Recent evidence indicates that formation of an intracoronary thrombus contributes directly to arrhythmogenesis in the ischemic heart.  Although the mechanism has not been established, increased production of thrombin during clot formation may play an important role.  Myocardial ischemia resulting from acute occlusive thrombus formation is also accompanied by accelerated phospholipid catabolism resulting in the accumulation of lysophospholipids in the sarcolemma which directly contributes to arrhythmogenesis.  Dr. McHowat and colleagues have found that stimulation of isolated myocytes with thrombin results in increased lysophosphatidylcholine (LPC) production which appears to be mediated by activation of a novel calcium-independent phospholipase A2 (PLA2) with a distinct preference for ether-linked phospholipid substrates, including plasmalogens.  They have recently identified plasmalogens as principal phospholipid constituents in isolated rabbit ventricular myocytes and have shown that activity of membrane-associated, plasmalogen-selective PLA2 in isolated myocytes is increased during hypoxia.  Thus, accelerated PLA2-catalyzed plasmalogen catabolism during hypoxia and in response to thrombin is expected to contribute to accumulation of amphiphilic lysoplasmenylcholine (LPlasC) products, which bear a striking degree of structural similarity to LPC.  In addition, preliminary data from this group support the conclusion that LPlasC production increases during ischemia and contributes to production of electrophysiologic (EP) abnormalities.  The increase in cardiac myocyte PLA2 activity was found to be completely blocked by a selective inhibitor of calcium-independent PLA2, thereby defining a pharmacologic strategy to investigate the role of calcium-independent PLA2 and accompanying lysophospholipid production in the pathogenesis of ischemic cell injury.  The hypothesis to be tested is that thrombin contributes to PLA2 activation within hypoxic myocytes which leads to increased LPlasC production and EP dysfunction.  The Specific Aims are: 1) to define the role of thrombin in the activation of PLA2 and subsequent lysophospholipid accumulation in isolated normoxic and hypoxic cardiac myocytes; 2) to determine the metabolic pathways that participate in the catabolism of LPlasC under both normoxic and hypoxic conditions; and 3) to characterize the electrophysiologic effects of exogenous LPlasC in isolated cardiac myocytes.  Dr. McHowat suggests that these studies will provide important information to help determine which metabolic pathways could be selected as appropriate targets for novel therapeutic strategies aimed at reducing the incidence of arrhythmias in the setting of acute myocardial ischemia.