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1 Department of Physiology, Mahidol University, Bangkok, Bangkok, Thailand; Department of Physiology, University of Arizona, Tucson, Arizona, USA
2 Department of Physiology, Mahidol University, Bangkok, Bangkok, Thailand
3 Department of Physiology, University of Arizona, Tucson, Arizona, USA
* To whom correspondence should be addressed. E-mail: shwright{at}u.arizona.edu.
A strategy was developed to determine the distribution of activity mediated by the organic cation (OC) transporters OCT1 and OCT2 in rabbit renal proximal tubule (RPT). Both transporters displayed similar affinities for TEA (in CHO-K1 cells, Kts of 19.9 and 34.5 µM for OCT1 and OCT2, respectively). Similarly, some OCs showed little capacity to discriminate between the two processes (IC50s for ephedrine of 13.6 and 24.2 µM for OCT1 and OCT2, respectively). However, OCT2 had a higher affinity for cimetidine and NBD-TMA (1.3 and 1.4 µM) than did OCT1 (97.3 and 108 µM, respectively). Conversely, OCT1 had a higher affinity for tyramine and pindolol than did OCT2 (21.2 and 2.4 µM vs. 361 and 50 µM, respectively). We designated these as 'discriminatory inhibitors'and used them to determine the relative contribution of OCT1 and OCT2 for TEA transport in single S2 segments of rabbit RPT. Cimetidine and NBD-TMA were high affinity inhibitors of TEA transport in S2 segments (median IC50s of 12.3 µM and 1.4 µM, respectively); by comparison, tyramine and pindolol were low affinity inhibitors (265 µM and 69.3 µM, respectively). These IC50 values were sufficiently close to those for OCT2 to support the conclusion that TEA transport in the S2 segment of rabbit RPT is dominated by OCT2. However, the profile of inhibition of tyramine (an OCT1-selective substrate) transport in single S2 segments indicated that, despite a comparatively low level of expression, OCT1 can play a dominant role in uptake of selected OC substrates.
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