Selected metabolic alterations in the ischemic heart and their contributions to arrhythmogenesis

Abstract

Myocardial ischemia in vivo is associated with dramatic electrophysiologic alterations which occur within minutes of cessation of coronary flow and are rapidly reversible with reperfusion. This suggests that subtle and reversible biochemical and/or ionic alterations within or near the sarcolemma may contribute to the electrophysiologic derangements. Our studies have concentrated on 2 amphipathic metabolites, long-chain acylcarnitines and lysophosphatidylcholine (LPC) which have been shown to increase rapidly in ischemic tissue in vivo and to elicit electrophysiologic derangements in normoxic tissue in vitro. Incorporation of these amphiphiles into the sarcolemma at concentrations of 1 to 2 mol%, elicits profound electrophysiologic derangements analogous to those observed in ischemic myocardium in vivo. LPC is produced in endothelial cells and myocytes in response to thrombin. Thus, activation of the coagulation system during ischemia may result in extracellular production and accumulation of LPC. The pathophysiological effects of the accumulation of both amphiphiles are thought to be mediated by alterations in the biophysical properties of the sarcolemmal membrane, although there is a possibility of a direct effect on ion channels. Inhibition of carnitine acyltransferase I in the ischemic cat heart was found to prevent the increase in both long-chain acylcarnitines and LPC and to significantly reduce the incidence of malignant arrhythmias including ventricular tachycardia and fibrillation. This review focuses on the influence of these amphiphiles on cardiac ionic currents observed during early ischemia and presents data supporting the concept that accumulation of these amphiphiles within the sarcolemma contributes to changes in ionic conductances leading to electrophysiological derangements. The contribution and the accumulation of these amphiphiles to alterations in intracellular Ca2+ as related to changes in Na/K-ATPase activity and intracellular Na+ are examined. Other alterations occur during early myocardial ischemia in addition to the events reviewed here; however, the results of multiple studies over the past 2 decades indicate that accumulation of these amphiphiles contributes importantly to arrhythmogenesis and that development of specific inhibitors of carnitine acyltransferase I or phospholipase A2 may be a promising therapeutic strategy to attenuate the incidence of lethal arrhythmias associated with ischemic heart disease in man.