CALCIUM-TRIGGERED EXOCYTOSIS AND ENDOCYTOSIS IN AN ISOLATED PRESYNAPTIC CELL - CAPACITANCE MEASUREMENTS IN SACCULAR HAIR-CELLS

Depolarization of isolated frog saccular hair cells caused Ca2+-depend ent increases in membrane capacitance that we interpret as the fusion of synaptic vesicles with the plasma membrane. During a maintained dep olarization to -10 mV, the capacitance increased at a rate correspondi ng to the fusion of similar to 500 vesicles per second at each active zone. Release continued at this high rate for up to 2 s, long enough t o exhaust >5 times the number of vesicles initially in close appositio n to the plasma membrane at active zones. We therefore propose that ha ir cells are specialized for rapid replenishment of vesicles at releas e sites. Upon repolarization to -70 mV, the capacitance returned expon entially (time constant, similar to 14 s) to near the prestimulus leve l in perforated-patch recordings, but not in whole-cell recordings, su ggesting that a mobile intracellular factor is required for membrane r etrieval.