The H2-receptor antagonist cimetidine is efficiently excreted by the kidneys. In vivo studies indicated an interaction of cimetidine not only with transporters for basolateral uptake of organic cations but also with those involved in excretion of organic anions. We therefore tested cimetidine as a possible substrate of the organic anion transporters cloned from winter flounder (fROAT) and from human kidney (hOAT1). Uptake of [3H]cimetidine into fROAT-expressing Xenopus laevis oocytes exceeded uptake into control oocytes. At −60-mV clamp potential, 1 mM cimetidine induced an inward current, which was smaller than that elicited by 0.1 mM PAH. Cimetidine concentrations exceeding 0.1 mM decreased PAH-induced inward currents, indicating interaction with the same transporter. At pH 6.6, no current was seen with 0.1 mM cimetidine, whereas at pH 8.6 a current was readily detectable, suggesting preferential translocation of uncharged cimetidine by fROAT. Oocytes expressing hOAT1 also showed [3H]cimetidine uptake. These data reveal cimetidine as a substrate for fROAT/hOAT1 and suggest that organic anion transporters contribute to cimetidine excretion in proximal tubules.
- winter flounder renal organic anion transporter
- organic anion transport
- organic cation transport
as a specific antagonist of histamine H2 receptors, cimetidine acts on gastric parietal cells to inhibit HCl secretion stimulated by histamine, pentagastrin, and acetylcholine. Between 50 and 80% of the dose administered intravenously was recovered in urine as unchanged cimetidine, and elimination half-life was ∼2 h in healthy volunteers with normal kidney and hepatic function (24). The mean steady-state plasma concentration on a standard 1,000-mg daily dose was 1 μg/ml (range 0.64–1.64 μg/ml). A renal clearance of 600 ml/min (24), exceeding the glomerular filtration rate by approximately fivefold, indicated an efficient tubular secretion of cimetidine. Therefore, cimetidine must interact with renal transporters, most probably with those located in the proximal tubules.
Because at physiological pH part of the cimetidine is positively charged, the organic cation transport systems of the proximal tubule cell are expected to be involved in cimetidine secretion. The two-step model of transcellular organic cation secretion consists of electrogenic uptake at the basolateral side, moving organic cations down their electrochemical gradient from blood into proximal tubular cells, and organic cation/proton exchange mediating uphill exit into the lumen. The cloned organic cation transporters, OCT1, OCT2, and OCT3, are electrogenic and likely contribute to organic cation uptake across the basolateral membrane (for reviews, see Refs. 6and 18). In accordance with their function, rat OCT1 and OCT2 were localized to the basolateral membrane of proximal tubule cells (16, 25, 29). However, the precise location of OCT3 is not yet known. Tetraethylammonium uptake by rat (29) and human (34) OCT1 was inhibited by unlabeled cimetidine in the medium with apparent K i values of 5.7 and 166 μM, respectively. Similarly, transport of 1-methyl-4-phenylpyridinium by rabbit OCT1 was also inhibited by cimetidine (26). Translocation of radiolabeled cimetidine, however, was slow or negligible in human and rat OCT1 (10, 34). Moreover, OCT1−/− mice did not show reduced renal transport of cimetidine compared with their wild-type littermates (14), suggesting that OCT1 may not play a significant role in renal cimetidine secretion. In contrast, rat OCT2 showed significant, saturable transport of labeled cimetidine (10). TheK m was 21 μM, a value close to theK i of 9.4 μM determined earlier for the inhibition of rat OCT2-mediated tetraethylammonium uptake by cimetidine (29). In a chronic renal failure model, nephrectomy, OCT2 expression and renal clearance of cimetidine were reduced, whereas the expression of OCT1 was unchanged in the remnant kidney (13). These data collectively suggested that OCT2 plays a dominant role in cimetidine secretion in the rat. Tetraethylammonium transport by human OCT2 was inhibited by cimetidine, with a K i of 10.9 μM (at pH 7.0) (1). The same K i (11 μM) was found for the inhibition of organic cation transport by isolated human proximal tubules (22). Because the expression of OCT1 in the human kidney is low (9, 33), OCT2 may well dominate OCT1 with respect to proximal tubular cimetidine transport. An interaction of cimetidine with mouse (31) and rat (17, 32) OCT3 has also been demonstrated, and hence OCT3 may add to overall cimetidine secretion, if it is indeed located in the basolateral membrane of proximal tubule cells.
