Inhibition of Ovarian Estradiol-17/3 Secretion by Luteinizing Hormone in Prepubertal, Pregnant Mare Serum-Treated Rats11

Abstract

Serum estradiol-17.beta. 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[National Institute of Health]-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-17.beta. concentrations fell detectably by 4 h, and reached levels around 20% of the controls by 8 h after i.v. injection of 10 .mu.g LH. Following an initial rise, concomitant decreases occurred in ovarian androgen concentrations and in the in vitro ovarian testosterone aromatizing enzyme activity. The LH-induced inhibition of the aromatase activity was of a non-competitive type. It is proposed that 2 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.