1. The membrane capacitance of mast cells was monitored under voltage clamp, using sinusoidal excitation and a lock‐in amplifier. 2. Degranulation was accompanied by stepwise capacitance increases that presumably represent the fusion of single secretory granules with the cell membrane. Besides capacitance steps, we also observed gradual changes in capacitance that occurred even in the absence of degranulation, were independent of the presence of nucleotides in the pipette, and were steeply dependent on cytoplasmic [Ca2+]. 3. Cytoplasmic Ca2+ at concentrations of 0.3‐3 microM stimulated a decline in capacitance, with a dose‐response curve suggesting control by the binding of Ca2+ to high‐affinity intracellular sites. When maximally activated, this mechanism could lead to a loss of about 6% of the cell membrane capacitance, at an average rate of 0.1‐0.2% s‐1. 4. At even higher cytoplasmic [Ca2+] (greater than 3 microM), the reverse effect was observed. The capacitance increased gradually by up to 40%, at an average rate of 0.4% s‐1. Evidently gradual changes in membrane capacitance can occur by two mechanisms, and both are influenced by cytoplasmic [Ca2+]. 5. Ca2+ frequently stimulated an inward current accompanied by an increase in membrane conductance. 6. The effects described above were observed also when only trace amounts of Ca2+ and chelator were added to the cytosol, and when increases in cytosolic [Ca2+] could have occurred only by endogenous mechanisms. It is suggested that these effects occur also in intact cells during the large [Ca2+] increases known to occur before and during degranulation.
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