Role of glucocorticoids in the response of rat leg muscles to reduced activity.

Adrenalectomy did not prevent atrophy of rat soleus muscle during 6 days of tail cast suspension. Cortisol treatment enhanced this atrophy and caused atrophy of the weight-bearing soleus and both extensor digitorum longus (EDL) muscles. Unloading led to increased sarcoplasmic protein concentration in the soleus, but cortisol administration increased the myofibrillar (+stromal) protein concentration in both muscles. Suspension of hindlimbs of adrenalectomized animals led to faster protein degradation, slower sarcoplasmic protein synthesis, and faster myofibrillar protein synthesis in the isolated soleus, whereas with cortisol-treated animals, the difference in synthesis of myofibrillar proteins was enhanced and that of sarcoplasmic proteins was abolished. Both soleus and EDL of suspended, cortisol-treated animals showed faster protein degradation. It is unlikely that any elevation in circulating glucocorticoids was solely responsible for atrophy of the soleus in this model, but catabolic amounts of glucocorticoids could alter the response of muscle to unloading.