0022-3565/98/2861-0157$03.00/0
THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS
Copyright © 1998 by The American Society for Pharmacology and Experimental Therapeutics
JPET 286:157–162, 1998
Vol. 286, No. 1
Printed in U.S.A.
Detailed Mapping of Ochratoxin A Reabsorption Along the Rat
Nephron In Vivo: The Nephrotoxin Can Be Reabsorbed in All
Nephron Segments by Different Mechanisms1
ANKE DAHLMANN, WILLIAM H. DANTZLER, STEFAN SILBERNAGL and MICHAEL GEKLE
Physiologisches Institut, Universität Würzburg (A.D., S.S., M.G.), D-97070 Würzburg, Germany and Department of Physiology, College of
Medicine, University of Arizona (W.H.D.), Tucson, Arizona
Accepted for publication March 9, 1998
This paper is available online at http://www.jpet.org
The kidney is the main target of OTA toxicity and, at the
same time, plays an important role in OTA excretion (Delacruz and Bach, 1990; Kuiper-Goodman and Scott, 1989). OTA
impairs renal hemodynamics (Gekle and Silbernagl, 1993),
urine concentrating mechanisms (Krogh et al., 1974; Gekle et
al., 1993), and secretion of organic anions (Gekle and Silbernagl, 1994), and it increases the incidence of renal adenoma
and carcinoma (Kuiper-Goodman and Scott, 1989; NTP,
1989). Because of the ubiquitous occurrence of OTA in improperly stored food and animal chow there is a high risk of
exposure for humans and animals (Marquardt and Frohlich,
1992). Its role in human renal disease is still not determined
Received for publication October 17, 1997.
1
This study was supported by the Deutsche Forschungsgemeinschaft (DFG
Si 170/7–2 and Ge 905/3–3) and by U.S. National Institutes of Health Research
Grant DK 16294.
a substrate of the apical organic anion carrier. L-Phenylalanine
did not reduce OTA reabsorption. After intravenous injection of
unlabeled OTA, resulting in a plasma concentration of ;1025
mol/l, the FR of [3H]OTA during early proximal microinfusion
was reduced slightly. From our results we conclude: 1) OTA can
be reabsorbed in all nephron segments investigated. 2) Under
physiological conditions the predominant sites of reabsorption
are PST, ALH and terminal CD. 3) Reabsorption in PST and ALH
is not pH-dependent. 4) pH-independent reabsorption in PST is
mediated by the apical organic anion transporter (OAT-K1),
whereas pH-dependent reabsorption in PCT is mediated by
H1-dipeptide cotransporter(s). 5) Reabsorption also takes
place during natural exposure, i.e., when OTA is present in
plasma and renal tissue. 6) The high FR in ALH and CD explains, at least in part, the preferential impairment of postproximal functions and the accumulation in renal inner medulla and
papilla.
completely. Yet, three forms of human renal disease, at least
in part, seem to be caused by enhanced OTA exposure: Balkan Endemic Nephropathy, chronic interstitial nephritis and
karyomegalic interstitial nephritis (Simon, 1996). Thus,
studies on the toxokinetics and toxodynamics of OTA are
relevant for human and animal health, because it is becoming more and more evident, that the complete avoidance of
OTA exposure is impossible because of its ubiquitous occurrence (Simon, 1996; Marquardt and Frohlich, 1992; KuiperGoodman and Scott, 1989).
Once ingested, OTA is absorbed very effectively from the
gastrointestinal tract and reaches the circulation (Kumagai
and Aibara, 1982). In blood, more than 99% of OTA is bound
to serum proteins (mainly albumin), which contributes to its
long half-life in the body (Chu, 1971, 1974). Renal elimination of OTA contributes at least 50% to its total clearance
(Kuiper-Goodman and Scott, 1989). Because effective filtra-
ABBREVIATIONS: OTA, ochratoxin A; FE, fractional excretion; FR, fractional reabsorption; TES, N-tris(hydroxymethyl)methyl-2-aminomethane
sulfonic acid; MES, 2-(N-morpholino)ethanesulfonic acid; PCT, proximal convoluted tubule; PST, proximal straight tubule; ALH, ascending limb
of Henle9s loop; DT, distal tubule; CD, collecting duct; EP, early proximal; LP, late proximal; ED, early distal; LD, late distal; LH, loop of Henle; BSP,
sulfobromophthalein; OAT-K1, organic anion transporter K1.
