH, and Dantzler WH. Method for measuring luminal efflux of fluorescent organic compounds in isolated,
perfused renal tubules. Am J Physiol Renal Physiol 279: F960 –
F964, 2000.
398. Silverman JA. Multidrug-resistance transporters. Pharm Biotechnol 12: 353–386, 1999.
399. Simonson GD, Vincent AC, Roberg KJ, Huang Y, and Iwanij V.
Molecular cloning and characterization of a novel liver-specific
transport protein. J Cell Sci 107: 3–72, 1994.
400. Simpson DP and Hager SR. pH and bicarbonate effects on mitochondrial anion accumulation. Proposed mechanism for changes in
renal metabolite levels in acute acid-base disturbances. J Clin
Invest 63: 704 –712, 1979.
401. Simpson DP, McSherry N, Scarbrough E, and Zweifel K. Substrate responses to acid-base alterations in dog renal proximal
tubules. Am J Physiol Renal Fluid Electrolyte Physiol 262: F1039 –
F1046, 1992.
402. Slitt AL, Cherrington NJ, Hartley DP, Leazer TM, and Klaassen CD. Tissue distribution and renal developmental changes in rat
organic cation transporter mRNA levels. Drug Metab Dispos 30:
212–219, 2002.
403. Smit JW, Duin E, Steen H, Oosting R, Roggeveld J, and Meijer
DK. Interactions between P-glycoprotein substrates and other cationic drugs at the hepatic excretory level. Br J Pharmacol 123:
361–370, 1998.
404. Smit JW, Schinkel AH, Weert B, and Meijer DK. Hepatobiliary
and intestinal clearance of amphiphilic cationic drugs in mice in
which both mdr1a and mdr1b genes have been disrupted. Br J
Pharmacol 124: 416 – 424, 1998.
405. Smit JW, Weert B, Schinkel AH, and Meijer DK. Heterologous
expression of various P-glycoproteins in polarized epithelial cells
induces directional transport of small (type 1) and bulky (type 2)
cationic drugs. J Pharmacol Exp Ther 286: 321–327, 1998.
406. Smith PM, Pritchard JB, and Miller DS. Membrane potential
drives organic cation transport into teleost renal proximal tubules.
Am J Physiol Regul Integr Comp Physiol 255: R492–R499, 1988.
407. Sokol PP and McKinney TD. Mechanism of organic cation transport in rabbit renal basolateral membrane vesicles. Am J Physiol
Renal Fluid Electrolyte Physiol 258: F1599 –F1607, 1990.
408. Somogyi A and Heinzow B. Cimetidine reduces procainamide
elimination. N Engl J Med 307: 1080, 1982.
409. Somogyi A, McLean A, and Heinzow B. Cimetidine-procainamide pharmacokinetic interaction on man: evidence of competition for tubular secretion of basic drugs. Eur J Clin Pharmacol 25:
339 –345, 1983.
410. Somogyi AA, Rumrich G, Fritzsch G, and Ullrich KJ. Stereospecificity in contraluminal and luminal transporters of organic
cations in the rat renal proximal tubule. J Pharmacol Exp Ther 278:
31–36, 1996.
411. Staal RG, Yang JM, Hait WN, and Sonsalla PK. Interactions of
1-methyl-4-phenylpyridinium and other compounds with P-glycoprotein: relevance to toxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Brain Res 910: 116 –125, 2001.
1045
1046
STEPHEN H. WRIGHT AND WILLIAM H. DANTZLER
Physiol Rev • VOL
447. Tang NL, Ganapathy V, Wu X, Hui J, Seth P, Yuen PM, Wanders RJ, Fok TF, and Hjelm NM. Mutations of OCTN2, an organic
cation/carnitine transporter, lead to deficient cellular carnitine uptake in primary carnitine deficiency. Hum Mol Genet 8: 655– 660,
1999.
448. Tanigawara Y, Okamura N, Hirai M, Yasuhara M, Ueda K,
Kioka N, Komano T, and Hori R. Transport of digoxin by human
P-glycoprotein expressed in a porcine kidney epithelial cell line
(LLC-PK1). J Pharmacol Exp Ther 263: 840 – 845, 1992.
449. Taniguchi K, Wada M, Kohno K, Nakamura T, Kawabe T,
Kawakami M, Kagotani K, Okumura K, Akiyama S, and Kuwano M. A human canalicular multispecific organic anion transporter (cMOAT) gene is overexpressed in cisplatin-resistant human
cancer cell lines with decreased drug accumulation. Cancer Res 56:
4124 – 4129, 1996.
450. Taylor CA, Stanley KN, and Shirras AD. The Orct gene of
Drosophila melanogaster codes for a putative organic cation transporter with six or 12 transmembrane domains. Gene 201: 69 –74,
1997.
