title: Taurocholate--sodium co-transport by brush-border membrane vesicles isolated from rat ileum.
abstract: Uptake of taurocholate into brush-border membrane vesicles isolated from rat small intestine by a Ca(2+) -precipitation method was investigated by using a rapid-filtration technique. Uptake of taurocholate by ileal brush-border membranes consisted of three phenomena: binding to the outside of the vesicles, transfer across the vesicle membrane and binding to the intravesicular compartment. The transport of taurocholate across the brush-border membranes was stimulated in the presence of Na(+) compared with the presence of K(+); stimulation was about 11-fold in the presence of a NaCl gradient (Na(o)>Na(i)), where the subscripts refer to ;outside' and ;inside' respectively, and 4-fold under equilibrium conditions for Na(+) (Na(o)=Na(i)). In the presence of a Na(+) gradient a typical ;overshoot' phenomenon was observed. Membranes preloaded with unlabelled taurocholate showed an accelerated entry of labelled taurocholate (tracer exchange) in the presence of Na(+) compared with the presence of K(+). The stimulation by Na(+) was observed only in membrane preparations from the ileum. Addition of monactin, an ionophore for univalent cations, decreased the Na(+)-gradient-driven taurocholate uptake. The Na(+)-dependent taurocholate transport showed saturation kinetics and the phenomenon of counterflow and was inhibited by glycocholate. Other cations such as Li(+), Rb(+) and Cs(+) could not replace Na(+) in its stimulatory action. When the electrical potential difference across the vesicle membrane was altered by establishing different diffusion potentials (anion replacement; K(+) gradient+/-valinomycin) a more-negative potential inside stimulated Na(+)-dependent taurocholate transport. These data demonstrate the presence of a rheogenic (potential sensitive) Na(+)-taurocholate co-transport system in ileal brush-border membranes and support the hypothesis that the reabsorption of bile acids in the ileum is a secondary active uptake.
mesh_terms: D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D001692:Biological Transport / Q000187:drug effects; D004058:Diffusion; D004386:Duodenum / Q000378:metabolism; D007082:Ileum / Q000378:metabolism*; D066298:In Vitro Techniques; D007583:Jejunum / Q000378:metabolism; D008297:Male; D008628:Mercury / Q000494:pharmacology; D008871:Microvilli / Q000378:metabolism; D011188:Potassium / Q000378:metabolism; D051381:Rats; D012964:Sodium / Q000378:metabolism*; D013656:Taurocholic Acid / Q000378:metabolism*
chemical_list: D000900:Anti-Bacterial Agents; D013656:Taurocholic Acid; D012964:Sodium; D008628:Mercury; D011188:Potassium