We compared the leucine (Leu) and phenylalanine (Phe) tracer-determined response
of forearm protein synthesis (PS) before and after stimulation of protein
anabolism by intravenous infusion of Leu-enriched, Phe-deficient amino acids and
insulin (increased to approximately 100 microU/mL) with the euglycemic clamp. Six
healthy subjects received primed-constant infusions of L-[ring-2H5]-Phe and
L-[1-(14)C]-Leu, and steady-state forearm Phe and Leu kinetics were determined.
Following the combined infusion, the arterial Leu concentration increased
approximately 70% (P < .001), whereas Phe decreased about 15% (P < .01). Forearm
PS and net balance (NB) increased (P < .05 or less v basal) using both amino acid
tracers. However, the relative increments observed with the Leu tracer were more
than 75% larger (P < .05 or less) than those observed with the Phe tracer, even
when the data were corrected for the standard relative abundance of these two
amino acids in forearm protein(s). Thus, the calculated changes of forearm PS and
NB in response to an unbalanced amino acid infusion with hyperinsulinemia were
affected by the plasma level of leucine and phenylalanine, whose tracers were
used to estimate forearm protein turnover. Since these two essential amino acids
share the same transport system, a competition at this level cannot be excluded
