Cholate uptake in basolateral rat liver plasma membrane vesicles and in liposomes

The mechanism(s) and driving force(s) for hepatocellular uptake of the unconjugated bile acid cholate were investigated in isolated basolateral (sinusoidal) rat liver plasma membrane (blLPM) vesicles and in protein free liposomes. In blLPM vesicles both an inwardly directed Na+ gradient and a transmembrane pH difference (8.0 in/6.0 out) stimulated cholate uptake 2-3-fold above equilibrium uptake values (overshoot). While Na+ gradient driven cholate uptake could be inhibited by the anion transport inhibitor 4,4'-diisothiocyanato-2,2'-disulfonic acid stilbene (DIDS), the pH gradient dependent portion of cholate uptake was insensitive to DIDS, but could be inhibited by furosemide. Furthermore, initial rates (1-s values) of the pH gradient stimulated cholate uptake were linear with increasing substrate concentrations (no saturability). In liposomes a similar inside alkaline pH gradient also induced a transient DIDS insensitive/furosemide inhibitable intravesicular accumulation (approx. 2-fold) of cholate (overshoot). These findings confirm that hepatocellular uptake of cholate occurs in part via the common Na+/bile acid cotransport system. In addition, the data strongly indicate that in isolated membrane vesicles pH gradient driven cholate uptake represents nonionic diffusion rather than a carrier mediated process (Blitzer, B.L., Terzakis, C. and Scott, K.A. (1986) J. Biol. Chem. 261, 12042-12046). Since in the perfused liver DIDS inhibited uptake of both cholate and taurocholate to a similar extent, DIDS-insensitive pH gradient dependent membrane diffusion appears to be of minor significance for cholate uptake in the intact organ.