EFFECTS OF GONADOTROPIN-RELEASING-HORMONE AGONIST ON STEROIDOGENESIS IN THE RAT OVARY

To assess the regulatory roles of gonadotropin-releasing hormone (GnRH) in ovarian function, the kinetics of the ovarian GnRH receptor and the effects of the GnRH superagonist buserelin on steroidogenesis in ovarian cell culture were examined. Scatchard analysis of buserelin-binding to crude ovarian cell membrane revealed a specific high-affinity GnRH receptor. Buserelin together with follicle-stimulating hormone stimulated estradiol (E(2)) production in immature follicles in hypophysectomized and DES-treated rats. On the other hand, applied to developing follicles of rats treated with pregnant-mare-serum gonadotropin buserelin suppressed E(2) production to terminate follicle maturation and simultaneously stimulated progesterone (P-4) production to induce luteinization. With ovarian cells luteinized by human chorionic gonadotropin in vitro, buserelin suppressed production of both P-4 and E(2), leading to luteolysis. Buserelin affected steroid production by modulating activities of key enzymes in steroid synthesis. These findings indicate that buserelin action depended on the gonadotropin priming of ovarian cells, and suggest the possible involvement of GnRH in the regulation of steroidogenesis throughout the ovulatory cycle.