Evaluation of the isotopic equilibration between lactate and pyruvate.

When an isotopic tracer is infused for the purpose of determining the rate of turnover or oxidation of a substrate, it is assumed that the resulting isotopic enrichment by the tracer will reflect the kinetics of only the pool of interest. However, this may not be the case when carbon-labeled lactate is infused, since rapid isotopic exchange with the intracellular pyruvate and alanine pools could potentially occur. Therefore we have determined the extent of isotopic exchange occurring during the infusion of [3-13C]lactate into six anesthetized dogs. In the steady state, pyruvate enrichment was 91 +/- 2.2% (means +/- SE) of the lactate enrichment, and alanine enrichment was 81 +/- 3.3% of the pyruvate enrichment and 72 +/- 2.6% of the lactate enrichment. In contrast, when [3-13C]alanine was infused (n = 2), pyruvate (and lactate) enrichment was 9.9% of the alanine enrichment. We therefore conclude that there is rapid isotopic equilibration between lactate and pyruvate but that interaction with alanine reflects the true metabolic flux rates, rather than isotopic exchange. Consequently, lactate kinetics, as traditionally determined, more accurately reflect whole body pyruvate kinetics.