Synthesis and Degradation of Mixed Proteins, and Composition Changes in Skeletal Muscle of Malnourished and Refed Rats

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Abstract
Selected compositional parameters of hind leg skeletal muscle and the rates of synthesis and degradation of mixed proteins were measured in rats (154 g initial weight) after ad libitum feeding a low protein diet for 17 weeks, and after 2 to 9 days and 59 to 66 days of refeeding with an adequate diet. The half-life for degradation of the mixed muscle proteins was determined from the change in total hind leg radioactivity between 2 and 7 days following an intravenous injection of 4-14C-aspartic acid. From these data and the size of the hind leg muscle protein pool, the rate of muscle protein synthesis was calculated. The prolonged protein-depletion phase resulted in loss of body weight, increase in the water, chloride and DNA concentrations and a decrease in lipid and RNA in muscle. Refeeding rapidly reversed these changes. The half-life for catabolism of mixed proteins in muscle of depleted rats was 10.2 days as compared (P > 0.1) with a value of 13.1 days for well-nourished weight-control rats (74 g). During the first 9 days of refeeding, the synthesis of mixed muscle proteins proceeded at a greater rate (P < 0.01) than in the weight control rats. In contrast, the catabolism of muscle protein was abolished during this period. After 2 months of refeeding, the half-life for catabolism of mixed muscle proteins was 12.6 days. It is concluded that increased synthesis and decreased catabolism of protein play major roles in bringing about a rapid accumulation of muscle protein during early protein refeeding.