Kaewmokul Santi, Chatsudthipong Varanuj, Evans Kristen K, Dantzler William H, Wright Stephen H
Department of Physiology, Mahidol University, Bangkok, 10700 Thailand.
Am J Physiol Renal Physiol. 2003 Dec;285(6):F1149-59. doi: 10.1152/ajprenal.00112.2003. Epub 2003 Aug 26.
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 tetraethylammonium (TEA; in CHO-K1 cells, TEA concentrations that resulted in half-maximal transport were 19.9 and 34.5 microM for OCT1 and OCT2, respectively). Similarly, some OCs showed little capacity to discriminate between the two processes (IC50 values for ephedrine of 13.6 and 24.2 microM for OCT1 and OCT2, respectively). However, OCT2 had a higher affinity for cimetidine and [2-(4-nitro-2,1,3-benzoxadiazol-7-yl) aminoethyl]trimethylammonium (NBD-TMA; 1.3 and 1.4 microM, respectively) than did OCT1 (97.3 and 108 microM, respectively). Conversely, OCT1 had a higher affinity for tyramine and pindolol than did OCT2 (21.2 and 2.4 vs. 361 and 50 microM, 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 IC50 values of 12.3 and 1.4 microM, respectively); in comparison, tyramine and pindolol were low-affinity inhibitors (265 and 69.3 microM, 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 the uptake of selected OC substrates.
已制定一项策略，以确定有机阳离子（OC）转运体OCT1和OCT2介导的活性在兔肾近端小管（RPT）中的分布。两种转运体对四乙铵（TEA）表现出相似的亲和力（在CHO-K1细胞中，导致转运达到半最大速率的TEA浓度，OCT1为19.9 microM，OCT2为34.5 microM）。同样，一些OCs对这两个过程的区分能力较弱（麻黄碱对OCT1和OCT2的IC50值分别为13.6和24.2 microM）。然而，OCT2对西咪替丁和[2-(4-硝基-2,1,3-苯并恶二唑-7-基)氨基乙基]三甲基铵（NBD-TMA）的亲和力高于OCT1（分别为1.3和1.4 microM，而OCT1分别为97.3和108 microM）。相反，OCT1对酪胺和吲哚洛尔的亲和力高于OCT2（分别为21.2和2.4 microM，而OCT2分别为361和50 microM）。我们将这些称为“区分性抑制剂”，并用它们来确定OCT1和OCT2对兔RPT单个S2节段中TEA转运的相对贡献。西咪替丁和NBD-TMA是S2节段中TEA转运的高亲和力抑制剂（中位IC50值分别为12.3和1.4 microM）；相比之下，酪胺和吲哚洛尔是低亲和力抑制剂（分别为265和69.3 microM）。这些IC50值与OCT2的IC50值足够接近，以支持兔RPT的S2节段中TEA转运由OCT2主导的结论。然而，单个S2节段中酪胺（一种OCT1选择性底物）转运的抑制曲线表明，尽管表达水平相对较低，但OCT1在某些OC底物的摄取中可发挥主导作用。
