A switch from continuous to episodic testicular testosterone release in response to pulsatile LH stimulation in juvenile rhesus monkeys (Macaca mulatta).

This study examined the ontogeny of the testicular testosterone response to precocious pulsatile LH stimulation in the juvenile rhesus monkey. LH stimulation was achieved with an i.v. infusion (one pulse every 3 h) of either single-chain human (sch)LH, administered alone or in combination with recombinant human (rh)FSH, or recombinant monkey (rm)LH in combination with rmFSH. Homologous gonadotropin treatment resulted in an adult profile of circulating mLH concentrations. The schLH infusions produced a similar pulsatile pattern in circulating LH with peak concentrations of approximately 5 IU/l. Although a robust testicular testosterone response was observed after 24 h of intermittent LH stimulation, surprisingly testosterone release at this time was continuous. The apulsatile mode of testosterone secretion, however, did not persist, and a switch to an unequivocal episodic mode of secretion, comparable to that observed in adult monkeys, occurred by day 4 of LH stimulation. FSH did not influence the pattern of the testosterone response. We conclude from these findings that progenitor Leydig cells in the primate testis are able to respond rapidly to a physiological LH stimulus. While the cell biology underlying the switch from a continuous to a pulsatile mode of testosterone secretion remains unclear, we suggest that this phenomenon may be related to the hypothesis that episodic testosterone secretion is required for the operation of the neuroendocrine axis governing testicular function.