In the intact rat kidney, cimetidine inhibited the uptake not only ofN 1-methylnicotinamide, a prototypical substrate of organic cation transporters, but also of PAH, the classic substrate of the renal organic anion transporter (27, 28). Similarly, the uptake of radiolabeled cimetidine from capillaries into proximal tubule cells was inhibited by tetraethylammonium and probenecid, compounds known to interact with organic cation and organic anion transporters, respectively (27, 28). Analogous to organic cations, secretion of organic anions is a two-step process. For organic anions, uptake across the basolateral membrane occurs against an electrochemical potential difference, followed by an energetically downhill exit across the brush-border membrane (3). Of the cloned organic anion transporters, rat and human OAT1 and OAT3 have been localized to the basolateral membrane of proximal tubule cells, with OAT1 being expressed mainly in the S2 segment and OAT3 in all three segments (4, 12, 15). Human (4) and rat (7, 20) OAT3 transported radiolabeled cimetidine withK m values of 40 (rat OAT3) and 57 μM (human OAT3). Unlabeled cimetidine inhibited the uptake of labeled organic anions by human (4, 15) and rat OAT3 (5, 7,20), and a K i of 46.8 μM has been determined for rat OAT3 (21). Thus there is no doubt that cimetidine is a substrate of human and rat OAT3. With regard to OAT1, conflicting results have been reported. Human OAT1 was inhibited by cimetidine (15), whereas rat OAT1 was not significantly affected (21). So far, a translocation of labeled cimetidine by any OAT1 has not been shown, leaving open whether OAT1 contributes to cimetidine secretion in proximal tubules.
PAH and cimetidine are small, polar compounds, as indicated by their structure and their octanol-water partition coefficients (Fig.1). At physiological pH, the COOH group of PAH is negatively charged, whereas cimetidine is present as an uncharged as well as a positively charged molecule. Given a pK a of 6.9 (1), 76% of cimetidine is uncharged, and only 24% is positively charged at pH 7.4. Human OCT2 has recently been shown to interact preferentially with positively charged cimetidine (1). Secretion of the uncharged cimetidine should then be mediated by another transporter, possibly OAT1 and OAT3. Here, we report that flounder renal organic anion transporter, fROAT, and human OAT1 translocate cimetidine. These data indicate that both organic cation and anion transporters are involved in proximal tubular cimetidine secretion and allow for an efficient renal excretion of this drug.
MATERIALS AND METHODS
In vitro cRNA transcription.
fROAT (30) and human OAT1 (23) cDNAs were used as templates for cRNA synthesis. Plasmids were linearized withNotI, and in vitro cRNA transcription was performed using a T7 mMessage mMachine Kit (Ambion, Huntingdon, UK). The resulting cRNAs were resuspended in purified, RNAse-free water to a final concentration of 1–1.5 μg/μl.
Oocyte preparation and storage.
Stage V and VI oocytes from Xenopus laevis (Nasco, Fort Atkinson, WI) were treated with collagenase (type CLS II, Biochrom, Berlin, Germany) and maintained at 16–18°C in control solution (in mM): 90 NaCl, 3 KCl, 2 CaCl2, 1 MgCl2, and 5 HEPES/Tris, pH 7.6. One day after removal, oocytes were injected either with 30 nl of cRNA (1 μg/μl) coding for fROAT or with 23 nl of cRNA (1.5 μg/μl) encoding for human OAT1 and maintained at 16–18°C in control solution supplemented with 100 kU/IU penicillin, 0.1 mg/l streptomycin, and 2.5 mM sodium pyruvate. After 4 days of incubation with daily medium changes, oocytes were used for electrophysiological as well as for tracer uptake studies. Oocytes injected with water served as controls.
Uptake and electrophysiological studies.
Expression of fROAT and human OAT1 in oocytes was confirmed by comparing the uptake of radiolabeled PAH between cRNA-injected oocytes and water-injected oocytes. Uptake of [3H]PAH (5 μCi/ml; p-[glycyl-2-3H] aminohippuric acid) and [3H]cimetidine (10 μCi/ml; [N-methyl-3H]cimetidine) in oocytes was assayed for 30 (fROAT) or 60 min (human OAT1) at room temperature in control solution additionally containing either 0.099 mM unlabeled and 0.001 mM labeled PAH or 0.99 mM unlabeled and 0.01 mM labeled cimetidine. The incubation was stopped by aspiration of the incubation medium and several washes with ice-cold control solution, and the oocytes were assayed for radioactivity as described by Wolff et al. (30).
