Protein Turnover in Skeletal Muscle

Rates of protein catabolism were compared in nongrowing skeletal muscle, in muscles undergoing work-induced hypertrophy, and in muscles growing in response to treatment with pituitary growth hormone. Hypophysectomized rats were injected initially with 3H-leucine, and on subsequent days received unlabeled leucine and a high protein diet to minimize reutilization of the 3H-leucine. Rates of protein degradation were estimated from the loss of radioactive proteins. Rates of protein synthesis were estimated from the amount of dilution of previously labeled proteins with newly synthesized unlabeled material (decrease in specific activity). Two days after injection of 3H-leucine, hypertrophy of the soleus of one limb was induced by sectioning the tendon of the gastrocnemius muscle. During compensatory growth, the soleus retained more of the labeled proteins than its contralateral control muscle. Nevertheless, the specific activity of the proteins decreased. It is concluded that during hypertrophy there is decreased protein catabolism as well as increased synthesis of new proteins. In addition, the degradation of sarcoplasmic (soluble) proteins decreased more markedly than that of myofibrillar proteins, resulting in a relative increase in the sarcoplasmic proteins during compensatory growth. Treatment of the hypophysectomized rats with bovine growth hormone (1 mg per day) was initiated 2 days after injection of 3H-leucine. Ten days later, the content of labeled proteins in the soleus of hormone-treated animals was indistinguishable from that in the smaller muscles of control rats. However, the specific activity of protein from muscles was significantly lower in the hormone-treated rats. These results indicated that unlike work-induced hypertrophy, growth hormone increased protein synthesis in muscle without changing protein degradative rates.