We investigated the effects of estradiol on bioactive luteinizing hormone (LH) release in normal men using two complementary strategies: (i) steady state intravenous infusions of estradiol at its endogenous production rate, and (ii) oral administration of the antiestrogen, tamoxifen HCl. Immunoreactive and biologically active LH were monitored by radioimmunoassay and the rat interstitial cell testosterone bioassay, respectively. Estradiol infusions significantly suppressed mean plasma bioactive LH concentrations and decreased the bio/immuno LH ratio. Conversely, antiestrogen treatment enhanced spontaneous bioactive LH pulse frequency, increased bioactive LH pulse amplitude, and augmented plasma intrapulse and interpulse bio/immuno LH ratios. Low-dose pulsed injections of exogenous gonadotropin-releasing hormone (GnRH) also increased plasma bio/immuno LH ratios. However, tamoxifen attenuated the ability of exogenous GnRH to further enhance the bio/immuno LH ratio, which suggests that endogenous LH release was already maximally enriched in LH bioactivity during antiestrogen administration. We conclude that estradiol modulates the pulsatile secretion of LH molecules enriched in biological activity in man.
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