Redistribution of canalicular organic anion transport activity in isolated and cultured rat hepatocytes

Abstract

The hepatocanalicular transport of a large number of organic anions, such as bilirubin glucuronides and glutathione conjugates in the rat, is mediated by an adenosine triphosphate (ATP)-dependent transport system, which is termed canalicular multispecific organic anion transporter (cMOAT). This system is mainly defined by its deficiency in mutant TR− rats. We have previously reported that in cultured hepatocytes the fluorescent organic anion glutathione-bimane (GS-B) accumulates in intracellular vesicles and that this transport is mediated by cMOAT. We now show that this intracellular accumulation of fluorescent organic anion is largely absent in freshly isolated hepatocytes but appears when cells are incubated in suspension at 37°C or cultured for periods of 1 to 24 hours. The appearance of intracellular cMOAT activity coincides with the disappearance of 70% of cMOAT activity from the plasma membrane as measured by the transport activity of the cells for the organic anion dinitrophenyl-glutathione (GS-DNP). Both the appearance of intracellular cMOAT and the disappearance of transport activity from the plasma membrane were completely inhibited at temperatures below 20°C. Residual cMOAT activity in 24-hour cultured hepatocytes could be further diminished by incubation of the cells with 1 μmol/L monensin or 10 mmol/L methylamine. We conclude that after disruption of the cell polarity by collagenase isolation of the hepatocytes, remnants of apical membrane containing cMOAT are rapidly endocytosed when the cells are kept at 37°C. Evidence suggests that at least part of the transporters may recycle back to the plasma membrane after endocytosis. These observations may be relevant for the understanding of regulation of canalicular transport.