| Abstract: | In the pregnant rat, luteinizing hormone (LH) stimulates the ovarian production of testosterone (T) which is aromatized to estradiol (E2). E2 promotes progesterone (P) synthesis by the ovary. To determine if the administration of gonadotropin-releasing hormone (GnRH) disrupts pregnancy by suppressing ovarian steroid production, rats were treated on days 7-12 of pregnancy with 25, 50 or 100 micrograms/day of GnRH or 0.2, 1 or 5 micrograms/day of a GnRH agonist (GnRH-Ag). The higher two doses of GnRH or GnRH-Ag within 24 h suppressed peripheral levels of plasma P and terminated pregnancy within 48 h. By day 12, P levels in the ovarian vein in rats treated with GnRH or GnRH-Ag in respective doses were 2098 +/- 261, 732 +/- 437, 110 +/- 15, and 2575 +/- 463, 49 +/- 9, 43 +/- 8 compared to 1833 +/- 433 ng/ml in controls. Daily treatment of P (4 mg) and E2 (0.5 microgram) simultaneously with GnRH-Ag at its maximum dose reversed the abortifacient effect of GnRH-Ag and maintained pregnancy. Peripheral levels of Plasma LH in all groups were higher than controls on days 10 and 12. Ovarian vein levels of T on days 10 or 12 of pregnancy were either not significantly different from controls (at 2703 +/- 607 or 3249 +/- 690 pg/ml, respectively) or increased dramatically to 9547 +/- 1769 on day 10 and to 5985 +/- 1426 pg/ml on day 12 in rats treated with 0.2 microgram of GnRH-Ag. Similarly, ovarian vein levels of E2 on days 10 or 12 were either not significantly different from controls (at 2022 +/- 227 or 2793 +/- 184 pg/ml, respectively) or increased dramatically to 2980 +/- 58 pg/ml on day 10 in rats treated with 25 micrograms of GnRH or to 3296 +/- 241 on day 10 and to 3420 +/- 325 pg/ml on day 12 in rats treated with 0.2 microgram of GnRH-Ag. These results indicate that the abortifacient effect of GnRH administration in rats is not due to its effect on the uterus, but to its suppressive effects on ovarian P secretion. There was no evidence to show that a GnRH-induced fall in ovarian secretion of either T or E2 were involved in this process. |
