Cardioprotective effects of hyperkalemia during simulated ischemia/reperfusion in neonatal rat cardiomyocytes-Preservation of Na⁺/K⁺-ATPase activity-

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Background: Hyperkalemia has multimodal effects on myocardial protection during ischemia/reperfusion. The preservation of Na⁺/K⁺-ATPase activity induced by hyperkalemia may have critical impact on myocardial protection. Methods: To elucidate the roles of hyperkalemia (16 mM) and Na⁺/K⁺-ATPase inhibition (100 µM ouabain) in myocardial protection during simulated ischemia (5 mM NaCN and 5.5 mM 2-deoxyglucose)/reperfusion, we measured loss of membrane integrity and bleb formation using a vital dye calcein AM in cultured neonatal rat cardiomyocytes. The control perfusate was switched to treatment solution for 15 min, followed by reperfusion for 30 min. In a second set of experiments, myocardial excitability and diastolic intracellular calcium ion concentration ([Ca²⁺]_i) were measured during a 45-min treatment using a calcium-sensitive fluorescent dye fluo-4 AM. Results: Simulated ischemia/reperfusion under ouabain treatment induced loss of membrane integrity, which was suppressed by hyperkalemia. Simulated ischemia/reperfusion induced bleb formation, which was accelerated by ouabain. Hyperkalemia delayed and inhibited the increase in diastolic [Ca²⁺]_i induced by simulated ischemia. Furthermore, hyperkalemia almost completely inhibited the effects of ouabain on the diastolic [Ca²⁺]_i during ischemia. Conclusions: These results suggest that hyperkalemia during ischemia is cardioprotective against ischemia/reperfusion insults and that hyperkalemia inhibits the effects of ouabain during ischemia. J. Med. Invest. 60: 66-76, February, 2013