Moseley R H, Zugger L J, Van Dyke R W
Department of Internal Medicine, Veterans Affairs Medical Center and University of Michigan School of Medicine, Ann Arbor 48105, USA.
J Pharmacol Exp Ther. 1997 Apr;281(1):34-40.
Hepatic organic cation transport consists, in part, of carrier-mediated sinusoidal uptake stimulated by an inside-negative membrane potential and canalicular excretion driven by electroneutral organic cation/H+ exchange. Intracellular organic cation transport involves sequestration into acidified organelles, also mediated by organic cation/H+ exchange. A sinusoidal organic cation transporter has been cloned; however, canalicular organic cation transport has not been characterized at the molecular level. On the assumption that hepatic organic cation/H+ exchange resembles monoamine transport in synaptic vesicles, we examined, using canalicular rat liver plasma membrane vesicles, the transport of 1-methyl-4-phenylpyridinium (MPP+), a neurotoxin taken up by a synaptic vesicular monoamine transporter that has been cloned. Under voltage-clamped conditions, an outwardly directed H+ gradient stimulated [3H]MPP+ uptake, compared with uptake under pH-equilibrated conditions, consistent with electroneutral MPP+/H+ exchange. Substrates for canalicular organic cation/H+ exchange cis-inhibited pH-dependent MPP+ uptake. Equilibrium exchange of [14C]tetraethylammonium was inhibited by MPP+ in a concentration-dependent manner, consistent with a direct interaction of MPP+ with the organic cation carrier. Carrier-mediated MPP+ uptake exhibited saturability, with kinetic parameters similarto those described for canalicular tetraethylammonium+/H+ exchange. Canalicular [3H]MPP+ uptake was ATP-independent and, thus, distinct from P-glycoprotein-mediated efflux. The finding that MPP+ is a substrate for canalicular organic cation/H+ exchange is applicable to studies, using degenerate oligonucleotides complementary to sequences conserved in neurotransmitter transporters, aimed at cloning this transporter.
肝脏有机阳离子转运部分由载体介导的、受膜内负电位刺激的窦状隙摄取以及由电中性有机阳离子/氢离子交换驱动的胆小管排泄组成。细胞内有机阳离子转运涉及被酸化细胞器隔离，这同样由有机阳离子/氢离子交换介导。一种窦状隙有机阳离子转运体已被克隆；然而，胆小管有机阳离子转运在分子水平上尚未得到表征。基于肝脏有机阳离子/氢离子交换类似于突触小泡中单胺转运的假设，我们使用大鼠肝脏胆小管质膜囊泡研究了1-甲基-4-苯基吡啶鎓（MPP⁺）的转运，MPP⁺是一种被已克隆的突触小泡单胺转运体摄取的神经毒素。在电压钳制条件下，与pH平衡条件下的摄取相比，外向的氢离子梯度刺激了[³H]MPP⁺的摄取，这与电中性的MPP⁺/氢离子交换一致。胆小管有机阳离子/氢离子交换的底物顺式抑制了pH依赖性的MPP⁺摄取。[¹⁴C]四乙铵的平衡交换受到MPP⁺的浓度依赖性抑制，这与MPP⁺与有机阳离子载体的直接相互作用一致。载体介导的MPP⁺摄取表现出饱和性，其动力学参数与描述的胆小管四乙铵⁺/氢离子交换的参数相似。胆小管[³H]MPP⁺摄取不依赖于ATP，因此与P-糖蛋白介导的外排不同。MPP⁺是胆小管有机阳离子/氢离子交换底物这一发现适用于使用与神经递质转运体中保守序列互补的简并寡核苷酸进行的研究，旨在克隆这种转运体。
