Nitrogen-sparing mechanisms of singly administered branched-chain amino acids in the injured rat.

Previous studies have demonstrated that the in vivo nitrogen-conserving quality of amino acid solutions might be improved by increasing the percentage of branched-chain amino acids (BCAA) and that infusion of a solution consisting only of the three BCAA-valine, leucine, and isoleucine-in the postoperative period resulted in nitrogen equilibrium. To clarify which of the properties ascribed to the BCAA is responsible for the improved postoperative nitrogen conserving quality, we infused rats undergoing laparotomy and jugular vein cannulation with each of the BCAA or alanine separately. Twenty-four hours before the were killed 5 microCi 14C-tyrosine was added to the infusate to determine total body protein degradation and fractional synthesis rate in liver and muscle. All four amino acid-containing solutions conserved nitrogen as compared with 6.5% dextrose. Fractional synthesis of rate mixed liver protein was significantly increased in all groups receiving BCAA. Only the infusion of valine significantly increased muscle protein synthesis. Total body protein breakdown rate was similarly decreased in all groups receiving amino acids (alanine, valine, leucine, or isoleucine). Total body protein breakdown correlated significantly with the nitrogen balance. The protein-sparing mechanisms of the BCAA in the post-traumatic period are mediated through reduction in whole body protein breakdown, as well as by increasing protein synthesis in both liver and muscle protein. These results appear specific for the BCAA as isonitrogenous amounts of alanine do not give similar results.