Stimulation of taurocholate and glycocholate efflux from the rat hepatocyte by arginine vasopressin

Vasopressin induces alterations in the transmembrane distribution of the bile salts taurocholate and glycocholate but not of cholate, chenodeoxycholate, or chenodeoxycholate derivatives in isolated rat hepatocytes in suspension. Studies were conducted to define the specific transport events modulated by vasopressin. Unidirectional uptake of cholate, taurocholate, and glycocholate, monitored within a 15-s time frame, is not altered by vasopressin. Km values for cholate, taurocholate, and glycocholate influx were found to be 57, 12, and 26 microM, respectively. Vmax values for influx of cholate, taurocholate, and glycocholate were 1.7, 1.8, and 2.4 nmol.mg protein-1.min-1, respectively. At half-maximal effective concentrations, arginine vasopressin increases unidirectional efflux of taurocholate by 34% (EC50 = 1.5 X 10(-9) M) and glycocholate by 17% (EC50 = 5 X 10(-9) M). In the presence of 0.05 microM arginine vasopressin, the apparent Km value for taurocholate efflux decreases from 1.57 mM (control) to 0.94 mM; for glycocholate, the apparent Km decreases from 5.19 mM (control) to 3.22 mM. Vasopressin does not significantly change the Vmax values for taurocholate and glycocholate efflux (0.40 and 1.05 nmol.mg protein-1.min-1, respectively). These studies suggest that modulation of bile salt efflux by vasopressin may be utilized to probe bile salt transport pathways in the rat hepatocyte.