Altered cardiac mitochondrial dynamics with excessive fission is a predominant cause of cardiac dysfunction during ischemia/reperfusion (I/R) injury. Although pre-ischemic inhibition of mitochondrial fission has been shown to improve cardiac function in I/R injury, the effects of this inhibitor given at different time-points during cardiac I/R injury are unknown. Fifty male Wistar rats were subjected to sham and cardiac I/R injury. For cardiac I/R injury, rats were randomly divided into pre-ischemia, during-ischemia, and upon onset of reperfusion group. A mitochondrial fission inhibitor, Mdivi-1 (mitochondrial division inhibitor 1) (1.2 mg/kg) was used. During I/R protocols, the left ventricular (LV) function, arrhythmia score, and mortality rate were determined. Then, the heart was removed to determine infarct size, mitochondrial function, mitochondrial dynamics, and apoptosis. Our results showed that Mdivi-1 given prior to ischemia, exerted the highest level of cardioprotection quantitated through the attenuated incidence of arrhythmia, reduced infarct size, improved cardiac mitochondrial function and fragmentation, and decreased cardiac apoptosis, leading to preserved LV function during I/R injury. Mdivi-1 administered during ischemia and upon the onset of reperfusion also improved cardiac mitochondrial function and LV function, but at a lower efficacy than when it was given prior to ischemia. Taken together, mitochondrial fission inhibition after myocardial ischemic insults still exerts cardioprotection by attenuating mitochondrial dysfunction and dynamic imbalance, leading to decreased infarct size and ultimately improved LV function after acute cardiac I/R injury in rats. These findings indicate its potential clinical usefulness.

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