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ABSTRACT
Ochratoxin A (OTA) is a widespread nephrotoxin excreted to a
substantial degree via the kidney. Previously we showed that
[3H]OTA can be reabsorbed along the rat nephron in vivo
(Zingerle et al., 1997). In this study we investigated in detail the
contribution of different nephron segments to [3H]OTA reabsorption and determined the possible mechanisms involved by
microinfusion and microperfusion experiments. At pH 6 (;94%
of OTA neutral), OTA is reabsorbed in all nephron segments
investigated. The estimated fractional reabsorptions (FR) at a
tubular load of 20 fmol/min are: proximal convoluted tubule
(PCT), 14.8%; proximal straight tubule (PST), 27.4%; ascending
limb of Henle9s loop (ALH), 13.6%; distal tubule (DT), 11.6%;
collecting duct (CD), 24.6%; terminal CD, 22.0%. At pH 8
(;10% of OTA neutral) FR are as follows: PCT, 0%; PST,
25.9%; ALH, 14.0%; DT, 3.2%; CD, 8.2%. Thus, OTA reabsorption in PST and ALH is pH-independent. Reabsorption in
PST but not in DT or CD was inhibited by sulfobromophthalein,
158
Dahlmann et al.
Materials and Methods
Preparation of animals for microinfusion and microperfusion experiments in vivo. Male Wistar rats (Charles River, Sulzfeld, Germany) weighing 210 to 290 g were fed an Altromin standard diet and had free access to water. The rats were anesthetized
with 120 mg/kg body weight Inactin (Byk-Gulden, Constance, Germany). After inserting a polyethylene tube into the trachea and two
catheters into the jugular vein, the animals were infused intravenously with Ringer’s solution at a rate of 20 ml/min. The kidney was
prepared for micropuncture as described elsewhere (Gekle and Silbernagl, 1994).
Microinfusion experiments on superficial cortical
nephrons. After identification of the nephron section by intravenous injection of lissamine green SF (Chroma-Gesellschaft, Köngen,
Germany) at a bolus dose of 0.02 ml of a 100 g/l solution titrated with
NaOH to pH 7.4, the tubule was micropunctured by glass capillaries.
They had ground tips (outer tip diameter, 10 –12 mm) and were
mounted on a microperfusion pump (Sonnenberg and Deetjen, 1964).
Tubules were punctured at the specific sites, OTA infused and the
urine collected. Excreted OTA and inulin were determined and the
fractional reabsorption was calculated (see below). Then this procedure was repeated at another tubular site. Three to five punctures
could be performed on one rat. Puncture sites were 1) the earliest
superficial loop of the proximal tubule (EP), 2) the last superficial
loop of the proximal tubule (LP), 3) the first superficial loop of the
distal tubule (ED) and 4) the last superficial loop of the distal tubule
(LP). The microinfused solution was a Ringer’s solution (see below)
containing [14C]inulin and [3H]OTA (1026 mol/l). Microinfusion at a
rate of 20 nl/min lasted for 10 min. Starting shortly before microinfusion, the ipsilateral urine was collected from a uretheral catheter
in 15-min fractions for 60 min, and the radioactivity of each fraction
was determined by counting in a liquid scintillation spectrometer
(Packard Instruments, Frankfurt, Germany). Urinary [3H]OTA recovery (FE) was calculated from ([3H]urine z [14C]perfusion)/([14C]urine z [3H]perfusion). FR is 1 2 FE. As a control, the urine of the
contralateral kidney was collected from a bladder catheter. Here,
radioactivity did not exceed the background level.
Microperfusion of the proximal convoluted tubule. The
proximal convoluted tubule was microperfused with the same solution (1026 mol/l [3H]OTA, pH 6) as used for microinfusion (Sonnenberg and Deetjen, 1964) at a rate of 20 nl/min. Sudan black-stained
castor oil was microinjected with a micropipette into the first superficial loop of the proximal convolution, and the endogenous tubule
fluid subsequently was drained into the same pipette. The tubule
was microperfused with a second micropipette between the second
superficial loop and the last accessible loop of the proximal convolution, where the perfusate was recollected, and the fractional late
proximal recovery was determined.
