The relationship between the rat of testosterone infusion and gonadotropin secretion

Prepubertal and young adult male rats were castrated and continuous intravenous infusions of testosterone were administered for 2-3 days. Blood samples were obtained at various intervals and serum concentrations of FSH and LH were determined. In 43-day-old males a dose of approximately 57 mug/day was required to suppress serum LH to levels observed in intact controls, and FSH was also suppressed at this dose. Within one day after termination of infusions serum LH and FSH concentration had returned to (or exceeded) the levels observed in castrates infused with vehicle only. This was true even after a very large dose of testosterone had been administered (918 mug/day). In 59-day-old males serum LH was suppressed somewhat by 40 mug testosterone/day and further by 200 mug/day. FSH did not appear to be suppressed as readily in these rats as compared to the younger animals. Further experiments in which castrated rats were injected with testosterone or testosterone propionate revealed marked differences in the magnitude and time course of the action of these compounds upon LH release. It is suggested that the rate of metabolism of these steroids is important in limiting their effects, especially when administered by single daily injections. Furthermore, it appears that "recovery" of the hypothalamic-pituitary system from exposure to high concentrations of testosterone is rapid following clearance of the steroid from the blood.