Direct inhibitory effect of gonadotropin-releasing hormone upon luteal luteinizing hormone receptor and steroidogenesis in hypophysectomized rats.

The effects of gonadotropin-releasing hormone (GnRH) and its potent agonist [des-Gly10, D-Leu6-N alpha Me) Leu7, Pro9,NHEt-GnRH (GnRH-A)] on ovarian luteal functions maintained by PRL were studied in vivo and in vitro. Hypophysectomized, diethylstilbestrol-treated female rats were primed with FSH for 2 days, followed by an ovulating dose of LH or hCG. Two days later, ovarian luteal functions were maintained by daily injections of 250 microgram PRL for 3 days. PRL treatment increased the serum progesterone level from 13.0 +/- 0.5 to 298 +/- 24 ng/ml and increased the ovarian hCG-binding capacity from 5.8 +/- 1.3 to 584 +/- 86 ng bound hCG/ovary. In contrast, concomitant treatment with GnRH or GnRH-A resulted in dose-dependent decreases in the PRL-induced increase of serum progesterone and ovarian LH/hCG receptor content. GnRH at 100 microgram/day caused a 60% decrease in serum progesterone and an 80% decrease in ovarian LH receptor content, whereas GnRH-A was effective at a 1-microgram dose level. Neither GnRH nor GnRH-A affected the binding affinity (Kd) of ovarian LH receptor. The direct inhibitory effects of GnRH and GnRH-A upon granulosa-luteal cell function were also tested in vitro. FSH treatment for 2 days induced functional LH and PRL receptors in cultured PRL, increased (by approximately 3-fold) progesterone production by these granulosa-luteal cells, whereas concomitant treatment with GnRH-A inhibited progesterone production in a dose-dependent manner. Thus, these studes demonstrated that GnRH and GnRH-A exert direct inhibition on ovarian luteal functions by decreasing LH receptor and progesterone production in vivo as well as inhibiting progesterone production by cultured granulosa-luteal cells in vitro.