Rapid exocytosis and endocytosis in nerve terminals of the rat posterior pituitary.

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Abstract
1. Ca(2+)-induced exocytosis and endocytosis were studied by measuring the membrane capacitance of voltage-clamped peptidergic nerve terminals in slices prepared from the rat posterior pituitary. 2. Depolarizing pulses produced rapid increases in capacitance. These increases varied in parallel with Ca2+ current as voltage was varied. Elimination of Ca2+ current blocked depolarization-induced capacitance changes. 3. Depolarization-induced capacitance changes increased with pulse duration. Capacitance changes also increased with integrated Ca2+ influx, but saturated at high levels of Ca2+ entry. This saturation allowed us to estimate a pool size of 190 vesicles, assuming each vesicle has a capacitance of 1 fF. Vesicles from this pool fused with a time constant of 0.43 s. The capacitance change increased with the first power of integrated Ca2+ influx. 4. Experiments with briefer pulses revealed a rapid component of exocytosis comprising a pool of forty vesicles that fuse with a time constant of 14 ms. This rapid process may reflect a final Ca(2+)-regulated triggering step, which is distinct from the slower kinetic step revealed by longer duration pulses. The slower step may reflect a priming of vesicles prior to exocytosis. 5. Depolarization-induced capacitance increases in most cases were followed by a rapid decay in capacitance, reflecting membrane reuptake tightly coupled to exocytosis. A variable amount of rapid endocytosis followed depolarization-induced capacitance increases. The time constant for rapid endocytosis to baseline was 0.44 s. Excess endocytosis was occasionally observed, with capacitance decaying below the pre-stimulus baseline with a time constant of 2.1 s. 6. Rapid endocytosis was slower after pulses that produced greater increases in intracellular Ca2+, consistent with the hypothesis that intracellular Ca2+ inhibits rapid endocytosis. 7. Exocytosis follows depolarization with no detectable delay, indicating that Ca2+ triggers neuropeptide secretion from nerve terminals with kinetics comparable to that observed in other rapidly secreting systems.