THE NEUROTOXIN 1-METHYL-4-PHENYLPYRIDINIUM IS A SUBSTRATE FOR THE CANALICULAR ORGANIC CATION H+ EXCHANGER

Hepatic organic cation transport consists, in part, of carrier-mediate d sinusoidal uptake stimulated by an inside-negative membrane potentia l and canalicular excretion driven by electroneutral organic cation/H exchange. Intracellular organic cation transport involves sequestrati on into acidified organelles, also mediated by organic cation/H+ excha nge. A sinusoidal organic cation transporter has been cloned; however, canalicular organic cation transport has not been characterized at th e molecular level, On the assumption that hepatic organic cation/H+ ex change 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 syna ptic vesicular monoamine transporter that has been cloned. Under volta ge-clamped conditions, an outwardly directed H+ gradient stimulated [H -3]MPP+ uptake, compared with uptake under pH-equilibrated conditions, consistent with electroneutral MPP+/H+ exchange. Substrates for canal icular organic cation/H+ exchange cis-inhibited pH-dependent MPP+ upta ke. Equilibrium exchange of [C-14]tetraethylammonium was inhibited by MPP+ in a concentration-dependent manner, consistent with a direct int eraction of MPP+ with the organic cation carrier. Carrier-mediated MPP + uptake exhibited saturability, with kinetic parameters similar to th ose described for canalicular tetraethylammonium(+)/H+ exchange. Canal icular [H-3]MPP+ uptake was ATP-independent and, thus, distinct from P -glycoprotein-mediated efflux. The finding that MPP+ is a substrate fo r canalicular organic cation/H+ exchange is applicable to studies, usi ng degenerate oligonucleotides complementary to sequences conserved in neurotransmitter transporters, aimed at cloning this transporter.