Testicular Metabolism and Serum Testosterone in Aging Male Rats

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Abstract
Male Long-Evans rats, 4 and 18 mo. old, were studied for age differences in competence of androgen production. Testes from these animals were incubated in vitro with labeled progesterone. In the older rats, testosterone production was significantly lower than in the young rats. Aging also resulted in a sharp decline in 5.alpha.-androstane-3.alpha.,17.beta.-diol production and a concomitant increase in a polar compound tentatively identified as 7.alpha.-hydroxytestosterone, but had no effects on the formation of 17.alpha.-hydroxyprogesterone, androstenedione and 5.alpha.-dihydrotestosterone. Basal levels of serum testosterone were measured to a circadian time course in both 4 and 18 mo. old rats. Serum testosterone was significantly greater at all times in the younger animals with the exception of 2000 h and of 2400 h, the latter being the nadir for both age groups. Peak levels of serum testosterone were found at 0800 h and 1600 h in the young and at 0800 h and 2000 h in the older animals. The older rats exhibited reduced testosterone levels but circadian rhythmicity was retained in a manner that mimicked that of the young rats. Testicular responsivity to human chorionic gonadotropin (hCG) was compared in the 2 age groups. Whereas both groups of animals responded to 50 IU of hCG with a rise in serum testosterone, a delayed responsiveness of lower magnitude was noted in aging. There is an inherent loss in steroidogenic function of the rat testis in aging.