The effect of amino acid infusion on leg protein turnover assessed by L‐[15N]phenylalanine and L‐[1‐13C]leucine exchange

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Abstract
A stable isotope technique depending on the use of [15N]phenylalanine and [l-13C]leucine to assess exchange was utilized to measure the components of protein turnover of the human leg and the effects of amino acid infusion. Eight healthy subjects (28.5 .+-. 2.5 years) were studied when post-absorptive in the basal state and again during infusion of a mixed amino acid solution (55 g l-1, 1.52 ml kg-1 h-1). During the basal period leucine oxidation by the leg was 4.4 .+-. 2.0 nmol 100 g-1 min-1 and this increased threefold during amino acid infusion (13.6 .+-. 3.1 nmol 100 g-1 min-1, mean .+-. SEM, P = 0.003). Amino acid infusion abolished the net negative balance between incorporation of leucine into, and release from, protein (basal, -31.8 .+-. 5.8; during infusion, +3.1 .+-. 7.1 nmol 100 g-1 P = 0.001). Phenylalanine exchange showed a similar pattern (basal, -13.7 .+-. 1.8; during infusion, -0.8 .+-. 3.0 nmol 100 g-1 min-1, P = 0.003). Basal entry of leucine into leg protein (i.e. protein synthesis) was 70.0 .+-. 10.8 nmol 100 g-1 min-1 and this increased during amino acid infusion to 87.3 .+-. 14.1 nmol 100 g-1 min-1 (P = 0.11). Phenylalanine entry to protein also increased with amino acid infusion (29.1 .+-. 4.5 vs. 38.3 .+-. 5.8 nmol 100 g-1 min-1, P = 0.09). Release from protein of leucine (101.8 .+-. 9.1 vs. 84.2 .+-. 9.1 nmol 100 g-1 min-1, P = 0.21) and of phenylalanine (42.8 .+-. 4.2 vs. 39.1 .+-. 4.2 nmol 100 g-1 min-1, P = 0.50) was unchanged by amino acid infusion. The results suggest that, in the post-absorptive state in man, infusion of mixed amino acids, without additional energy substrates; reverses amino acid balance by a mechanism which includes stimulation of muscle protein synthesis but which does not alter protein break-down. Interpretation of the results obtained concurrently on whole-body protein turnover suggests that the increase in muscle protein synthesis contributes substantially to the whole-body increase, but the fall in whole-body breakdown with exogenous amino acids is independent of changes in muscle.