Morphologic and functional characterization of caveolae in rat liver hepatocytes.

Caveolae are small pits on the plasma membrane found in several, if not all, differentiated cells. They are involved in potocytosis, endocytosis, transcytosis, membrane trafficking, and signal transduction. Although caveolin has recently been identified in subcellular fractions from rat liver there is no clear-cut morphologic evidence for the presence of prototypical caveolae on the surface of hepatocytes. In this study the presence of caveolae at the cell surface of hepatocytes was directly shown by rapid-freeze, deep-etching electron microscopy. Moreover, combined deep-etching and immunogold techniques revealed caveolin in caveolae of the dorsal membrane of primary culture hepatocytes. Using reagents that perturb membrane cholesterol and interfere with endocytosis through the caveolae, a caveolae-dependent internalization of cholera toxin B and retinol-binding protein by hepatocytes in primary culture was shown. Finally, immunocytochemical analysis of caveolin in nonparenchymal cells of the rat liver showed its presence in Kupffer and stellate cells, however no caveolin could be detected in endothelial cells.