451. Terashita S, Dresser MJ, Zhang L, Gray AT, Yost SC, and
Giacomini KM. Molecular cloning and functional expression of a
rabbit renal organic cation transporter. Biochim Biophys Acta
1369: 1– 6, 1998.
452. Terlouw SA, Graeff C, Smeets PH, Fricker G, Russel FG,
Masereeuw R, and Miller DS. Short- and long-term influences of
heavy metals on anionic drug efflux from renal proximal tubule.
J Pharmacol Exp Ther 301: 578 –585, 2002.
453. Terlouw SA, Masereeuw R, and Russel FG. Modulatory effects
of hormones, drugs, and toxic events on renal organic anion transport. Biochem Pharmacol 65: 1393–1405, 2003.
454. Terlouw SA, Masereeuw R, Russel FG, and Miller DS. Nephrotoxicants induce endothelin release and signaling in renal proximal tubules: effect on drug efflux. Mol Pharmacol 59: 1433–1440,
2001.
455. Thiebaut F, Tsuruo T, Hamada H, Gottesman MM, and Pastan
I. Cellular localization of multidrug resistance gene product Pglycoprotein in normal human tissues. Proc Natl Acad Sci USA 84:
7735–7738, 1987.
456. Tirona RG and Kim RB. Pharmacogenomics of organic aniontransporting polypeptides (OATP). Adv Drug Deliv Rev 54: 1343–
1352, 2002.
457. Tojo A, Sekine T, Nakajima N, Hosoyamada M, Kanai Y,
Kimura K, and Endou H. Immunohistochemical localization of
multispecific renal organic anion transporter 1 in rat kidney. J Am
Soc Nephrol 10: 464 – 471, 1999.
458. Tomita Y, Otsuki Y, Hashimoto Y, and Inui K. Kinetic analysis
of tetraethylammonium transport in the kidney epithelial cell line,
LLC-PK1. Pharm Res 14: 1236 –1240, 1997.
459. Tsuda M, Sekine T, Takeda M, Cha SH, Kanai Y, Kimura M,
and Endou H. Transport of ochratoxin A by renal multispecific
organic anion transporter 1. J Pharmacol Exp Ther 289: 1301–1305,
1999.
460. Tsuruoka S, Sugimoto KI, Fujimura A, Imai M, Asano Y, and
Muto S. P-glycoprotein-mediated drug secretion in mouse proximal tubule perfused in vitro. J Am Soc Nephrol 12: 177–181, 2001.
461. Tune BM, Burg MB, and Patlak CS. Characteristics of p-aminohippurate transport in proximal renal tubules. Am J Physiol 217:
1057–1063, 1969.
462. Uchino H, Tamai I, Yamashita K, Minemoto Y, Sai Y, Yabuuchi
H, Miyamoto K, Takeda E, and Tsuji A. p-Aminohippuric acid
transport at renal apical membrane mediated by human inorganic
phosphate transporter NPT1. Biochem Biophys Res Commun 270:
254 –259, 2000.
463. Ujhelyi MR, Bottorff MB, Schur M, Roll K, Zhang H, Stewart
J, and Markel ML. Aging effects on the organic base transporter
and stereoselective renal clearance. Clin Pharmacol Ther 62: 117–
128, 1997.
464. Ullrich KJ. Specificity of transporters for “organic anions” and
“organic cations” in the kidney. Biochim Biophys Acta 1197: 45– 62,
1994.
465. Ullrich KJ. Renal transporters for organic anions and cations.
Structural requirements for substrates. J Membr Biol 158: 95–107,
1997.
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
428. Takami K, Saito H, Okuda M, Takano M, and Inui KI. Distinct
characteristics of transcellular transport between nicotine and tetraethylammonium in LLC-PK1 cells. J Pharmacol Exp Ther 286:
676 – 680, 1998.
429. Takano M, Inui KI, Okano T, Saito H, and Hori R. Carriermediated transport systems of tetraethylammonium in rat renal
brush-border and basolateral membrane vesicles. Biochim Biophys Acta 773: 113–124, 1984.
430. Takano M, Kato M, Takayama A, Yasuhara M, Inui KI, and
Hori R. Transport of procainamide in a kidney epithelial cell line
LLC-PK1. Biochim Biophys Acta 1108: 133–139, 1992.
431. Takano M, Nagai J, Yasuhara M, and Inui KI. Regulation of
p-aminohippurate transport by protein kinase C in OK kidney epithelial cells. Am J Physiol Renal Fluid Electrolyte Physiol 271:
F469 –F475, 1996.
