BOTH INHIBIN-A AND INHIBIN-B RESPOND TO EXOGENOUS FOLLICLE-STIMULATING-HORMONE IN THE FOLLICULAR PHASE OF THE HUMAN MENSTRUAL-CYCLE

To ascertain whether changes in the concentrations of the dimeric inhi bins A and/or B (INH-A and INH-B) contributed to the previously descri bed dose-dependent increase in immunoreactive inhibin (INH) in respons e to FSH during the follicular phase of the human menstrual cycle, bot h dimers were measured by specific two-site assays in stored serum sam ples 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-d ependent 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 ob served between INH and each of the specific inhibin dimers (A: r = 0.7 9, P < 0.001; B: r = 0.76, P < 0.001), and the responses of the two di mers were also highly correlated (r = 0.59, P < 0.001). The response o f each inhibin was also highly correlated with the response of serum e stradiol (A: r = 0.45, P < 0.001; B: r = 0.40, P < 0.001). When analyz ed by ANOVA, the INH response of INR-B was significantly above the con trol value at 36 h after treatment with both 100 and 200 IU FSH, where as the response of INH-A was significant only at 200 IU. It is conclud ed that the concentrations of both dimeric INH-A and INH-B are stimula ted by increases in FSH within the physiological range in the follicul ar phase of the human menstrual cycle and that both contribute to the previously observed rise in INH.