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Are we ready for a new mechanism of action underlying digitalis toxicity?

It is said that the only certainties in life are death, taxes and tetrodotoxin. However, just when we think we understand the cellular mechanisms of a drug that has been used in the treatment of cardiac disease for over two centuries, along comes something completely different. Such is the case with the report in a recent issue of The Journal of Physiology by Ho et al. that cardiac glycoside (CG) toxicity in the form of spontaneous Ca2+ waves arises in response to altered function of cardiac ryanodine receptors (RyRs) induced by reactive oxygen species (ROS). The authors found that ROS generation oxidized thiol groups on RyRs, increasing their functional activity by increasing sensitivity to Ca2+ in the sarcoplasmic reticulum (SR) (Terentyev et al. 2008). The result is increased frequency of Ca2+ waves which, through sodium–calcium exchange (NCX), depolarize cardiac myocytes and induce triggered arrhythmias during CG toxicity. They also suggest that this effect of CGs occurs secondary to an interaction with (but not inhibition of) the Na+,K+-ATPase (NKA) that induces intracellular signalling through Src kinase, causing ROS production and opening of the mitochondrial ATP-dependent K+ channel (mito-KATP) (Tian et al. 2003; Pasdois et al. 2007). 
 
It is generally accepted that NKA inhibition by CGs causes both inotropic and toxic actions since the resulting intracellular Na+ accumulation, through NCX, also increases Ca2+. The raised Ca2+ increases contraction (resulting in positive inotropy) but eventually leads to toxicity in the form of Ca2+ waves (and triggered arrhythmias) as SR Ca2+ storage capacity is exceeded and Ca2+ overload develops. Since the 1960s, however, it has been known that different CGs have different abilities to produce both inotropy and toxicity, questioning the singular NKA hypothesis. One possible explanation involves direct CG actions on the RyR to increase its sensitivity to Ca2+ (Wasserstrom & Aistrup, 2005). Ho et al. suggest that an effect on RyRs might also occur indirectly, with ROS as the intermediary. CG binding to the NKA induces ROS generation through Src kinase signalling which does not involve increased Na+ and Ca2+. Rather, the authors suggest that NADPH oxidase causes release of mitochondrial ROS since increased ROS, RyR oxidation and Ca2+ wave frequency were reversed by inhibition of NADPH oxidase and mito-KATP channels but not by inhibition of xanthine oxidase. The authors are careful to point out that this mechanism may be distinct from the traditional mechanisms involving NKA inhibition and resulting intracellular Na2+ and Ca2+ changes as well as from potential direct actions on the RyR itself. Their results suggest a novel mechanism for the increased spontaneous Ca2+ release that underlies the toxic, arrhythmogenic effects of CGs and not their inotropic effects. 
 
Another interesting feature of their studies was the fact that they found that Ca2+ waves occurred despite a reduced SR Ca2+ load. Thus, RyR activation occurred at lower SR load, presumably because RyRs were sensitized to luminal Ca2+ after thiol modification. This is an unusual view of Ca2+ wave generation which ordinarily occurs through increased SR Ca2+ load which then increases RyR sensitivity, causing spontaneous release in the form of a wave. In contrast, Ho et al. present compelling evidence that CG induction of ROS with subsequent RyR modification increases functional activity despite a lower SR Ca2+ load. This mechanism may also provide a novel explanation for why toxicity might occur more easily with some CGs than with others; if one agent is more able to induce increased ROS than another, this might explain the differences in toxicity to therapeutic ratio that have been reported among different CGs for many years. 
 
An increase in RyR sensitivity to Ca2+, independent of changes in Na+ and Ca2+, suggests a unique mechanism for CG toxicity in the absence of increased SR load. Clinically, arrhythmias might then be susceptible to treatment with agents that reduce RyR sensitivity, obviating the use of generally ineffective conventional antiarrhythmic agents. Cardiac ROS suppression might also provide a fruitful therapeutic approach to treatment of CG toxicity. Consequently, CG effects on RyRs through a ROS intermediary might provide an entirely new and fresh view of how to deal therapeutically with the extremely common and highly dangerous effects of one of the world's most widely used medications for the treatment of heart disease.