Background: The mechanism through which ischemic preconditioning causes cardioprotection is unknown. The present study investigated the role of stunning in preconditioning.
Methods and results: We studied three different protocols of preconditioning: two cycles of 2-minute ischemia separated by 5-minute reperfusion (2'PC), one cycle of 5-minute ischemia by 5-minute reperfusion (5'PC1), and two cycles of 5-minute ischemia separated by 5-minute reperfusion (5'PC2). In the first series of experiments, the stunning associated with 2'PC, 5'PC1, or 5'PC2 was assessed using an epicardial Doppler transducer in anesthetized open-chest rabbits. The thickening fraction (percent baseline) of the preconditioned region was 76.8 +/- 7.2% (mean +/- SEM) after 2'PC but 31.4 +/- 9.2% and 34.3 +/- 9.7% after 5'PC1 and 5'PC2, respectively, which were significantly lower, thus indicating more severe stunning than that after 2'PC. In the second series of experiments, a branch of the left circumflex artery was occluded for 30 minutes and then reperfused for 72 hours in four groups of rabbits. One group was not preconditioned and three groups were preconditioned with 2'PC, 5'PC1, or 5'PC2 protocols before the 30-minute ischemia. In contrast to the differences observed in the stunning in the first series of experiments, histological infarct size was similar in the three preconditioned groups (21.1 +/- 3.0% of area at risk after 2'PC, 20.1 +/- 3.4% after 5'PC1, 16.4 +/- 4.2% after 5'PC2), all of which were significantly smaller than that in the unpreconditioned group (43.9 +/- 5.0%). The third series of experiments examined the degree of stunning by 2'PC, 5'PC1, or 5'PC2 and the size of infarct (tetrazolium staining) in the same animal after 30-minute ischemia/3-hour reperfusion; again, the results showed no significant correlation between degree of stunning and infarct size.
Conclusions: The myocardial infarct size-limiting effect of preconditioning did not correlate with the degree of myocardial stunning accompanying preconditioning. Thus, it is unlikely that myocardial stunning contributes to the cardioprotective effect of ischemic preconditioning.