Background: Hyperkalemic depolarized cardiac arrest has been the cornerstone of myocardial protection during cardiac surgery for more than than 30 years. Many of the advances in myocardial protection seek to minimize the cellular damage and to reduce the ongoing metabolic processes occurring as a direct consequence of the depolarized state. Ideally, cardiac arrest at hyperpolarized cellular membrane potentials-the natural resting state of the heart-will meet all the requirements of modern cardioplegia, namely, electromechanical asystole and cardiac relaxation, while preserving the vital integrity of the heart itself. Methods and results: To determine whether activation of adenosine triphosphate-sensitive potassium channels by pharmacologic agents could produce hyperpolarized cardiac arrest, we tested the ability of aprikalim, a known adenosine triphosphate-sensitive potassium channel opener, to arrest the intact beating heart. In a normothermic (37° C) isolated rabbit heart preparation, aprikalim was found to rapidly shorten the action potential duration and produce cardiac asystole that was maintained during 20 minutes of 'no-flow' global ischemia without a rise in end-diastolic pressure. Cardiac rhythm and function were fully restored by reperfusion alone (developed pressure was 100.6% ± 7.9% of prearrest value after 30 minutes of reperfusion). In contrast, 20 minutes of unprotected normothermic global ischemia resulted in a 2.7 ± 0.55 mm Hg rise in end- diastolic pressure and only 58.2% ± 3.8% recovery of developed pressure after 30 minutes of reperfusion. By way of comparison, 20 minutes of standard hyperkalemic depolarized normothermic rest was accompanied by a 1.2 ± 0.6 mm Hg rise in end-diastolic pressure and only 80.8% ± 2.6% recovery of developed pressure after 30 minutes of reperfusion. To directly compare hyperkalemic depolarized cardiac arrest to hyperpolarized cardiac arrest induced by potassium channel openers and to better define the characteristics of such hyperpolarized arrest, we studied a fixed (4 mm Hg rise in end- diastolic pressure-contracture) ischemic injury model. The time to development of the contracture was prolonged by hyperkalemic arrest (35.8 ± 1.7 minutes) and significantly more so by hyperpolarized arrest (47.0 ± 3.3 minutes) when compared with that of unprotected hearts (24.0 ± 1.2 minutes). Moreover, aprikalim resulted in significantly better postischemic recovery of function (developed pressure was 69.0% ± 6.7% of prearrest value after 30 minutes of reperfusion) than after no cardioplegia (45.4% ± 7.5%) or standard hyperkalemic cardioplegia (44.3% ± 5.7%). Conclusions: Pharmacologic activation of adenosine triphosphate-sensitive potassium channels can result in predictable and sustainable hyperpolarized cardiac arrest that is reversible by reperfusion. This method of myocardial protection was found to fully preserve cardiac electromechanical function after a 20-minute period of global normothermic ischemia. Furthermore, hyperpolarized arrest induced by potassium channel openers significantly prolonged the period to the development of contracture and afforded a significantly better postischemic recovery of function than obtained in either hearts protected with hyperkalemic depolarized arrest or those not protected by any form of cardioplegia.