Membrane recapture after hormone release from nerve endings in the neural lobe of the rat pituitary gland

Abstract

The magnocellular neurosecretory system of mammalia, which produces oxytocin and vasopressin and releases these hormones at a neurohaemal contact area in the neural lobe of the pituitary gland, provides a useful model for the study of release of granule-packaged material from both neurones and endocrine cells. Isolated neural lobes from rats were stimulated to release hormone by incubation for 15min in a depolarizing sodium-free medium containing 56 mM potassium ions. Controls were incubated in a sodium-free medium containing 5.6 mM potassium ions. The amounts of vasopressin and oxytocin released by depolarization together comprised ~83 mU. At the end of the stimulation the glands were fixed for electron microscopy and stereological analysis. Loss of neurosecretory granules from stimulated glands was restricted to the nerve endings. From knowledge of the amount of hormone stored in a single neurosecretory granule, the number of granules lost from the gland could be calculated to contain approximately the amount of hormone released. After stimulation, the nerve endings were unaltered in size or membrane area. There was also no change in the total microvesicle population of the endings, though the microvesicles were redistributed towards the basement membrane contact zone. The vacuole population of the endings was, however, increased three-fold after stimulation. We conclude that, after acute stimulation of hormone release from the neural lobe, neurosecretory granules are lost specifically from the nerve endings and that the excess membrane that results from their exocytosis is to be found primarily in vacuoles.