THE PROTECTION OF ISCHEMIC PRECONDITIONING CAN BE REINSTATED IN THE RABBIT HEART AFTER THE INITIAL PROTECTION HAS WANED

Objective: Preconditioning the heart with 5 min of ischaemia followed by reperfusion renders the myocardium resistant to infarction from subsequent ischaemia. This protection lasts for about 1 h. The aim of this study was to test whether the protection could be reinstated with a second episode of preconditioning after protection from an initial episode had worn off. Methods: To induce infarction animals experienced 30 min of coronary artery occlusion and then 3 h reperfusion. Ischaemic preconditioning was accomplished with 5 min of coronary branch occlusion. In one group of rabbits the subsequent reperfusion period was prolonged to 2 h to permit protection to wear off before the 30 min coronary occlusion was initiated. In another group a second 5 min coronary occlusion was performed at the end of the 2 h reperfusion. After 10 min of reperfusion the 30 min ischaemic period began. Control animals experienced only the 30 min ischaemia and 3 h reperfusion. Infarct volume was measured with tetrazolium and expressed as a percentage of the ischaemic zone volume. Results: Average infarct size in seven control rabbits [36.0(SEM 2.0)% of the ischaemic zone] was not significantly different from that in eight rabbits with the prolonged coronary occlusion occurring 2 h after the preconditioning stimulus [28.6(2.9)% infarction]. In contrast infarcts were significantly smaller in the re-preconditioned group of seven rabbits [8.3(4.2)%] and comparable to those previously seen with a single preconditioning stimulus followed 10 min later by the 30 min occlusion [6.1(1.8)%]. Conclusions: The rabbit heart can be protected with a second preconditioning stimulus after protection from an initial period of preconditioning has subsided.