Depolarization of isolated frog saccular hair cells caused Ca2+-dependent increases in membrane capacitance that we interpret as the fusion of synaptic vesicles with the plasma membrane. During a maintained depolarization to -10 mV, the capacitance increased at a rate corresponding to the fusion of ∼500 vesicles per second at each active zone. Release continued at this high rate for up to 2 s, long enough to exhaust >5 times the number of vesicles initially in close apposition to the plasma membrane at active zones. We therefore propose that hair cells are specialized for rapid replenishment of vesicles at release sites. Upon repolarization to -70 mV, the capacitance returned exponentially (time constant, ∼14 s) to near the prestimulus level in perforated-patch recordings, but not in whole-cell recordings, suggesting that a mobile intracellular factor is required for membrane retrieval.
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