Release of Neurophysin Together with Vasopressin by a Vasopressin by a Ca $^{2+}$ Dependent Mechanism

Oxytocin and vasopressin are stored with their binding proteins, the neurophysins, within neurosecretory vesicles in the nerve endings of the mammalian neurohypophysis. Depolarization of the nerve terminals, either by the arrival of a nerve impulse in vivo or by immersion of the gland in solutions of high K+ concentration in vitro , brings about a release of the hormones into the extracellular space. Douglas & Poisner (1964) have shown that this release is dependent on the entry of Ca2+ into the nerve endings, and have proposed that Ca2+ is necessary for coupling the stimulus of depolarization to the secretory process. Whereas Douglas (1967) suggests that Ca2+ plays a part in emptying the neurosecretory vesicles by an exocytotic mechanism, the finding of Smith & Thorn (1965) that Ca2+ dissociates the hormone-neurophysin complex suggests that secretion may take place by diffusion of the hormones through the vesicular and roteins. A biochemical method of distinguishing between these two mechanisms is to study whether other macromolecular constituents of the neurosecretory vesicles are specifically released by depolarizing stimuli. Fawcett, Powell & Sachs (1968) have previously shown by tracer experiments in dogs that a protein cross-reacting with a rabbit antiserum to bovine neurophysin is released from neurohypophyses stimulated by high K+ solutions in vitro or by haemorrhage in vivo , but their technique did not allow a quantitation of the protein in relation to the amount of hormone released. A parallel release of neurophysin and hormone would be expected if exocytosis plays a part in the secretory mechanism.