Microinfusion into long loops of Henle. The experiments on
long loops of Henle were performed as described previously (Silbernagl et al., 1997). The papilla of the left kidney was exposed, and a
single ascending limb of a long loop of Henle was punctured near the
hairpin bend with a glass micropuncture pipette with an external tip
diameter of 6 mm. The infusion generally was maintained at 10
nl/min. After the infusion was well established, collections of urine
emerging from the ducts of Bellini were made with a second micropuncture pipette. The radioactivity emanating from [14C]inulin
and [3H]OTA in the collected fluid and the initial infusate was
determined, and the fractional reabsorption was calculated as described above. Because quantitative collecting of urine is not possible
with this preparation we had to use a higher concentration of
[3H]OTA (5z1024 mol/l) to obtain enough radioactivity for reliable
measurements. Because FR of OTA in postproximal parts of the
nephron is concentration-independent up to 1023 mol/l (Zingerle et
al., 1997), the use of 5z1024 mol/l [3H]OTA did not affect the investigation of OTA reabsorption.
Materials. The Ringer’s solution consisted of (in mmol/l): 156.4,
Na1; 5.4, K1; 1.7, Ca11; 162.8, Cl2; and 2.4 HCO32, pH 7.4. The
Ringer’s solution was buffered with either 50 mmol/l TES (pKa 5 7.5)
or 10 mmol/l MES (pKa 5 6.15). The solution was titrated to pH 8.0,
7.4 or 6.0. The changes in osmolality caused by the addition of buffer
did not affect OTA reabsorption (data not shown). [14C]Inulin (0.4
GBq/g) was obtained from Du Pont de Nemours (Dreieich, Germany),
[3H]OTA (1.37z1014 Bq/mol) from Moravek Biochemicals (Brea, CA)
and TES and MES from Serva (Heidelberg, Germany). All other
chemicals were purchased from Sigma (St. Louis, MO). The purity of
[3H]OTA was checked by high-performance liquid chromatography
as described previously (Gekle and Silbernagl, 1994).
Statistics. The data are presented as mean values 6 S.E.M.. The
n value is the number of tubules studied. Data for the different
puncture sites were obtained from at least three different animals.
Significance was tested by the unpaired t test. Differences were
considered significant if P , .05.
Results
Because the reabsorption of OTA along the nephron is
sensitive to luminal pH (Zingerle et al., 1997), we determined
FR at defined pH values with buffered infusion solutions (see
Methods). To assess the contribution of different nephron
segments to reabsorption, we microinfused OTA at different
sites along the nephron, as shown in figure 1A, and determined the respective FR values (%) at pH 6 and pH 8.
Comparison of the FR values of two consecutive puncture
sites permits an estimation of the contribution of the nephron
segment in between those puncture sites to OTA reabsorption (5 segmental FR). Thus, reabsorption in PCT, for example, can be estimated from the difference in FR during EP
and LP microinfusion. If two consecutive FR values are significantly different it can be concluded that the respective
nephron segment contributes significantly to OTA reabsorption.
To validate the estimation of segmental FR by the micro-
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tion is hindered by the binding to albumin, the main route for
OTA entry into the tubular lumen is secretion via the basolateral organic anion carrier in the proximal tubule (Bahnemann et al., 1997; Sokol et al., 1988; Stein et al., 1984; Gekle
and Silbernagl, 1994). The concentration of free OTA in final
urine is ;10% the concentration of total OTA in plasma
(Gekle and Silbernagl, 1994).
The fate of OTA that has reached the tubular lumen needs
to be investigated to know which epithelial cells suffer enhanced exposure and to develop strategies for accelerated
OTA excretion. Recently, we showed that the mycotoxin is
reabsorbed along the nephron, partially via the H1-dipeptide
cotransporter (Zingerle et al., 1997; Silbernagl et al., 1987).
This reabsorption of OTA is of toxicological importance because it contributes to the long half-life of the mycotoxin in
the body (Kuiper-Goodman and Scott, 1989). To determine
the contribution of different nephron segments to OTA reabsorption we performed in vivo microinfusion and microperfusion experiments with [3H]OTA and estimated the FR in
PCT, PST, ALH, DT and CD. Furthermore, we determined
the pH dependence of FR and the contribution of the proximal tubular apical organic anion transporter OAT-K1 to reabsorption (Masuda et al., 1997). Finally, we tested whether
[3H]OTA reabsorption also takes place under conditions
when unlabeled OTA is present in plasma and renal tissue,
as would occur after ingestion.