432. Takeda M, Babu E, Narikawa S, and Endou H. Interaction of
human organic anion transporters with various cephalosporin antibiotics. Eur J Pharmacol 438: 137–142, 2002.
433. Takeda M, Khamdang S, Narikawa S, Kimura H, Hosoyamada
M, Cha SH, Sekine T, and Endou H. Characterization of methotrexate transport and its drug interactions with human organic
anion transporters. J Pharmacol Exp Ther 302: 666 – 671, 2002.
434. Takeda M, Khamdang S, Narikawa S, Kimura H, Kobayashi Y,
Yamamoto T, Cha SH, Sekine T, and Endou H. Human organic
anion transporters and human organic cation transporters mediate
renal antiviral transport. J Pharmacol Exp Ther 300: 918 –924, 2002.
435. Takeda M, Narikawa S, Hosoyamada M, Cha SH, Sekine T,
and Endou H. Characterization of organic anion transport inhibitors using cells stably expressing human organic anion transporters. Eur J Pharmacol 419: 113–120, 2001.
436. Takeda M, Sekine T, and Endou H. Regulation by protein kinase
C of organic anion transport driven by rat organic anion transporter 3 (rOAT3). Life Sci 67: 1087–1093, 2000.
437. Takeda M, Tojo A, Sekine T, Hosoyamada M, Kanai Y, and
Endou H. Role of organic anion transporter 1 (OAT1) in cephaloridine (CER)-induced nephrotoxicity. Kidney Int 56: 2128 –2136,
1999.
438. Takeuchi A, Masuda S, Saito H, Abe T, and Inui K. Multispecific substrate recognition of kidney-specific organic anion transporters OAT-K1 and OAT-K2. J Pharmacol Exp Ther 299: 261–267,
2001.
439. Takeuchi A, Masuda S, Saito H, Doi T, and Inui K. Role of
kidney-specific organic anion transporters in the urinary excretion
of methotrexate. Kidney Int 60: 1058 –1068, 2001.
440. Takeuchi A, Masuda S, Saito H, Hashimoto Y, and Inui K.
Trans-stimulation effects of folic acid derivatives on methotrexate
transport by rat renal organic anion transporter, OAT-K1. J Pharmacol Exp Ther 293: 1034 –1039, 2000.
441. Tamai I, China K, Sai Y, Kobayashi D, Nezu J, Kawahara E,
and Tsuji A. Na⫹-coupled transport of L-carnitine via high-affinity
carnitine transporter OCTN2 and its subcellular localization in
kidney. Biochim Biophys Acta 1512: 273–284, 2001.
442. Tamai I, Nezu J, Uchino H, Sai Y, Oku A, Shimane M, and
Tsuji A. Molecular identification and characterization of novel
members of the human organic anion transporter (OATP) family.
Biochem Biophys Res Commun 273: 251–260, 2000.
443. Tamai I, Ohashi R, Nezu J, Sai Y, Kobayashi D, Oku A, Shimane M, and Tsuji A. Molecular and functional characterization
of organic cation/carnitine transporter family in mice. J Biol Chem
275: 40064 – 40072, 2000.
444. Tamai I, Ohashi R, Nezu J, Yabuuchi H, Oku A, Shimane M,
Sai Y, and Tsuji A. Molecular and functional identification of
sodium ion-dependent, high affinity human carnitine transporter
OCTN2. J Biol Chem 273: 20378 –20382, 1998.
445. Tamai I, Yabuuchi H, Nezu JI, Sai Y, Oku A, Shimane M, and
Tsuji A. Cloning and characterization of a novel human pH-dependent organic cation transporter, OCTN1. FEBS Lett 419: 107–111,
1997.
446. Tanaka K, Hirai M, Tanigawara Y, Ueda K, Takano M, Hori R,
and Inui K. Relationship between expression level of P-glycoprotein and daunorubicin transport in LLC-PK1 cells transfected with
human MDR1 gene. Biochem Pharmacol 53: 741–746, 1997.
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
Physiol Rev • VOL
481. Ullrich KJ, Rumrich G, and Klöss S. Contraluminal para-aminohippurate (PAH) transport in the proximal tubule of the rat kidney.
Pflügers Arch 413: 134 –146, 1988.
482. Ullrich KJ, Rumrich G, and Klöss S. Contraluminal organic
anion and cation transport in the proximal renal tubule. V. Interaction with sulfamoyl- and phenoxy diuretics, and with ␤-lactam
antibiotics. Kidney Int 36: 78 – 88, 1989.
483. Ullrich KJ, Rumrich G, Papavassiliou F, and Hierholzer K.
Contraluminal p-aminohippurate transport in the proximal tubule
of the rat kidney. VIII. Transport of corticosteroids. Pflügers Arch
418: 371–382, 1991.
