Am J Physiol Renal Physiol
282: F1–F9, 2002.
invited review
Return of the secretory kidney
JARED J. GRANTHAM AND DARREN P. WALLACE
Kidney Institute, Departments of Internal Medicine, Biochemistry, and Molecular Biology,
University of Kansas Medical Center, Kansas City, Kansas 66160
tubule; glomerulus; fluid secretion; salt secretion; renal evolution; fluid
balance
THE LIVER, PANCREAS, SALIVARY and lacrimal glands secrete
relatively large amounts of Na⫹, Cl⫺, K⫹, and HCO3⫺
coupled with the isosmotic movement of water (51). Commonly, this secretate is generated in the proximal portions or acini of a branching tubular arcade, and the
chemical composition is modified by converging downstream segments before it is finally discharged from the
gland. Sweat glands exhibit similar properties and are
counted among the organs of secretion. With the exception of Heidenhain (32), Marshall (44), Fine (21), and a
few other heretics who have observed net fluid secretion
in isolated tubule segments in vitro (7, 24, 64), the mammalian kidney has not been counted among the organs
that produce a watery secretion.
The urine that emerges from mammalian kidneys is
widely believed to represent fluid that is left over from
incomplete tubular reabsorption of glomerular filtrate
(2, 17). In the renal field, “secretion” is a term that is
usually reserved to describe the deposition into urine of
organic anions and cations, largely by proximal tubules, and of potassium, ammonium, and protons by
Address for reprint requests and other correspondence: J. J.
Grantham, Kidney Institute, Univ. of Kansas Medical Ctr., 3901
Rainbow Blvd., Kansas City, KS 66160 (E-mail: [email protected]).
http://www.ajprenal.org
distal tubules and collecting ducts (45, 49, 57). However, recent evidence demonstrating that isolated nonperfused outer and inner medullary collecting ducts
(OMCD and IMCD, respectively) dissected from rat
kidneys secrete fluid has reopened the possibility that
segmental salt and fluid secretion by mammalian renal
tubules may contribute to the formation of the final
urine (73, 75). In light of this new information, it is not
unreasonable to suppose that the secretory transport of
organic and inorganic solutes into the proximal tubules
(22), coupled with the downstream secretion of salt and
fluid into the collecting ducts, contributes to the formation and the modification of urine by mammalian kidneys. In this brief review, we reexamine the evolutionary basis of renal tubule secretory mechanisms that
couple solute and fluid transport into the urine formed
by mammalian kidneys and how this process may
participate in normal and pathological states.
EVIDENCE OF RENAL TUBULE SOLUTE AND
FLUID SECRETION
Protovertebrates
In his brilliant treatise, From Fish to Philosopher,
Homer Smith (60) considered how the kidney may have
evolved in the sea from early excretory structures. The
0363-6127/02 $5.00 Copyright © 2002 the American Physiological Society
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Grantham, Jared J., and Darren P. Wallace. Return of the secretory kidney. Am J Physiol Renal Physiol 282: F1–F9, 2002.—The evolution of the kidney has had a major role in the emigration of vertebrates from
the sea onto dry land. The mammalian kidney has conserved to a remarkable extent many of the molecular and functional elements of primordial
apocrine kidneys that regulate fluid balance and eliminate potentially toxic
endogenous and xenobiotic molecules in the urine entirely by transepithelial secretion. However, these occult secretory processes in the proximal
tubules and collecting ducts of mammalian kidneys have remained underappreciated in the last half of the twentieth century as investigators focused, to a large extent, on the mechanisms of glomerular filtration and
tubule sodium chloride and fluid reabsorption. On the basis of evidence
reviewed in this paper, we propose that transepithelial salt and fluid
secretion mechanisms enable mammalian renal tubules to finely regulate
extracellular fluid volume and composition day to day and maintain urine
formation during the cessation of glomerular filtration.
