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Both Inhibin A and B Respond to Exogenous Follicle-Stimulating Hormone in the Follicular Phase of the Human Menstrual Cycle¹

Prince Henry’s Institute of Medical Research, Monash Medical Center (H.G.B., D.M.R.), Clayton, Victoria 3168, Australia; and Oxford Brookes University (N.P.G.), Oxford, OX3 O8P United Kingdom

Address all correspondence and requests for reprints to: Dr. Henry G. Burger, Prince Henry’s Institute of Medical Research, P.O. Box 5152, Monash Medical Center, Level 4, Block 4, 246 Clayton Road, Clayton, Victoria 3168, Australia. E-mail: henry.burger{at}med.monash.edu.au

To ascertain whether changes in the concentrations of the dimeric inhibins A and/or B (INH-A and INH-B) contributed to the previously described dose-dependent increase in immunoreactive inhibin (INH) in response to FSH during the follicular phase of the human menstrual cycle, both dimers were measured by specific two-site assays in stored serum samples from regularly cycling normal volunteers who had received saline as a control (n = 5) or FSH [100 IU (n = 6) or 200 IU (n = 5)] between days 3–5 of the menstrual cycle. Both INH-A and INH-B showed a dose-dependent increase in response to administered FSH; INH-A rose from 13.5 to 35.9 ng/L (P < 0.01), and INH-B rose from 77.8 to 205 ng/L (P < 0.05) at 36 h after 200 IU FSH. Highly significant correlations were observed between INH and each of the specific inhibin dimers (A: r = 0.79, P < 0.001; B: r = 0.76, P < 0.001), and the responses of the two dimers were also highly correlated (r = 0.59, P < 0.001). The response of each inhibin was also highly correlated with the response of serum estradiol (A: r = 0.45, P < 0.001; B: r = 0.40, P < 0.001). When analyzed by ANOVA, the INH response of INH-B was significantly above the control value at 36 h after treatment with both 100 and 200 IU FSH, whereas the response of INH-A was significant only at 200 IU. It is concluded that the concentrations of both dimeric INH-A and INH-B are stimulated by increases in FSH within the physiological range in the follicular phase of the human menstrual cycle and that both contribute to the previously observed rise in INH.