IDENTIFICATION AND CHARACTERIZATION OF A BASOLATERAL DICARBOXYLATE CHOLATE ANTIPORT SYSTEM IN RAT HEPATOCYTES

The mechanisms and driving forces for the uptake of the unconjugated bile acid cholate were investigated both in cultured rat hepatocytes and in rat liver basolateral (sinusoidal) plasma membrane (BLPM) vesicles. Determination of initial uptake rates of [H-3]cholate (0.1 mu M) into cultured hepatocytes confirmed that the majority (75%) of the transmembrane transport was mediated by Na+-independent mechanisms. This portion of cholate uptake consisted of a pH-sensitive moiety representing nonionic diffusion, which may become quantitatively important at low pH and high cholate concentrations, as well as of a saturable (Michaelis constant 7.4 mu M), 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS)-sensitive transport moiety, suggesting the involvement of a carrier. This latter transport system was functionally characterized by 1) inhibition of cellular cholate uptake in the absence of extracellular sodium by the dicarboxylic acid alpha-ketoglutarate (alpha-KG; 1 mM) and by the organic anion p-aminohippurate (PAH; 1 mM); 2) stimulation of cellular cholate uptake by alpha-KG (10 mu M) or PAH (1 mM) in the presence of an inwardly directed sodium gradient; 3) lack of sensitivity toward lithium in BLPM vesicles; 4) trans-stimulation of vesicular cholate uptake by alpha-KG or PAH, but not by benzoate; and 5) cis-inhibition of alpha-KG/alpha-KG self-exchange by extravesicular cholate (400 mu M), PAH (5 mM), probenecid, or DIDS. Collectively, these data indicate the presence of a Na+-dicarboxylate cotransport-coupled organic anion exchanger in the hepatocyte basolateral plasma membrane that may be involved in cholate uptake in the liver. In conjunction with the previously reported Na+-dependent dicarboxylate transport, these findings support the concept that organic anion transport mechanisms in the hepatocytes bear striking similarity to an anion exchange mechanism in the renal proximal tubules.