Increases in metabolic rate and core temperature are common responses to severe injury. We have investigated the hypothesis that these responses are due to increases in substrate cycling. A substrate cycle exists when opposing, nonequilibrium reactions catalyzed by different enzymes are operating simultaneously. At least one of the reactions must involve the hydrolysis of ATP. Thus, a substrate cycle both liberates heat and increases energy expenditure, yet there is not net conversion of substrate to product. In studies in volunteers (n = 18) and in patients with severe burns who were in a hypermetabolic state (n = 18), we used stable-isotope tracers to quantify substrate cycling in the pathways of glycolysis and gluconeogenesis and a cycle involving the simultaneous breakdown and synthesis of stored triglyceride (triglyceride-fatty acid cycle). The total rates of triglyceride-fatty acid and glycolytic-gluconeogenic cycling were elevated in the patients by 450 and 250 percent, respectively (P less than 0.01). An infusion of propranolol in the patients greatly reduced triglyceride-fatty acid cycling but did not affect gluconeogenic-glycolytic cycling. We conclude that increased substrate cycling contributes to the increased thermogenesis and energy expenditure following severe burns and that the increased triglyceride-fatty acid cycling is due to beta-adrenergic stimulation.

We have used the primed constant infusion of di-[15N]urea and [1-13C]leucine to determine the effects of mild exercise (approx 30% Vo2max for 105 min) on urea production and leucine metabolism in human subjects. The oxidation of plasma leucine was distinguished from the oxidation of leucine that never entered the plasma pool ("intracellular" leucine) by means of determining the enrichment of alpha-ketoisocaproic acid (alpha-KICA). Total leucine oxidation increased from 0.38 +/0 0.05 to 1.41 +/- 0.14 micromol . kg-1 . min-1 during exercise due to increases in the oxidation of plasma leucine (150%) and intracellular leucine (600%). Plasma leucine flux decreased slightly, but not significantly (0.1 greater than P greater than 0.05), and the percent of alpha-KICA derived from plasma leucine dropped significantly (P less than 0.05) from 79.5 +/- 4.3 at rest to 62.0 +/- 5.3% over the last 30 min of exercise. Despite the increase in leucine oxidation during exercise, urea concentration and production did not change. Thus in exercise urea production does not accurately reflect all aspects of amino acid metabolism.

In metabolic tracer studies it is frequently useful to infuse tracers that are differently labeled variants of the same molecule. These tracers are known as isotopomers. Analysis of the enrichment of each isotopic analogue can be accomplished by gas chromatography-mass spectrometry (GC-MS). However, the raw GC-MS data must be corrected to give the information required. This paper addresses how to transform the raw GC-MS data, consisting of relative abundance ratios at specific ion masses, into relative molar ratios of tracer and tracee molecules. Several correction factors are necessary. First, the background must be measured and corrected for, since it is always present in the sample. Second, the abundances in the spectrum of the labeled molecule are different from those in the unlabeled molecule, and this proportionality "skew" is corrected. A third correction factor accounts for the overlapping spectra of two or more isotopomers that cannot be measured independently. The final correction removes the "double vision" effect that may appear in some spectra due to the presence of (M - H)+ species.

The effects of two levels of protein intake on protein metabolism in six severely burned adult patients were studied (means of 70% BSA burned). A crossover experimental design enabled the authors to study each patient at the end of two three-day dietary regimens. All diets were isocaloric and provided approximately 25% more calories than the measured energy expenditure (means = 40.8 Kcal/kg X day). In one regimen, each patient received 2.2 g protein/kg X day, while during the other treatment period they received 1.4 g protein/kg X day. The patients were studied in the fed state and after 10 to 12 hours of fasting. Leucine kinetics were determined by means of the primed-constant infusion of [1--13C]--leucine. The authors were able to distinguish the oxidation of plasma leucine from the oxidation of leucine derived from intracellular protein at the site of the deamination of leucine (predominantly muscle) by simultaneously determining both leucine and alpha-ketoisocaproic acid enrichment. Also, rates of whole-body protein synthesis and catabolism were calculated from the leucine flux and oxidation data. Net protein synthesis was also calculated by means of another stable-isotope technique involving the infusion of [15N2]--urea. Finally, a third means of estimating net protein catabolism based on urinary N-excretion data was used at the same time that the isotopic studies were performed. The 13C leucine-data and the N-excretion data indicated that a balance between protein synthesis and catabolism could be achieved with a protein intake of 1.4 protein/kg X day. When protein intake was increased to 2.2 g protein/kg X day, neither isotopic method indicated a further beneficial effect on net protein synthesis, although the absolute rates of protein synthesis and catabolism were stimulated. The N-excretion data, on the other hand, indicated a significant improvement in net protein synthesis with higher protein intake. Regardless of the level of protein intake, the underlying alterations in protein metabolism that occurred as a response to burn injury persisted.