Bilirubin diglucuronide transport by rat liver canalicular membrane vesicles: Stimulation by bicarbonate ion



The purpose of this study was to provide further insight into the mechanism of bilirubin diglucuronide excretion through the hepatocyte canalicular membrane by investigating the uptake of (3H)bilirubin diglucuronide by purified canalicular membrane vesicles of rat liver. The uptake was analyzed by a rapid filtration technique. The difference between vesicle-associated (3H)bilirubin diglucuronide at 37° C and at 0° C during the initial 1 min was regarded as uptake. Twenty second uptake was saturated by increasing the (3H)bilirubin diglucuronide concentration at a vesicle-inside-directed 100 mmol/L KCl gradient (Km = 75 μmol/L, Vmax = 320 pmol/mg protein 20 sec at 37° C). No sodium dependency was observed. When canalicular membrane vesicles were preincubated with nonlabeled bilirubin diglucuronide, the uptake increased 1.3-fold (transstimulation). Vesicle-inside-positive potential induced by valinomycin and potassium caused a 1.4-fold increase in the uptake. When Cl was replaced by equivalent ion concentrations of SOmath image, HCOmath image, NOmath image and SCN, the uptake was 78%, 244%, 68% and 50%, respectively, and specific stimulation by HCOmath image was observed (Km = 75 μmol/L, Vmax = 700 pmol/mg protein 20 sec at a vesicle-inside-directed 100 mmol/L KHCO3 gradient at 37° C). The uptake was inhibited in a dose-dependent manner by the addition of 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid. The uptake was ATP independent. From these results, it was concluded that bilirubin diglucuronide transport through the canalicular membrane is carrier mediated, electrogenic and stimulated by HCOmath image. (HEPATOLOGY 1991;14:1251–1258.)