484. Ullrich KJ, Rumrich G, Papavassiliou F, Klöss S, and Fritzsch
G. Contraluminal p-aminohippurate transport in the proximal tubule of the rat kidney. VII. Specificity: cyclic nucelotides, eicosanoids. Pflügers Arch 418: 360 –370, 1991.
485. Ullrich KJ, Rumrich G, Wieland T, and Dekant W. Contraluminal para-aminohippurate (PAH) transport in the proximal tubule of
the rat kidney. VI. Specificity: amino acids, their N-methyl, Nacetyl-, and N-benzoyl-derivatives; glutathione- and cysteine conjugates, di- and oligopeptides. Pflügers Arch 415: 342–350, 1989.
486. Urakami Y, Akazawa M, Saito H, Okuda M, and Inui K. cDNA
cloning, functional characterization, and tissue distribution of an
alternatively spliced variant of organic cation transporter hOCT2
predominantly expressed in the human kidney. J Am Soc Nephrol
13: 1703–1710, 2002.
487. Urakami Y, Nakamura N, Takahashi K, Okuda M, Saito H,
Hashimoto Y, and Inui K. Gender differences in expression of
organic cation transporter OCT2 in rat kidney. FEBS Lett 461:
339 –342, 1999.
488. Urakami Y, Okuda M, Masuda S, Akazawa M, Saito H, and Inui
K. Distinct characteristics of organic cation transporters, OCT1
and OCT2, in the basolateral membrane of renal tubules. Pharm
Res 18: 1528 –1534, 2001.
489. Urakami Y, Okuda M, Masuda S, Saito H, and Inui KI. Functional characteristics and membrane localization of rat multispecific organic cation transporters, OCT1 and OCT2, mediating tubular secretion of cationic drugs. J Pharmacol Exp Ther 287: 800 –
805, 1998.
490. Urakami Y, Okuda M, Saito H, and Inui K. Hormonal regulation
of organic cation transporter OCT2 expression in rat kidney. FEBS
Lett 473: 173–176, 2000.
491. Uwai Y, Okuda M, Takami K, Hashimoto Y, and Inui K. Functional characterization of the rat multispecific organic anion transporter OAT1 mediating basolateral uptake of anionic drugs in the
kidney. FEBS Lett 438: 321–324, 1998.
492. Uwai Y, Saito H, Hashimoto Y, and Inui K. Inhibitory effect of
anti-diabetic agents on rat organic anion transporter rOAT1. Eur
J Pharmacol 398: 193–197, 2000.
493. Uwai Y, Saito H, Hashimoto Y, and Inui KI. Interaction and
transport of thiazide diuretics, loop diuretics, and acetazolamide
via rat renal organic anion transporter rOAT1. J Pharmacol Exp
Ther 295: 261–265, 2000.
494. Van Aubel RA, Hartog A, Bindels RJ, van Os CH, and Russel
FG. Expression and immunolocalization of multidrug resistance
protein 2 in rabbit small intestine. Eur J Pharmacol 400: 195–198,
2000.
495. Van Aubel RA, Koenderink JB, Peters JG, van Os CH, and
Russel FG. Mechanisms and interaction of vinblastine and reduced glutathione transport in membrane vesicles by the rabbit
multidrug resistance protein Mrp2 expressed in insect cells. Mol
Pharmacol 56: 714 –719, 1999.
496. Van Aubel RA, Masereeuw R, and Russel FG. Molecular pharmacology of renal organic anion transporters. Am J Physiol Renal
Physiol 279: F216 –F232, 2000.
497. Van Aubel RA, Peters JG, Masereeuw R, van Os CH, and
Russel FG. Multidrug resistance protein Mrp2 mediates ATP-dependent transport of classic renal organic anion p-aminohippurate.
Am J Physiol Renal Physiol 279: F713–F717, 2000.
498. Van Aubel RA, Smeets PH, Peters JG, Bindels RJ, and Russel
FG. The MRP4/ABCC4 gene encodes a novel apical organic anion
transporter in human kidney proximal tubules: putative efflux
pump for urinary cAMP and cGMP. J Am Soc Nephrol 13: 595– 603,
2002.
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
466. Ullrich KJ, Fasold H, Rumrich G, and Klöss S. Secretion and
cotraluminal uptake of dicarboxylic acids in the proximal convolution of rat kidney. Pflügers Arch 400: 241–249, 1984.
467. Ullrich KJ, Fritzsch G, Rumrich G, and David C. Polysubstrates: substances that interact with renal contraluminal PAH,
sulfate, and NMeN transport: sulfamoyl-, sulfonylurea-, thiazideand benzeneamino-carboxylate (nicotinate) compounds. J Pharmacol Exp Ther 269: 684 – 692, 1994.
