PATHOGENESIS OF ACUTE MYOCARDIAL-INFARCTION - NOVEL REGULATORY SYSTEMS OF BIOACTIVE SUBSTANCES IN THE VESSEL WALL

Rupture of the lipid-rich atheromatous plaque, intraplaque hemorrhage, and intraluminal thrombus are three pathological hallmarks most commonly recognized in the infarct-related coronary artery at the site of acute myocardial infarction. Rupture of the atheromatous plaque is closely related to but does not fully explain the genesis of occlusive intracoronary thrombus formation and thus the development of acute myocardial infarction. Besides a variety of hematologic disorders, one should emphasize the role of the platelet-derived mediators that promote an environment where thrombosis and vasoconstriction occur, including TXA2, serotonin, ADP, platelet-derived growth factor, tissue factor, and the diminished availability of those natural endogenous substances that inhibit platelet aggregation, such as EDRF, tissue plasminogen activator, and PGI2. PGI2 released from vascular endothelial cells is extremely unstable. Our group provided the first evidence that HDL stabilizes PGI2 through the newly discovered function of Apo A-I, which is associated with the surface of HDL particles and identified as PGI2 stabilizing factor. Decrease in HDL-associated Apo A-I in patients with unstable angina and during the acute phase of myocardial infarction indicates that HDL plays an important role in preventing coronary atherosclerosis and intracoronary thrombus formation by stabilizing PGI2 in addition to the generally accepted biochemical property of HDL to prevent the accumulation of cholesterol by mobilizing free cholesterol from tissues or macrophages. There is also a PGI2 synthesis-stimulating factor in serum that has not yet been identified chemically. EDRF or nitric oxide provides another important regulating system in the vessel wall. Lipoproteins are inhibitors of endothelium-dependent relaxation of rabbit aorta. The role of plasma Lp(a) in inhibiting intrinsic fibrinolysis is still to be elucidated. Damage to the vascular endothelium at the time of atheromatous plaque rupture could provoke the breakdown of the exquisite homeostasis regulating platelet aggregation, prostaglandins, EDRF, and as yet unknown bioactive substances in the vessel wall, resulting in occlusive intracoronary thrombus formation and acute myocardial infarction. The present review clearly demonstrates the importance of the new discoveries to the regulatory systems in the vessel wall for intracoronary thrombus formation.