Electrophysiological studies were performed by the conventional two-microelectrode, voltage-clamp method (2). Oocytes were superfused with control solution followed by the same medium additionally containing PAH or cimetidine. The membrane potential of the oocytes was clamped at −60 mV, and the current induced by 0.1 mM PAH was measured to demonstrate functional expression of the fROAT protein. Voltage pulses between −90 and +10 mV, in 10-mV increments, were applied for 5 s each, and steady-state currents were recorded to obtain current-voltage (I-V) relationships. In general, the I-V protocol was applied first under control conditions and then 30 s after the superfusion solution was changed to the test solution. The difference between the steady-state currents measured in the presence and absence of substrates was considered as substrate-induced current.
All chemicals, including PAH and cimetidine were of analytic grade and purchased from Merck (Darmstadt, Germany) or Sigma (Deisenhofen, Germany). [3H]PAH and [3H]cimetidine were from New England Nuclear (Cologne, Germany) and from Amersham Pharmacia Biotech (Buckinghamshire, UK), respectively. The concentrations of uncharged and positively charged cimetidine were calculated using the Henderson-Hasselbalch equation and the pK alisted in Fig. 1.
PAH- and cimetidine uptake into fROAT-expressing oocytes.
To test whether the fROAT translocates cimetidine, X. laevisoocytes were injected with cRNA coding for fROAT. Four days later, these oocytes mediated uptake of 132.1 ± 15.7 pmol [3H]PAH · 30 min−1 · oocyte−1 and 14.4 ± 4.23 pmol [3H]cimetidine · 30 min−1 · oocyte−1. Water-injected control oocytes showed uptakes of 5.5 ± 0.8 pmol [3H]PAH · 30 min−1 · oocyte−1 and 2.7 ± 0.1 pmol [3H]cimetidine · 30 min−1 · oocyte−1 (Fig.2; representative experiment of a total of 3 independent experiments, each with an average of 6–12 oocytes/experimental condition). From these data, the fROAT-mediated uptake rates for [3H]PAH and [3H]cimetidine are 126.6 and 11.7 pmol · 30 min−1 · oocyte−1, respectively. Despite a 10 times lower substrate concentration, PAH uptake exceeded that of cimetidine by a factor of 10.8. Nevertheless, cimetidine was translocated in fROAT-expressing oocytes significantly more quickly than in water-injected control cells, indicating that this compound is a substrate for the organic anion transporter.
Because the uptake of PAH through fROAT generated an inward current (2), we tested next whether cimetidine is also able to induce such a current. The two-electrode, voltage-clamp technique allows a direct comparison of PAH- and cimetidine-induced currents in the same oocyte. As measured at a holding potential of −60 mV, in paired experiments on 20 oocytes from 14 donors, application of 0.1 mM PAH and of 1 mM cimetidine resulted in inward currents of −22 ± 2 (data from Fig. 3 C) and −4 ± 1 nA (data from Fig. 3 D), respectively. Despite a 10 times lower concentration, the average PAH-induced current exceeded that of cimetidine by a factor of 5.5. No such currents were observed in water-injected control oocytes (data not shown).
The small inward currents observed on application of cimetidine could be due either to a low affinity of cimetidine for fROAT or to low rates of translocation. To distinguish between these two possibilities, the concentration whereby half-maximal saturation of the cimetidine-induced current was observed (K 0.5) was determined. At −60 mV and pH 7.6, cimetidine-induced currents tended to saturate at concentrations >250 μM. In three experiments,K 0.5 was 45 ± 9 μM, and the maximal cimetidine-induced current (ΔI max) approached 2.92 ± 0.03 nA.
Steady-state currents in fROAT-expressing oocytes were measured in the absence and presence of PAH and are plotted as a function of membrane potential in Fig. 3 A. Respective currents in the absence and presence of cimetidine are shown in Fig. 3 B. PAH and, to a much lesser extent, cimetidine increased the slope conductance, indicating electrogenic uptake. When the substrate-induced currents for PAH (Fig. 3 C) and cimetidine (Fig. 3 D) were plotted as a function of membrane potential between −90 and +10 mV, a linear I-V relationship was obtained for PAH as well as for cimetidine. Again, at all potentials, the cimetidine-induced currents were smaller than the PAH-induced currents. At pH 7.4, the reversal potential (E rev) for the PAH-induced current was −11.2 ± 2.4 mV (Fig. 3 C) and that for the cimetidine-induced current was −27.8 ± 4.6 mV (Fig.3 D).
