Differences in estimates of forearm protein synthesis between leucine and phenylalanine tracers following unbalanced amino acid infusion

We compared the leucine (Leu) and phenylalanine (Phe) tracer-determined res ponse of forearm protein synthesis (PS) before and after stimulation of pro tein anabolism by intravenous infusion of Leu-enriched, Phe-deficient amino acids and insulin (increased to approximate to 100 mu U/mL) with the eugly cemic clamp. Six healthy subjects received primed-constant infusions of L-[ ring-H-2(5)]-Phe and L-[1-C-14]-Leu, and steady-state forearm Phe and Leu k inetics 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 le ss 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) th an those observed with the Phe tracer, even when the data were corrected fo r the standard relative abundance of these two amino acids in forearm prote in(s). Thus, the calculated changes of forearm PS and NE in response to an unbalanced amino acid infusion with hyperinsulinemia were affected by the p lasma level of leucine and phenylalanine, whose tracers were used to estima te forearm protein turnover. Since these two essential amino acids share th e same transport system, a competition at this level cannot be excluded. Co pyright (C) 1999 by W.B. Saunders Company.