Partitioning of pyruvate between oxidation and anaplerosis in swine hearts

The goal of this study was to measure flux through pyruvate carboxylation a nd decarboxylation in the heart in vivo. These rates were measured in the a nterior wall of normal anesthetized swine hearts by infusing [U-C-13(3)]lac tate and/or [U-C-13(3)] pyruvate into the left anterior descending (LAD) co ronary artery. After 1 h, the tissue was freeze-clamped and analyzed by gas chromatography-mass spectrometry for the mass isotopomer distribution of c itrate and its oxaloacetate moiety. LAD blood pyruvate and lactate enrichme nts and concentrations were constant after 15 min of infusion. Under near-n ormal physiological concentrations of lactate and pyruvate, pyruvate carbox ylation and decarboxylation accounted for 4.7 +/- 0.3 and 41.5 +/- 2.0% of citrate formation, respectively. Similar relative fluxes were found when ar terial pyruvate was raised from 0.2 to 1.1 mM. Addition of 1 mM octanoate t o 1 mM pyruvate inhibited pyruvate decarboxylation by 93% without affecting carboxylation. The absence of M1 and M2 pyruvate demonstrated net irrevers ible pyruvate carboxylation. Under our experimental conditions we found tha t pyruvate carboxylation in the in vivo heart accounts for at least 3-6% of the citric acid cycle flux despite considerable variation in the flux thro ugh pyruvate decarboxylation.