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INVITED REVIEW
Fig. 1. Schematic view of tubule NaCl and fluid secretion over the
course of evolution. The protovertebrate tubule provided for the
elimination of organic “waste” or xenobiotics through a simple canal
mechanism. Urine flow was generated by the influx of liquid along
the tubule due to the secretion of organic molecules and salt and,
possibly, by the sweeping action of cilia. The glomerular apparatus
was added to the mesonephric tubule to facilitate the elimination of
relatively large amounts of salt-free water that diffused through the
skin or was absorbed from the gut. These tubules greatly increased
the reabsorptive mechanisms for conserving NaCl, thereby obfuscating any roles the less powerful secretory processes for organic anion/
cation and salt secretion might have had in the generation and
modification of urine. The return of some animals to the sea diminished the importance of glomerular filtration for the elimination of
water and the conservation of salt, leading to atrophy or complete
loss of glomeruli. The metanephric tubule retained the proximal and
collecting duct capacities for solute and fluid secretion; however, the
relative dominance of glomerular filtration appears to vary depending on whether the animals live in a freshwater aquatic or desert
environment. There are no known examples of aglomerular mammals living on land or in the sea. It is important to note that,
although the metanephric collecting ducts arise from a different
analage (ureteric bud) than either the pronephric or mesonephric
tubules, they harbor molecular machinery for the secretion and
reabsorption of salt and water.
AJP-Renal Physiol • VOL
gian tubules, which have no glomeruli attached to
them, organic anions and cations are secreted into the
lumens of blind-end tubules against steep concentration gradients. The fluid elaborated by these tubules
appears to depend on the contemporaneous secretion of
electrolyte and nonelectrolyte solutes. In recent work,
it has been observed in Malphigian tubules of Aedes
aegypti that hormonally regulated chloride secretion
through selective anion channels has a significant role
in the elimination of massive amounts of solutes and
fluid (6, 47).
Elements of these relatively simple protovertebrate
kidneys can still be found in the hagfish and lamprey
(60) and are transiently represented in the pronephros
of early-stage embryonic renal development in reptiles,
birds, and mammals (72).
The Vertebrate Kidney
The upheaval of continents from the sea and the
consequent exposure to fresh water provided “the theater of evolution of the early vertebrates” kidney (60).
Herein, the simple tubular “kidney” assumed additional roles to preserve the composition of the “internal
environment” in this radically different medium
through the elimination of excess water and the conservation of salt. To this end, a rudimentary filtering
device, the glomerulus, a latecomer in the quiet ritual
of urine formation and excretion, was introduced at the
blind end of the tubule (Fig. 1). The exuberant filtration of extracellular fluid (ECF) into the tubules by the
newly acquired glomeruli solved the problem of getting
rid of excess free water, but it created another. Mechanisms for the recapture from tubule fluid of precious
filtered solutes such as NaCl were required to preserve
homeostasis. Mechanisms for the avid reabsorption of
Na⫹ and Cl⫺ had to be upgraded along the tubule
segments and added to the tubular secretory mechanisms for the elimination of potentially toxic organic
anions and cations. Any contribution of tubular NaCl
secretion to urine formation carried from the protovertebrate era was buried in the glutinous reabsorption of
filtered solutes imposed by the acquisition of glomeruli.
Smith opined (60)
What engineer, wishing to regulate the composition
of the internal environment of the body on which the
function of every bone, gland, muscle, and nerve depends, would devise a scheme that operated by throwing the whole thing out sixteen times a day and rely on
grabbing from it, as it fell to earth, only those precious
elements which he wanted to keep?
The bony fishes fully exhibit the mesonephric agenda,
where precious solutes in the glomerular filtrate are
reclaimed for the ECF and the extraneous water is
released to the sea. However, on close inspection, the
proximal tubule segments of marine and freshwater
fishes have also retained mechanisms for transepithelial net solute secretion that can be coupled to fluid
secretion. Beyenbach and colleagues (5, 7, 8, 16) dissected proximal tubules from marine- and freshwateradapted fish and observed that in some, but not all,
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leading electrolyte constituents in seawater are (in
meq/l) chloride (537), sodium (461), magnesium (53),
and calcium (10) (35). Smith reasoned that, with free
access to both water and salt, primordial kidneys
evolved primarily to discharge potentially harmful
chemicals that could not be eliminated by simple diffusion through the integument of increasingly complex
multicellular animals. These waste products included
polar organic compounds that, in early protovertebrates, were secreted into the lumens of simple tubular
structures open to the coelom at one end and to the sea
at the other. In this context, the secretory transport of
solutes, including NaCl and organic molecules, coupled
with the osmotic flow of water, also served to propel
liquid along the tubule for eventual discharge into the
sea. This primitive apocrine kidney appears to represent the framework on which additional tubular excretory processes were added in the course of evolution
(Fig. 1).
