Moseley R H, Smit H, Van Solkema B G, Wang W, Meijer D K
Department of Internal Medicine, Veterans Affairs Medical Center, Ann Arbor, Michigan, USA.
J Pharmacol Exp Ther. 1996 Feb;276(2):561-7.
Hepatic organic cation transport in vitro, using tetraethylammonium (TEA) as a substrate, consists of at least two steps: sinusoidal uptake is stimulated by an inside-negative membrane potential and canalicular membrane transport is mediated by organic cation:H+ exchange (Moseley et al., 1992b). In vivo, however, TEA is poorly excreted into bile. In contrast, larger, more hydrophobic organic cations, such as tributylmethylammonium (TBuMA), undergo significant hepatobiliary excretion. To better characterize hepatic organic cation transport, TBuMA transport was examined in rat canalicular liver plasma membrane (cLPM) and basolateral liver plasma membrane (bILPM) vesicles. In cLPM vesicles, under voltage-clamped conditions, an outwardly directed H+ gradient stimulated [3H]TBuMA uptake consistent with electroneutral TBuMA:H+ exchange; H(+)-dependent [3H]TBuMA uptake was not the result of a H+ diffusion potential. In the absence of a H+ gradient, intravesicular TBuMA trans-stimulated [3H]TBuMA uptake. Substrates for renal and hepatic organic cation:H+ exchange cis-inhibited H(+)-dependent [3H]TBuMA uptake. No ATP-dependent [3H]TBuMA uptake was detected in cLPM vesicles, and the P-glycoprotein substrate, daunomycin, did not cis-inhibit H(+)-dependent [3H]TBuMA uptake. Carrier-mediated [3H]TBuMA uptake exhibited saturability (Km of 0.5 mM and Vmax of 0.5 nmol/mg prot/5 s). In bILPM vesicles, in contrast, a valinomycin-induced intravesicular-negative K+ diffusion potential stimulated [3H]TBuMA uptake. These findings suggest that hepatic transport of TBuMA is similar to TEA but fundamentally different from that of P-glycoprotein substrates, indicating the involvement of at least two separate processes for the hepatobiliary excretion of organic cationic drugs.
以四乙铵（TEA）为底物的肝有机阳离子体外转运至少包括两个步骤：窦状隙摄取受膜内负电位刺激，胆小管膜转运由有机阳离子：H⁺交换介导（莫斯利等人，1992b）。然而，在体内，TEA经胆汁排泄的能力较差。相比之下，更大、疏水性更强的有机阳离子，如三丁基甲基铵（TBuMA），则会发生显著的肝胆排泄。为了更好地表征肝有机阳离子转运，对大鼠胆小管肝质膜（cLPM）和基底外侧肝质膜（bILPM）囊泡中的TBuMA转运进行了研究。在cLPM囊泡中，在电压钳制条件下，外向的H⁺梯度刺激了[³H]TBuMA摄取，这与电中性的TBuMA：H⁺交换一致；H⁺依赖性的[³H]TBuMA摄取并非H⁺扩散电位的结果。在没有H⁺梯度的情况下，囊泡内的TBuMA反刺激了[³H]TBuMA摄取。肾和肝有机阳离子：H⁺交换的底物顺式抑制了H⁺依赖性的[³H]TBuMA摄取。在cLPM囊泡中未检测到ATP依赖性的[³H]TBuMA摄取，P-糖蛋白底物柔红霉素也未顺式抑制H⁺依赖性的[³H]TBuMA摄取。载体介导的[³H]TBuMA摄取表现出饱和性（Km为0.5 mM，Vmax为0.5 nmol/mg蛋白/5秒）。相比之下，在bILPM囊泡中，缬氨霉素诱导的囊泡内负K⁺扩散电位刺激了[³H]TBuMA摄取。这些发现表明，TBuMA的肝转运与TEA相似，但与P-糖蛋白底物的转运有根本区别，这表明有机阳离子药物的肝胆排泄至少涉及两个独立的过程。
