Role of inorganic electrolytes in bile acid-independent canalicular bile formation

Ion-replacement studies were carried out in the isolated perfused rat liver to obtain insight into the role played by inorganic electrolytes in bile acid-independent canalicular bile flow (BAICF). The BAICF decreased significantly when Na+ (146 mM) was replaced by 120 mM K+, Rb+, Cs+, or choline and when Cl- (127 mM) was replaced by 120 mM acetate or isethionate; there was no reduction in BAICF when Na+ was replaced by Li+ (146 mM) and Cl- by NO-3. K+, Rb+, and Cs+, however, also caused a simultaneous decline in the perfusion rate. The BAICF decreased by 50% when HCO-3 was replaced by equimolar tricine; under this condition replacement of Cl- by NO-3, but not Na+ by Li+, decreased BAICF by 45%. Thus the hepatic transport of Cl- cannot be explained by simple diffusion only, and a special mechanism, probably Na+-coupled Cl- transport, may contribute about 30% of the BAICF. With Li+ replacing Na+ in the medium, the intracellular concentration of Li+ in isolated rat hepatocytes was less than that calculated for electrochemical equilibrium and was increased by 2 mM KCN, indicating active extrusion of this ion. Li+ was unable to activate Mg2+-ATPase of isolated rat liver plasma membranes, and 1 mM ouabain did not affect the Li+ distribution. These results suggest the potential importance of ion pumps other than Na+-K+-ATPase in BAICF.