Uptake of radioactive calcium, 45Ca efflux, and hormone release from the isolated rat neurohypophysis were monitored in vitro after the addition of veratridine to the incubation medium. Veratridine dramatically increased hormone release, but the release was not sustained and had declined by about 90% after 2 h. Removal of external Na+ prevented hormone release as did addition to the incubation medium of tetrodotoxin or the calcium antagonists D600 and Mn2+ ions. Veratridine increased 45Ca uptake into the isolated neurohypophysis and the increase could be prevented by addition of tetrodotoxin or D600 to the medium. Efflux of 45Ca was not changed by addition of veratridine. The results underline the importance of both Na+ and Ca+2 channels in the regulation of secretion of neurosecretory products.

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