Cardioprotection by ischemic and nonischemic myocardial stress and ischemia in remote organs. Implications for the concept of ischemic preconditioning.

Ischemic preconditioning studies employ one or more brief total coronary artery occlusions separated by complete reperfusion to limit infarct size during a subsequent prolonged coronary artery occlusion. We now present evidence that in anesthetized pigs a partial coronary artery occlusion without intervening reperfusion between the partial and prolonged total occlusions can also precondition the myocardium provided that the reduction in coronary blood flow is sufficiently severe. Thus infarct size was reduced after a 60 min total coronary artery occlusion when the total occlusion was preceded by a partial coronary occlusion that reduced coronary blood flow by 70% but not when the flow reduction was only 30%. In this two-stage coronary occlusion model the degree of protection appears greater in the epicardial than in the endocardial half. In view of evidence that brief occlusions of a coronary artery also protect myocardium outside its perfusion territory, we subsequently investigated whether ischemia in remote organs can protect myocardium. Because of reports that development of infarct size may be temperature dependent, we also investigated whether the cardioprotection by remote organ ischemia was temperature dependent. In anesthetized rats a 15 min coronary artery occlusion was more effective in reducing infarct size produced by a subsequent 60 min total coronary artery occlusion when the experiments were performed at a body core temperature of 30-31 degrees C than at 36-37 degrees C, while infarct size of animals which were subjected to only the 60 min total coronary artery occlusion was the same for the two body core temperatures. In rats with a body core temperature of 36-37 degrees C a 15 min mesenteric artery occlusion, but not a 15 min renal artery occlusion, reduced infarct size produced by a subsequent 60 min coronary artery occlusion. When the experiments were performed at 30-31 degrees C both the mesenteric and renal artery occlusions were protective. These observations indicate the local myocardial ischemia is not required to protect the myocardium during a prolonged coronary occlusion. We further investigated whether myocardium could also be protected by a cardiac stimulus which does not produce ischemia at all. For this purpose we electrically paced the left ventricle of anesthetized pigs to produce heart rates of 200 bpm (which did not lead to ischemia as assessed by a number of functional and biochemical variables) and found that 30 min of ventricular pacing reduced myocardial infarct size produced by a subsequent 60 min coronary artery occlusion. The protection by ventricular pacing involved activation of K+ATP channels as pretreatment with glibenclamide abolished the protection by ventricular pacing. We conclude that a number of distinctly different stimuli can protect the myocardium suggesting that ischemic myocardial preconditioning could be just one feature of a more general protection phenomenon.