Does enteral glutamine modulate whole-body leucine kinetics in hypercatabolic dogs in a fed state?

To determine whether enteral glutamine alters whole-body leucine metabolism in a state of hypercatabolism, 6 dogs adapted to a normocaloric, low-protein diet received intramuscular dexamethasone (0.44 mg. kg(-1). d(-1)) for 1 week, during 2 separate study periods. On the last day of each period, intravenous infusions of L-[1-(13)C]leucine and L-[2-(15)N]glutamine were performed to assess whole-body leucine and glutamine metabolism, and duodenal biopsies were obtained to determine gut protein fractional synthesis rate (FSR), while dogs were receiving enteral nutrition. The nutrient mixture supplied 6.2 kcal. h(-1) nonprotein energy per kg(0.75) of body weight (84% glucose, 16% fat) and 0.2 g amino acid per kg(-0.75). h(-1); the nutrient mixture was glutamine-free on the "control day," and supplemented with 1,150 micromol. kg(-1). h(-1) natural L-glutamine on the "glutamine day." Glutamine supplementation induced an approximately 56% rise in plasma glutamine appearance rate (P <.05), and was associated with an approximately 26% reduction in leucine oxidation (P <.05) with no change in leucine release from protein breakdown or nonoxidative leucine disposal, an index of whole-body protein synthesis. Glutamine supplementation improved net leucine balance (protein synthesis-protein breakdown) (-26 +/- 4 v -48 +/- 11 micromol. kg(-1). h(-1); P <.05). In addition, glutamine enhanced intestinal protein FSR by approximately 22% in the 4 dogs where it was assessed. We conclude that, in hypercatabolic adult dogs in the fed state, enteral glutamine supplementation acutely decreases leucine oxidation and improves net leucine balance, and may thus preserve body protein.