Final steps in exocytosis observed in a cell with giant secretory granules

Secretion by single mast cells was studied in normal and beige mice, a mutant with grossly enlarged secretory vesicles or granules. During degranulation, the membrane capacitance increased in steps, as single secretory vesicles fused with the cell membrane. The average step size was 10 times larger in beige than in normal mice, in agreement with the different granule sizes measured microscopically in the two preparations. Following individual capacitance steps in beige mice, individual granules of the appropriate size were observed to swell rapidly. Capacitance steps are frequently followed by the stepwise loss of a fluorescent dye loaded into the vesicles. Stepwise capacitance increases were occasionally intermittent before they became permanent, indicating the existence of an early, reversible, and incomplete state of vesicle fusion. During such "capacitance flicker," loss of fluorescent dye from vesicles did not occur, suggesting that the earliest aqueous connection between vesicle interior and cell exterior is a narrow channel. Our results support the view that the reversible formation of such a channel, which we term the fusion pore, is an early step in exocytosis.