Kiss-and-run exocytosis is a rapid and efficient method of releasing secretory products to the extracellular medium through the transient formation of a fusion pore. During kiss and run, the vesicle membrane does not collapse into the plasma membrane, maintaining the secretory vesicle’s position, identity and potentiality to perform a second round of exocytosis. In a previous report, we found that the incidence of kiss-and-run increases as the extracellular calcium concentration rises in the millimolar range (Ales et al. 1999). These kiss-and-run events lasted only for milliseconds (average of 50 ms, called ‘fast kiss-and-run’) and released the same amount of transmitter as a conventional exocytosis. However, we did not demonstrate whether calcium mediated this effect extracellularly or by an indirect cytosolic calcium increase. To answer this question, we have done whole cell membrane capacitance measurements in peritoneal mast cells of rats that had been humanely killed in which we could experimentally control both the extracellular and the intracellular calcium concentrations. This preparation also allows resolution of individual fusion events. To monitor release, we have made simultaneous measurements of serotonin release with amperometry by positioning a carbon fibre electrode over the cell surface.

We analysed well-defined single exocytotic fusion events from capacitance and amperometric measurements. The incidence of kiss-and-run increased when the rate of degranulation was decreased either by lowering the amount of secretagogue (GTPλS) or by dialysing the cells with lower calcium concentrations. At similar amounts of GTPλS, the number of kiss-and-run events decreased as the cytosolic calcium concentration was raised from 100 nM to 1 µM. Simultaneous amperometric measurements revealed that these kiss-and-run events were slow (lasting on average 500 ms), and release only a small fraction of the vesicle contents. By contrast, when the extracellular calcium concentration was raised from 2 mM up to 90 mM, the incidence of kiss-and-run increased and the amount of serotonin was similar to the quantal contents of a rat mast cell granule, indicating the occurrence of fast kiss-and-run events.

These data reveal that fast kiss-and-run events are evoked by calcium acting from the extracellular medium and that these events represent a different kinetic process from the slow kiss-and-run events. We propose a kinetic model of exocytosis that accounts for slow and fast kiss-and-run exocytosis.