Mechanisms involved in postconditioning protection of cardiomyocytes against acute reperfusion injury

Experimental and clinical studies demonstrated that postconditioning confers protection against myocardial ischemia/reperfusion injury. However the underlying cellular mechanisms responsible for the beneficial effect of postconditioning are still poorly understood. The aim of the present study was to examine the role of cytosolic and mitochondrial Ca2+-handling. For this purpose adult rat cardiomyocytes were subjected to simulated in vitro ischemia (glucose-free hypoxia at pH 6.4) followed by simulated reperfusion with a normoxic buffer (pH 7.4; 2.5 mmol/L glucose). Postconditioning, i.e., 2 repetitive cycles of normoxic (5 s) and hypoxic (2.5 min) superfusion, was applied during the first 5 min of reoxygenation. Mitochondrial membrane potential (Delta Psi(m)), cytosolic and mitochondrial Ca2+ concentrations, cytosolic pH and necrosis were analysed applying JC-1, fura-2, fura-2/manganese, BCECF and propidium iodide, respectively. Mitochondrial permeability transition pore (MPTP) opening was detected by calcein release. Hypoxic treatment led to a reduction of Delta Psi(m), an increase in cytosolic and mitochondrial Ca2+ concentration, and acidification of cardiomyocytes. During the first minutes of reoxygenation, Delta Psi(m) transiently recovered, but irreversibly collapsed after 7 min of reoxygenation, which was accompanied by MPTP opening. Simultaneously, mitochondrial Ca2+ increased during reperfusion and cardiomyocytes developed spontaneous cytosolic Ca2+ oscillations and severe contracture followed by necrosis after 25 mm of reoxygenation. In postconditioned cells, the collapse in Delta Psi(m) as well as the leak of calcein, the increase in mitochondrial Ca2+, cytosolic Ca2+ oscillations, contracture and necrosis were significantly reduced. Furthermore postconditioning delayed cardiomyocyte pH recovery. Postconditioning by hypoxia/reoxygenation was as protective as treatment with cyclosporine A. Combining cyclosporine A and postconditioning had no additive effect. The data of the present study demonstrate that postconditioning by hypoxia/reoxygenation prevents reperfusion injury by limiting mitochondrial Ca2+ load and thus opening of the MPTP in isolated cardiomyocytes. These effects seem to be supported by postconditioning-induced delay in pH recovery and suppression of Ca2+ oscillations. This article is part of a Special Issue entitled "Calcium Signaling in Heart". (C) 2013 Elsevier Ltd. All rights reserved.