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.