Immunoreactive inhibin in human follicular fluid in an ovarian hyperstimulation programme for in vitro fertilization: correlations and different forms

The inhibin concentration in 131 samples of human follicular fluid obtained from 31 women undergoing ovarian hyperstimulation for in vitro fertilization was measured using specific double antibody radioimmunoassay. We used the synthetic 1-32-α-inhibin as standard and radioiodinated 1-32-Tyr-α-inhibin as tracer. Antibodies were raised in rabbits by immunization with the synthetic peptide. Estradiol and progesterone concentrations were measured using commercial radioimmunoassays. Results: The inhibin concentration correlated with the estradiol (r = 0.57, N = 88.p<0.0001) and progesterone (r=0.82, N=88. p<0.0001) concentrations in human follicular fluid. The dosage of human menopausal gonadotropin given to individual patients correlated with the average inhibin concentration measured in their follicles (r = 0.72, N = 23, p<0.0001). Similarly, the size of follicles correlated with their inhibin content (r = 0.75, N = 131, p<0.0001). Nineteen samples of human follicular fluid originating from follicles of different size and volume were examined using gel-chromatography. In each human follicular fluid the main form of inhibin (32 kDa) was recovered. In small follicles (3 ml) we found 12.8±9.1% (mean±sd) of the whole immunoreactive inhibin eluating in the area of V0 (≤80 kDa). In the larger follicles (4-7 ml), however, only 4.4±4.2% of this large inhibin form could be found. Conclusions: Our data confirm that human menopausal gonadotropin stimulates ovarian inhibin production. In addition to the estradiol and progesterone concentrations, the inhibin concentration may be an index of granulosa cell function and follicular maturation. The occurrence of large molecular weight forms of inhibin in small follicles remains unclear. They may represent large precursor molecules which are proteolytically cleaved in more mature follicles.