Effects of amino acids on synthesis and degradation of skeletal muscle proteins in humans.

Synthesis and degradation of globular and myofibrillar proteins across arm and leg muscles were examined during stepwise increased intravenous infusion of amino acids (0.1, 0.2, 0.4, and 0.8 g N.kg-1.day-1) to healthy volunteers. Protein dynamics were measured by a primed constant infusion of L-[ring-2H5]phenylalanine and the release of 3-methylhistidine from skeletal muscles. Arterial concentrations and flux of glucose, lactate, and free fatty acids were unchanged despite increasing concentrations of plasma amino acids from 2.6 to 5.7 mM. Plasma insulin, insulin-like growth factor I (IGF-I), and plasma concentrations of IGF-I-binding proteins-1 and -3 remained at fasting levels throughout the investigation. Amino acid infusion caused a significant uptake of the majority of amino acids across arm and leg tissues, except tyrosine, tryptophan, and cysteine, probably due to low concentrations of these amino acids in the formulation. The balance of globular proteins improved significantly (P < 0.01) due to stimulation of synthesis and attenuation of degradation across arm and leg tissues, despite insignificant uptake of tyrosine, tryptophan, and cysteine. Degradation of myofibrillar proteins was uninfluenced by provision of amino acids. The results demonstrate that neither insulin nor circulating IGF-I explained improved protein balance in skeletal muscles after elevation of plasma amino acids. Rather, some amino acids in themselves trigger cellular reactions that initiate peptide formation. Limited availability of some extracellular amino acids was overcome by increased reutilization of the intracellular amino acid.