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

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Abstract
During the days preceding the 1st ovulation the ovary of the rat exhibits a remarkable increase in estradiol (E2) and progesterone (P) release in response to gonaodtropins. No such increase is observed in the case of androgen (A, testosterone + dihydrotestosterone). The possibility of reproducing these developmental events by stimulating the ovary with a gonadotropin that has substantial FSH-ike activity was examined. 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 1st 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 [human chorionic gonadotropin] was not enhanced by PMSG. In fact the response was similar to that of juvenile ovaries and markedly less than that of 1st proestrous rats. This decreased P response was not due to a greater conversion of P to its less active metabolite 20.alpha.-hydroxy-4-pregnen-3-one (20.alpha.-OH-P). Apparently, 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.

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