Vol. 286
1998
infusion method described above, we compared the value
obtained for reabsorption in PCT with the value obtained by
microperfusion (solution buffered to pH 6). FR in PCT was
14.8 6 4.2% (n 5 11) as determined by microperfusion and
11% as determined by subtracting FR during LP microperfusion from FR during EP microperfusion. The two values are
not significantly different, which indicates the validity of our
method to estimate segmental FR.
Figure 1, B and C, shows the FR values obtained during
microinfusion at the puncture sites. During microinfusion at
pH 6 all nephron segments contributed significantly to reabsorption (fig. 1B). This was clearly not the case during microinfusion at pH 8 (fig. 1C). At this pH significant reabsorption was observed only between the LP and LH infusion sites
(PST), LH and ED infusion sites (ALH) and during LD infusion (CD). The calculated segmental FR values during microinfusion at pH 6 and pH 8 are shown in figure 2, A and B. As
already mentioned, at pH 6 OTA is reabsorbed in all nephron
segments investigated (fig. 2A): PCT, PST, ALH, DT and CD.
The highest FR were observed in PST (;27%) and CD
(;24%), whereas FR in PCT, ALH and DT amounted to 12 to
15%. In an additional series of experiments we determined
the contribution of the terminal collecting duct (1.2 6 0.3 mm
from the tip of the papilla), located in the inner medulla. FR
in this segment was 22 6 5% (n 5 6), which indicates that the
major part of collecting duct reabsorption takes place in the
inner medullary collecting duct. At pH 8 the pattern of seg-
159
Fig. 2. Calculated fractional reabsorption in different parts of the
nephron (segmental reabsorption; for details see “Results”) at pH 6 (A)
and pH 8 (B). * indicates significant reabsorption as defined under “Results.” (C) Bar graph of the differences of segmental reabsorption at pH 6
and 8. b.d.l., below detection limit.
mental FR changed dramatically compared with pH 6 (fig.
2B): No significant reabsorption could be determined in PCT
and DT. FR along CD was reduced dramatically from ;25%
to ;8%. By contrast, FR in PST and ALH remained almost
unchanged. Figure 2C illustrates again the effect of alkalinization on FR. From this figure we can deduce that OTA
reabsorption shows strong pH dependence in some parts of
the nephron (PCT, DT and CD), whereas it seems to be
virtually pH-independent in PST and ALH. Of course we
have to consider that the comparison of the FR values yields
only a crude estimate of the pH dependence of OTA reabsorption. Yet, it certainly allows us to obtain qualitative information about the influence of luminal pH.
From our previous study, we know that OTA reabsorption
in nephron segments before DT is mediated in part by H1dipeptide cotransporters (Zingerle et al., 1997). Together
with the data of the present study we can assign this form of
reabsorption to PCT, the predominant site of H1-dipeptide
cotransporter (Pept2) expression (Daniel and Herget, 1997).
To determine the transport system involved in pH-insensitive OTA reabsorption in PST we tested the effect of BSP, a
substance that inhibits transport via the apical organic anion
transporter OAT-K1 located in PST (Kontaxi et al., 1996;
Masuda et al., 1997; Saito et al., 1996). These experiments
were performed at pH 8 to minimize transport via the H1-
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Fig. 1. (A) Schematic drawing of a nephron with collecting duct, which
indicates the different puncture sites. G, glomerulum; EP, early proximal;
LP, late proximal; LH, loop of Henle; ED, early distal; LD, late distal. The
arrows indicate the direction of urine flow. (B, C) FR in percent of the
amount infused during microinfusion at the different sites indicated in A
at pH 6 (B) and pH 8 (C). The numbers in parentheses give the number
of punctured nephrons. *P , .05 versus zero.
Renal Reabsorption of Ochratoxin A
160
Dahlmann et al.
Discussion
Studies on the toxicokinetics and toxicodynamics of OTA
are relevant for human and animal health because it is
becoming more and more evident that the complete avoidance of OTA exposure is impossible, because of its ubiquitous
occurrence (Simon, 1996; Marquardt and Frohlich, 1992;
Kuiper-Goodman and Scott, 1989). It is therefore necessary
to obtain detailed information on the characteristics of OTA
handling and its mechanisms of action. In the present study
we focused on renal toxokinetics and performed a detailed in
vivo mapping of OTA reabsorption along the nephron.
