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Figure 1 (A) Cell survival of rat cardiac myocytes versus cardiac fibroblasts after hydrogen peroxide (H₂O₂) (300 to 800 µM). Neonatal rat cardiac myocytes and fibroblasts were cultured for 5 days before exposure to H₂O₂ for 4 h. n = 6 experiments at each time point (two-way analysis of variance [ANOVA], p < 0.01). Results are expressed as mean ± SEM. t test was used for individual comparisons. **p < 0.01. (B) Graph showing time course of the effect of H₂O₂ (100 µM) on survival of neonatal cardiac myocytes and fibroblasts. Cells were exposed to H₂O₂ (100 µM) for a period of up to 72 h before cell survival analysis was performed. n = 4 experiments for both cardiac myocytes and fibroblasts at each time point (two-way ANOVA, p < 0.01). Results are expressed as mean ± SEM. t test was used for individual comparisons. *p < 0.05, **p < 0.01. (C) Deoxyribonucleic acid (DNA) ladder electrophoresis of neonatal cardiac myocytes treated with H₂O₂ (in concentrations from 25 to 800 µM) for 60 min. The DNA fragmentation ladder is most intense at 100 and 300 µM H₂O₂, and becomes much less visible at lower concentrations (25 and 50 µM) as well as higher (500 and 800 µM) concentrations.