Stimulation of In Vitro Rat Muscle Protein Synthesis by Leucine Decreases with Age

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Abstract
Aging is characterized by a decrease of muscle mass associated with a decrease in postprandial anabolism. This study was performed to gain a better understanding of the intracellular mechanisms involved in the stimulation of muscle protein synthesis by amino acids and their role in the decrease of muscle sensitivity to food intake during aging. The effects of amino acids or leucine alone were assessed in vitro on epitrochlearis muscle from young, adult and old rats. Protein synthesis was assessed by incorporation of radiolabeled phenylalanine into protein and p70 S6 kinase activity by incorporation of 32P into a synthetic substrate. Amino acids, at physiologic concentrations, stimulated muscle protein synthesis (P < 0.05) and leucine reproduced this effect. The intracellular targets of amino acids were phosphatidylinositol 3′ kinase and the rapamycin-sensitive pathways mammalian target of rapamycin (mTOR)/p70 S6 kinase. In old rats, the sensitivity of muscle protein synthesis to leucine was lower than in adults (P < 0.05) and this paralleled the lesser ability of leucine to stimulate the rapamycin-sensitive pathways (P < 0.05). We demonstrated that amino acids and leucine stimulate muscle protein synthesis and that aging is associated with a decrease in this effect. However, because aged rats are still able to respond normally to high leucine concentrations, we hypothesize that a nutritional manipulation increasing the availability of this amino acid to muscle could be beneficial in maintaining the postprandial stimulation of protein synthesis.