RESPONSE OF PROTEIN-SYNTHESIS IN HUMAN SKELETAL-MUSCLE TO INSULIN - AN INVESTIGATION WITH L-[H-2(5)]PHENYLALANINE

The role of insulin in the regulation of muscle protein synthesis in a dult humans has been investigated with intravenous infusion of insulin at levels comparable with those observed after normal feeding. Glucos e was also infused to maintain euglycemia. Muscle protein synthesis wa s measured in six healthy subjects before and during insulin and gluco se infusion from the incorporation of L-[H-2(5)]phenylalanine into the protein of vastus lateralis sampled by percutaneous biopsy. L-[H-2(5) ]phenylalanine was given as a single injection of a flooding amount (4 5 mg/kg). The relatively low levels of enrichment of phenylalanine in protein (0.005 atom%) were measured by modified gas chromatography-mas s spectrometry and verified by comparison with incorporation of L-[2,6 -H-3]phenylalanine. Similarity of enrichment in tissue-free and plasma pools (flooding) and linear incorporation over the period of measurem ent were also verified. The fractional rate of muscle protein synthesi s in the group of postabsorptive subjects was 1.65 +/- 0.11% (SE)/day. The rate was unaltered by insulin and glucose infusion, 1.66 +/- 0.16 %/day.