Cardiac protection by ischaemic preconditioning is not mediated by myocardial stunning.

Objective: Previous studies have shown that cardiac protection by ischaemic preconditioning wanes before contractile function recovers; thus stunning is insufficient to cause preconditioning. To test whether reduced contractile effort is necessary for preconditioning induced protection, the effect on myocardial infarct size of restoring contractile function with dobutamine was examined in preconditioned and control dogs.
Methods: In two experimental groups (groups P and P+D), preconditioning was produced by four 5 min occlusions of the left anterior descending coronary artery, each separated by 5 min of reperfusion. Contractile function was assessed by sonomicrometry 5 min after completion of the preconditioning protocol. In group P+D, dobutamine (average dose = 5 micrograms.kg-1.min-1) was then infused intravenously to restore systolic shortening to baseline. The artery then was reoccluded for 40 min of sustained ischaemia followed by 4 d of reperfusion. Two additional groups of non-preconditioned control dogs (groups C and C+D) also underwent 40 min of coronary occlusion and 4 d of reperfusion. Group C+D received a dobutamine infusion beginning 15 min before and during the 40 min occlusion to match the dobutamine received in group P+D, whereas group C received normal saline.
Results: Preconditioning caused mild postischaemic contractile dysfunction (50% decrease in systolic shortening) which was easily reversed by dobutamine treatment. Dobutamine also increased both the rate-pressure product and the left ventricular dP/dt in both treated groups (C+D and P+D). Histological infarct size was 12.3(SEM 2.0)% of the area at risk in the untreated control group (n = 11), and was reduced to 4.4(1.7)% in the untreated preconditioning group (n = 8; p < 0.05). Dobutamine increased non-preconditioned infarct size (group C+D) to 22.1(3.4)% (n = 7; p < 0.05). Infarct size in the dobutamine treated preconditioning group (P+D) was not significantly different from infarct size in group P (n = 8), at 6.1(2.5%).
Conclusions: In preconditioned hearts, dobutamine restored postischaemic contractile function but did not increase infarct size significantly. Thus reduced contractile effort is not required for the cardioprotective effect on ischaemic preconditioning.