STIMULATORY EFFECTS OF ESTROGEN ON GONADOTROPIN-RELEASING HORMONE-INDUCED PHOSPHOINOSITIDE TURNOVER IN GRANULOSA-CELLS

Gonadotropin-releasing hormone (Gn-RH) stimulates phosphoinositide metabolism in granulosa by cells by binding to its specific receptor, and suppresses gonadotropin-induced steroidogenesis. Incubation of immature rat granulosa cells with Gn-RH stimulated time-sequential [P-32]phosphate incorporation into phosphatidic acid (PA) and phosphatidylinositol (PI) in a dose-dependent manner; EC50 was at 10 nM. Concurrent exposure to estradiol-17-beta (E2) (100 nM) and Gn-RH (1-mu-M) augmented P-32-labeling of PI by 5-fold, while Gn-RH alone induced 3.5-fold increase in PI-labeling. In cells preincubated with E2 for 48 h, Gn-RH provoked a 7-fold [P-32]phosphate incorporation into PI, suggesting the induction by E2 of Gn-RH-responsible phosphoinositide turnover. E2 alone provoked a low but significant increase in basal labeling rate of PA and PI. Progesterone failed to mimic the action of E2. Essentially similar results were also obtained in mature rat granulosa cells. These results indicate that E2 augments Gn-RH-stimulated phospholipid turnover in granulosa cells, and suggest that estrogens within the microenvironment of the ovary may exert a local autoregulatory effect on their own production pathway through accelerating Gn-RH action to attenuate steroidogenesis.