Influence of insulin and contractile activity on muscle size and protein balance.

Although the biochemical mechanism by which insulin and contractile activity affect protein turnover in muscle are still unclear, certain physiologic conclusions can be made; (a) Increased work can induce muscle hypertrophy even in the diabetic or starving animal. Thus, work-induced growth differs from normal growth of muscle in not requiring insulin. (b) Like insulin, repeated contractions stimulate the transport of amino acids into muscle. Contractile activity or passive tension can also reduce the rate of protein degradation in this tissue. These effects can be shown with isolated muscles in vitro, but the mechanisms coupling contractile activity to these anabolic processes are unknown. (c) Insulin also reduces overall protein breakdown in skeletal muscle, heart, and liver, apparently by regulating lysosomal function. (d) Insulin reduces selectively the breakdown of cell proteins with relatively long half-lives. This hormone does not affect the rapid breakdown of abnormal proteins which probably does not occur within the lysosome. (c) Normally, liver and muscle degrade large, acidic cell proteins quite rapidly, but this selectivity is lost in the tissues of diabetic or starved organisms, in which proteolysis is accelerated.