Physiological Peripubertal Activation of the Ovary Is Not Reproduced by Pregnant Mare Serum Gonadotropin (PMSG) Administration 1

Abstract

Abstract

During the days preceding the first ovulation the ovary of the rat exhibits a remarkable increase in estradiol (E₂) and progesterone (P) release in response to gonadotropins. No such increase is observed in the case of androgens (A, testosterone + dihydrotestosterone). The present experiments were undertaken to examine the possibility of reproducing these developmental events by stimulating the ovary with a gonadotropin that has substantial FSH-like activity. In vivo administration of pregnant mare serum gonadotropin (PMSG) to juvenile 29-day-old rats greatly increased the in vitro E₂ and A response to human chorionic gonadotropin (hCG) measured 2 days later in the morning. The magnitude of the A response was significantly larger than that of ovaries from juvenile animals or rats in first proestrus. The E₂ response was much greater than that of juvenile ovaries but similar to that of ovaries from late proestrous rats. In contrast, the P response to hCG was not enhanced by PMSG. In fact the response was similar to that of juvenile ovaries and markedly less than that of first proestrous rats. This decreased P response was not due to a greater conversion of P to its less active metabolite 20α-hydroxy-4-pregnen-3-one (20α-OH-P). The results suggest that PMSG enhances the E₂ and A response of immature ovaries to hCG at the expense of that of P. Treatment of immature rats with PMSG may represent a useful model to study E₂ release from preovulatory ovaries, but it cannot be used to reproduce in its entirety the developmental changes in steroidal response to gonadotropins associated with normal puberty.