Nuclear magnetic resonance studies of cationic and energetic alterations with oxidant stress in the perfused heart. Modulation with pyruvate and lactate.

Abstract

The postischemic generation of oxygen-derived free radicals may contribute to myocardial reperfusion injury by affecting sarcolemmal ion transport. Recent evidence indicates that exposure to reactive oxygen intermediates induces rapid increases in myocardial cytosolic free Ca2+ (Ca2+i). The mechanism is undetermined but may involve disturbances in Na+ homeostasis. We tested this hypothesis by interleaving 23Na and 31P nuclear magnetic resonance (NMR) measurements of Na+i and high-energy phosphates in glucose-perfused rat hearts exposed to hydroxyl radicals generated from H2O2 and Fe3+. In separate experiments, K+i and Ca2+i were measured with 39K and 19F NMR, respectively. The hearts rapidly exhibited contracture. Threefold Na+i increases and substantial K+i depletion were observed. Glycolytic inhibition was indicated by rapid sugar phosphate accumulation and cellular energy depletion. Notably, however, severe functional and energetic deterioration and substantial elevation of Ca2+i occurred before substantial Na+i accumulation or K+i depletion was observed. Further experiments investigated the ability of pyruvate to scavenge H2O2 and to protect the myocardium from oxidant stress. Pyruvate (1 or 2.5 mmol/L) dramatically attenuated functional and energetic alterations and alterations in Na+i and K+i, whereas acetate (2.5 mmol/L) offered no protection. Unlike pyruvate, lactate (5 mmol/L) has little or no capacity to scavenge H2O2 but has similar protective effects. In conclusion, pyruvate effectively protects against H2O2/Fe3+, largely by direct H2O2 scavenging. Protection with lactate may involve intracellular pyruvate augmentation. Without exogenous pyruvate or lactate, myocardial Na+ homeostasis can be substantially altered by oxidant stress, possibly via cellular energy depletion. Excess Na+i accumulation may, in turn, hasten metabolic and functional deterioration, but a causal link with the initial alterations in function or Ca2+i was not supported.