Superposition of PAH- and cimetidine-induced currents.
To test whether PAH and cimetidine utilize the same transporter, fROAT, we added increasing cimetidine concentrations in the presence of 0.1 mM PAH to fROAT-expressing oocytes. PAH plus cimetidine-induced currents (I) were normalized to the PAH-induced current in the absence of cimetidine (I 0) as shown in Fig.4. The smallest cimetidine concentration applied (0.1 mM) increased the total current, whereas a further increase in cimetidine concentration decreased it. At 5 mM cimetidine, <40% of the current elicited by PAH alone was found. Thus PAH- and cimetidine-induced currents are additive only at low, and nonadditive at higher, cimetidine concentrations. The nonadditive behavior indicates that PAH and cimetidine utilize the same transporter rather than two separate, electrogenic transporters in oocytes.
Cimetidine-induced currents at pH 6.6 and 8.6.
To examine the influence of uncharged cimetidine (CIM0) on fROAT, cimetidine-induced currents at pH 6.6 and 8.6 were compared. At pH 6.6 (Fig. 5 A, ●), 0.1 mM total cimetidine did not induce a detectable current over the tested voltage range, indicating that 0.033 mM CIM0 was insufficient to induce measurable currents. At 1 mM, total cimetidine (0.333 mM CIM0) evoked inward currents at membrane potentials more negative than −20 mV and outward currents at more depolarized potentials. At pH 8.6 (Fig. 5 B), comparable currents were induced by 0.1 (0.098 mM CIM0) as well as by 1 mM total cimetidine (0.98 mM CIM0).
[3H]PAH and [3H]cimetidine uptake by human OAT1-expressing oocytes.
To explore whether cimetidine is also a substrate of human OAT1, oocytes were injected with cRNA coding for human OAT1 and assayed 4 days later. In two-electrode voltage-clamp studies, neither PAH nor cimetidine induced an inward current. However, oocytes expressing human OAT1 exhibited [3H]PAH and [3H]cimetidine uptake. Within 60 min, the [3H]PAH- and the [3H]cimetidine-associated uptakes were 43.6 ± 4.3 and 20.1 ± 4.3 pmol/oocyte, respectively, whereas water-injected oocytes showed uptakes of 9.7 ± 1.1 and 6.8 ± 0.3 pmol/oocyte (Fig. 6; representative experiment of a total of 4, each with an average of 6–15 oocytes/experimental condition). Thus human OAT1-mediated uptake rates were 33.9 and 13.3 pmol · 60 min−1 · oocyte−1 for [3H]PAH and [3H]cimetidine, respectively. The ratio of human OAT1-mediated PAH over cimetidine uptake was 2.5 in these experiments.
In the rat kidney in situ, cimetidine uptake from the peritubular capillaries into proximal tubule cells involved the organic cation and an organic anion transport system. Uptake of the organic cationN 1-methylnicotinamide was inhibited by cimetidine with a K i of 0.16 mM and uptake of the organic anion PAH with a K i of 1.7 mM (28). Thus the affinity of the organic cation transporter for cimetidine was 10 times higher than that of the organic anion transporter. The uptake of radiolabeled cimetidine from the capillaries was inhibited by tetraethylammonium and probenecid, proving translocation by organic cation and anion transporters. As expected from uptake through parallel systems, the inhibitions by tetraethylammonium and probenecid were additive. Interestingly, probenecid inhibited a larger portion of cimetidine uptake in situ than did tetraethylammonium (28).
Also in the intact kidney, it was noted that organic cation and organic anion transport systems “do not see the degree of substrate ionization,” because interaction with cimetidine with both systems was rather unaffected, when capillary pH was changed between 6.0 and 8.0 (27). Moreover, the uptake of labeled cimetidine was the same at pH 6.0, 7.4, and 8.0, suggesting that the transporters possibly interacted with both positively charged and uncharged cimetidine. These and related findings with dissociable primary, secondary, and tertiary amines led Ullrich and colleagues (28) to postulate that substrates qualify for transport by the presence of a hydrophobic core and by their ability to form hydrogen bonds.
