RESPONSE OF GLUTAMINE-METABOLISM TO EXOGENOUS GLUTAMINE IN HUMANS

To determine whether exogenous glutamine affects whole body glutamine metabolism, preliminary experiments were performed to verify that L-[1-C-13]-, L-[U-C-14]-, and L-[3,4-H-3]glutamine given simultaneously by vein provided similar estimates of glutamine appearance rates [R(a); 355 +/- 24, 373 +/- 19, and 393 +/- 24 (SE) mu mol . kg(-1)h(-1), respectively, P = NS] in six healthy men; glutamine oxidation accounted for 32 +/- 3 and 51 +/- 5% (P < 0.01) of glutamine R(a) when it was measured using L-[U-C-14]- and L-[1-C-13]glutamine, respectively. Five subjects received two 5-h intravenous infusions of L-[3,4-H-3]glutamine and a simultaneous nasogastric infusion of L-[1-C-13]glutamine on 2 separate days in the postabsorptive state, along with saline on 1 day and natural L-glutamine (856 +/- 45 mu mol . kg(-1). h(-1)) on another day in a randomized order. Splanchnic glutamine extraction (deter mined from [C-13]glutamine appearance into systemic blood) reached 74 +/- 4 and 53 +/- 5% during the enteral infusion of tracer alone and in combination with a large load of glutamine, respectively. Glutamine infusion was associated with increased plasma glutamine concentration (from 630 +/- 50 to 1,297 +/- 75 mu M), R(a) (from 258 +/- 20 to 589 +/- 45 mu mol . kg(-1). h(-1)), and oxidation (from 179 +/- 20 to 477 +/- 47 mu mol . kg(-1). h(-1), all P < 0.01), no change in glutamine release from proteolysis, and a decline in glutamine de novo synthesis (from 156 +/- 15 to 93 +/- 13 mu mol . kg(-1). h(-1)). We conclude that the response to exogenous glutamine includes 1) a stimulation of glutamine oxidation and splanchnic extraction and 2) suppression of glutamine de novo synthesis.