Evidence that androgen negative feedback regulates hypothalamic gonadotropin-releasing hormone impulse strength and the burst-like secretion of biologically active luteinizing hormone in men.

Abstract

Testosterone injections or pharmacological amounts of dihydrotestosterone infused in men, and androgen-secreting tumors in women, can suppress plasma LH bioactivity assessed in an in vitro rat Leydig-cell bioassay. However, such observations do not define the physiological nature of endogenous androgen feedback actions on the hypothalamo-pituitary axis. To explore the feedback role of endogenous androgen on the male gonadotropic axis, we used a potent, selective, nonsteroidal competitive antagonist of the androgen receptor, flutamide HCl. Eight young men (ages 21-30) each received flutamide (750 mg orally daily x 3 days) and placebo and underwent blood sampling at 10-min intervals for 28 h, the last 2 h of which included two consecutive iv pulses of GnRH (10 micrograms). Plasma bioactive LH concentrations were measured in the rat Leydig cell bioassay. Deconvolution analysis was used to evaluate the number, amplitude, mass, and duration of bioactive LH secretory bursts and simultaneously estimate the half-life of endogenous LH. Flutamide treatment increased mean plasma bioactive LH concentrations from 27 +/- 2.3 to 54 +/- 9.1 IU/L (P = 0.018). Increased LH concentrations were achieved by a significantly amplified mass of LH secreted per burst, which rose from 14 +/- 1.8 (control) to 24 +/- 2.8 (flutamide) IU/L distribution volume. The amplitude (maximal secretion rate) of bioactive LH release episodes also increased from 1.3 +/- 0.21 (control) to 2.2 +/- 0.29 (flutamide) IU/L/min. These responses were specific, since flutamide did not influence bioactive LH half-life [49 +/- 6.5 (control) vs 52 +/- 4.1 (flutamide) min], LH secretory burst duration, frequency, or interburst interval. The total 24-h production rate of bioactive LH rose significantly from 310 +/- 35 (control) to 570 +/- 82 (flutamide) IU/L.day. In contrast, no features of LH secretory bursts evoked by exogenous GnRH pulses were altered significantly by antiandrogen. In summary, in vivo blockade of endogenous androgen negative feedback actions in normal men selectively amplifies the mass and amplitude of bioactive LH secretory bursts without altering their number or duration, or the half-life of LH, or the amount of LH released in response to exogenous GnRH. Therefore, we infer that in the steroid milieu of normal men endogenous androgen acting via the androgen receptor can negatively regulate hypothalamic GnRH stimulus strength, and hence the rate and mass of biologically active LH secretion in vivo.