Effect of Luteinizing Hormone on the Pattern of Steroid Production by Preovulatory Follicles of Pregnant Mare's Serum Gonadotropin-injected

Abstract

Preovulatory follicles were explanted on the day before ovulation from immature rats given a single injection of pregnant mare''s serum gonadotropin (PMS) 2 days earlier. The follicles were incubated for 4 h in modified Krebs bicarbonate buffer containing glucose and albumin in absence or presence of ovine luteinizing hormone (NIH[National Institute of Health]-LH-S18; 0.1-10 .mu.g/ml). The accumulation of progesterone, androstenedione and 17.beta.-estradiol in the medium was determined by radioimmunoassay. As an indicator of LH exposure the meiotic stage of the follicle-enclosed oocyte was determined at recovery by interference contrast microscopy. The 1st group of follicles was explanted in the morning, before the endogenous gonadotropin surge. In hormone-free medium the oocytes remained in the dictyate stage, whereas addition of LH induced oocyte maturation. These follicles, when incubated in hormone-free medium, secreted predominantly androstenedione and estradiol and only low amounts of progesterone. In the presence of LH the secretion of all steroids was enhanced. The 2nd group of follicles was explanted in the evening, 2-4 h after the endogenous gonadotropin surge. After incubation in hormone-free medium the follicle-enclosed oocytes had matured. The steroid secretion by the follicles was different from that of the 1st group. In hormone-free medium they secreted predominantly progesterone and low amounts of androstenedione and estradiol. Addition of LH to the medium caused further enhancement of progesterone secretion, but had no effect on androstenedione and estradiol secretion. The 3rd group of follicles was explanted in the evening from rats in which the preovulatory gonadotropin surge had been prevented by Nembutal treatment. Oocyte maturation and steroid secretion did not differ from that found for the 1st group of follicles explanted in the morning. The results agree with the hypothesis that LH, after a transitory stimulation, inhibits androgen and estrogen secretion and stimulates progesterone secretion by the preovulatory ovarian follicle.