Transport of sodium, chloride, and taurocholate by cultured rat hepatocytes

Abstract

Transport of Na, Cl and taurocholate was studied in primary cultures of adult rat hepatocytes incubated in a balanced electrolyte solution containing 150 mM NaCl, various concentrations of taurocholate and 22Na, 36Cl, [3H]taurocholate and 3-O-[3H]methyl-D-glucose. LiCl, choline chloride or Na2SO4 and mannitol were substituted isotonically for NaCl in selected studies. The steady-state intracellular concentrations of exchangeable Na and Cl averaged 6.5 and 30.1 mM, respectively. Ouabain reversibly increased intracellular Na concentration. Cl entry rate was .apprx. double that of Na. Unlike Na entry, Cl entry rate increased nonlinearly with increasing extracellular concentration. Taurocholate entry exhibited both saturable and nonsaturable components; the former accounted for virtually all taurocholate uptake at concentrations comparable to those found in vivo. Taurocholate was actively concentrated by the cultured cells, with the steady-state intracellular-to-extracellular concentration ratio decreasing from > 50 to .apprx. 1 as extracellular taurocholate concentration was increased from 10-4 mM. Both the saturable uptake component and concentrative taurocholate transport were virtually abolished by substitution of choline or Li for Na or by addition of ouabain. Taurocholate entry rate first increased in a sigmoid fashion and then decreased as extracellular Na concentration was increased from 0-150 mM. Na entry rate increased in the presence of added taurocholate with an average of 1 Na ion accompanying each taurocholate molecule into the cell. Na and Cl differ strikingly in their mechanism and rate of entry into cultured rat hepatocytes and in their intracellular concentration. Hepatocytes concentrate taurocholate by a Na-coupled mechanism with an apparently equimolar transport stoichiometry.