Isotopic determination of glycolytic flux during intense exercise in humans

We used a new stable isotope tracer approach incorporating muscle intracellular lactate enrichment to determine the flux of glucose/glucosyl toward lactate [i.e., nonoxidized pyruvate (Pyr) production (Pyrno)] in moderately trained cyclists exercising at approximately 80% (259 +/- 16 W; n = 6) and approximately 100% (341 +/- 9 W; n = 8) maximal O2 uptake (VO2max). Primed constant infusions of [6,6-2H2]glucose and [13C]lactate or [13C]Pyr tracers were given, and rapid achievement of plateau was obtained during exercise by increasing the infusion rates at exercise onset to correspond with expected increases in production. The accumulated O2 deficit was simultaneously determined over the 1st 3 min of exercise as an indirect means of quantifying glycolytic flux for comparison with our tracer-determined values and was significantly greater at the higher intensity (38 +/- 3 vs. 30 +/- 3 ml O2.kg-1.3 min-1; P < 0.02). Pyrno was also significantly higher (6.38 +/- 0.91 vs. 4.38 +/- 0.65 mmol.kg-1.min-1 over 3 min at 100 and 80% VO2max, respectively). The blood lactate rate of appearance at approximately 100% VO2max (828 +/- 69 mumol.kg-1.min-1) represented a higher percentage of Pyr rate of appearance (RaPyr; 31 +/- 3%) than that at approximately 80% VO2max (416 +/- 36 mumol.kg-1.min-1; 22 +/- 2%; P < 0.02). Although only approximately 27 +/- 2% of RaPyr was oxidized, this provided 78 +/- 2% of the total energy demand during the 1st 3 min of exercise at either intensity. Our new method provided values for Pyrno that were in the expected range and were highly correlated with respective accumulated O2 deficit values (r = 0.87, P < 0.0001). In conclusion, our new tracer method appears to be valid for the measurement of RaPyr and Pyrno during high-intensity exercise lasting even < 10 min.