Effect of Testosterone Implants on Serum Gonadotropin Concentrations in the Male Rat

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Abstract
Subcutaneous implantation of testosterone in Silastic capsules in castrated male rats results in sustained elevations of serum testosterone. In rats castrated and implanted with capsules 2cm long at 30 days of age, serum testosterone concentrations were elevated within 24 h and then gradually declined during the following nine days. This decline appears related to the animals’ growth. In rats castrated and implanted at 30 days of age with capsules ranging in length from 0.5 to 2cm, serum testosterone concentrations at autopsy 5 days after implantation were comparable to those of intact control animals of the same age. The capsules suppressed the castration induced rise in serum gonadotropin concentrations. Similar results were obtained with rats castrated and implanted with either one or two capsules 4cm long at 80 days of age. Although these results suggest that serum testosterone concentrations in either sexually immature or mature male rats can regulate serum gonadotropin concentrations, the question arises whether the sustained release of testosterone from Silastic capsules mimics the pattern of endogenous testosterone secretion.