Do nitroxides protect cardiomyocytes from hydrogen peroxide or superoxide?

The aim of the research was to study the role played by extracellular O₂^.- radicals, which are implicated in cardiac cell damage and the protective effect by cell-permeable, nitroxide, superoxide dismutase-mimics. Cardiomyocytes cultures from 1-day-old rats served as the test-system. Experiments were performed since 5th day in culture when >80% of the cells were beating myocardial cells. Oxidative damage was induced by 0.5 mM hypoxanthine and 0.06 U/ml xanthine oxidase or by 10 mM glucose and 0.15 U/ml glucose oxidase. The parameters used to evaluate damages were spontaneous beating, lactate dehydrogenase release and ATP level. The rhythmic pulsation was followed microscopically. To determine the kinetics of cytosolic enzyme release from the cells, media samples were collected at various points of time and assayed for enzyme activity. To determine the cellular ATP, cells were washed with sodium phosphate buffer, scraped off and boiled for 3 min with sodium phosphate buffer. Following centrifugation the supernatant was collected and ATP was determined by the chemiluminogenic assay using firefly tails. The present results indicate that nitroxide stable free radicals, in the millimolar concentration range, provide full protection without toxic side-effect. Unlike exogenously added SOD that failed to protect, exogenous catalase provided almost full protection. In addition, the metal-chelating agent dipyridyl, but not diethylene-triamine-pentaacetate or desferrioxamine, protected the cultured cells. The present results suggest that H₂O₂ is the predominant toxic species mediating the oxidative damage whereas extracellular superoxide radical does not contribute to cultured cardiomyocyte damage. Since nitroxides do not remove H₂O₂ they can protect the cells possibly by oxidizing the metal ions and inhibiting the Fenton reaction. The superoxide dismutase-mimic activity of nitroxides does not seem to underlie their protective effect, however, the involvement of intracellular O₂^.- cannot be excluded.