Ischaemic preconditioning is not mediated by oxygen derived free radicals in rats

Although several studies have shown that ischaemic preconditioning greatly limits myocardial infarct size in the rat, the mechanisms of the beneficial effect of preconditioning in this species are not known. Experiments in dogs and rabbits have suggested that this effect could be related in part to the production of oxygen derived free radicals, as free radical scavengers partially prevent the limitation of infarct size induced by preconditioning. This study was designed to assess the contribution of oxygen derived free radicals in the infarct size limiting effects of preconditioning in rats, using the cell diffusible free radical scavenger N-2-mercaptopropionyl glycine (MPG).

Open chest rats underwent 20 minutes of coronary occlusion followed by one hour of reperfusion. Preconditioning was elicited by three cycles of five minutes ischaemia and five minutes reperfusion. MPG (20 mg.kg-1) was infused for 60 minutes starting 30 minutes before preconditioning. Control hearts (with or without MPG) were treated identically but without ischaemic preconditioning. Area at risk and infarct size were determined by India ink injection and triphenyltetrazolium chloride stain, with computerised analysis of enlarged sections after colour video acquisition.

During preconditioning, reperfusion after the first episode of ischaemia was associated with a high occurrence of severe ventricular arrhythmias, and this was reduced by MPG. Preconditioning, however, reduced arrhythmias and mortality during subsequent episodes of ischaemia and reperfusion. In the absence of MPG, preconditioning greatly limited infarct size (from (mean (SEM)) 59.8(3.9)% to 1.2(0.6)% of the area at risk; p < 0.01). MPG alone did not affect infarct size (60.5(6.1)%), and did not modify the infarct size limiting effect of preconditioning (infarct size: preconditioning 1.2(0.6)%; preconditioning + MPG 2.9(1.2)%.

Preconditioning greatly reduced infarct size, and this was not affected by MPG. These experiments suggest that production of oxygen derived free radicals does not contribute to preconditioning in the rat heart.