Presence and possible function of activin-like substance in human follicular fluid

We assessed the presence of an activin-like substance in human follicular fluid that was obtained from women undergoing in-vitro fertilization using a bioassay for activin A. Activin activity was not detected in crude follicular fluids; the bioactivity of standard activin A was inhibited by the addition of follicular fluid. After the follistatin (binding protein of activin A) was removed from follicular fluid using a purification procedure, activin activity was detected in the follicular fluids (mean concentration: 131 +/- 40 ng/ml). Activin activity was inhibited by the addition of follistatin to fluid. The concentration of activin activity was substantially higher (approximately 100-fold) than that reported in serum. The concentration negatively and significantly correlated with the number of developed follicles in the ovary (r = 0.501, P < 0.01). These results suggest that activin A and its binding protein are present in follicular fluid in large amounts and that they may have a role in local ovarian regulation.