Is inhibin B a pharmacodynamic parameter for FSH in normal men?

Objective: This study aims to investigate the pharmacodynamic effect of FSH on inhibin B serum levels in normal men in order to elucidate the physiolo gical regulation of inhibin B secretion in more detail. Design and Methods: Injections of 3000 IU recombinant, human FSH (rhFSH) we re followed by single-blinded injections of placebo, 1000 and 2000 IU rhFSH spaced by at least 28 days between injections. Results: After injection of 3000 IU rhFSH. inhibin B values were significan tly elevated above baseline for 24, 96 and 120 h (maximal increase after 96 h, mean +/- S.E.M. 303 +/- 18 pg/ml). Injection of 2000 IU rhFSH led to a significant increase in inhibin B (maximum mean +/- S.E.M. 318 +/- 20 pg/ml ) from 24 to 120 h. Injection of 1000 IU rhFSH led to a significant increas e in inhibin B after 96 h (maximum mean +/- S.E.M. 300 +/- 16 pg/ml). The i nhibin B areas under the curve after injection of 2000 and 3000 IU rhFSH we re significantly higher than those following the placebo and 1000 IU rhFSH. In the 12 fertile men investigated, at baseline a strong diurnal rhythm of inhibin B parallel to that of testosterone was observed, Conclusions: Serum inhibin B can be considered only a partial pharmacodynam ic parameter of FSH in vivo, since the integrity of the spermatogenic proce ss appears to be a second fundamental component in the regulation of its se cretion from the testis.