Ultrastructural characterization of cholesterol distribution in toad bladder using filipin.

The polyene antibiotic filipin was used to characterize the cholesterol distribution in the membranes of the toad bladder epithelium in freeze-fracture replicas. The apical membranes of granular and mitochondria-rich cells incorporate moderate amounts of filipin while the basolateral membranes of both cell types incorporate substantially greater amounts. Intracellular membranes, in general, take up very little filipin. The major exception to this is the granule membrane, which appears to be rich in cholesterol. An inverse correlation was found between the density of filipin-sterol complexes in the apical membrane and the incidence of granules in the cytoplasm. This suggests that fusion of granules with the apical membrane may be responsible for variation in the concentration of cholesterol in the apical membrane. Thirty minutes following vasopressin exposure, there is no consistent change in the cholesterol content of the apical membrane of granular cells as measured by the incidence of filipin-sterol complexes. The lack of change in the amount of membrane cholesterol indicates that the vasopressin-induced increase in transepithelial water permeability is not mediated by a change in cholesterol content of the apical membrane.