A new method to measure muscle protein synthesis in humans by endogenously introduced d₉-leucine and using blood for precursor enrichment determination

Enrichment from the easily accessible blood amino acid pool is commonly used as precursor enrichment to calculate rates of muscle protein fractional synthesis in relevant human studies in lieu of the less accessible muscle fluid amino acid pool. However, the accuracy of this approach depends largely on the extent to which there is low discrepancy in free amino acid enrichment between blood and muscle. Steady-state gradient (i.e., ratio) of amino acid enrichment between blood and muscle fluid in the basal state and in response to amino acid infusion were determined in five healthy subjects, and in association with two separate tracers: d₉-leucine, introduced endogenously by the metabolism of d₁₀-leucine (i.e., L-[2,3,3,4,5,5,5,6,6,6-²H₁₀]leucine) infused in blood, and¹³C₆-phenylalanine introduced/infused in blood. The blood-tomuscle fluid amino acid enrichment ratio was lower (P < 0.05) for d₉-leucine compared to s¹³C₆-phenylalanine both before (1.5 ± 0.1 vs. 2.5 ± 0.1) and during (1.1 ± 0.1 vs. 1.2 ± 0.1) amino acid infusion. Importantly, the decrease in this ratio in association with the amino acid infusion was considerably less for the d₉-leucine than the₁₃C₆-phenylalanine (-0.38 ± 0.03 vs. -1.29 ± 0.07; P < 0.05). In conclusion, blood d₉-leucine enrichment introduced endogenously by intravenous infusion of d₁₀-leucine provides a closer estimate of the muscle fluid amino acid enrichment, and its associated changes, than blood phenylalanine enrichment to calculate rates of muscle protein synthesis in humans.