INHIBIN-A, INHIBIN-B AND ACTIVIN-A IN THE FOLLICULAR-FLUID OF REGULARLY CYCLING WOMEN

Recent measurements of circulating inhibin A and inhibin B concentrati ons indicate that inhibin B map play an important role in the selectio n of dominant follicles. The concentrations of inhibin A, inhibin B an d activin A were measured in the follicular fluids of 61 individual fo llicles (4.8-20 mm in diameter) from 47 regularly cycling women using specific two-site enzyme-linked immunosorbent assays, The microenviron ment of each follicle was characterized by measuring follicular fluid androstenedione and oestradiol concentrations, The mean activin A conc entrations were <8 ng/ml for follicles of all sizes (4-17 mm). Inhibin A concentrations were <1 ng/ml in follicles <6 mm? and progressively increased to concentrations >50 ng/ml in follicles greater than or equ al to 13 mm, Follicles with androstenedione/oestradiol ratios less tha n or equal to 4 had higher concentrations of inhibin A than follicles with androstenedione/oestradiol ratios >4. Inhibin B concentrations we re higher than inhibin A concentrations in all follicles, increasing f rom 19.2 +/- 8.3 ng/ml in 4 mm follicles to 409 +/- 9.6 ng/ml in 13 mm follicles and then declining to 275 +/- 47 ng/ml in 17 mm follicles. These results support the hypothesis that inhibin B may play a more im portant paracrine role in developing follicles and a greater regulator y role with respect to follicle stimulating hormone (FSH) secretion th an inhibin A.