Taurocholate uptake by isolated skate hepatocytes: effect of albumin

Taurocholate transport was analyzed in a well characterized, polarized hepatocyte preparation from the small skate (Raja erinacea), an analbuminemic species that does not synthesize bile acids. In addition, the effect of sodium omission, bovine albumin, ovalbumin, and bovine gamma-globulin on the uptake of taurocholate was studied. Uptake could be divided into nonsaturable (0.48 pmol X min-1 X mg protein-1. microM-1) and saturable components (Km, 32.5 microM and Vmax 110 pmol X min-1 X mg protein-1). No evidence for sodium dependence could be obtained. Glycocholate competitively inhibited taurocholate uptake. The initial rate of taurocholate uptake was also inhibited by sulfobromophthalein, N-(4-azido-2-nitrophenyl)-2-aminoethyl sulfonate, and 4,4'-diisothiocyanostylbene-2,2'-disulfonic acid but not lactate or alanine. Hepatic uptake of taurocholate in albumin solutions (2.5%) was twice as great as expected from estimates of the free taurocholate concentration. In contrast, equimolar concentrations of ovalbumin or bovine gamma-globulin had no effect on measured rates of taurocholate uptake and no evidence for a specific albumin receptor could be found on these hepatocytes by 125I-albumin binding. These studies indicate that a carrier-mediated, sodium-independent transport system for taurocholate uptake is present in skate hepatocytes that is not driven solely by the free concentration of taurocholate.