Arachidonate metabolism in myocardial ischemia and reperfusion

Circulatory blood corpuscles have enzymes catalyzing arachidonic acid. Platelets have cyclo-oxygenase system which produce highly vasoconstrictive and thrombogenic thromboxane A2 (TXA2). Neutrophils have another type of arachidonate metabolism system, lipoxygenase enzymes, which produce hydroxyeicosatetraenoic acids (HETE) and leukotrienes (LT), mediating inflammatory reactions. These arachidonate metabolites were found to play important roles in the evolution of myocardial ischemia. Thromboxane B2 (TXB2) a stable metabolite of TXA2, was elevated in peripheral blood of patients with angina pectoris. This elevation of TXB2 was supposed to be derived from platelet activation in coronary circulation due to altered production of TXA2 and prostacyclin (PGI2). Augmentation of TXA2 was also observed in patients with acute myocardial infarction. TXA2 synthetase inhibitors decreased plasma levels of TXB2 in these patients accompanied by attenuation of infarct size. Neutrophils were found to accumulate in ischemic myocardium and were augmented at reperfusion phase especially at interface between infarcted and risk zone. These infiltrated neutrophils may also provide deleterious effects on myocardial cells by producing lipoxygenase metabolites. In fact, a chemotactic and vasoconstrictive lipoxygenase product, 12-HETE, was produced selectively in ischemic myocardial tissue of an occlusion-reperfusion model. During evolution of myocardial cell damage, platelets and neutrophils, accumulated in ischemic tissue, may contribute to the exacerbation of microcirculatory disorders by producing vasoactive prostanoids, leading to expansion of myocardial necrosis. We should gain insights into these cellular interactions through arachidonate metabolism under normal and catastrophic conditions of coronary circulation.