Summary. The effects of s.c. administration of oil, testosterone, 5\g=a\-dihydrotestosterone, 5\g=a\-androstane-3\g=a\,17\g=b\-diol and oestradiol-17\g=b\ on plasma concentrations of LH and FSH were determined in 5 orchidectomized dogs. The dosages for the androgens and oestradiol-17\g=b\ were 500 and 50 \g=m\g/kgbody weight, respectively. Testosterone and oestradiol-17\g=b\significantly reduced plasma gonadotrophin concentrations, although the onset and duration of their suppressive effects differed. Dihydrotestosterone and oil had no effect on either gonadotrophin. Administration of androstanediol had no effect on plasma concentrations of LH but did cause a temporary and significant reduction in FSH. It is concluded that testosterone and oestradiol-17\g=b\ are major regulators of gonadotrophin secretion in the male dog, but the 5\g=a\-reductionof testosterone seems to play only a minor role in this control.
To characterize the effects of testosterone (T) pretreatment on the resenous gonadotropin‐releasing hormone (GnRH), intact male dogs were injected subcutaneously with either oil or 500 μMg/kg of T in oil at 1, 3, 6, 12, or 24 hours prior to intravenous GnRH administration (50 ng/kg). The pre‐GnRH levels of plasma LH were reduced in all groups of T‐treated dogs except in animals given T 1 hour before GnRH. The concentrations of plasma LH during both the peak‐response period and the recovery period following GnRH administration in animals injected with T did not differ from those in animals injected with oil. These results indicate that T pretreatment has no effect on the ability of the pituitary to respond to exogenous GnRH at all time periods tested, and imply that direct feedback of T on the pituitary may not be acutely involved in steroid negative feedback in the male dog. Unexpectedly, however, there was some indication that the time of injection of either oil or T could affect the response of the pituitary to GnRH, and this may represent a stress phenomenon.
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