Relationship between follicle size and gonadotrophin surge attenuating factor (GnSAF) bioactivity during spontaneous cycles in women

BACKGROUND: We have previously demonstrated that follicles less than or equal to 11 mm diameter from women undergoing IVF contain higher concentrations of gonadotrophin surge attenuating factor (GnSAF) bioactivity than large follicles from the same ovaries. METHODS: To determine whether this finding is relevant to spontaneous cycles, follicular fluid aspirated from 37 follicles between 3 and 25 mm in diameter from 14 pairs of ovaries from regularly cycling women undergoing total abdominal hysterectomy and bilateral salpingoophorectomy for benign gynaecological disease was pooled into size categories (3 + 4, 5 + 6, 7 + 8, 9 + 10, 11 + 12, 14 + 15, 18 and 25 mm), These pools were bioassayed for GnSAF and inhibin-A, inhibin-B and activin-ii concentrations were determined, RESULTS: Follicles of 5 + 6 mm diameter contained the highest concentrations of GnSAF bioactivity (reducing GnRH-induced LH secretion to 38 +/- 8% of control, P < 0.001), while those of 25 mm diameter contained one quarter of this concentration (reducing GnRH-induced LH secretion to 72 +/- 2% of control, P < 0.05). GnSAF bioactivity was closely related to follicle size (r = -0.836, P < 0.01), but not to inhibin-A, inhibin-B or activin-A concentrations. CONCLUSIONS: The finding that small follicles contain high concentrations of GnSAF bioactivity, which fall as folliculogenesis progresses during spontaneous cycles, support the hypothesis that GnSAF has a role in preventing the premature onset of the LH surge in women.