ATP-dependent organic anion transport system in normal and TR- rat liver canalicular membranes

Many non-bile acid organic anions enter hepatocytes, where they are converted into polar conjugates and preferentially secreted into bile. ATP-dependent and membrane potential-dependent transport of non-bile acid organic anions across the plasma membrane of rat hepatocytes was studied in canalicular (CMV) or sinusoidal membrane vesicles (SMV) using sulfobromophthalein (BSP), a representative non-bile acid organic anion. Temperature-dependent, osmotically sensitive, saturable, ATP-dependent, and unidirectional (inside to outside) BSP transport was observed in CMV but not in SMV. Apparent Michaelis constants for ATP and BSP were 0.26 mM and 31 microM, respectively. ATP-dependent BSP transport was inhibited by oxidized glutathione, dinitrophenyl-glutathione (GSDNP), BSP glutathione, and bilirubin diglucuronide but not by daunomycin, taurocholate, and reduced glutathione. Inhibition by GSDNP and bilirubin diglucuronide was competitive, with apparent inhibitor constants of 41 and 4.2 microM, respectively. CMV from normal rats demonstrated ATP-dependent and membrane potential-dependent BSP transport that was additive. CMV from TR- rats lacked ATP-dependent BSP transport but retained membrane potential-dependent transport of non-bile acid organic anions as well as ATP-dependent transport mechanisms for organic cations (i.e., daunomycin) and bile acids (i.e., taurocholate).