In representatives of the early life forms available
for modern study, orthologs of the brush-border-laden
proximal tubules exhibit mechanisms for the transepithelial secretion of organic solutes, including polar
anions and cations (43). For example, in insect Malphi-
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The Metanephric Kidney
As emphasized by Smith (60), the kidney did not
finally assume full responsibility for regulating body
fluid and salt balance until animals emerged permanently onto dry land (Fig. 1). Amphibians, followed by
reptiles, acquired mechanisms for excreting dilute
fluid created in a unique segment just beyond the
proximal tubule that absorbed salt without water,
which eventually became the ascending limb of Henle.
In this way, the volume of dilute urine could be hormonally regulated in relation to the availability of
water. Reptiles, some living away from a source of
water for long periods, reduced the importance of glomerular filtration, depending once again on tubular
mechanisms to regulate solute and fluid balance to a
significant extent (18). But because the osmolality of
urine could not be concentrated above that of the body
fluids, severely limiting habitats for survival, additional mechanisms were required to conserve water
and to regulate the excretion of NaCl. To this end, the
collecting duct system arose from the metanephric
blastema, and the loop of Henle arrangement of tubule
segments and blood vessels in the medulla were interposed between the proximal and distal tubules. With
the creation of the countercurrent mechanism for concentrating solutes in the renal medulla, the mammalian kidney expanded the capability of animals to live
away from pools of water. For example, the desert rat
concentrates urine to more than 4,000 mosmol/kgH2O,
in the face of relatively high rates of glomerular filtration, subsisting only on the water derived from the
metabolism of relatively dry food.
An easier solution to mammalian survival under
extremely arid conditions might have been to reduce
glomerular filtration to rates as some desert reptiles
and birds have done (10–12). However, as Smith (60)
surmised, “the filtration-reabsorption system (of mammals) is now so firmly established that there is no easy
way to overhaul it and to convert it to a purely tubular
kidney, as the marine fishes have done.” He became
resigned to the filtration-reabsorption view of urine
AJP-Renal Physiol • VOL
formation in mammals but overlooked the collecting
duct system in the regulation of NaCl balance, relegating this segment to a relatively passive role in the
reabsorption of water under the control of the antidiuretic hormone (60, 61). However, these terminal segments do much more than reabsorb water. In the last
two decades, direct studies have established that the
cortical and medullary collecting ducts participate in
the regulation of Na⫹, Cl⫺, K⫹, NH4⫹, and H⫹ excretion. Despite this, the possibility that the proximal
tubule and collecting duct systems may contribute to
the regulation of solute and fluid balance in normal
and pathological states by secreting electrolytes and
fluid is not included in modern textbooks of renal
physiology and nephrology.
Solute and Fluid Secretion in Mammalian
Proximal Tubules
The capacity of mammalian proximal tubules to secrete as well as to reabsorb solutes and fluid was
discovered inadvertently. My colleagues and I were
surprised to find that the inclusion of p-aminohippurate (PAH) or human uremic serum containing increased levels of hippurate in the external medium
bathing an isolated perfused S2 proximal tubule segment dissected from the rabbit kidney caused a sustained
reversal in the net transport of fluid from absorption to
secretion (28) (Fig. 2). Hippurate, a normal aryl anion
metabolite, was as effective in causing fluid secretion
as PAH, and probenecid, which blocked peritubular
hippurate transport into the cells, inhibited fluid secretion. The net flux of hippurate was relatively low compared with net sodium transport in this segment; thus
the effect of the organic anion on net fluid transport
could only be seen at very low rates of tubule perfusion.
Fig. 2. Secretion of fluid by isolated mammalian renal tubules.
Proximal tubule: a nonperfused rabbit S2 segment was incubated in
control medium (top), and then human uremic serum containing
hippurate was added for 15 min (bottom). A prominent lumen formed
as fluid was secreted secondary to the active transport of hippurates
from the serum into the urine (adapted from Ref. 28). Inner medullary collecting duct: a nonperfused, rat inner medullary collecting
duct (IMCD1) was incubated in control medium (top), and then
8-bromoadenosine 3⬘,5⬘-cyclic monophosphate, a permeable derivative of cAMP, was added for several hours (bottom). A prominent
lumen formed as fluid was secreted secondary to the active transport
of salt and water.
