DECREASED INHIBIN-B SECRETION IS ASSOCIATED WITH THE MONOTROPIC FSH RISE IN OLDER, OVULATORY WOMEN - A STUDY OF SERUM AND FOLLICULAR-FLUID LEVELS OF DIMERIC INHIBIN-A AND INHIBIN-B IN SPONTANEOUS MENSTRUAL CYCLES

This study sought to compare circulating and follicular fluid (FF) con centrations of dimeric inhibin A and B utilizing specific two-site ELI SAs for these hormones in normal older and younger ovulatory women. Me thods: Normally ovulating women age 40-45 (n=10) and 20-25 (n=13) were studied throughout the follicular phase with daily blood sampling, tr ansvaginal ultrasound examinations, and dominant follicle aspiration. When the dominant follicle reached a mean diameter of 16 mm or serum e stradiol (E(2)) was greater than or equal to 550 pmol/L, 10,000 IU of hCG was administered intramuscularly followed 32 hours later by transv aginal follicle aspiration. Serum and FF samples were analyzed for E(2 ), FSH, and inhibin A and B. Daily hormone levels were compared by ANO VA, and mean results were compared using t-tests. Results: Older women developed a dominant follicle sooner, meeting criteria for hCG cycle day 10.6+/-0.4 vs. 14.5+/-1.0 p<0.001. As expected, the older group ha d higher maximal serum FSH concentrations compared to the younger wome n (11.4+/-0.5 vs. 8.0+/-0.4 IU/L, p<0.001). We compared hormone concen trations from days - 1 to 3 (where day 0 = day of maximal FSH concentr ation). E(2) concentration was higher in the older women (p=0.002), an d there was no significant difference in inhibin A secretion (p=0.61). In contrast, mean inhibin B concentration was significantly lower in the older women (p=0.04). On the day of aspiration of the dominant fol licle, serum inhibin B was decreased in the older subjects (42.6+/-6.5 vs. 153.1+/-53 pg/ml, p=0.02), whereas older subjects had higher leve ls of inhibin A (106+/-16 vs. 60.4+/-9.4 pg:ml, p=0.04) and similar E( 2) levels (665+/-35.2 vs. 687+/-92 pmol/L, p=0.83). There were no diff erences in FF concentrations of inhibin B (164+/-31 vs. 174+/-37 ng/ml , p=0.85), inhibin A (317.7+/-38 vs. 248+/-57 ng/ml, p=0.16), or E(2) (2074+/-294 vs. 2474+/-338 nmol/L, p=0.82) in the older and younger wo men. Conclusion: Follicular phase inhibin B secretion is decreased in older ovulatory women who demonstrate a monotropic FSH rise, whereas i nhibin A secretion is similar to that in younger women. The dominant f ollicle in these older women appears to be normal in terms of FF E(2) and inhibin content. We speculate that decreased inhibin B secretion m ost likely reflects a diminished follicular pool in older women and ma y be an important regulator of the monotropic FSH rise.