Distribution of filipin-cholesterol complexes at sites of exocytosis - a freeze-fracture study of degranulating mast cells.

The distribution of membrane cholesterol at sites of exocytosis was studied with the cytochemical probe, filipin, in freeze-fractured rat peritoneal mast cells. When degranulation was induced in mast cells by polymyxin B, the membrane of all secretory granules, as well as the plasma membrane, including bulged areas signaling incipient exocytosis, appeared heavily labeled with filipin-sterol complexes (recognizable as 25-nm protuberances). This is evidence that cholesterol depletion is not an absolute requirement for membrane fusion.