Studies on the regulation of tissue protein sythesis in chickens given various nutrients

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The protein synthesis rate and mRNA levels were measured in the liver and muscle of chicks under various nutritional conditions. Fractional synthesis rate (FSR) of protein was measured using a large dose injection of L-[4-_3H] phenylalanine. Poly (A)+RNA was extracted as mRNA using poly (U)-Sepharose 4B affinity chromatograpy.The influence of varying dietary protein levels (0 to 60%) was examined. The FSRs of both liver and muscle protein incereased with increasing dietary protein levels from 0 to 20% (the protein requirement), whereas when dietary protein increased from 20 to 40%, both rates decreased significantly. The change in liver FSR was accounted for by changes in RNA concentration (RAN : protein ratio) and hepatic mRNA content. The response of muscle FSR to varying dietary protein levels below the requirement is partially regulated by the change in the efficiency of RNA in synthesizing protein, whereas above the requirement by not only the efficiency of RNA but also RNA : mRNA ratio (ribosome number relative to mRNA), mRNA : DNA ratio (mRNA availability) and DNA : protein ratio (DNA concentration).Secondly, the time course changes in liver and muscle protein synthesis and mRNA contents were measured in force-fed chicks followed by refeeding. Liver and muscle FSRs were significantly greater 30 min after refeeding compared to that of unfed chicks.Thirdly, experimental diets containing individual nutrients (protein, carbohydrate, lipid) were force-fed to chicks after 2 d food-deprivation, and no changes in liver FSR were observed. However, muscle FSR returned to the control level 0.5h after consumption of each nutrient. The increase in muscle protein synthesis was explained by the change in the efficiency of RNA in synthesizing protein.