Increased concentration of tracee affects estimates of muscle protein synthesis

Muscle protein synthesis was measured by infusion of L-[H-2(5)]phenylalanin e in two groups of anesthetized dogs, before and during infusion of insulin with euaminoacidemia, and with differing concentrations of unlabeled pheny lalanine (tracee). With the infusion of insulin, muscle protein synthesis i ncreased 39 +/- 12% based on phenylalanyl-tRNA. Calculation with plasma phe nylalanine enrichment overestimated insulin stimulation by 40% (56 +/- 12 v s. 39 +/- 12%). Raising the concentration of plasma phenylalanine twofold d uring infusion of insulin further increased the apparent stimulation of mus cle protein synthesis based on plasma relative to phenylalanyl-tRNA by 225% (65 +/- 19 vs. 20 +/- 14%, P < 0.001). In both experiments, the stimulatio n of synthesis rates calculated from phenylalanine enrichment within the mu scle was closer to that from phenylalanyl-tRNA (48 +/- 19%, experiment 1; 3 0 +/- 14%, experiment 2). Results indicate that the enrichment of a labeled amino acid within plasma and tissue amino acid pools is affected by the co ncentration of tracee infused. Increasing the concentration of tracee overe stimates the insulin-mediated stimulation of muscle protein synthesis when amino acid pools other than aminoacyl-tRNA are used as the precursor enrich ment.