Kaler L W, Neaves W B
Endocrinology. 1981 Feb;108(2):712-9. doi: 10.1210/endo-108-2-712.
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)
在3至24月龄的雄性Sprague-Dawley大鼠中，血浆睾酮浓度下降了50%以上，而血浆中促黄体生成素（LH）的浓度保持相对恒定。在同一时期，体重增长了近50%，这表明，假设血浆容量成比例增加，随着年龄增长，睾酮的总循环量保持相对恒定，并且在老年大鼠中循环中的LH总量实际上增加了。通过证明在本研究中大鼠所代表的年龄和体重范围内，血浆容量与体重成比例增加，证实了这一假设。在最老的大鼠中，注射促性腺激素后达到的血浆睾酮水平显著较低，但在根据血浆容量增加进行校正后，响应注射促性腺激素而添加到循环中的总睾酮量并未随年龄减少。在对照条件下或在孵育培养基中添加促性腺激素后，去包膜大鼠睾丸的睾酮分泌均未检测到与年龄相关的变化。随着年龄增长，单个睾丸间质细胞的平均体积保持在接近年轻成年大鼠的值，而在最老的大鼠中，每个睾丸间质细胞的总数略有增加。因此，老年大鼠雄激素状态的降低不能归因于睾丸间质细胞群体的功能或数量缺陷。相反，血浆睾酮水平低可能是由两个相互关联的睾丸外现象导致的，即分泌的激素在明显更大的体重中被扩张的血浆容量稀释，以及LH水平未能充分升高以刺激额外的睾酮分泌。（《内分泌学》108：712，1981）
