A possible mechanism of release of posterior pituitary hormones involving adenosine triphosphate and an adenosine triphosphatase in the neurosecretory granules.

Neurosecretory granules were isolated from bovine posterior pituitary glands and incubated at 30 or 37° in a medium containing KCl and MgCl2. On exposure to ATP the granules released vasopressin, oxytocin, and protein. Vasopressin release in response to ATP was potentiated by phosphoenolpyruvate + pyruvate kinase, was inhibited by AMP, and was accompanied by a fall in the optical density (at 540 mµ) of the granule suspension. The neurosecretory granules possessed ATPase activity and ATP. It is suggested that ATP and ATPase may participate in the release of hormones from the intact posterior pituitary gland.