In vivo patterns of circulating testosterone following castration and intramuscular testosterone propionate injections of adult male rats.

In vivo patterns of circulating testosterone (T) were investigated in castrated and in intact male rats following a single intramuscular injection of 1mg testosterone propionate (TP). In addition, the in vivo clearance pattern of T was investigated following castration. The injection of TP into intact rats resulted in a rapid, statistically significant increase in circulating T which lasted for 12h and returned to normal by 24h post-injection. Circulating T decreased to nondetectable levels by 6h following orchiectomy. Injection of TP into castrated rats caused a rapid increase in circulating T which reached highest levels 60 min after administration. The concentrations of T in TP-treated orchiectomized rats remained elevated for 12h with a gradual decline which persisted for the experimental period of 6 days post-injection and provided moderate maintenance of sex accessory organ weights. TP injections increased circulating T levels in both groups of rats, but the magnitude of elevation was greater in intact animals where the normal T diurnal variations were disturbed. The elevation in circulating T persisted longer in the castrate-TP-treated animals than in intact animals, suggesting a difference in clearance, distribution and metabolism of T in orchiectomized rats.