Seasonal Changes in Steroidogenesis in the Testis of the Rhesus Monkey (Macaca mulatto)

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Abstract
To investigate the influence of seasonal changes from breeding to nonbreeding season on testicular steroidogenesis in the adult rhesus monkey, two animals were hemiorchidectomized in November and the second testis of each was removed the following April. Intratesticular steroid concentrations related to the formation of testosterone from pregnenolone were measured by radioimmunoassay. Related steroidogenic enzyme activities were also assayed and dynamic studies of the bioconversion of radiolabeled precursor steroids by testicular microsomes were performed.Testosterone was synthesized from pregnenolone through both dehydroepiandrosterone and 17α‐hydrox‐yprogesterone. No distinct differences were found in the testosterone biosynthetic pathway between testes in breeding and nonbreeding seasons. In the breeding season, intratesticular testosterone concentrations, the sum of intratesticular steroids measured, and the ratio of testosterone to the steroid sum were increased when compared to those in the nonbreeding season. No major seasonal changes in microsome enzyme activities were found, although enzyme activities, except for Δ5‐3β‐hydroxysteroid dehydrogenase for C21‐steroids, were slightly higher in the breeding season. We conclude that increased testosterone production in the testis during the breeding season is due primarily to increased pregnenolone supply from Leydig cell mitochondria and, to a lesser extent, to an enhanced pregnenolone bioconversion to testosterone in the microsomes.