FACTORS CONTROLLING MUSCLE PROTEIN-SYNTHESIS AND DEGRADATION

Insulin is a well-known anabolic hormone. The mechanism of insulin's p rotein anabolic effect remains controversial. Although insulin undoubt edly inhibits protein degradation, its effect on protein synthesis is incompletely defined. Recent studies reviewed in this article highligh t the methodologic limitations in studying the effect of insulin on pr otein synthesis. These methodological issues are related to the hypoam inoacidemia that ensues after insulin administration and to the diffic ulty in measuring the obligatory precursor pool (aminoacyl tRNA) label . Differential responses to unweighing in different muscle proteins ha s been demonstrated. The protein loss during unweighing is due to the loss of myofibrillar proteins, although sarcoplasmic proteins are spar ed. A recent study has found that lipid emulsion has no effect on whol e protein degradation but decreases forearm protein degradation and sy nthesis. Age-related muscle wasting in humans has been shown to be rel ated to a decline in fractional myofibrillar protein synthesis rate. A lthough some progress has been made by recent studies, refined methodo logies are needed to define the regulation of muscle protein turnover in humans.