468. Ullrich KJ, Papavassiliou F, David C, Rumrich G, and Fritzsch
G. Contraluminal transport of organic cations in the proximal
tubule of the rat kidney. I. Kinetics of N1-methylnicotinamide and
tetraethylammonium, influence of K⫹, HCO3, pH: inhibition by
aliphatic primary, secondary and tertiary amines, and mono- and
bisquaternary compounds. Pflügers Arch 419: 84 –92, 1991.
469. Ullrich KJ and Rumrich G. Renal contraluminal transport systems for organic anions (paraaminohippurate, PAH) and organic
cations (N1-methyl-nicotinanide, NMeN) do not see the degree of
substrate ionization. Pflügers Arch 421: 286 –288, 1992.
470. Ullrich KJ and Rumrich G. Morphine analogues: relationship
between chemical structure and interaction with proximal tubular
transporters-contraluminal organic cation and anion transporter,
luminal H⫹/organic cation exchanger, and luminal choline transporter. Cell Physiol Biochem 5: 290 –298, 1995.
471. Ullrich KJ and Rumrich G. Luminal transport system for choline⫹ in relation to the other organic cation transport systems in
the rat proximal tubule. Kinetics, specificity: alkyl/arylamines, alkylamines with OH, O, SH, NH2, ROCO, RSCO and H2PO4-groups,
methylaminostyryl, rhodamine, acridine, phenanthrene and cyanine compounds. Pflügers Arch 432: 471– 485, 1996.
472. Ullrich KJ and Rumrich G. Luminal transport step of paraaminohippurate (PAH): transport from PAH-loaded proximal tubular cells into the tubular lumen of the rat kidney in vivo. Pflügers
Arch 433: 735–743, 1997.
473. Ullrich KJ, Rumrich G, Burke TR, Shirazi-Beechey SP, and
Lang H. Interaction of Alkyl/Arylphosphonates, phosphonocarboxylates and diphosphonates with different anion transport systems in the proximal renal tubule. J Pharmacol Exp Ther 283:
1223–1229, 1997.
474. Ullrich KJ, Rumrich G, David C, and Fritzsch G. Bisubstrates:
substances that interact with both, renal contraluminal organic
anion and organic cation transport systems. II. Zwitterionic substrates: dipeptides, cephalosporins, quinolone-carboxylate gyrase
inhibitors and phosphoamide thiazine carboxylates; nonionizable
substrates: steroid hormones and cyclophosphamides. Pflügers
Arch 425: 300 –312, 1993.
475. Ullrich KJ, Rumrich G, David C, and Fritzsch G. Bisubstrates:
substances that interact with renal organic anion and organic cation transport systems. I. Amines, piperidines, piperazines, azepines, pyridines, quinolines, imadazoles, thiazoles, guanidines,
and hydrazines. Pflügers Arch 425: 280 –299, 1993.
476. Ullrich KJ, Rumrich G, and Fritzsch G. Contraluminal transport
of organic cations in the proximal tubule of the rat kidney. II.
Specificity: anilines, phenylalkylamines (catecholamines), heterocyclic compounds (pyridines, quinolines, acridines). Pflügers Arch
420: 29 –38, 1992.
477. Ullrich KJ, Rumrich G, and Fritzsch G. Substrate specificity of
the organic anion and organic cation transport systems in the
proximal renal tubule. In: Progress in Cell Research, edited by E.
Bamberg and H. Passow. Amsterdam: Elsevier, 1992, p. 315–321.
478. Ullrich KJ, Rumrich G, Fritzsch G, and Klöss S. Contraluminal
para-aminohippurate (PAH) transport in the proximal tubule of the
rat. I. Kinetics, influence of cations, anions, and capillary preperfusion. Pflügers Arch 409: 229 –235, 1987.
479. Ullrich KJ, Rumrich G, Fritzsch G, and Klöss S. Contraluminal
para-aminohippurate transport in the proximal tubule of rat kidney. II. Specificity: aliphatic dicarboxylic acids. Pflügers Arch 408:
38 – 45, 1987.
480. Ullrich KJ, Rumrich G, and Klöss S. Contraluminal para-aminohippurate transport in the proximal tubule of the rat kidney. III.
Specificity: monocarboxylic acids. Pflügers Arch 409: 547–554,
1987.
1047
1048
STEPHEN H. WRIGHT AND WILLIAM H. DANTZLER
Physiol Rev • VOL
519. Werner A, Moore ML, Mantei N, Biber J, Semenza G, and
Murer H. Cloning and expression of cDNA for a Na/Pi cotransport
system of kidney cortex. Proc Natl Acad Sci USA 88: 9608 –9612,
1991.
