Introduction: Cardiac remodeling after myocardial infarction, is associated with progressive ventricular dysfunction and cardiovascular death. The purpose of this study was to examine the effect of endurance exercise training on morphological changes in rat heart tissue following experimental myocardial infarction.

Materials and methods: Rats used in this experiment (8-10 weeks old, 235 ± 5 g body weight) after homogenization according to body weight, were randomly divided to three groups: Healthy (Sham), control (Sedentary) and endurance training groups. Myocardial infarction was induced by subcutaneous injection of isoprenaline (150 mg/kg) in two consecutive days with an interval of 24 hours. Endurance training initiated 2 days after infarction and continued for 4 weeks. In order to assess the necrosis lesion and fibrosis tissue, Hematoxylin–Eosin and Masson's trichrome staining were used, respectively. Data were analyzed by using one-way analysis of variance (ANOVA) at P< 0.05 level.

Results: The results indicated that subcutaneous injection of isoprenaline, induced myocardial infarction in rat heart tissue. Rate of necrotic lesion in the Endurance training group was significantly higher than Sedentary (control) group (P=0.005).        Also, the size of fibrosis tissue in the Endurance training group was significantly higher than the sedentary         (control) group (P= 0.001).

Conclusion: In general, results of this study showed that 4 weeks of endurance exercise training after myocardial infarction exacerbate tissue injury in rat heart.