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nonperfused segments, the lumens opened in the dissecting dish despite the absence of glomeruli. Sustained net secretion of fluid isosmotic with the ambient
medium was documented by direct measurement, and
cAMP accelerated the rate of net chloride and fluid
secretion. These studies proved that mesonephric tubules retained the capacity to secrete solutes and water. It is not a stretch of logic to suppose that under in
situ conditions, in which glomerular filtration might be
halted, these same tubule segments would generate
urine by transepithelial secretion of solutes and water.
Indeed, studies of intact Southern Flounder in which
glomerular filtration was reduced to zero demonstrated
unequivocal secretion of urine containing Mg2⫹, Na⫹,
Cl⫺, and SO42⫺ (33, 34). Proximal tubules isolated from
frog kidneys have been observed to secrete liquid in
response to the addition of phenol red to the external
medium (35).
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Solute and Fluid Secretion in Mammalian
Collecting Ducts
Collecting ducts, the last intrarenal tubular segments through which urine flows, were also the last
tubular segments in which transepithelial NaCl transport was rigorously quantified. The potential role of
collecting ducts in the regulation of NaCl balance was
hotly debated in the 1970s (20, 36, 41, 48, 64, 65, 67,
68). The majority view, which holds to this day, supposes that collecting ducts reabsorb, under the control
of aldosterone, an amount of NaCl equivalent to ⬃3–
5% of the filtered load (45, 76–78). Na⫹ reabsorption,
linked to the epithelial sodium channel (ENaC) (54),
has been demonstrated in the cortical collecting ducts
(CCD) (2, 26, 27); however, OMCD and IMCD have not
been found in in vitro perfused tubule experiments to
transport much NaCl one way or the other, with some
notable exceptions. Rocha and Kudo (52) reported that
rat IMCD absorbed NaCl under normal perfusion conAJP-Renal Physiol • VOL
ditions, but this could be reversed to net secretion on
the addition of cGMP to the external medium or the
addition of atrial natriuretic peptide, which stimulates
the intracellular production of cGMP. However, efforts
in other laboratories to reproduce these results have
not been successful. On the other hand, in support of
Rocha and Kudo’s findings, cultured rat CCD and
IMCD appear to absorb Na⫹ under basal conditions
but can be induced to secrete Cl⫺ on the addition of
certain agonists (19, 38–40). Recently, Wall (73) has
observed net Cl⫺ and fluid secretion in isolated perfused OMCD of the rat, although the effect of cyclic
nucleotides on the rate of secretion was not tested.
Sonnenberg and colleagues (62–64) used a retrograde catheterization technique to measure NaCl and
fluid transport in rat IMCD in situ and found that
massive ECF volume expansion with saline converted
net NaCl and fluid absorption to net secretion. Results
consistent with bidirectional Na⫹ transport in the
IMCD have been reported by using classic in situ
micropuncture methods (3); however, there is a concern that admixture of urine from juxtamedullary longlooped nephrons and superficial nephrons may have
complicated the interpretation of these results (20, 36,
41). Net NaCl secretion in IMCD is further obfuscated
by the relatively low rates of net solute transport,
together with the fact that renal pelvic contraction (56)
and the concentration of interstitial solutes are disturbed by in situ microcatheterization and micropuncture and in vitro microperfusion methods.