520. Werner D, Martinez F, and Roch-Ramel F. Urate and p-aminohippurate transport in the brush border membrane of the pig
kidney. J Pharmacol Exp Ther 252: 792–799, 1990.
521. Wieland A, Hayer-Zillgen M, Bonisch H, and Bruss M. Analysis
of the gene structure of the human (SLC22A3) and murine
(Slc22a3) extraneuronal monoamine transporter. J Neural Transm
107: 1149 –1157, 2000.
522. Wijnholds J, Scheffer GL, d. van V, Beijnen JH, Scheper RJ,
and Borst P. Multidrug resistance protein 1 protects the oropharyngeal mucosal layer and the testicular tubules against druginduced damage. J Exp Med 188: 797– 808, 1998.
523. Wolff NA, Grunwald B, Friedrich B, Lang F, Godehardt S, and
Burckhardt G. Cationic amino acids involved in dicarboxylate
binding of the flounder renal organic anion transporter. J Am Soc
Nephrol 12: 2012–2018, 2001.
524. Wolff NA, Thies K, Kuhnke N, Reid G, Friedrich B, Lang F,
and Burckhardt G. Protein kinase C activation downregulates
human organic anion transporter 1-mediated transport through
carrier internalization. J Am Soc Nephrol 14: 1959 –1968, 2003.
525. Wolff NA, Werner A, Burkhardt S, and Burckhardt G. Expression cloning and characterization of a renal organic anion transporter from winter flounder. FEBS Lett 417: 287–291, 1997.
526. Wong LT, Escobar MR, Smyth DD, and Sitar DS. Genderassociated differences in rat renal tubular amantadine transport
and absence of stereoselective transport inhibition by quinine and
quinidine in distal tubules. J Pharmacol Exp Ther 267: 1440 –1444,
1993.
527. Wong LTY, Smyth DD, and Sitar DS. Stereoselective inhibition
of renal organic cation transport in human kidney. Br J Clin
Pharmacol 34: 438 – 440, 1992.
528. Woodhall PB, Tisher CC, Simonton CA, and Robinson RR.
Relationship between para-aminohippurate secretion and cellular
morphology in rabbit proximal tubules. J Clin Invest 61: 1320 –
1329, 1978.
529. Wright EM. Renal Na⫹-glucose cotransporters. Am J Physiol Renal Physiol 280: F10 –F18, 2001.
530. Wright EM, Mircheff AK, Hanna SD, Harms V, van Os CH,
Walling MW, and Sachs G. The dark side of the intestinal epithelium: the isolation and charcterization of basolateral membranes.
In: Mechanisms of Intestinal Secretion. New York: Liss, 1979, p.
117–130.
531. Wright SH, Kippen I, Klinenberg JR, and Wright EM. Specificity of the transport system for tricarboxylic acid cycle intermediates in renal brush borders. J Membr Biol 57: 73– 82, 1980.
532. Wright SH and Wunz TM. Succinate and citrate transport in renal
basolateral and brush-border membranes. Am J Physiol Renal
Fluid Electrolyte Physiol 253: F432–F439, 1987.
533. Wright SH and Wunz TM. Transport of tetraethylammonium by
rabbit renal brush-border and basolateral membrane vesicles. Am J
Physiol Renal Fluid Electrolyte Physiol 253: F1040 –F1050, 1987.
534. Wright SH and Wunz TM. Influence of substrate structure on
substrate binding to the renal organic cation/H⫹ exchanger.
Pflügers Arch 437: 603– 610, 1999.
535. Wright SH, Wunz TM, and Wunz TP. A choline transporter in
renal brush-border membrane vesicles: energetics and structural
specificity. J Membr Biol 126: 51– 65, 1992.
536. Wright SH, Wunz TM, and Wunz TP. Structure and interaction of
inhibitors with the TEA/H⫹ exchanger of rabbit renal brush border
membranes. Pflügers Arch 429: 313–324, 1995.
537. Wu X, Fei YJ, Huang W, Chancy C, Leibach FH, and Ganapathy V. Identity of the F52F12.1 gene product in Caenorhabditis
elegans as an organic cation transporter. Biochim Biophys Acta
1418: 239 –244, 1999.
538. Wu X, George RL, Huang W, Wang H, Conway SJ, Leibach FH,
and Ganapathy V. Structural and functional characteristics and
tissue distribution pattern of rat OCTN1, an organic cation transporter, cloned from placenta. Biochim Biophys Acta 1466: 315–
327, 2000.
