LEUCINE KINETICS FROM [H-2(3)]- AND [C-13]LEUCINE INFUSED SIMULTANEOUSLY BY GUT AND VEIN

In amino acid tracer kinetic studies of the fed state, ingested amino acid may be taken up during its initial transit through splanchnic tissues and thus not enter the plasma compartment where tracer is infused. To investigate this possibility, adult human subjects received simultaneous intravenous (iv) and intragastric (ig) leucine tracer infusions, first during a postabsorptive (PA) 4-h primed continuous ig infusion of L-[1-C-13]-leucine and L-[5,5,5-H-2(3)]leucine iv, followed on a separate day by a fed infusion, in which an ig infusion of a liquid formula was started 2 h before the tracer infusion and continued throughout the tracer study. Subjects were accustomed to a constant experimental diet supplying 1.5 g protein.kg-1.day-1 and 41-45 kcal.kg-1.day-1 for 7 and 12 days before the PA and fed studies, respectively. For the PA study, plasma enrichment for the ig tracer was 3.34 +/- 0.27 (SE) mol %excess and for the iv tracer it was 4.18 +/- 0.10 (P < 0.02). Enrichments of alpha-ketoisocaproic acid (KIC) were 3.24 +/- 0.16 (ig) and 3.02 +/- 0.14 (iv), respectively [not significant (NS)]. For the fed study, plasma leucine enrichment for the ig tracer was 2.15 +/- 0.14 and for the iv tracer was 2.84 +/- 0.09 (P < 0.02). KIC enrichments were 2.02 +/- 0.08 (ig) and 2.24 +/- 0.08 (iv), respectively (NS). In the PA study, the ratio of the plasma leucine enrichments for the ig and iv tracers was 0.80 +/- 0.06 and in the fed experiment, 0.76 +/- 0.05, respectively. In contrast, the ratio of the plasma KIC enrichments formed from the ig and iv tracers was 0.99 +/- 0.09 for the PA experiment and 0.90 +/- 0.10 for the fed experiment. This dual-tracer infusion approach demonstrates that ig-administered leucine tracer is removed in a first pass through splanchnic tissues (PA experiment 19 +/- 7%, fed experiment 24 +/- 5%) but may be transminated and released as KIC.