Arrhythmogenic adverse effects of cardiac glycosides are mediated by redox modification of ryanodine receptors

Abstract

Non-technical summary Cardiac glycosides (CGs) have been routinely used in the treatment of congestive heart failure (HF). Unfortunately, the therapeutic use of CGs in treating HF is limited by their adverse side effects, including cardiac arrhythmias. The arrhythmic side effects of CGs have been traditionally ascribed to excessive cellular Ca 2+ retention (Ca 2+ overload) leading to spontaneous discharges of intracellular Ca 2+ stores, or Ca 2+ waves, in turn causing oscillations of the cardiac membrane potential. In the present study, we demonstrate that the proarrhythmic effects of CGs on Ca 2+ cycling in cardiac myocytes involve alterations in the function of ryanodine receptor calcium channels caused by oxidative changes in the channel structure by reactive oxygen species. Our findings reveal a new mechanism for CG-induced Ca 2+ waves and suggest a potential target for antiarrhythmic therapy in HF patients treated with CGs.Abstract The therapeutic use of cardiac glycosides (CGs), agents commonly used in treating heart failure (HF), is limited by arrhythmic toxicity. The adverse effects of CGs have been attributed to excessive accumulation of intracellular Ca 2+ resulting from inhibition of Na + /K + -ATPase ion transport activity. However, CGs are also known to increase intracellular reactive oxygen species (ROS), which could contribute to arrhythmogenesis through redox modification of cardiac ryanodine receptors (RyR2s). Here we sought to determine whether modification of RyR2s by ROS contributes to CG-dependent arrhythmogenesis and examine the relevant sources of ROS. In isolated rat ventricular myocytes, the CG digitoxin (DGT) increased the incidence of arrhythmogenic spontaneous Ca 2+ waves, decreased the sarcoplasmic reticulum (SR) Ca 2+ load, and increased both ROS and RyR2 thiol oxidation. Additionally, pretreatment with DGT increased spark frequency in permeabilized myocytes. These effects on Ca 2+ waves and sparks were prevented by the antioxidant N -(2-mercaptopropionyl) glycine (MPG). The CG-dependent increases in ROS, RyR2 oxidation and arrhythmogenic propensity were reversed by inhibitors of NADPH oxidase, mitochondrial ATP-dependent K + channels (mito-K ATP ) or permeability transition pore (PTP), but not by inhibition of xanthine oxidase. These results suggest that the arrhythmogenic adverse effects of CGs involve alterations in RyR2 function caused by oxidative changes in the channel structure by ROS. These CG-dependent effects probably involve release of ROS from mitochondria possibly mediated by NADPH oxidase.

Muscarinic Stimulation Facilitates Sarcoplasmic Reticulum Ca Release by Modulating Ryanodine Receptor 2 Phosphorylation Through Protein Kinase G and Ca/Calmodulin-Dependent Protein Kinase II

“…Accordingly, the fraction of stored Ca that was released from the SR during a Ca transient (i.e. fractional release, FR, the ratio of the Ca transient amplitude to total SR Ca content 20,21 ) was increased by CCH (Fig 1F). …”

Cardiomyocyte Ca 2+ homeostasis as a therapeutic target in heart failure with reduced and preserved ejection fraction

“…By partially inhibiting the Na + /K + -ATPase, glycosides elevate intracellular Na + , which extends the brief period of the action-potential when NCX “reverses” and contributes to Ca 2+ influx (Figure 3, Table 1). Glycosides may also directly [32] or indirectly [33] activate RyR 2 and facilitate SR Ca 2+ release. While such actions may further augment contractility they also likely contribute to the arrhythmogenic side-effects of cardiac glycosides.…”

Redox Control of Cardiac Excitability

“…Thus, in presence of ROS, the same amount of CTS will cause toxicity and arrhythmogenesis. The effect of CTS and ROS on the Na/K-ATPase activity and RYR to cause arrhythmogenesis has been recently studied by Ho et al (111) (here CTS were shown to increase ROS).…”