EFFECTS OF SEX STEROIDS ON SECRETORY GRANULE FORMATION IN GONADOTROPES OF CASTRATED MALE RATS WITH RESPECT TO GRANIN EXPRESSION

Pituitary gonadotropes show sex-related differences in their ultrastru cture. Typical gonadotropes of male rats exhibit both large granules, which contain chromogranin A (CgA), and small granules, which contain secretogranin II (SgII). In contrast, typical female rat gonadotropes show only a very few large granules among the numerous small granules. To clarify the nature of the biogenesis of these secretory granules a nd the effects of sex steroids, the ultrastructural and immunocytochem ical changes in gonadotropes were examined in castrated male rats supp lied with a testosterone or estradiol implant. In castrated rats, pitu itary expression and plasma levels of LH increased drastically, but th e pituitary content of CgA decreased. The majority of gonadotropes the n showed features of ''castration cells'' containing many small secret ory granules. A testosterone implant to castrated rats remarkably supp ressed the expression and circulating levels of LH and increased the C gA content in the pituitary to near-normal levels. In this situation, immunocytochemical studies demonstrated that gonadotropes again exhibi ted large and small secretory granules with the respective localizatio n of CgA and SgII. On the contrary, in castrated rats supplied with an estradiol implant, the expression and content of CgA in the pituitary were remarkably suppressed, and large secretory granules disappeared from gonadotropes. These results suggest that the expression of CgA in gonadotropes is regulated differently by male and female sex steroids . These different effects of androgen and estrogen on the expression l evel of CgA are closely associated with the sex-related differences in the ultrastructure of secretory granules within gonadotropes.