Wallace and colleagues (75) have recently addressed
the issue of collecting duct solute and fluid transport
using a novel strategy to quantify net absorption and
secretion. IMCD were dissected from the renal medullas of rats and maintained in vitro at 37°C for several
hours. With the use of methods developed for the study
of net fluid secretion in proximal tubules, the lumens of
nonperfused collecting ducts were examined after addition to the incubation media of substances that had
been shown in a variety of secretory tissues, including
human renal cyst epithelial cells, to promote the net
secretion of Cl⫺ and fluid (Fig. 2). The results were
striking and unambiguous. The addition of cAMP to
the collecting ducts caused previously collapsed lumens to open widely with fluid, albeit at a relatively
low rate. Inclusion of benzamil, a high-affinity amiloride derivative that inhibits ENaC channels in IMCD,
potentiated the net secretion of fluid caused by cAMP,
indicating that reduction of competing Na⫹ absorption
unmasked a significant degree of solute and fluid secretion. After inhibition with benzamil, a small amount
of residual absorption implicated nonspecific solute
transport mechanisms in addition to ENaC. The solutes secreted into the lumens were osmometrically
active and, more than likely, principally comprised
Na⫹ and Cl⫺, although contributions of K⫹, NH4⫹, and
unspecified osmolytes have not been excluded. Inhibitors of Cl⫺ secretion [bumetanide, diphenylamino-2carboxylic acid (DPC), and DIDS] significantly reduced
net fluid secretion promoted by cAMP. In preliminary
studies, epinephrine was found to be a potent agonist
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A luminal PAH concentration of 40 mM had to be
reached in the luminal fluid to override the active
reabsorptive transport of sodium (22, 28). The peritubular mechanism for cellular uptake is powerful
enough to raise intracellular hippurate to levels manyfold greater than in the external medium, thereby
favoring hippurate entry into the lumen. This hyperpolarizes the transepithelial electrical potential, promoting Na⫹ transport through a paracellular pathway.
In this way, the net reabsorption of NaCl is converted
to net secretion of Na-hippurate, NaCl, and fluid. The
mechanism is similar in most respects to other tissues
that can either absorb or secrete Na⫹, owing to changes
in the amount of secreted anion (58, 70, 71).
We calculated that as much as 1 liter of secreted fluid
[0.5% of glomerular filtration rate (GFR)] could be
generated by hippurate transport within a 24-h period
(22, 24). During periods of normal glomerular filtration, this secretory contribution of hippurate to urine
formation would be masked by the robust reabsorption
of 179 liters of Na⫹-rich glomerular filtrate. Thus it is
not difficult to appreciate why fluid secretion driven by
organic anion transport would be overlooked by even
the best clearance methods for quantifying fractional
and absolute levels of tubule fluid absorption and secretion. Indeed, an attempt to detect in humans a
contribution of transtubular PAH secretion to urine
formation failed because the reabsorption of glomerular filtrate was many times greater than the amount of
fluid that could be secreted into the urine coupled to
the transport of organic anions (4). To demonstrate
fluid secretion coupled to hippurate transport by clearance methods, GFR would have to be reduced to ⬍10%
of normal to diminish the obscuring effect of competing
NaCl-driven tubule fluid reabsorption, i.e., a situation
more closely approximating that of the apocrine protovertebrate kidney. Thus the extent to which organic
anion secretion may contribute to the entry of fluid into
the lumens of mammalian proximal tubules in situ
remains to be determined.
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Regulation of NaCl and Fluid Balance
Fig. 3. Illustration of the potential role of IMCD NaCl secretion in
intact kidneys. In the classic view (A), most of the NaCl is reabsorbed
in the proximal tubules, loop of Henle, and distal convoluted tubules,
leaving a small portion to be reabsorbed by the cortical collecting
duct (CCD) system. In an extreme hypothetical alternative (B), Na⫹
reabsorption (similar to K⫹ reabsorption) could be nearly complete
by the end of the distal tubule or CCD, leaving the outer medullary
collecting duct (OMCD) and the IMCD to add NaCl to the final urine.
Axial blending of the NaCl absorptive and secretory processes along
the CCD, OMCD, and IMCD would accomplish the same outcome to
finely regulate NaCl balance in the terminal portions of the kidney
tubular system.
Under normal conditions in which there is free access to water and salt, the kidneys of a normal person
filter ⬃125 ml/min of plasma containing 140 and 105
meq/l (25,200 and 18,900 meq/day) of Na⫹ and Cl⫺,
respectively (69). Approximately 100 meq of Na⫹ and
Cl⫺ are excreted daily in the urine. Of the 180 liters of
water filtered daily, ⬃1.0 liter is excreted in the urine.
Thus ⬎99% of the filtered NaCl and 179 liters of water
are normally reabsorbed in the steady state. The extent to which relatively small rates of tubular NaCl
and water secretion might contribute to this urine
formation has been impossible to quantify and, consequently, easy to overlook.
