ORGANIC CATION-TRANSPORT BY RAT HEPATOCYTE BASOLATERAL MEMBRANE-VESICLES

Hepatocyte basolateral membrane possesses transport systems for mediated uptake of organic cations, the first step in the subsequent biliary excretion and/or metabolism of these compounds. The purpose of these studies was to evaluate potential mechanisms for transport of this class of solutes across this membrane by measuring H-3-labeled tetraethylammonium ([H-3]TEA) transport into rat hepatocyte basolateral membrane vesicles. [H-3]TEA uptake was stimulated by an outwardly directed proton gradient consistent with TEA-proton exchange. Proton gradient-stimulated [H-3]TEA uptake was inhibited by quinidine and by the combination of valinomycin and carbonyl cyanide m-chlorophenylhydrazone (CCCP) but not by CCCP alone or by N1-methylnicotinamide (NMN). An outwardly directed TEA gradient also stimulated uptake of [H-3]TEA with values at early time points exceeding those at equilibrium. This trans-stimulation or countertransport was saturable with an apparent Michaelis constant of 106 muM and maximal velocity of 434 pmol.mg-1.15 s-1. TEA countertransport was cis-inhibited by quinidine, cimetidine, and thiamine and by low temperature, but not by NMN. Thiamine was also capable of trans-stimulating [H-3]TEA uptake. An outwardly directed potassium gradient enhanced and an inwardly directed potassium gradient reduced TEA countertransport but had no effect on [H-3]TEA uptake occurring in the absence of other electrochemical driving forces. These studies indicate that there are at least two potential mechanisms in the hepatocyte basolateral membrane for transport of organic cations; organic cation-organic cation exchange (countertransport) and organic cation-proton exchange. Furthermore, the results are consistent with the existence of more than one transporter with different substrate affinities in each of these categories. Finally, these results also suggest potential mechanisms for drug-to-drug interactions occurring at the level of transport across the hepatocyte basolateral membrane.