Effects of Free Radicals on the Fluidity of Myocardial Membranes

Free radicals, including superoxide anions (O2.-), hydroxyl radical (HO.), and hypohalite radical (OCl.), as well as oxidants such as hydrogen peroxide (H2O2) and hypochlorous acid (HOCl), have been indicated in the pathogenesis of myocardial ischemic and reperfusion injury. In this report, we compared the integrity of the myocardial membrane when exposed to these free radicals/oxidants. Isolated rat heart membrane preparations were exposed to chemically generated free radicals with or without their respective scavengers. Membrane fluidity was monitored by fluorescence polarization using the diphenylhexatriene probe, as well as by electron spin resonance (ESR) spectroscopy using 2,2,6,6-tetramethyl piperidine-n-oxyl as the spin labeling agent. HO., H2O2, and OCl. + HOCl increased the fluorescence polarization (FP) and microviscosity significantly by 1.7-fold, 1.8-fold, and 1.7-fold, respectively, as compared to an only 1.2-fold increase in FP by O2.-. O2.- did not alter the fatty acid profiles of the membrane phospholipids. However, HO. and H2O2 reduced the arachidonic acid contents in phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylinositol (PI). These radicals also stimulated the lipid peroxidation by several-fold, while that by O2.- was only insignificant. These results suggest that HO. and H2O2 decreased the membrane fluidity and induced lipid peroxidation by releasing the arachidonic acid from PC, PE, and PI.