Hormonal regulation of skeletal muscle hypertrophy in rats: the testosterone to cortisol ratio

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Abstract
This study determined the influence that the catabolic hormone, corticosterone (C), and the anabolic hormone, testosterone (T), had in regulating skeletal muscle hypertrophy using the rat hind limb ablation model. Specifically, the ratio of T : C (TCR) was manipulated via hormone implants and injections and concentrations measured to evaluate the relative contribution of each hormone to skeletal muscle protein balance. Skeletal muscle growth was measured 16 days after gastrocnemius muscle ablation. Elevations in plasma concentrations of C (via daily C injections, 50 mg · kg−1 body mass) resulted in TCR of 0.007 that was less than the control group TCR of 0.249. In this C-injected group, whole body and skeletal muscle atrophy was elicited-this being greater in the fast-twitch plantaris muscle than in the slow-twitch soleus muscle. The overloaded leg resisted the C-induced atrophy. Castration of animals (TCR 0.024) resulted in less whole body and skeletal muscle growth. However, elevations in plasma concentrations of T (two groups, with TCR of 1.35 and 1.64) did not result in significantly greater muscle growth. Furthermore, T was also ineffective in antagonizing the C-induced atrophy in a group that received both T implants and C injections. This group had a TCR of 0.175 that was similar to the control group ratio of 0.249 that received no manipulations. We concluded that glucocorticoids were able to induce pronounced atrophy, but at the same time overloaded muscles were able to over-ride the glucocorticoid signal. Plasma concentrations of C were a better predictor of muscle growth/atropy than T and/or the TCR. In addition, it is suggested that the volume of contractile activity of the muscle is perhaps an important determinant of C-induced atrophy, because less atrophy occurs in the more active slow twitch muscles.