Effects of increasing the frequency of low doses of gonadotropin-releasing hormone (GnRH) on gonadotropin secretion in GnRH-deficient men.

The effects of increasing the frequency of pulsatile GnRH administration on LH and FSH responsiveness were studied in five GnRH-deficient men who had achieved normal sex steroid levels during prior long term GnRH replacement. Intravenous doses of GnRH were employed that had previously been demonstrated to produce LH and FSH levels in each subject similar to those in normal men. Both acute and chronic changes in pituitary responses were studied after progressive increases in GnRH frequency (from every 120 to 60 min, from 60 to 30 min, and from 30 to 15 min) during three 12-h admissions, each separated by 7 days. During the two intervals between the studies GnRH frequency was 60 and 30 min, respectively. Pituitary responses were characterized by determining the mean serum LH and FSH levels, LH pulse amplitudes, and mean LH and FSH levels which were normalized for the frequency of GnRH administration (nLH and nFSH). As the frequency of GnRH stimulation was increased acutely, mean serum LH levels rose progressively, in contrast to both LH pulse amplitude and nLH levels which decreased, while serum testosterone (T) concentrations remained constant. No further evidence of gonadotroph desensitization occurred after chronic GnRH administration at either 60- or 30-min intervals. At higher frequencies of GnRH stimulation, discrete pulses of LH were not always apparent after injections of GnRH, and in two men, marked destabilization of the gonadotroph responses occurred. Even without detectable LH pulses, serum T levels did not decline during administration of GnRH at intervals as rapid as 15 min. In contrast, there was no change in mean FSH concentrations, although nFSH values decreased progressively as the GnRH frequency was increased. nFSH levels fell to a greater degree than nLH after each increase in GnRH frequency. Thus, pituitary gonadotroph responsiveness to a fixed dose of GnRH decreased as the frequency of GnRH stimulation increased. FSH responsiveness decreased to a greater degree than did LH. Gonadotropin secretory responses are destabilized at higher frequencies of GnRH administration. Pulsatile LH stimulation of the testes does not appear necessary to maintain T secretion.