Inhibition of ovarian estradiol-17beta secretion by luteinizing hormone in prepubertal, pregnant mare serum-treated rats.

Serum estradiol-17beta levels, elevated prior to the luteinizing hormone (LH) surge, decline abruptly following the release of endogenous LH or the injection of exogenous LH. To investigate the mechanism of this decline, bovine LH (NIH-LH-B8) was administered to immature rats, in which follicular maturation and estrogen biosynthesis were induced by a non-ovulating dose of pregnant mare serum gonadotropin (PMS). Serum and ovarian estradiol-17beta concentrations fell detectably by 4h, and reached levels around 20% of the controls by 8h after iv injection of 10 mug LH. Concomitant decreases occurred in ovarian androgen concentrations, following an initial rise, and in the in vitro ovarian testosterone aromatizing enzyme activity. The LH-induced inhibition of the aromatase activity was found to be of a non-competitive type. It is proposed that two enzyme systems are inhibited as a result of the LH treatment: the C17,20-lyase and the C19 androgen aromatase, thereby leading to decreased concentrations of estrogens in the ovaries and blood.