Regulation of secretory granule formation in chronically hypersecretory melanotrophs in the rat pituitary.

The formation of secretory granules in chronically hypersecretory melanotrophs in the rat pituitary was studied. Hypersecretion was induced by treatment with the dopamine antagonist haloperidol (1.5 mg/kg daily for 7 days), which releases the normal neural dopaminergic inhibition of secretion from the melanotroph. Morphometric analysis showed a 100% increase in the volume fraction of granular endoplasmic reticulum after haloperidol treatment, while the volume fractions of electron-dense granules, electron-lucent granules and the Golgi apparatus were unaltered. The mean diameter of the mature secretory granules was increased by 10%, indicating a 30% increase in mean granule volume. A similar increase in diameter was observed in condensing granules within the Golgi area. With earlier results on the effect of chronic inhibition the study shows that a main adaptive response of the melanotroph to altered secretory conditions is a change in the volume of the secretory granules, regulated by a mechanism that operates at an early stage of granule formation.