Maintaining muscle protein anabolism after a metabolic stress: Role of dextrose vs amino acid availability

The effect of chronic hypocaloric parenteral infusions of amino acids (AA) vs. dextrose (D) on protein homeostasis after a generalized metabolic stress was examined. Multicatheterized mongrel dogs were metabolically challenged by a 4-day fast and then administered a 4-day intravenous infusion of saline (S, n = 8), D (n = 8), or isocaloric AA (n = 7). Although nitrogen balance (g . kg(-1) . day(-1)) was similarly negative with S (-0.37 +/- 0.05), D (-0.28 +/- 0.03), and AA (-0.31 +/- 0.04) during the fasting period, it was less negative (P less than or equal to 0.05) with AA (-0.06 +/- 0.04) than with D (-0.20 +/- 0.03) or S (-0.23 +/- 0.04) during nutrient infusion. AA resulted in net hindlimb uptake and D in net hindlimb release of essential AA (570 +/- 261 vs. -248 +/- 59 nmol . kg(-1) . min(-1)). Whereas S and D infusions led to net hindlimb muscle protein loss (-37 +/- 24 and -89 +/- 33 mu g . kg(-1) . min(-1), respectively, P less than or equal to 0.05 vs. AA), parenteral AA resulted in net deposition (169 +/- 62 mu g . kg(-)1 . min(-1)) as measured using L-[ring-H-2(5)]phenylalanine. Thus hypocaloric parenteral D infusion after a metabolic stress does not favor nitrogen conservation, because net whole body nitrogen loss, skeletal muscle protein catabolism, and hindlimb AA release were not blunted compared with S infusion. Conversely, hypocaloric AA infusion preserves whole body and muscle protein stores.