Assuming for the moment that certain renal tubule
segments do secrete NaCl under normal conditions,
how might this have a role in the formation of the final
urine and the regulation of salt and extracellular fluid
balance? Salt secretion by tubules distal to segments
with high Na⫹ absorptive capacity would seem to have
the greatest impact on the economy of salt and water
balance. Were collecting ducts to add NaCl to the
tubule fluid, it is likely that this contribution would be
reflected in the solute content of the final urine. For the
purpose of illustration, an extreme example of this
hypothesis is shown in Fig. 3, where it is assumed that
all of the filtered NaCl is reabsorbed before the urine
enters the medullary collecting ducts. In this case, the
NaCl in the final urine would be derived entirely from
collecting duct secretion. Alternatively, salt absorptive
and secretory processes may be distributed to varying
degrees along the collecting duct system from cortex to
papilla.
How then might we assess the possible contribution
of salt secretion to urine formation? Clinicians have
observed for decades that an imbalance between the
intake and the excretion of salt and fluid often occurs
insidiously, although eventually leading to massive
edema. For example, the daily retention of salt and
water equal to 100 ml of ECF (equivalent to only
0.055% of the GFR) could in 1 mo lead to the accumulation of 3 liters of fluid. In other words, changes in net
tubule reabsorption equal to ⬍1% of the filtered NaCl
load would be sufficient to cause large changes in ECF
volume over extended periods of time. Similarly, a
decrease in NaCl secretion equal to ⬍1% of the filtered
load could have an effect as important as an increase in
the fractional reabsorption of a similar magnitude, and
we would be unable to tell one mechanism from the
other. Conversely, were renal tubules to secrete each
day an additional amount of NaCl equal to 100 ml
of glomerular filtrate, neglecting compensatory responses, in 1 mo there would be a decrease in ECF
volume of 3 liters and, more than likely, a lowering of
the blood pressure. Thus seemingly inconsequential
changes in tubule salt secretion could ultimately be
reflected by changes in ECF volume manifested as
edema or conversely as orthostatic hypotension.
Vasoactive intestinal peptide, a powerful Cl⫺ secretogogue in the rat that activates adenylyl cyclase (59),
increased the renal excretion of NaCl in isolated perfused rat kidneys without causing changes in renal
hemodynamics or GFR (53). This finding is consistent
with a contribution of net NaCl secretion to the final
PHYSIOLOGICAL IMPLICATIONS OF RENAL
TUBULAR SOLUTE AND FLUID SECRETION
IN TERRESTRIAL MAMMALS
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of Cl⫺-dependent fluid secretion, working through
␤-adrenergic receptor stimulation of adenylate cyclase (74).
Evidence of cAMP-mediated Cl⫺ secretion has been
observed in cultured monolayers enriched in IMCD
cells obtained from rat and mouse (39, 40, 66). In
preliminary studies (74), cultured cell monolayers enriched in human IMCD cells from the initial region of
the inner medulla generated an apically negative
transepithelial potential difference averaging 5 mV
and positive short-circuit current (SCC). This basal
SCC was diminished by the apical application of benzamil, implicating ENaC-dependent cation absorption
as a contributor to the basal SCC. The addition of
cAMP to benzamil-treated membranes strikingly increased positive SCC, consistent with the stimulation
of anion transport. Bumetanide, DPC, and DIDS inhibited the cAMP-mediated current, further implicating
net secretory Cl⫺ transport by these cells. These preliminary studies in cultured human cells support the
view that IMCD have the intrinsic capacity to secrete
Cl⫺ and Na⫹ and thereby osmotically drive water into
the tubule lumen.
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Fig. 4. When mammalian nephrons “return to the sea.” Under normal conditions, the reabsorption of glomerular filtrate in conjunction
with the secretion of solutes by proximal tubules and collecting ducts
contributes to the volume and composition of the final urine. In acute
renal failure and chronic renal failure states, glomerular filtration
may be strikingly reduced or abolished, leaving only tubular secretory mechanisms in surviving nephrons to form the urine. Under
conditions leading to dessication without salt depletion, glomerular
filtration may be strikingly reduced to conserve water, and tubule
salt secretion may contribute to the avoidance of profound hypernatremia. In all of these pathological states, secretory mechanisms
contribute to the elimination of potentially toxic substances (uremic
waste products) in a sparingly small volume of liquid, thereby resembling aglomerular animals that live in the sea.
