Evidence for Calcium Inactivation During Hormone Release in the Rat Neurohypophysis

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Abstract
A study has been made of the relationship between 45Ca uptake into and hormone release from isolated rat neurohypophyses incubated in vitro. Hormone secretion is triggered by high-K (56 mm) but long exposure to the stimulus does not generate a maintained release of hormone. When hormone release began to wane, addition of Ba or La increased hormone output which suggests that the decline in output did not result from depletion of the neurosecretory granules at the nerve terminals. 45Ca uptake is enhanced in the presence of high-K concentration, but the initial high rate declines during long exposure to the potassium stimulus with a time constant similar to that of the decline in hormone release. After a period of incubation in a K-rich, calcium-free medium, addition of calcium to the medium induced hormone release. The magnitude of this release was dependent on the time of exposure to excess potassium. After inactivation of secretion, mobilization of internal calcium by means of a calcium ionophore increased hormone release.