Exocytosis of neurosecretory granules in the corpus cardiacum of the blowfly Calliphora erythrocephala was induced both by electrical stimulation and by depolarization with K+. This secretory activity was quantitatively determined by the frequency of omega figures and vesicle clusters, which are the ultrastructural indications of two distinct phases of the exocytotic process. Experimentally elicited exocytosis was reduced when Ca2+ were omitted from the bathing medium. This result supports the idea that the coupling between excitation (depolarization) and exocytosis involves Ca2+ influx through the axolemma. Barium effectively substitutes for calcium in the secretory process. High levels of magnesium in the bathing medium, however, decreased exocytosis, possibly by interfering with calcium influx.

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