AJP - Endocrinology and Metabolism, Vol 271, Issue 4 E718-E724, Copyright © 1996 by American Physiological Society
Effects of amino acids on synthesis and degradation of skeletal muscle proteins in humans
E. Svanberg, A. C. Moller-Loswick, D. E. Matthews, U. Korner, M. Andersson and K. Lundholm
Department of Surgery, Sahlgrenska University Hospital, University of Goteborg, Sweden.
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.
