Different activity of ATP dependent transport across the canalicular membrane for tributylmethylammonium and triethylmethylammonium as a potential mechanism of the preferential biliary excretion for tributylmethylammonium in the rat.

Abstract

The mechanism(s) responsible for the significantly higher biliary excretion of tributyl methyl ammonium (TBuMA) than of triethyl methyl ammonium (TEMA) was investigated in canalicular liver plasma membrane vesicles (cLPM). The uptake of [3H]TBuMA and [3H]TEMA into cLPM in the presence of a pH gradient or ATP was measured by a rapid filtration technique. The uptake of substrates into the vesicle was significantly increased by an outwardly directed pH gradient. The pH dependent uptake was saturable and cross-inhibited by the other organic cation, indicating that TEMA and TBuMA share a common transport mechanism. Kinetic analysis revealed the two compounds show similar characteristics for the pH-gradient dependent uptake. Thus, the organic cation/H+ exchange mechanism does not appear to explain the significant difference in biliary excretion of the organic cations. In the presence of ATP, however, uptake into cLPM was readily observed for TBuMA while TEMA uptake was negligible. Inhibition studies with typical P-glycoprotein substrates indicated the uptake may be mediated by the P-glycoprotein. Differences between TBuMA and TEMA in reactivity for an ATP dependent transport process, rather than for an organic cation/H+ exchanger, may be responsible for the markedly different biliary excretion of TBuMA and TEMA.