Measurement of hepatobiliary transport

A major hepatic function involves the removal of xenobiotics from portal blood and their subsequent storage, metabolism, and excretion into bile, or delivery of various products back into the bloodstream. Transport across the liver cell basolateral (or sinusoidal) plasma membrane is a key step in the overall metabolism of many compounds, whereas transport across the apical (or canalicular) plasma membrane into bile is often the principal pathway for their elimination. Although significant progress has been made in the molecular identification of the transport proteins that mediate these processes, much remains to be learned about regulation and physiological integration. The critical limiting factor in studying biliary excretion is that the bile canaliculus is very small (~1 µm in diameter) and is generally inaccessible to sampling by conventional approaches. The bile canalicular membrane is a specialized part of the hepatocellular plasma membrane, such that bile is separated from blood plasma only by the tight junctions that encircle each hepatocyte. Because hepatocyte polarity is rapidly lost during cell isolation, most cell culture models provide only limited information on mechanisms of biliary excretion. Thus, biliary secretion has been studied using four major experimental models: canalicular plasma membrane vesicles, cultured hepatocyte couplets, perfused liver, and in vivo (bile duct-cannulated animals). This unit describes basic methods for collecting bile from anesthetized mice and rats, for carrying out isolated perfused rat liver studies, and for the simultaneous isolation of plasma membrane vesicles derived from the basolateral and canalicular domains of rat liver.