Hormonal regulation of protein degradation and synthesis in skeletal muscle

A variety of factors have been shown to influence rates of protein breakdown in rat skeletal muscles. Muscles isolated from hypophysectomized rats show lower rates of protein synthesis and breakdown than those from normal controls. The lack of growth hormone is primarily responsible for the lower rates of protein synthesis, but this hormone does not affect overal protein catabolism. The lack of thyroid hormones is responsible for reduced protein breakdown in muscle after hypophysectomy or thyroidectomy. Treatment with triiodothyronine or thyroxine stimulates protein breakdown as well as synthesis in muscle after a lag time of 2 days. This acceleration of protein catabolism accounts for the decrease in weight of muscle and liver in hyperthyroidism. Thyroid hormones also increase the content of lysosomal proteases and other hydrolases in muscle and liver, and this effect may be responsible for the concomitant increase in protein breakdown in these tissues. In fasting, proteolysis in muscle rises and protein synthesis falls to provide the organism with amino acid precursors for gluconeogenesis and protein synthesis. However, in muscles of adrenalectomized rats, protein breakdown does not increase and may even decrease on fasting. Consequently these muscles show no net loss of protein content. Treatment of these animals with glucocorticoids leads to an increase in protein breakdown and increased release of amino acids. This effect is evident in muscles of fasted but not fed animals. The ability of glucocorticoids to promote protein catabolism in muscle during fasting complements their actions in stimulating hepatic gluconeogenesis.