The transport of bile acid was studied in basolateral membrane vesicles isolated from rat small intestine. Taurocholate transport into an osmotically reactive intravesicular space was Na+ independent. The uptake of taurocholate in jejunal and ileal vesicles preloaded with sulfate was stimulated with respect to uptake in unpreloaded vesicles. Glycocholate inhibited the transstimulation of taurocholate uptake by sulfate. Sulfate and taurocholate uptake in ileal vesicles preloaded with bicarbonate was stimulated with respect to uptake in unpreloaded vesicles. Taurocholate inhibited the transstimulation of sulfate uptake by bicarbonate.When ileal vesicles were loaded withp-aminohippurate, an early transstimulation of taurocholate was found that exceeded equilibrium uptake, was insensitive to a K+ diffusion potential, and was cis-inhibited by taurocholate, glycocholate, pyruvate, paminohippurate, probenecid, chloride, sulfate, and bicarbonate. These data indicate the presence of an anion exchanger in intestinal basolateral membrane vesicles that may be involved in the exit of bile acids from the enterocyte.

Bile acid-binding polypeptides were examined using basolateral membrane vesicles and enterocytes isolated from rat ileum. The uptake of a photolabile taurocholate derivative, (7,7,-azo-3 alpha,12 alpha-dihydroxy-5 beta[3 beta-3H]cholan-24-oyl)-2- aminoethanesulfonate,7,7-azo-TC, in ileal vesicles preloaded with paraaminohippurate (PAH) was stimulated with respect to uptake in unpreloaded vesicles. The PAH-transstimulated uptake of 7,7-azo-TC was inhibited by taurocholate and vice versa. Irradiation of membrane vesicles in the presence of 7,7-azo-TC irreversibly inhibited PAH-transstimulated taurocholate uptake. Photoaffinity labeling of basolateral membrane vesicles directly with [3H] 7,7-azo-TC and separation of proteins by SDS-PAGE revealed incorporation of radioactivity into several polypeptides. Photoaffinity labeling of vesicles in the presence of taurocholate inhibited the labeling of 54,000 and 59,000 mol. wt. polypeptides. The efflux of taurocholate from ileal enterocytes was cis-inhibited by 7,7-azo-TC and transstimulated by PAH. Irradiation of enterocytes in the presence of 7,7-azo-TC inhibited taurocholate efflux greater than the presence of 7,7-azo-TC in the dark. When enterocytes that were irradiated in the presence of [3H] 7,7-azo-TC were fractionated and the resultant basolateral membrane fraction was subjected to SDS-PAGE, incorporation of radioactivity into the 54,000 and 59,000 mol. wt. polypeptides was seen. In contrast, when the brush-border membrane fraction was subjected to SDS-PAGE, greatest incorporation of radioactivity was seen in the previously described 99,000 mol. wt. polypeptide. These studies suggest that 7,7-azo-TC shared transporters with natural bile acid and identified polypeptides that may be involved in bile acid transport across the basolateral membrane and differ from that seen in the brush-border membrane of the ileal epithelial cell.