Response of muscle protein synthesis to parenteral administration of amino acid mixtures in growing rats.

Previous studies have shown that 1-hour infusions of neither glucose nor an amino acid mixture alone stimulates muscle protein synthesis in postabsorptive rats (Biosci Rep 1986;6:177-183). We have therefore investigated whether longer periods of infusion are required to initiate responses. The effects of intravenous infusions of various substrates for 6 hours on rates of skeletal muscle protein synthesis in fasted rats were studied. Fractional rates of protein synthesis (ks, the percent of muscle protein renewed each day) and the ribonucleic acid activity (kRNA, the amount of protein synthesis per unit of ribonucleic acid [milligrams of protein per day per milligram of ribonucleic acid]) were measured with a flooding dose of L-[4-3H]phenylalanine. Infusion of mixed amino acids (15 mg of nitrogen per hour per rat) increased ks and kRNA by approximately 50% in comparison with saline-infused rats, but rates of muscle protein synthesis remained below those observed in fed animals. Doubling the infusion rate of amino acids to 30 mg of nitrogen per hour per rat had no additional effect on ks and kRNA, and addition of glucose (0.3 g/h per rat) or glucose plus lipid also had no significant additive effect. However, infusion of glucose alone (0.3 g/h per rat) increased ks and kRNA by 25% when compared with saline-infused controls. Infusion of the branched-chain amino acids in the same amounts as present in the amino acid mixture increased ks and kRNA by 20% compared with saline-infused controls, but the rate remained significantly lower than that obtained by infusion of mixed amino acids.