The ins and outs of membrane movement in biliary epithelia.

We have developed a novel technique for the isolation from normal rat liver of morphologically polar, intrahepatic bile duct epithelial cells which exhibit clathrin-coated pits. Using electron microscopic cytochemistry, we demonstrated receptor-mediated endocytosis of EGF by cultured IBDEC. Also, using freshly isolated polar couplets of IBDEC, we demonstrated that these cells participate in fluid-phase endocytosis. Finally, using a novel fluorescence unquenching assay and our isolated bile duct epithelial cell model, we showed that secretin stimulates exocytosis in IBDEC, a finding compatible with the possibility that secretin-induced changes in ductular bile flow may occur by an exocytic process. The availability of a reproducible and reliable technique to prepare liver cell fractions highly enriched in intrahepatic bile duct epithelial cells with morphologic polarity has made it possible to do direct experiments on the functions of intrahepatic bile duct epithelial cells, including the study of plasma membrane movement (i.e., endocytosis and exocytosis). With the availability of this technique, other studies previously impossible to carry out in IBDEC are now feasible. Such studies are too numerous to mention, but would include experiments on ligand binding, transport of macromolecules, assessment of metabolic activities and toxicity studies, to name just a few. Indeed, virtually any question that has been asked about hepatocytes and addressed using isolated hepatocytes can now be directed toward isolated intrahepatic bile duct epithelial cells. Finally, the methodology described here is theoretically applicable to human liver. Indeed, intrahepatic bile duct epithelial cells are considered to be involved in the pathogenesis of several kinds of drug and immunologically induced liver diseases, including allograft rejection, primary biliary cirrhosis, and primary sclerosing cholangitis. The availability of the technology described here should make feasible direct experimental approaches to questions in all of these areas.