We examined for oxidative stress in the myocardium using 3-nitrotyrosine (3-NT) staining, a marker of oxidant stress (Fig. 4B). We found a significant increase in the grayscale intensity of 3-NT staining of the myocardium in female mice consuming a WD compared to CD fed mice (p < 0.01). Daily volitional exercise reduced 3-NT accumulation in the myocardium in WD fed mice (p < 0.01 for WD+Ex compared to WD group). There was no significant difference in the intensity of 3-NT staining between WD fed mice treated with exercise and sedentary CD fed mice (p = 0.72). 3.6. Daily Volitional Exercise Preserved Heme Oxygenase-1 (HO-1) Protein Levels in the Heart of WD Fed Female Mice Finally, we assessed HO-1 protein levels in the heart of WD fed mice. We found a marked reduction in HO-1 protein levels in hearts of sedentary mice fed a WD compared to CD fed sedentary mice (p < 0.001 for sedentary WD vs CD). WD fed female mice treated with daily volitional exercise had a significantly higher level of HO-1 protein in the heart compared to sedentary WD fed mice (p < 0.004 for WD+Ex compared to WD group). There was no significant difference in the HO-1 protein levels in the heart between sedentary CD fed mice and WD fed mice treated with exercise (p = 0.48) (Fig. 5).