CARDIAC RESPONSES TO INDUCED LACTATE OXIDATION - NMR ANALYSIS OF METABOLIC EQUILIBRIA

The role of lactate as a source of pyruvate oxidation in supporting cardiac work, energetics, and formation of oxidative metabolites was examined in normal myocardium. C-13 and P-31-nuclear magnetic resonance (NMR) spectra were acquired from isolated rabbit hearts supplied 2.5 mM [3-C-13]lactate or [3-C-13]pyruvate with or without stimulation of pyruvate dehydrogenase (PDH) by dichloroacetate (DCA). Similar workloads determined by rate-pressure products were noted with pyruvate (21,700 +/- 2,400; mean +/- SE) and lactate (18,970 +/- 1,510). Oxygen consumption was similar in all four groups with means between 19.0 and 22.2 mu mol . min(-1). g dry weight(-1) (SE = 1.6-2.0) as was the ratio of phosphocreatine to ATP with means between 1.8 and 2.1 (SE = 0.1-0.6). Intracellular pH, determined from P-31-NMR spectra, was essentially the same with pyruvate (1.06 +/- 0.02) and lactate (7.05 +/- 0.04). C-13 enrichment of glutamate was higher with lactate (92%) than with pyruvate (70%). Pyruvate plus DCA induced no change in glutamate content at 9-10 mu mol/g, but C-13 enrichment increased to 83%, while lactate plus DCA maintained enrichment at 90%. Levels of alpha-ketoglutarate were lower with lactate (1.81 mu mol/g) than with pyruvate (2.36 mu mol/g). Lactate plus DCA elevated glutamate by 60% with a proportional increase in alpha-ketoglutarate. Thus the balance between glutamate and alpha-ketoglutarate was affected by substrate supply only and not by PDH activation. The results suggest that the equilibrium between alpha-ketoglutarate and glutamate is sensitive to cytosolic redox state, an important consideration for C-13-NMR analyses that rely on glutamate. In conclusion, lactate and pyruvate are able to support similar function and energetics through differences in the balance of oxidative, intermediary metabolites.