FOLLICULAR DYNAMICS AND SECRETION OF INHIBIN AND ESTRADIOL-17-BETA DURING THE ESTROUS-CYCLE OF THE HAMSTER

Plasma and ovarian levels of inhibin were determined by a radioimmunoa ssay (RIA) at 3-h intervals throughout the 4-day oestrous cycle of ham sters. Plasma concentrations of FSH, LH, progesterone, testosterone an d oestradiol-17 beta were also determined by RIAs. In addition, hamste rs were injected at various times with human chorionic gonadotrophin ( hCG) to determine the follicular development. The changes in plasma co ncentrations of FSH after injection of antisera to oestradiol-17 beta (oestradiol-AS) and inhibin (inhibin-AS) on the morning of day 2 (day 1 = day of ovulation) were also determined. Plasma concentrations of i nhibin showed a marked increase on the afternoon of day 1, remained at plateau levels until the morning of day 4, then increased abruptly on the afternoon of day 4 when preovulatory LH and FSH surges were initi ated. A marked during the process of ovulation after the preovulatory gonadotrophin surges. An inverse relationship between plasma levels of FSH and inhibin was observed when the secondary surge of FSH was in p rogress during the periovulatory period. Plasma concentrations of oest radiol-17 beta showed three increase phases and these changes differed from those of inhibin. Changes in plasma concentrations of oestradiol -17 beta correlated well with the maturation and regression of large a ntral follicles. Follicles capable of ovulating following hCG administ ration were first noted at 2300 h on day 1. The number of follicles ca pable of ovulating reached a maximum on the morning of day 3 (24.8 +/- 0.6), and decreased by 0500 h on day 4 (15.0 +/- 1.1), corresponding to the number of normal spontaneous ovulations. Plasma concentrations of FSH were dramatically increased within 6 h after inhibin-AS, though no increase in FSH levels was observed after oestradiol-AS. These fin dings suggest that changes in the plasma levels of inhibin during the oestrous cycle provide a precise indicator of follicular recruitment, and that the changes in plasma concentrations of oestradiol-17 beta ar e associated with follicular maturation. These findings also suggest t hat inhibin may play a major role in the inhibition of FSH secretion d uring the oestrous cycle of the hamster.