ATP-dependent taurocholate transport by rat liver canalicular membrane vesicles

We conducted an experimental study to examine the possibility that ATP is involved in the mechanism by which bile acids are excreted through the liver canalicular membrane in opposing the concentration gradient. Canalicular membrane vesicles were purified from the livers of Sprague-Dawley rats, and the uptake of tritiated sodium taurocholate into canalicular membrane vesicles was determined by rapid filtration technique. Vesicle-associated sodium taurocholate was increased in the presence of ATP and ATP-regenerating system. This was also observed at a voltage-clamped condition. ATP-dependent uptake into the osmotically reactive intravesicular space was saturated with increasing concentrations of sodium taurocholate (Km = 47 mumol/L, Vmax = 270 pmoles/mg protein.20s). ATP-dependent uptake increased to the point of saturation when the sodium taurocholate concentration was 50 mumol/L and the ATP concentration was increased from 0 to 1 mmol/L (Km = 64 mumol/L). Among the several nucleotides used, ATP was a potent stimulator of transport, whereas a nonhydrolyzable analogue (i.e., adenosine 5'-[beta,gamma-imino]triphosphate) showed no effect. In addition, ATP-dependent transport was inhibited by vanadate in a dose-dependent manner. From these results it was concluded that the primary active transport of sodium taurocholate is present in hepatocellular canalicular membranes. This transport is directly dependent on ATP, and hydrolysis of gamma-phosphate of ATP is required.