THE SEPARATE AND COMBINED EFFECT OF LEUCINE AND INSULIN ON MUSCLE FREE AMINO-ACIDS

The effect of insulin and leucine on amino acid and protein metabolism in muscle is not fully understood. To characterize their separate and combined effects on free amino acids in muscle and plasma, 11 volunte ers received an infusion of either leucine (1 g h(-1), Group 1) or glu cose (20 g h(-1), Group 2) for 2 h followed by a combination of the tw o infusions for an additional 2-h period. In muscle both the leucine i nfusion and the leucine plus glucose infusion increased the concentrat ion of free leucine significantly, while the sum of the other branched chain amino acids (BCAA), of the aromatic amino acids and of the basi c amino acids decreased. Glucose infusion alone decreased the sum of t he essential amino acids, the BCAA and the aromatic amino acids. The c ombination of leucine and glucose augmented the decreases, while the c oncentrations of glutamate, glutamine and alanine were unaffected. In plasma the leucine infusion doubled the leucine concentration-and-decr eased alanine, valine, methionine, tyrosine, phenylalanine and the sum of the aromatic amino acids. Glucose infusion decreased methionine, s erine, isoleucine and the sum of the essential amino acids and of the BCAA. The combination of leucine infusion and hyperinsulinaemia augmen ted the decreases. The plasma concentrations of the keto acids of vali ne and isoleucine decreased by the leucine infusion while the concentr ations of the keto acid of leucine and isoleucine decreased by glucose infusion. The combination of leucine and glucose had an additive effe ct. These effects are attributed to a specific effect of leucine on th e other two BCAA and a depression of muscle proteolysis by both leucin e and insulin, resulting from glucose infusion.