Bidirectional transport of cholesterol between gallbladder epithelial cells and model bile

Lipids in hepatic bile may be modified by the gallbladder epithelium. The purpose of this study was to demonstrate a bidirectional exchange of cholesterol between biliary lipid carriers and gallbladder epithelial cells and to determine the factors regulating this cholesterol transfer. Gallbladder epithelial cells were cultured to confluent monolayers, their membranes were labeled with endogenously synthesized [C-14]cholesterol, and the cells were incubated with model bile introduced into the apical membrane compartment. Similarly, model bile with different lipid composition containing [H-3]cholesterol was incubated with the unlabeled monolayers. We found that cholesterol in the apical membrane bilayer of the epithelial cells exchanged readily with that in bile, but only in the presence of bile salts. The rate of exchange is dependent on the concentration and species of bile salts. The net gain of cholesterol (absorption) or net loss of cholesterol (cytotoxicity) exhibited by the epithelial cells was regulated by the thermodynamic stability of cholesterol and the detergent effect of mixed micelles in bile. It is also possible that the physicochemical composition of lipids in bile may modify the cellular function of the gallbladder epithelium.