Meanwhile, it is known that several transporters for organic ions exist in the kidneys. Of the cloned organic cation transporters, OCT1 and OCT2 have been localized to the basolateral membrane of rat kidney proximal tubule cells (16). Translocation of labeled cimetidine was slow or absent for human (34) and rat (10) OCT1 but clearly demonstrable for expressed rat OCT2 (10). Thus OCT2 may contribute significantly to cimetidine secretion, particularly in humans, where renal expression of OCT1 is low (6). Human OCT2 expressed in Chinese hamster ovary K1 cells preferred the positively charged cimetidine over uncharged cimetidine: at higher pH, i.e., at a lesser abundance of charged cimetidine (CIM+ ), human OCT2 showed a decreased affinity for total (CIM0 plus CIM+) cimetidine (1).
Of the cloned organic anion transporters, OAT1 and OAT3 have been localized to the basolateral membrane of proximal tubule cells in rat (19) and human (4, 12) kidneys. Because both transport PAH, they may have contributed to the PAH transport as determined in the intact rat kidney. There is no doubt that rat and human OAT3 transport cimetidine (4, 7, 20). The interaction with the organic cation cimetidine has been taken as a characteristic of OAT3. Mutational analysis of rat OAT3 revealed that the conserved cationic amino acids lysine 370 and arginine 454 are involved in the binding of PAH but not of cimetidine (7). Hydrophobic amino acid residues in the seventh transmembrane domain (tryptophan 334, phenylalanine 335, tyrosine 341, and other nearby residues) were found to be important for both PAH and cimetidine transport by allowing the formation of hydrogen bonds and hydrophobic interactions (8). Because these hydrophobic amino acids are fully conserved in OAT1, this transporter could accept cimetidine as well.
X. laevis oocytes expressing fROAT translocated labeled PAH ∼12 times faster than did water-injected control oocytes. Transport of labeled cimetidine was also faster in fROAT-expressing oocytes, but the transport ratio of fROAT-expressing to water-injected oocytes was only four. In absolute terms, fROAT-mediated PAH uptake, i.e., uptake in expressing minus uptake in water-injected oocytes was ∼10 times higher than uptake of cimetidine, although PAH concentration (0.1 mM) was one-tenth of the cimetidine concentration (1 mM). PAH and cimetidine induced inward currents. The current evoked by 0.1 mM PAH was approximately five times higher than that induced by 1 mM cimetidine. If PAH and cimetidine were transported at equal rates, CIM0/α-ketoglutarate2− exchange would produce an inward current twice as large as PAH−/α-ketoglutarate2− exchange. However, because cimetidine uptake was only one-tenth of PAH uptake, the cimetidine-induced current should be one-fifth of the PAH-induced current. This was actually observed. Taken together, these data show that fROAT-expressing oocytes translocate both PAH and cimetidine, albeit at considerably different rates.
To exclude that cimetidine uptake occurred through an endogenous transporter upregulated by the expression of fROAT, we added PAH and cimetidine simultaneously to oocytes and measured the resulting inward current. If cimetidine was taken up by a separate endogenous, electrogenic transporter, PAH- and cimetidine-induced currents should be additive at all cimetidine concentrations. If, however, PAH and cimetidine use the same carrier, increasing cimetidine concentrations should replace more and more PAH from fROAT. Because the maximum current induced by saturating cimetidine concentrations is lower than that induced by PAH, replacement of PAH by cimetidine should reduce the total current. This was actually observed at cimetidine concentrations exceeding 0.5 mM. Only at 0.1 mM cimetidine was a small increase in total current observed, most likely due to translocation of cimetidine by transporter units not saturated with PAH. Taken together, our experiments provide direct evidence for transport of cimetidine by fROAT.