According to previous studies (Bahnemann et al., 1997;
Gekle and Silbernagl, 1994; Sokol et al., 1988; Friis et al.,
1988), the basolateral organic anion transport system in the
proximal tubule is responsible for the major part of blood-tolumen translocation of OTA. Subsequent reabsorption of the
toxin reduces transepithelial net secretion and thereby its
elimination. Furthermore, reabsorption enhances the accumulation of OTA within tubular cells. Thus, knowledge of the
fate of OTA that has reached the tubular lumen is of crucial
importance to determine which epithelial cells suffer en-
Fig. 3. Inhibitory effect of BSP on OTA-reabsorption during EP microinfusion at pH 8. L-phenylalanine (L-Phe) did not inhibit reabsorption.
LH, loop of Henle; ED, early distal; FR, fractional reabsorption; con,
control. *P , .05. The number in parentheses is the number of punctured
nephrons.
Fig. 4. Effect of intravenous injection of unlabeled OTA on FR during
EP microinfusion (pH 6). OTA (0.3 mg/kg) was injected 60 min before
microinfusion, leading to a total plasma concentration of ;1025 mol/l and
to final urine concentration of ; 1026 mol/l (Gekle and Silbernagl, 1994).
con, control.
hanced exposure and to develop possible strategies for accelerated OTA excretion.
The results of this study show clearly that OTA, once it has
reached the tubular lumen, can be reabsorbed in any part of
the nephron investigated, depending on the luminal pH (fig.
5A). Yet, reabsorption is not distributed uniformly along the
nephron, but shows marked qualitative and quantitative differences. According to our data, the major sites of reabsorption under physiological conditions are PST, ALH and CD.
Because luminal pH in DT is close to 7 under physiological
conditions, as we determined previously (Kuramochi et al.,
1997), there should be little reabsorption under physiological
conditions. As already mentioned, reabsorption in PCT, DT
and CD is clearly pH-dependent. Furthermore, we could
show that dipeptides reduce OTA reabsorption along the
proximal tubule but not in DT or CD by 15 to 20% (Zingerle
et al., 1997). Thus, it can be concluded that reabsorption in
PCT is mediated by H1-dipeptide transporter(s). This conclusion is in good agreement with the fact that H1-dipeptide
transport is expressed predominantly in PCT (Daniel and
Herget, 1997). To gain more information about the pH dependence we determined FR in PCT and during ED microperfusion (DT 1 CD) at pH 7.4 (n 5 6 and 3, respectively)
and plotted FR versus pH, as shown in figure 5B. FR in PCT
was not a linear function of luminal pH, but showed a dramatic decrease between pH 7.4 and 8 (fig. 5B). This pattern
resembles the pH dependence of H1-dipeptide cotransport,
which shows the major loss of activity in the range of pH 7.5
to 8 (Ramamoorthy et al., 1995) and therefore supports the
hypothesis of reabsorption in the PCT by the H1-dipeptide
transporter. Thus, FR in PCT is in the range of 10% under
physiological conditions.
The pH dependence of reabsorption in DT and CD displayed a pattern different from that in PCT. Increasing luminal pH led to an almost linear decrease of FR (fig. 5B). In
parallel, the portion of nondissociated (neutral) OTA also
decreased in an almost linear way across the pH range investigated, as shown in figure 5B [pKa 5 7.1 (Kuiper-Goodman and Scott, 1989)]. Thus, FR in DT 1 CD is proportional
to the portion of nondissociated OTA, pointing to nonionic
diffusion as an important mechanism of reabsorption, as
proposed for intestinal absorption (Kumagai and Aibara,
1982). Furthermore, dipeptides did not affect reabsorption
during ED microinfusion (Zingerle et al., 1997). Because the
pH of collecting duct fluid is in the range of 6 (Kuramochi et
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dipeptide cotransporter. As shown in figure 3, BSP significantly reduced OTA reabsorption during EP microinfusion
but not during ED microinfusion, which indicates that OTA
is a substrate for OAT-K1 in PST. Because 5z1022 mol/l BSP
reduced OTA reabsorption to a value no longer significantly
different from FR during LH microinfusion, it seems that the
major part of FR in PST is caused by OAT-K1. By contrast,
5z1022 mol/l L-phenylalanine did not exert any significant
effect (fig. 3).
In a last series of experiments we determined the effect of
systemic application of unlabeled OTA on the reabsorption of
[3H]OTA. Sixty minutes before EP microinfusion of [3H]OTA,
we injected 0.3 mg/kg of unlabeled OTA intravenously, leading to a total plasma concentration of ;1025 mol/l (Gekle and
Silbernagl, 1994). Under these conditions the concentration
of OTA in the final urine is ;1026 mol/l, as determined
previously (Gekle and Silbernagl, 1994). As shown in figure
4, reabsorption of [3H]OTA is reduced only slightly, albeit
significantly, after intravenous injection of unlabeled OTA.
