Oxygen free radicals have been shown to play a major role in the development of perfusion abnormalities, contractile dysfunction, and irreversible injury in ischemic-reperfused myocardium. The aim of this study was to assess the direct protective effects of radical scavengers, calcium antagonists, and combination of these substances against free radical induced myocyte damage. Viability (% of rod-shaped cells) and adenine nucleotide content (AdN, high-pressure liquid chromatography) of isolated adult rat cardiomyocytes were measured after exposure to hypoxanthine (2 mM) and xanthine oxidase (25 mU/ml). After 90 min, viability of myocytes decreased to 4.2 +/- 3.4% (mean +/- SEM) of pre-exposure control, and AdN decreased from 28.2 +/- 1.8 to 8.09 +/- 1.1 nmol/mg protein. Addition of catalase (1500 U/ml) resulted in the preservation of viability (77 +/- 6% of pre-exposure control, n = 6, mean +/- SEM), and AdN 84 +/- 6%, p less than 0.001. These values are not significantly different from those measured in myocytes not exposed to free radicals (88 +/- 9% and 79 +/- 6%, respectively). Superoxide dismutase (2400 U/ml), dimethylthiourea (10 mM), and desferrioxamine (1 mM) did not preserve either viability or AdN. The calcium antagonist verapamil (10 microM) also preserved myocyte viability significantly (23 +/- 9.7%, p less than 0.05 vs unprotected cells), but failed to prevent the loss of AdN (13.2 +/- 4%, not significant as compared to unprotected cells). Viability and AdN in myocytes treated with nifedipine (10 microM) or diltiazem (10 microM) were not higher than in unprotected cells. All combined treatment forms which included catalase resulted in the preservation of myocyte viability as well as AdN. These data show that only the hydrogen peroxide scavenger catalase protects isolated cardiomyocytes against free radicals generated in the purine catabolic pathway.