Functional map of TEA transport activity in isolated rabbit renal proximal tubules

The organic cation (OC) transporters OCT1 and OCT2 are suspected of mediating substrate entry from the blood into proximal tubule cells as the first step in renal secretion of OCs. We examined the contribution of each process in different rabbit renal proximal tubule (RPT) segments, taking advantage of the fact that rabbit orthologs of OCT1 and OCT2 can be distinguished by the high affinity of the former for tyramine (TYR) and of the latter for cimetidine (CIM). We verified that TEA uptake, for which both transporters share a similar affinity, is relatively constant in all three segments (apparent inhibitory constant of 33, 74, and 30 μM and maximal rate of mediated TEA uptake of 0.8, 1.0, and 1.2 pmol·mm^-1 in S1, S2, and S3. respectively). In the S1 segment, TYR was a more effective inhibitor of TEA uptake than CIM (IC₅₀ values of 39 and 328 μM, respectively), implicating OCT1 as the predominant pathway for TEA transport. The opposite profiles were noted in the S2 segment (IC₅₀ values of 302 and 20 μM for TYR and CIM, respectively) and S3 segment (IC₅₀ values of 2,900 and 54 μM TYR and CIM, respectively), suggesting that OCT2 is the predominant TEA transporter in the later portion of RPT, TEA sufficient to saturate OCT1 and OCT2 blocked only 37% of mediated amantadine transport in the S2 segment, confirming the functional presence of at least one additional OC transporter (perhaps OCT3). These data indicate that renal OC transport involves the concerted activity of a suite of transport processes.