The role of glucose, long-chain triglycerides and amino acids for promotion of amino acid balance across peripheral tissues in man.

The role of amino acids, glucose and lipids in improving amino acid balance in peripheral tissues was evaluated. Primed constant infusion of L-[ring-2H5]phenylalanine in combination with flux measurements of glucose, free fatty acids (FFA) and amino acids across arm and leg tissues were applied in male volunteers after an overnight fast with subsequent primed constant infusions of amino acids (0.2 g N kg-1 body weight day-1), long-chain triglycerides (0.98-1.079 g kg-1 day-1) and glucose (3.13-3.62 g kg-1 day-1). Amino acids and phenylalanine tracer infusion continued for 6 h; the lipid infusion was provided during 2-6 h from the start, and glucose infusion was provided between 4 and 6 h. Flux measurements were performed at steady state before the next infusion started. Arterial concentrations of infused substrates increased during provision, but remained constant thereafter. Plasma insulin increased when glucose was provided, whereas insulin-like growth factor (IGF) I was unchanged during all infusions. Blood flow was unchanged in arm tissue during all infusions, while leg blood flow increased during fat and glucose infusion. FFA and glucose balance were unchanged during amino acid infusion but improved during lipid and glucose infusions. Amino acid balance was negative across arm and leg tissues in the fasted state, but reached balance during amino acid infusion. This effect was equally dependent on protein synthesis and protein degradation without any contribution from lipids and glucose. 3-Methylhistidine release from tissues was not influenced by any substrate. Our results suggest that extracellular amino acid concentrations determine amino acid balance across peripheral tissues independently of non-protein calories, insulin and IGF-I.