Duration and reinstatement of myocardial protection against infarction by ischemic preconditioning in open chest dogs

These studies were undertaken to determine the duration of protection against myocardial infarction provided by ischemic preconditioning in the canine heart, and to learn if cardioprotection can be restored by another preconditioning stimulus when the initial effect is lost. Control and four preconditioning groups of anesthetized, open-chest dogs were compared. All underwent a test 60 min episode of ischemia, induced by occlusion of the anterior descending (LAD) artery, followed by 3 h of reperfusion. Preconditioning was induced by one 10 min LAD occlusion, followed by either 10 min, 2, 3, or 5 h of reperfusion. In order to test whether preconditioning could be reinstated, another group of dogs with preconditioning plus 3 h reperfusion underwent a second 10 min preconditioning stimulus with 10 min reperfusion before the 60 min test-occlusion. Infarct size (as percent of area-at-risk) was analyzed (using analysis of covariance) with respect to coronary collateral blood flow measured with radioactive microspheres. Infarct size was limited markedly by preconditioning (23+/-6 v 6+/-2%, P<0.05) but the protective effect was dissipated partially after 2 h reperfusion and was dissipated completely after 3 h reperfusion (20+/-4%, non-significant v Control and significant P<0.05 v preconditioning). Protection was restored in three of six dogs with preconditioning +5 h reperfusion, suggesting that the second window of protection appears early in some canine hearts. When preconditioning was repeated after 3 h reperfusion, cardioprotection was reinstated fully (7+/-2%, P<0.05 v Control and NS v preconditioning). The results show that maximal preconditioning cardioprotection is present in the dog heart after 10 min of reperfusion and is dissipated totally following 3 h of reperfusion. However, a second preconditioning stimulus of 10 min of ischemia followed by 10 min of reperfusion to the dissipated preconditioned heart reinstates full preconditioning. Thus, this model provides a system to test for theoretical causes of the preconditioned state. Final mediators should be present when preconditioning is present and absent when preconditioning is dissipated. It is noteworthy that a second window of protection appeared in 50% of dogs when the period of reperfusion was extended to 5 h.