title: The neurotoxin 1-methyl-4-phenylpyridinium is a substrate for the canalicular organic cation/H+ exchanger.
abstract: 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.
authors.affiliation: Department of Internal Medicine, Veterans Affairs Medical Center and University of Michigan School of Medicine, Ann Arbor 48105, USA.
authors.forename: R H
authors.initials: RH
authors.lastname: Moseley
authors.forename: L J
authors.initials: LJ
authors.lastname: Zugger
authors.forename: R W
authors.initials: RW
authors.lastname: Van Dyke
mesh_terms: D015655:1-Methyl-4-phenylpyridinium / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D017920:Antiporters / Q000378:metabolism*; D001648:Bile Canaliculi / Q000378:metabolism*; D001692:Biological Transport; D006863:Hydrogen-Ion Concentration; D008297:Male; D008562:Membrane Glycoproteins / Q000378:metabolism; D026901:Membrane Transport Proteins; D009479:Neuropeptides; D051381:Rats; D013757:Tetraethylammonium Compounds / Q000494:pharmacology; D050492:Vesicular Biogenic Amine Transport Proteins; D050493:Vesicular Monoamine Transport Proteins
chemical_list: D017920:Antiporters; D008562:Membrane Glycoproteins; D026901:Membrane Transport Proteins; D009479:Neuropeptides; D013757:Tetraethylammonium Compounds; D050492:Vesicular Biogenic Amine Transport Proteins; D050493:Vesicular Monoamine Transport Proteins; D000255:Adenosine Triphosphate; D015655:1-Methyl-4-phenylpyridinium