Plasma-membrane KATP channel-mediated cardioprotection involves posthypoxic reductions in calcium overload and contractile dysfunction:: mechanistic insights into cardioplegia

Our recent data demonstrate that activation of pmK(ATP) channels polarizes the membrane of cardiornyocytes and reduces Na+/Ca2+ exchange-mediated Ca2+ overload. However, it is important that these findings be extended into contractile models of hypoxia/reoxygenation injury to further test the notion that pmK(ATP) channel activation affords protection against contractile dysfunction and calcium overload. Single rat heart right ventricular myocytes were enzymatically isolated, and cell contractility and Ca2+ transients in field-stimulated myocytes were measured in a cellular model of metabolic inhibition and reoxygenation. Activation of pmK(ATP) with P-1075 (5 mu M) or inhibition of the Na+/Ca2+ exchanger with KB-R7943 (5 mu M) reduced reoxygenation-induced diastolic Ca2+ overload and improved the rate and magnitude of posthypoxic contractile recovery during the first few minutes of reoxygenation. Moreover, diastolic Ca2+ overload and posthypoxic contractile dysfunction were aggravated in ventricular myocytes either subjected to specific blockade of pmK(ATP) with HMR1098 (20 mu M) or expressing the dominant-negative pmK(ATP) construct Kir6.2(AAA) in the presence of P-1075. Our results suggest that a common mechanism, involving resting membrane potential-modulated increases in diastolic [Ca2+](i), is responsible for the development of contractile dysfunction during reoxygenation following metabolic inhibition. This novel and highly plausible cellular mechanism for pmK(ATP)-mediated cardioprotection may have direct clinical relevance as evidenced by the following findings: a hypokalemic polarizing cardioplegia solution supplemented with the pmK(ATP) opener P-1075 improved Ca2+ homeostasis and recovery of function compared with hyperkalemic depolarizing St. Thomas' cardioplegia following contractile arrest in single ventricular myocytes and working rat hearts. We therefore propose that activation of pmK(ATP) channels improves posthypoxic cardiac function via reductions in abnormal diastolic Ca2+ homeostasis mediated by reverse-mode Na+/Ca2+ exchange.