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 membr ane fusion in rat mast cell grannies is followed by a variable, expone ntially distributed, delay before bulk release, This delay reflects th e time required for the expansion of the exocytotic fusion pore, lasti ng, 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 b etween fusion and release was reduced to 123 ms. The characteristics o f 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 transmitt er release.