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Abstract:
In male Sprague-Dawley rats between 3-24 months of age, plasma concentrations of testosterone declined by more than 50% while concentrations of LH in plasma remained relatively constant. During the same interval, body weight rose almost 50%, suggesting that total circulating amounts of testosterone, assuming a proportional expansion of plasma volume, remained relatively constant with increasing age and that total LH in the circulation actually increased in older rats. This assumption was justified by demonstrating that blood plasma volume increased in proportion to body weight over the range of ages and weights represented by rats in this study. Plasma testosterone levels achieved after the injection of gonadotropin were significantly lower in the oldest rats, but when adjusted for increased plasma volume, total testosterone added to the circulation in response to injected gonadotropin did not diminish with age. Age-related change was not detected in testosterone secretion by decapsulated rat testes, either under control conditions or after the addition of gonadotropin to the incubation medium. The average volume of individual Leydig cells remained near young adult values with advancing age, while the total number of Leydig cells per testis rose slightly in the oldest rats. Hence, diminished androgen status in old rats could not be attributed to functional or numerical deficits in the Leydig cell population. Instead, low plasma testosterone levels may have resulted from two interrelated extratesticular phenomena, dilution of secreted hormone with the expanded volume of plasma in a significantly larger body mass, and failure of LH levels to rise sufficiently to stimulate additional testosterone secretion. (Endocrinology 108: 712, 1981)