Secretory granules and granins in hyperstimulated male rat gonadotropes

Granins are acidic proteins co-localized with peptides in secretory granules of many endocrine cells. They are thought to participate in certain steps of the regulated secretory pathway. This is of particular interest in rat pituitary gonadotropes, which in most cases contain both gonadotropins (follicle-stimulating hormone, FSH and luteinizing hormone, LH) and two granins (chromogranin A, CgA and secretogranin II, Sg II). Therefore, we investigated male rat gonadotropes ultrastructurally and for the cellular and subcellular localization of gonadotropins/granins under normal conditions and after stimulation by luteinizing hormone-releasing hormone (LHRH) or castration. Typical gonadotropes of controls contained differently composed secretory granules: small granules showed immunoreactivity for LH and SgII and larger granules were immunoreactive for FSH and CgA and to a lesser extent, for LH. Stimulated gonadotropes showed hypertrophy or hyperplasia and RIA for plasma LH levels showed a 30-70-fold increase. In stimulated cells a third type of secretory granule became prominent. These "intermediate" granules had an electron-dense core immunoreactive for LH and SgII, whereas CgA labeling was confined to a less electron-dense outer region. (In stimulated gonadotropes, FSH immunoreactivity could be shown effectively only at the light microscopic level.) Intermediate granules developed from structures resembling condensing vacuoles. They began to exhibit their typical double structure as they budded off from the trans-Golgi network. It therefore appears that CgA and SgII participate in establishing two different routes of the regulated pathway in gonadotropes. Therefore, immunocytochemistry of the granins seems to be a suitable approach to investigating secretory pathways in these endocrine cells.