In vitro Activity of Ribosomes and RNA Content of Skeletal Muscle in Young Rats Fed Adequate or Low Protein ,

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Abstract
Young rats (89 g) were fed an adequate (18% casein) or low (3% casein) protein diet for 30 days. The content of RNA in thigh muscle, the sedimentation of ribosomes in sucrose gradients, and the synthetic activity of ribosomes in a cell-free system were studied at intervals during the experiment. The concentration of thigh-muscle RNA decreased during growth in the well-nourished rats, but the absolute amount of RNA increased almost threefold. The low protein diet decreased the concentration of RNA and the absolute amount in thigh muscle remained constant during the study. Ribosomes from muscle of rats fed the low protein diet were less active for protein synthesis in vitro than ribosomes from well-nourished rats. This difference was evident within the first 6 days and was maintained throughout the study. Sedimentation analysis in sucrose gradients revealed a higher proportion of lighter ribosome species in muscle of rats fed the low protein diet, but the differences in polysome profiles did not appear as marked as the consistent differences in the synthetic capacity of ribosomes obtained from muscle of the two dietary groups.