The purpose of the present study was to determine whether enhanced activation of myocardial ATP-dependent potassium channels (KATP) with a potassium channel opener, bimakalim, can reduce the time necessary to produce the protective effect of ischemic preconditioning and to determine whether this effect is mediated via accelerating the rate of action potential shortening during preconditioning. Barbital-anesthetized dogs were subjected to 60 min of left anterior descending coronary artery (LAD) occlusion followed by 4 h of reperfusion. Ten minutes of preconditioning was found to markedly reduce myocardial infarct size from 30.6 +/- 4.7 to 7.1 +/- 2.6%. Subsequently, it was observed that either 3 min of LAD occlusion or a 3-min intracoronary infusion with 0.3 micrograms/min of bimakalim did not reduce myocardial infarct size. However, intracoronary infusion with bimakalim during the 3-min preconditioning period markedly reduced myocardial infarct size to a similar extent as that of ischemic preconditioning (12.2 +/- 1.9%). In addition, it was observed that bimakalim markedly accelerated the ischemia-induced shortening of the action potential during preconditioning. These results are the first to demonstrate that activation of KATP channels with a potassium channel opener reduces the threshold of time necessary to produce preconditioning in anesthetized dogs. These data also suggest that KATP channel activation may produce this effect by enhancing the rate of ischemic myocardial action potential shortening during preconditioning.