These data show that reabsorption also takes place when
OTA is present in plasma and renal tissue, as after ingestion.
Vol. 286
1998
al., 1997), we can conclude that up to 25% of OTA is reabsorbed in CD under physiological conditions. Furthermore,
our data indicate that reabsorption along CD occurs mainly
in the inner medullary part.
Reabsorption in PST and ALH is pH-independent and
therefore not, or at least not predominantly, mediated by
H1-dipeptide cotransporter(s) or nonionic diffusion. Recently
it was shown that OTA uptake in liver cells occurs via the
cloned organic anion transporter and can be inhibited by BSP
(Kontaxi et al., 1996). A homologous (80%) and functionally
similar transporter, named OAT-K1, also was cloned from
rat kidney (Saito et al., 1996). This transporter is localized in
the brush-border membrane of renal PST (Masuda et al.,
1997). We applied BSP to determine the possible contribution
of OAT-K1 in pH-independent reabsorption in PST. Indeed,
BSP reduced reabsorption in PST dramatically. Because FR
during LP microinfusion in the presence of BSP was no
longer different from FR during LH microinfusion, we conclude that the major part of reabsorption is mediated by
OAT-K1. Thus, ; 25% of OTA can be reabsorbed along PST
by OAT-K1 under physiological conditions.
161
Surprisingly, there is also significant reabsorption in ALH,
which was not affected by pH. Unfortunately, there is very
little information available on transport mechanisms of organic anions in the loop of Henle. Thus, we do not know at
present which transport mechanisms are responsible for reabsorption in ALH. Nevertheless, our data show that ALH
contributes substantially (;14%) to OTA reabsorption under
physiological conditions. The high reabsorption in ALH and
inner medullary CD (in sum ;40%) yield a good explanation
for the accumulation of OTA in renal outer medulla and
papilla (Schwerdt et al., 1996) and for the high susceptibility
of postproximal nephron functions, such as electrolyte excretion and urine concentration (NTP, 1989; Gekle et al., 1993),
to its toxic effects.
When microinfusion or microperfusion is performed in experimental animals, virtually no OTA is present in renal
tissue or plasma. Yet, during dietary exposure to OTA, the
substance is present in all three compartments (lumen, cell,
plasma) and the gradients for carrier-mediated reabsorption
are different. Furthermore, the number of available OTA
binding sites in renal tissue is reduced (Schwerdt et al.,
1996). To determine whether reabsorption also occurs under
these “natural” conditions, we measured reabsorption in
OTA-exposed animals. As our data show, there still was
substantial reabsorption in the presence of OTA in renal
tissue and plasma. Thus, OTA reabsorption also plays an
important role in renal toxicokinetics during natural dietary
exposure. Because the contribution of the kidney to OTA
metabolism has not yet been characterized sufficiently, it is
possible that part of the radioactivity appearing in the urine
represents metabolites (Kuiper-Goodman and Scott, 1989).
Previous studies, however, showed that the major part of
OTA given to experimental animals appears as intact OTA in
the urine (Kuiper-Goodman and Scott, 1989; Gekle and Silbernagl, 1994). Thus, the data presented here should reflect
mainly the fate of intact OTA. In the case of metabolite
excretion the fractional reabsorptions determined would
even be an underestimation, because OTA has to be taken up
into the cells before metabolism.
In conclusion, our data show that reabsorption of OTA
along the nephron is an important event for renal toxicokinetics. All nephron segments have the ability to reabsorb
OTA. Under physiological conditions the predominant sites
of reabsorption are PST, ALH and inner medullary CD. Reabsorption in PCT and PST can be attributed to H1-dipeptide
cotransporter and OAT-K1, respectively. Reabsorption in CD
seems to be mediated to a substantial degree by nonionic
diffusion. Reduction of renal reabsorption of OTA, for example by urine alkalinization or BSP, could lead to a reduction
of its biological half-life and thereby prevent part of its toxic
action. These potential implications will be the subject of
future studies.
Acknowledgments
We thank Katharina Völker for introducing A.D. to the technique
of micropuncture.
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Fig. 5. (A) Schematic drawing of a nephron with collecting duct, which
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