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

Abstract

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.

Enhancement of Myofibrillar Proteolysis Following Infusion of Amino Acid Mixture Correlates Positively with Elevation of Core Body Temperature in Rats

“…Therefore, the composition of AA given may account for previous discrepancies in AA effects on myofibrillar proteolysis. In fact, we used a commercially available AA mixture (Table 1) as Svanberg et al did (20). Another plausible explanation is that insufficiency of non-protein energy might have caused the increment of plasma MeHis concentrations in this study.…”

“…Results from several previous reports support the present finding about enhanced myofibrillar proteolysis following AA administration. Svanberg et al noted that alterations in MeHis release from human arm and leg muscle tissues in response to increasing doses of AA mixture infusion showed an increasing trend (20). Furthermore, urinary MeHis output was elevated in response to increased dietary protein levels in rats under conditions of restricted food intake (21).…”

Amino Acids, Glutamine, and Protein Metabolism in Very Low Birth Weight Infants

“…Previous studies in human adults and newborn infants have only examined the responses during 3-6 h of amino acid infusions. These acute responses have been attributed to increased substrate availability and independent of plasma insulin and IGF (38). Although the data from our study cannot identify the mechanism of the transient response, we speculate that the adaptation to the higher amino acid concentrations may have resulted in inhibition of the amino acid transporter systems.…”

Síntese proteica em suínos: como fêmeas, machos não castrados e castrados respondem a este processo?

“…Com relação aos fatores nutricionais, sabe-se que os AA são necessários como precursores para a síntese proteica (Garlick & Grant, 1988), sendo que o aumento da disponibilidade de AA exógenos demonstrou elevar a síntese proteica no músculo (Svanberg et al, 1996, Biolo et al, 1997.…”