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

The mechanisms and driving forces for the uptake of the unconjugated b ile acid cholate were investigated both in cultured rat hepatocytes an d in rat liver basolateral (sinusoidal) plasma membrane (BLPM) vesicle s. Determination of initial uptake rates of [H-3]cholate (0.1 mu M) in to cultured hepatocytes confirmed that the majority (75%) of the trans membrane transport was mediated by Na+-independent mechanisms. This po rtion of cholate uptake consisted of a pH-sensitive moiety representin g nonionic diffusion, which may become quantitatively important at low pH and high cholate concentrations, as well as of a saturable (Michae lis constant 7.4 mu M), 4,4'-diisothiocyanostilbene-2,2'-disulfonic ac id (DIDS)-sensitive transport moiety, suggesting the involvement of a carrier. This latter transport system was functionally characterized b y 1) inhibition of cellular cholate uptake in the absence of extracell ular sodium by the dicarboxylic acid alpha-ketoglutarate (alpha-KG; 1 mM) and by the organic anion p-aminohippurate (PAH; 1 mM); 2) stimulat ion 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 sensi tivity toward lithium in BLPM vesicles; 4) trans-stimulation of vesicu lar cholate uptake by alpha-KG or PAH, but not by benzoate; and 5) cis -inhibition of alpha-KG/alpha-KG self-exchange by extravesicular chola te (400 mu M), PAH (5 mM), probenecid, or DIDS. Collectively, these da ta indicate the presence of a Na+-dicarboxylate cotransport-coupled or ganic anion exchanger in the hepatocyte basolateral plasma membrane th at 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 i n the hepatocytes bear striking similarity to an anion exchange mechan ism in the renal proximal tubules.