The temporal response of rat testes to hCG stimulation during sexual maturation

Serum testosterone responses to a single sc injection of hCG (25 IU/100 g body weight) were monitored for 5 days in rats throughout sexual maturation (22–70 days). Two hours after hCG injection serum testosterone levels rose in 22, 37 and 53 day‐old animals and remained elevated for 2 days, returning to control levels on day 3. This response differed markedly from the biphasic secretion of testosterone reported for adult animals. In 70 day‐old animals the serum testosterone response approached that seen in adult animals. Testosterone levels were elevated 2 h after hCG injection (25.4 ± 2.5 ng/ml) and declined significantly at 12 and 24 h to 17.1 ± 1.0 and 16.1 ± 3.4 ng/ml, respectively. Testosterone levels tended to increase again on days 2 and 3 (19.9 ± 2.8 and 21.1 ± 3.5 ng/ml, respectively) but the increase was not statistically significant. This response differed markedly to the biphasic secretion of testosterone reported for adult animals. In vitro patterns of basal and hCG‐stimulated testosterone secretion by decapsulated testes following a single hCG injection also changed during sexual maturation. In 22 day‐old animals the testes exhibited refractoriness to in vitro hCG stimulation at 12 h, but testes from 37 day old rats were refractory from 2 to 24 h. In vitro testosterone responses of testes from 53 and 70 day‐old rats were similar to that reported for adult rats with a period of refractoriness from 12 h to 2 days. This study demonstrates that during sexual maturation in the rat alterations occur in the temporal patterns of testosterone secretion in vivo and in vitro following hCG stimulation.