A new method to measure muscle protein synthesis in humans by endogenously introduced d(9)-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: d(9)-leucine, introduced endogenously by the metabolism of d(10)-leucine (i.e., l-[2,3,3,4,5,5,5,6,6,6-(2)H(10)]leucine) infused in blood, and (13)C(6)-phenylalanine introduced/infused in blood. The blood-to-muscle fluid amino acid enrichment ratio was lower (P < 0.05) for d(9)-leucine compared to (13)C(6)-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 d(9)-leucine than the (13)C(6)-phenylalanine (−0.38 ± 0.03 vs. −1.29 ± 0.07; P < 0.05). In conclusion, blood d(9)-leucine enrichment introduced endogenously by intravenous infusion of d(10)-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.