Human muscle amino acid transport kinetics in vivo

In vitro studies of amino acid transport indicate that phenylalanine (Phe) and leucine (Leu) apparently share a common transporter. We studied in vivo transport kinetics of Phe and Leu over a range of arterial concentrations. Healthy volunteers (3 male, 3 female) were studied on each of three occasions while drinking different amino acid solutions to vary arterial amino acid concentrations (Ca). Blood samples were taken from femoral arterial and venous catheters and muscle biopsies were taken from the vastus lateralis during a four h infusion of ring-d₅-Phe and 1-¹³C-Leu. A 3-compartment model of muscle amino acid kinetics was used to determine inward amino acid transport (AT) from Phe and Leu enrichments in artery (Ea) and muscle (Em), as well as Ca and muscle intracellular amino acid concentrations. Linear regression analysis revealed that, while both Phe and Leu AT were significantly (p=0.002 and p=0.03, respectively) related to Phe and Leu Ca (r=0.72 and r=0.54, respectively), the slope of the line was different for Phe (y=5.15x + 136) than for Leu (y=1.84x + 274). Further, Phe Em/Ea, which indicates the fractional contribution of the blood to the intracellular amino acid pool, is significantly (p=0 003) and positively related to Phe Ca, while Leu Em/Ea is not related to Leu Ca. We conclude that,under physiological conditions in vivo, Phe and Leu transport kinetics are different.