TIME-COURSE OF CA2+ CONCENTRATION TRIGGERING EXOCYTOSIS IN NEUROENDOCRINE CELLS

We have used the secretory response of chromaffin cells to estimate th e submembrane intracellular Ca2+ concentration ([Ca2+](i)) ''seen'' by secretory granules during short depolarizations. The rate of secretio n during a depolarization was assessed by combining the electrochemica l method of amperometry and electrical capacitance measurements. The f ate was then related to [Ca2+](i) based on a previous characterization of how Ca2+ affects the dynamics of vesicle priming and fusion in chr omaffin cells [Heinemann, C., Chow, R. H., Neher, E. and Zucker, R. S. (1994) Biophys. J. 67, in press]. Calculated [Ca2+](i) rose during th e depolarization to a peak of <10 mu M, then decayed over tens of mill iseconds. In synapses, vesicles are presumed to be located within nano meters of Ca2+ channels where [Ca2+](i) is believed to rise in only mi croseconds to near steady-state levels of hundreds of micromolar. Chan nel closure should lead to a decrease in [Ca2+]i also in microseconds. Our findings of the slower time course and the lower peak [Ca2+](i) s uggest that in chromaffin cells, unlike synapses, Ca2+ channels and ve sicles are not strictly colocalized. This idea is consistent with prev iously published data on dense-core vesicle secretion from diverse cel l types.