AJP-Renal Physiol • VOL
this regard, inhibition of prostaglandin synthesis in
normal rats enhanced Cl⫺ reabsorption in collecting
duct segments (37), a possible clue that eicosanoids
regulate salt transport in these segments through their
capacity to stimulate the formation of cAMP and,
thereby, stimulate Cl⫺ secretion pathways. As we come
to better understand the cellular mechanisms of renal
tubule NaCl secretion, it may be possible to find more
selective inhibitors or promoters of tubule solute and
fluid secretion that will make it possible to examine the
potential impact of this occult mechanism on long-term
ECF balance.
Potential Role of Tubule NaCl and Fluid Secretion
in Acute Renal Failure
The sudden reduction of blood volume through external hemorrhage or plasma sequestration leads to
changes in renal hemodynamics that ultimately lead to
the renal conservation of ECF. Similar effects on the
kidney can be produced by the intravascular infusion of
powerful renal vasoconstricting agents, or in some animals such as the diving seal, by a combination of
neural and humoral mediated factors. The net effect is
to reduce renal blood flow, decrease GFR, and minimize the renal loss of NaCl and water. Were glomerular filtration to cease, tubular lumens would collapse as
fluid was completely reabsorbed. However, do the tubule lumens remain completely collapsed under such
extreme circumstances? Most human plasma contains
sufficient hippurate to promote net fluid secretion in
isolated proximal tubules (28, 50). A similar secretory
mechanism has been suggested as a means to maintain
tubular excretory function in lower animals in which
glomerular filtration is intermittent (7, 13, 15).
It is conceivable that, under conditions of markedly
reduced or complete cessation of glomerular filtration,
mammalian proximal tubules could secrete hippurate
and similar substances in amounts sufficient to maintain patent tubule lumens and to propel urine through
the nephron, albeit at markedly reduced rates of flow
(Fig. 4). In the collecting duct segments, the net addition of NaCl and water by secretion could contribute
further to urine formation. In this way, elimination of
some of the potentially toxic products normally excreted by the kidneys, as well as NaCl and water,
would serve a useful survival function, which mirrors
to a large extent the formation of urine in aglomerular
teleosts.
The extent to which proximal tubule fluid secretion
may occur in intact kidneys under conditions defined
as acute renal failure is unknown. Widely dilated renal
tubules, commonly seen in photomicrographs of renal
biopsy sections, are consistent with a secretory process.
Robust secretion of hippurate in patients destined to
recover from acute renal failure has been well established and is a reliable indicator of prognosis (30).
Whether the scant amount of urine formed under such
circumstances is a product of incomplete reabsorption
of markedly reduced glomerular filtrate or a product of
tubule solute and fluid secretion is a question worth
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urine. Luke (42) reported in 1973 that restricting
drinking water for 24 h with a constant intake of salt
caused rats to increase the net excretion of NaCl, an
effect that was mimicked by the administration of
vasopressin to euvolemic animals. The authors suggested that the increased excretion of NaCl, which
seemed paradoxical at the time, may reflect a mechanism the kidneys use to maintain the tonicity of ECF
during dehydration. In this regard, it is important to
note that net NaCl secretion in the IMCD would most
likely be downhill, in which case salt excretion could be
regulated in part by hormonal effects on the permeability of collecting duct segments to Na⫹ and Cl⫺ (55). It
is conceivable that simple dehydration unmasks a
cAMP-mediated salt secretory mechanism in rat renal
tubules that contributes to the regulation of ECF tonicity, a process in the protovertebrate era that was
reinstated several million years ago in Lophius, a fish
that unloaded its glomeruli on returning to live in the
sea (44, 60) (Fig. 1).
Perhaps it is time to reexamine, in those patients
with mysterious forms of chronic edema or ECF contraction, the possibility that tubule NaCl secretion
might have a role to play. Could the impaired renal
tubular functions in some idiopathic edema states, e.g.,
congestive heart failure and hepatic insufficiency, reflect to some extent a diminution in the tubular secretion of NaCl? And, conversely, could excessive tubule
NaCl secretion aggravate the salt wastage of certain
renal diseases and orthostatic states associated with
ECF contraction? Nephrologists may recall that it was
not until indomethacin, a cyclooxygenase inhibitor,
was administered to intact animals that roles for endogenous eicosanoids were appreciated as determinants of water and salt balance (1). Furthermore, in
F7
INVITED REVIEW
Role of NaCl and Fluid Secretion in Tubule Cyst
Formation and Enlargement
SUMMARY AND CONCLUSIONS
Autosomal dominant polycystic kidney disease (ADPKD) (14) and acquired cystic kidney disease (23) have
provided important clues to the existence within tubular epithelial cells of mechanisms for the secretion of
solutes and fluid. In ADPKD, cysts develop in proximal, distal, and collecting tubules as a consequence of
the mutation of either of two genes, PKD1 or PKD2.