The next question was whether CIM+, CIM0, or both forms of cimetidine interact with fROAT. To discriminate between these possibilities, we changed bath pH from 7.6 to either 6.6 or 8.6. Given a pK a of 6.9, the relative abundance of CIM0 is 33.4, 83.4, and 98.0% at pH 6.6, 7.6, and 8.6, respectively. Conversely, CIM+ makes up 66.6, 16.6, and 2.0% of total cimetidine at these pH values. If fROAT would prefer CIM0 over CIM+, currents should increase at a constant cimetidine concentration when pH is raised. Unfortunately, fROAT itself is pH dependent, as seen with the anionic PAH as a substrate: PAH-induced currents strongly decreased with increasing pH (data not shown), a behavior found earlier for PAH and adefovir uptake by human OAT1 (11). Thus it is not possible to clearly discern the effects of pH on fROAT itself and on the availability of the suited substrate (CIM0 vs. CIM+). Taking a different approach, we added 0.1 or 1 mM cimetidine at pH 6.6 and 8.6 to fROAT-expressing oocytes and determined the current. At pH 6.6, only 1 mM cimetidine evoked a current, whereas at pH 8.6 both concentrations induced similar currents. This result is best explained by assuming that CIM0 is preferred over CIM+. At pH 6.6 and 0.1 mM total cimetidine, CIM0 concentration is 33 μM, which was obviously too low to elicit a measurable current. At 1 mM total cimetidine, 333 μM CIM0 amply sufficed to show inward currents. At pH 8.6, 98 μM CIM0 at 0.1 mM total cimetidine concentration produced measurable currents as did 980 μM CIM0 at 1 mM cimetidine. We emphasize that our data do not exclude that CIM+ may bind to, and be translocated by, fROAT with low affinity. However, the majority of cimetidine appears to be transported as uncharged CIM0. Thereby, fROAT does see the degree of ionization. However, in the intact rat kidney, inhibition of PAH uptake by cimetidine was pH independent (27). A possible explanation is that PAH uptake in situ occurred through OAT1 and OAT3 in parallel. If OAT1 and OAT3 show opposing preferences for CIM0 and CIM+, the overall pH dependence of cimetidine interaction with PAH transport should vanish.
The I-V relationships of PAH- and cimetidine-induced currents reverse at inside negative membrane potentials. Although a negative E rev has also been found in our earlier study (2), it remains to be clarified which ions contribute to PAH- and cimetidine-induced currents. Our first assumption was that the inward currents are due to the electrogenic exchange of one PAH− against one α-ketoglutarate2− molecule, causing the net efflux of one negative charge. However, assuming that the PAH concentration outside the oocyte is higher than inside, and α-ketoglutarate concentration outside lower than inside, E revshould be far on the positive side. Replacement of PAH− by CIM0 or CIM+ would theoretically shiftE rev to the left, i.e., to less positive values. Such a shift was indeed observed: E rev was around −10 mV for PAH-induced current and approximately −27 mV for cimetidine current (cf. Fig. 3). However, the problem remains thatE rev was negative under both conditions. It is possible that fROAT interacts, in addition to PAH (or cimetidine) and α-ketoglutarate, with chloride or hydroxyl ions. Chloride removal inhibited PAH transport and abolished PAH-induced currents (data not shown), suggesting that chloride ions may modify or even be translocated by fROAT. The large shift in E revobserved when bath pH was changed from 6.6 to 8.6 suggests that hydroxyl ions may be translocated by fROAT. However, more experiments are needed to elucidate the ionic basis for the substrate-induced currents generated in fROAT-expressing oocytes.
In the phylogenetic tree, fROAT is positioned between OAT1 and OAT3 and may, therefore, combine transport properties of OAT1, i.e., PAH/α-ketoglutarate exchange, and OAT3, i.e., interaction with both organic anions and cimetidine. Therefore, it was not clear a priori whether our findings can be extrapolated to mammals, particularly to human OAT1. We expressed human OAT1 in oocytes and found that it transports both radiolabeled PAH and cimetidine. Relative to PAH, cimetidine transport was even higher in human OAT1 than in fROAT. Thus translocation of cimetidine is no longer a specific feature of OAT3. We therefore propose that human OCT2, OAT1, and OAT3 contribute to cimetidine excretion. Most likely, OCT2 is responsible for the cationic form, whereas OAT1 takes care of the uncharged form of cimetidine, which actually predominates at pH 7.4. The preference of OAT3 is not yet known. Because probenecid inhibited a greater part of the cimetidine transport than did tetraethylammonium in the intact rat kidney (28), OAT1 and OAT3 may take the greater share. In summary, the efficient renal excretion of cimetidine is due to transport through both organic cation and anion transporters in the basolateral membrane of proximal tubule cells.
The authors thank G. Dallmeyer and I. Markmann for technical assistance and E. Thelen for the artwork.
This study was supported by Deutsche Forschungsgemeinschaft Grant Bu 998/2–1.
Address for reprint requests and other correspondence: B. C. Burckhardt, Zentrum Physiologie und Pathophysiologie, Abteilung Vegetative Physiologie und Pathophysiologie, Georg-August Universität Göttingen, Humboldtallee 23, 37073 Göttingen, Germany (E-mail:).
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First published November 12, 2002;10.1152/ajprenal.00290.2002
- Copyright © 2003 the American Physiological Society