Cytosolic calcium facilitates release of secretory products after exocytotic vesicle fusion.

We monitored single vesicle exocytosis by simultaneous measurements of cell membrane capacitance as an indicator of fusion and amperometric detection of serotonin release. We show here that vesicle-plasma membrane fusion in rat mast cell granules is followed by a variable, exponentially distributed, delay before bulk release. This delay reflects the time required for the expansion of the exocytotic fusion pore, lasting, on average, 231 ms in resting cytosolic calcium, [Ca2+]i (50 nM). In the presence of [Ca2+]i in the low micromollar range, the lag between fusion and release was reduced to 123 ms. The characteristics of the amperometric signals were unchanged by [Ca2+]i. These results show a novel site of regulation in the exocytotic process, the fusion pore, which may represent a different mechanism facilitating transmitter release.