The cysts begin as focal outgrowths of individual tubule segments and in a sense represent benign neoplastic tumors that are full of liquid rather than cells.
In the early stages of cyst formation, the fluid within
them derives from the afferent tubule segment in
which the cyst arose. Unreabsorbed glomerular filtrate
enters the cysts and collects there as the segment
expands, owing to the growth of mural epithelial cells.
Interestingly, when the cysts reach a diameter of ⬃200
␮m, most of them separate from the parent tubule and
become isolated sacs of liquid. In this circumstance, net
transepithelial fluid secretion is the only mechanism
for the sustained addition of liquid to the expanding
cyst.
The mechanisms of fluid secretion in human renal
cysts of ADPKD patients have recently been elucidated
(70, 71). The mural cells appear somewhat less mature
that the proximal and distal epithelium from which
they derived, and the capacity to absorb solutes and
water is greatly reduced. On the other hand, the epithelial cells derived from both proximal and distal
tubule elements retain the capacity to secrete NaCl
and fluid, as was demonstrated in tubules from normal
individuals. This fluid secretion process is regulated by
intracellular cAMP, which increases the permeability
of cystic fibrosis transmembrane conductance regulator-like Cl⫺ channels in apical membranes. This, together with the entry of Cl⫺ from the basolateral side
of the cells through a Na⫹-K⫹-2Cl⫺ cotransporter in
conjunction with basolateral K⫹ channels for recycling
this ion, leads to the net secretion of Cl⫺ into the urine
and an increase in lumen negativity. The negative
transepithelial potential increases the movement of
AJP-Renal Physiol • VOL
peritubular Na⫹ into the lumen through paracellular
pathways.
The amount of fluid secretion into cysts can be increased by substances that activate adenylate cyclase,
including arginine vasopressin, prostaglandin E2, secretin, vasoactive intestinal polypeptide, phosphodiesterase inhibitors, and an anonymous neutral lipid that
has been detected in cyst fluids of patients with ADPKD (25, 29). Fluid secretion can be inhibited by blocking the Na⫹ pump with ouabain, inhibiting the Na⫹K⫹-2Cl⫺ cotransporter with bumetanide, inhibiting a
basolateral K⫹ channel with glybenclamide, blocking
the apical cystic fibrosis transmembrane conductance
regulator with DPC, and blocking the action of protein
kinase A with H-89 and RpcAMP. Fluid secretion in
renal cysts, therefore, resembles in most respects the
secretion of NaCl and fluid by mammalian renal collecting ducts.
The mammalian kidney reflects its evolutionary precursors to a remarkable extent. It retains many of the
molecular and energy-efficient functional features of
simple primordial kidneys that regulate fluid balance
and eliminate potentially toxic molecules entirely by
secretion. However, the modern kidney must deal with
an exuberant glomerulus that was fused to the proximal tubule during the adaptive journey in a freshwater
environment, where radical expulsion of free water
was necessary for survival. Terrestrial reptiles and
desert birds have suppressed the impact of the glomerulus, but it has stubbornly flourished even in mammals
that prefer to live in arid climates. In the face of avid
solute and fluid reabsorption imposed by glomeruli,
occult secretory processes in the proximal and collecting tubules of the mammalian kidney, carried forward
during the evolution of vertebrates from the sea to dry
land, have persisted as mechanisms for the fine regulation of ECF volume and composition day to day, and
possibly for the continuance of a small stream of urine
even during the failure of glomerular filtration.
The authors thank Drs. Lawrence Sullivan and Thomas DuBose
for helpful discussions.
This work was supported by National Institute of Diabetes and
Digestive and Kidney Diseases Grants (P01-DK-53763 and P50-DK57301 from the Department of Health and Human Services to J. J.
Grantham), a National Research Service Award (to D. P. Wallace),
and the Polycystic Kidney Foundation.
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