RELEASABLE POOLS AND THE KINETICS OF EXOCYTOSIS IN ADRENAL CHROMAFFINCELLS

Using high resolution capacitance (C-m) measurement techniques, we map ped the kinetics of exocytosis evoked by brief (5-100 ms) depolarizati ons that activate voltage-dependent Ca2+ channels in rat adrenal chrom affin cells. After correcting the C-m signal for contributions from Na + channel-gating charge movements, the initial kinetics of exocytosis were consistent with depolarization-triggered release occurring initia lly from an immediately releasable pool of only similar to 17 secretor y vesicles. In contrast, repetitive application of longer depolarizati ons evoked release from a distinct, larger readily releasable pool of similar to 170 vesicles. Our results suggest that the secretory respon se of a chromaffin cell to an action potential is limited by the size of the immediately releasable pool rather than by a fusion mechanism t hat is stower than that at synapses.