Effects of thyroxine on protein turnover in rat skeletal muscle

The effects of thyroxine (T₄) on protein turnover in skeletal muscle were studied using normal, thyroidectomized (thyrex), and hypophysectomized (hypox) rats. Thyrex rats had a depressed growth rate that was accompanied by 50% reductions in the levels of RNA and the rate of protein synthesis in gastrocnemius muscle, as determined in the perfused hemicorpus. Protein synthetic efficiency (protein synthesis per unit RNA) was decreased by 18%. Daily treatment of thyrex rats with T₄ at different dose levels for up to 16 days led to improved growth rates, elevated RNA concentrations, and increased protein synthesis rates. The primary effect of T₄ was to increase the protein synthetic capacity of muscle. Protein degradation, determined in the perfused hemicorpus, and activity of a lysosomal protease, determined in unperfused muscle, were reduced in the thyrex condition. Treatment of thyrex rats with T₄ increased protein degradative rates, but not protease activity. Hypox rats, which also exhibited depressed skeletal muscle protein synthesis, responded to T₄ and combined T₄ and growth hormone with marked improvements in protein synthesis.