PLASMA TESTOSTERONE IN BULLS

Abstract

Injections of 375-6000 IU HCG [human chorionic gonadotropin] to 10 bulls, 7 mo. of age, gave no dose related response in plasma testosterone [T] measured in samples collected during the following 9 h. An increase in the dose of HCG was followed by an increase in maximal T values (r = +0.9), an extension of the period between injection and maximal value and an increased duration of the response. Single i.v. or i.m. injections of 6000 IU PMSG [pregnant mare''s serum gonadotropin] to 4 bulls led to initial elevations of plasma T to about the spontaneous diurnal maxima, followed by a prolonged additional elevation to approximately 2-3 times this level. Values decreased to pre-treatment levels on days 16-18 following i.m. injection and 20-22 days following i.v. injection. In dose-response studies 750-12,000 IU PMSG gave similar response in plasma testosterone measured in samples collected 4 and 5 h after the injection, while a dose-dependent response in plasma testosterone was found with regard to maximal values as well as duration of the effect. When comparing the plasma testosterone response following the 2 gonadotropins some differences were found. Injections of 6000 IU of the 2 gonadotropins showed that PMSG required longer time to cause any effect upon plasma testosterone than HCG. A dose of 375 IU HCG was sufficient to evoke a prolonged effect while 750 IU PMSG was not. When a sufficient PMSG dose to evoke a prolonged effect was used, the duration of the effect was markedly longer after PMSG administration than after a similar HCG-dose.