Free radicals and myocardial ischemia and reperfusion injury

There is a growing body of evidence for the role of free radicals in mediating myocardial tissue injury during myocardial ischemia and in particular during the phase of myocardial reoxygenation. Associated with myocardial ischemia and reperfusion is the generation of oxygen-derived free radicals from a variety of sources that include the mitochondrial electron transport chain; the biosynthesis of prostaglandins; the enzyme xanthine oxidase; and circulating elements in the blood, with the polymorphonuclear neutrophil assuming a primary focus of attention. Experimental studies have shown that free radical scavengers (e.g., N-[2-mercaptopropionyl]glycine) and enzymes that scavenge or degrade reactive species of oxygen (superoxide dismutase or catalase) can reduce the mass of myocardial tissue that undergoes irreversible injury. Additionally allopurinol, which inhibits the enzyme xanthine oxidase, reduces ultimate infarct size, putatively by reducing the xanthine oxidase generation of superoxide anion. Neutrophils that enter the ischemically injured myocardium under the influence of chemotactic attraction and activation of the complement system generate and release highly reactive and cytotoxic oxygen derivatives that are destructive to the vascular endothelium and to the cardiac myocytes. Studies have documented that neutrophil depletion or suppression of neutrophil function (ibuprofen, nafazatrom, BW 755C, or more recently with prostacyclin or iloprost) results in a significant salvage of myocardial tissue that is subjected to a period of regional ischemia followed by reperfusion. Our current understanding of the events associated with myocardial ischemia suggests that within the ischemic myocardial region or area at risk, there is a population of cells that are reversibly injured and that reperfusion within a specified period (less than 3 hours) of time is capable of restoring the majority of the jeopardized cells to a normal status, but that the act of reperfusion itself will lead to the sudden demise of a fraction of the cells because of the cytotoxic effects of reactive species of oxygen derived from one or more of the sources indicated above. The efforts to minimize the amount of tissue that undergoes cell death as a result of myocardial ischemia demand that early reperfusion be established. However, the reintroduction of molecular oxygen and the circulating elements of the blood will be associated with an "explosive" and self-limited destruction of some of the myocardial cells in the area at risk.(ABSTRACT TRUNCATED AT 400 WORDS)