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
498a.Van Der Sandt I, Blom-Roosemalen MC, de Boer AG, and
Breimer DD. Specificity of doxorubicin versus rhodamine-123 in
assessing P-glycoprotein functionality in the LLC-PK1, LLC-PK1:
MDR1 and caco-2 cell lines. Eur J Pharm Sci 11: 207–214, 2000.
499. Van Kuijck MA, van Aubel RA, Busch AE, Lang F, Russel FG,
Bindels RJ, van Os CH, and Deen PM. Molecular cloning and
expression of a cyclic AMP-activated chloride conductance regulator: a novel ATP-binding cassette transporter. Proc Natl Acad Sci
USA 93: 5401–5406, 1996.
500. Van Montfoort JE, Hagenbuch B, Fattinger KE, Muller M,
Groothuis GM, Meijer DK, and Meier PJ. Polyspecific organic
anion transporting polypeptides mediate hepatic uptake of amphipathic type II organic cations. J Pharmacol Exp Ther 291: 147–152,
1999.
501. Van Montfoort JE, Hagenbuch B, Groothuis GM, Koepsell H,
Meier PJ, and Meijer DK. Drug uptake systems in liver and
kidney. Curr Drug Metab 4: 185–211, 2003.
502. Van Montfoort JE, Muller M, Groothuis GM, Meijer DK, Koepsell H, and Meier PJ. Comparison of “type I” and “type II”
organic cation transport by organic cation transporters and organic
anion-transporting polypeptides. J Pharmacol Exp Ther 298: 110 –
115, 2001.
504. Villalobos AR and Braun EJ. Characterization of organic cation
transport by avian renal brush-border membrane vesicles. Am J
Physiol Regul Integr Comp Physiol 269: R1050 –R1059, 1995.
505. Villalobos AR and Braun EJ. Substrate specificity of organic
cation/H⫹ exchange in avian renal brush-border membranes.
J Pharmacol Exp Ther 287: 944 –951, 1998.
506. Villalobos AR, Dunnick CA, and Pritchard JB. Mechanisms
mediating basolateral transport of 2,4-dichlorophenoxyacetic acid
in rat kidney. J Pharmacol Exp Ther 278: 582–589, 1996.
507. Volk C, Gorboulev V, Budiman T, Nagel G, and Koepsell H.
Different affinities of inhibitors to the outwardly and inwardly
directed substrate binding site of organic cation transporter 2. Mol
Pharmacol 64: 1037–1047, 2003.
508. Wada S, Tsuda M, Sekine T, Cha SH, Kimura M, Kanai Y, and
Endou H. Rat multispecific organic anion transporter 1 (rOAT1)
transports zidovudine, acyclovir, and other antiviral nucleoside
analogs. J Pharmacol Exp Ther 294: 844 – 849, 2000.
509. Wadiche JI, Amara SG, and Kavanaugh MP. Ion fluxes associated with excitatory amino acid transport. Neuron 15: 721–728,
1995.
510. Wagner CA, Lukewille U, Kaltenbach S, Moschen I, Broer A,
Risler T, Broer S, and Lang F. Functional and pharmacological
characterization of human Na⫹-carnitine cotransporter hOCTN2.
Am J Physiol Renal Physiol 279: F584 –F591, 2000.
511. Wang H, Fei YJ, Kekuda R, Yang-Feng TL, Devoe LD, Leibach
FH, Prasad PD, and Ganapathy V. Structure, function, and
genomic organization of human Na⫹-dependent high-affinity dicarboxylate transporter. Am J Physiol Cell Physiol 278: C1019 –C1030,
2000.
512. Wang J and Giacomini KM. Serine 318 is essential for the pyrimidine selectivity of the N2 Na⫹-nucleoside transporter. J Biol Chem
274: 2298 –2302, 1999.
513. Wang Y, Meadows TA, and Longo N. Abnormal sodium stimulation of carnitine transport in primary carnitine deficiency. J Biol
Chem 275: 20782–20786, 2000.
514. Weiner IM. Transport of weak acids and bases. In: Handbook of
Physiology. Renal Physiology. Washington, DC: Am. Physiol. Soc.,
1973, sect. 8, p. 521–554.
515. Weiner IM. Organic acids and bases and uric acid. In: The Kidney:
Physiology and Pathophysiology, edited by D. W. Seldin and G.
Giebisch. New York: Raven, 1985, p. 1703–1724.
516. Welborn JR, Groves CE, and Wright SH. Peritubular transport
of ochratoxin A by single rabbit renal proximal tubules. J Am Soc
Nephrol 9: 1973–1982, 1998.
517. Welborn JR, Shpun S, Dantzler WH, and Wright SH. Effect of
␣-ketoglutarate on organic anion transport in single rabbit renal
proximal tubules. Am J Physiol Renal Physiol 274: F165–F174,
1998.
