Abstract 18186: Arrhythmogenic Adverse Effects of Cardiac Glycosides are Mediated by Redox Modification of Ryanodine Receptors
The therapeutic efficacy of cardiac glycosides (CG) for heart failure is limited by the narrow therapeutic index and possible arrhythmogenic effects at high concentrations. In general, the arrhythmic side effects of CGs are ascribed to Ca2+ overload due to inhibition of Ca2+ removal by Na+/Ca2+ exchange secondary to Na+/K+-ATPase inhibition and elevation of intracellular [Na+]. However, CGs have been also shown to increase reactive oxygen species (ROS) which are also known to contribute to cardiac arrhythmogenesis through redox modification of cardiac ryanodine receptors (RyR2s) channels. Therefore, we tested the hypothesis that the arrhythmic adverse effects of CGs are caused by redox modification of RyR2s. Intracellular Ca2+ cycling and ROS production in isolated rat ventricular myocytes were monitored by confocal Ca2+ imaging using the Ca2+— and ROS-sensitive indicators Fluo-3 and DCFDA, respectively. Exposure to the CGs, digitoxin and ouabain, resulted in a significant increase in the incidence of arrhythmogenic spontaneous Ca2+ waves in myocytes paced at 0.3–1 Hz. Preincubation with digitoxin produced similar effects to those of the oxidant DTDP in permeabilized myocytes: a marked reduction in the sarcoplasmic reticulum Ca content accompanied by decreased amplitude and frequency of spontaneous Ca2+ sparks, compared to controls. Exposure of myocytes to CGs significantly increased the rate of ROS production. CG-induced changes in Ca2+ handling were prevented by treatment with the ROS scavenger, N-(2-mercaptopropionyl)glycine. The increase in CG-induced ROS production was significantly slowed by the NADPH-oxidase inhibitor diphenyleneiodonium chloride (DPI). Additionally, DPI reversed the digitoxin-induced increases in oxidized thiols on RyR2s Allopurinol, an inhibitor of xanthine oxidase, and rotenone, an inhibitor of mitochondrial electron transport showed no significant effects on the rate of digitoxin-dependent ROS production. These results suggest that the arrhythmogenic adverse effects of cardiac glycosides are mediated by redox modification of ryanodine receptors caused by ROS. Strategies to inhibit NADPH-oxidase may reduce CG-induced ROS and may reduce CG-induced arrhythmogenic adverse events.
- © 2010 by American Heart Association, Inc.