Response of Plasma Testosterone to Human Chorionic Gonadotropin Stimulation in the Ram

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Abstract
The kinetics of responsiveness of sheep testes to hCG [human chorionic gonadotropin] stimulation was studied following injection of 6000 IU hCG. When 4 rams received a single i.v. injection each, plasma testosterone (T) increased sharply (0.81 .+-. 0.27 to 3.76 .+-. 0.41 ng/ml) within 2 h, decreasing thereafter to reach control levels at about 10 h, despite the fact that hCG levels were still high. Subsequently, T levels increased to reach the 2 h levels at 20 h. A 2nd delayed peak (7.16 .+-. 0.47), significantly higher (P < 0.001) than that observed at 2 h was seen between 96 and 120 h. T levels declined thereafter to reach the control levels 8-9 days after the 1st injection. Two rams received 2 i.v. injections at 24 h intervals. The profile of plasma T after the 1st injection was similar to that observed in the 1st experiment. The 2nd hCG injection did not induce a significant modification of T levels. The 2nd delayed peak of plasma T was seen at 120 h and was followed by a decline in plasma T levels. A 3rd hCG injection 8 days after the 1st induced a rapid increase of plasma T, the levels being similar to those of the 2nd peak, but significantly higher than those observed at 2 h. In the ram, as in other species, hCG apparently induces testicular desensitization; the pattern of plasma T between 2 and 20 h after hCG administration was not described before during hCG-induced desensitization.