518. Werner A, Dehmelt L, and Nalbant P. Na⫹-dependent phosphate
cotransporters: the NaPi protein families. J Exp Biol 201: 3135–
3142, 1998.
RENAL ORGANIC CATION AND ORGANIC ANION TRANSPORT
Physiol Rev • VOL
547. Zhang L, Brett CM, and Giacomini KM. Role of organic cation
transporters in drug absorption and elimination. Annu Rev Pharm
Toxicol 38: 431– 460, 1998.
548. Zhang L, Dresser MJ, Chun JK, Babbitt PC, and Giacomini
KM. Cloning and functional characterization of a rat renal organic
cation transporter isoform (rOCT1A). J Biol Chem 272: 16548 –
16554, 1997.
549. Zhang L, Dresser MJ, Gray AT, Yost SC, Terashita S, and
Giacomini KM. Cloning and functional expression of a human
liver organic cation transporter. Mol Pharmacol 51: 913–921,
1997.
550. Zhang L, Gorset W, Dresser MJ, and Giacomini KM. The interaction of n-tetraalkylammonium compounds with a human organic
cation transporter, hOCT1. J Pharmacol Exp Ther 288: 1192–1198,
1999.
551. Zhang L, Gorset W, Washington CB, Blaschke TF, Kroetz DL,
and Giacomini KM. Interactions of HIV protease inhibitors with a
human organic cation transporter in a mammalian expression system. Drug Metab Dispos 28: 329 –334, 2000.
552. Zhang L, Schaner ME, and Giacomini KM. Functional characterization of an organic cation transporter (hOCT1) in a transiently
transfected human cell line (HeLa). J Pharmacol Exp Ther 286:
354 –361, 1998.
553. Zhang X, Evans KK, and Wright SH. Molecular cloning of
rabbit organic cation transporter rbOCT2 and functional comparisons with rbOCT1. Am J Physiol Renal Physiol 283: F124 –
F133, 2003.
554. Zwart R, Verhaagh S, Buitelaar M, Popp-Snijders C, and Barlow DP. Impaired activity of the extraneuronal monoamine transporter system known as uptake-2 in Orct3/Slc22a3-deficient mice.
Mol Cell Biol 21: 4188 – 4196, 2001.
84 • JULY 2004 •
www.prv.org
Downloaded from http://physrev.physiology.org/ by 10.220.33.4 on July 4, 2017
539. Wu X, Huang W, Ganapathy ME, Wang H, Kekuda R, Conway
SJ, Leibach FH, and Ganapathy V. Structure, function, and
regional distribution of the organic cation transporter OCT3 in the
kidney. Am J Physiol Renal Physiol 279: F449 –F458, 2000.
540. Wu X, Huang W, Prasad PD, Seth P, Rajan DP, Leibach FH,
Chen J, Conway SJ, and Ganapathy V. Functional characteristics and tissue distribution pattern of organic cation transporter 2
(OCTN2), an organic cation/carnitine transporter. J Pharmacol
Exp Ther 290: 1482–1492, 1999.
541. Wu X, Kekuda R, Huang W, Fei YJ, Leibach FH, Chen J,
Conway SJ, and Ganapathy V. Identity of the organic cation
transporter OCT3 as the extraneuronal monoamine transporter
(uptake2) and evidence for the expression of the transporter in the
brain. J Biol Chem 273: 32776 –32786, 1998.
542. Wu X, Prasad PD, Leibach FH, and Ganapathy V. cDNA sequence, transport function, and genomic organization of human
OCTN2, a new member of the organic cation transporter family.
Biochem Biophys Res Commun 246: 589 –595, 1998.
543. Yabuuchi H, Tamai I, Nezu JI, Sakamoto K, Oku A, Shimane
M, Sai Y, and Tsuji A. Novel membrane transporter OCTN1
mediates multispecific, bidirectional, and pH-dependent transport
of organic cations. J Pharmacol Exp Ther 289: 768 –773, 1999.
544. Yanagawa N, Shih RN, Jo OD, and Said HM. Riboflavin transport
by isolated perfused rabbit renal proximal tubules. Am J Physiol
Cell Physiol 279: C1782–C1786, 2000.
545. You G, Kuze K, Kohanski RA, Amsler K, and Henderson S.
Regulation of mOAT-mediated organic anion transport by okadaic
acid and protein kinase C in LLC-PK(1) cells. J Biol Chem 275:
10278 –10284, 2000.
546. Zagryadskaya EI, Ostretsova IB, and Nikiforov AA. Inhibitory
effect of valproate on weak organic acid uptake in rat renal proximal tubules. Biochem Pharmacol 58: 1371–1377, 1999.
1049