Protein degradation and the regulation of protein balance in muscle

The rate of of protein degradation in muscle changes in many states but the nature of these changes is often paradoxical. Thus there can be increases during growth (anabolic increases) as well as during wasting (catabolic increases). Decreases can occur during growth (anabolic decreases) as well as during wasting (catabolic decreases). These changes are observed in man (as judged by changes in 3-methylhistidine excretion) and in experimental animals. The nature of the changes is not understood but it is possible that muscle growth induces increased degradation as part of the accompanying myofibre remodelling. The rate of protein degradation can also be influenced by thyroid status, since in thyroid deficiency degradation is reduced and can be stimulated by triiodothyronine. This response is independent of changes in muscle growth. Finally, acute exercise suppresses protein degradation in vivo in man as well as suppressing protein synthesis (in vivo in rats). When protein degradation rates change, acid proteinase activities also change in muscle. The anabolic increase in degradation appears to involve increases in mainly cathepsin D whereas catabolic increases in degradation are associated with an increase mainly in pepstatin-insensitive acid autolytic activity.