Linking ryanodine receptor Ca2+ leak and Na+ current in heart: a day in the life of flecainide

In this issue of Acta Physiologica, Salvage et al. uncover a possible role for spontaneous Ca release from the cardiac Ca release channel, RyR2, in modulating cardiac voltage-gated Na channel (NaV1.5) activity (Salvage et al. 2015). Using a mouse model of increased spontaneous RyR2-mediated Ca release (RyR2), the authors report a significant decrease in NaV1.5-dependent current (INa) at baseline in intact atria. The central finding of this investigation is that when low concentrations of the Class IC anti-arrhythmic agent, flecainide, are applied to intact RyR2 atria, the INa paradoxically recovers to normal levels. This occurs despite the expected inhibitory effect of flecainide on INa. These new data suggest that increased spontaneous Ca release from the sarcoplasmic reticulum, or ‘SR Ca leak’, may directly modulate INa current in vivo. There has been a recent rekindling of clinical interest in flecainide fuelled by the findings of Watanabe et al. that indicated the anti-arrhythmic effects of flecainide may, in part, be attributed to its ability to act as a potent RyR2 inhibitor (Watanabe et al. 2009). After this initial finding, several other lines of investigation have supported this conclusion (Hilliard et al. 2010, Hwang et al. 2011, van der Werf et al. 2011). The inhibitory effect on RyR2 is reported to be especially robust when this drug is applied at low concentrations. However, this property of flecainide has been met with controversy. Several groups have attributed the anti-arrhythmic effects of flecainide rather to its inhibitory effect on INa in the heart, in line with its Class IC designation, and minimize the impact the drug may have on the RyR2 (Liu et al. 2011, Sikkel et al. 2013, Bannister et al. 2015). These divergent lines of thought on the dominant mechanism of flecainide are far from resolved. It is interesting to note that the published ranges for the IC50 of flecainide for RyR2 and NaV1.5 are 2–7 lM and 5–11 lM respectively (Galimberti & Knollmann 2011, Heath et al. 2011). With this in mind, experimentally differentiating between the substrate-specific effects of flecainide in the intact physiological system poses substantial difficulties. It is certainly possible that flecainide exerts effects on both the RyR2 and NaV1.5, simultaneously. The work by Salvage et al. adds a potentially new layer of functionality to flecainide. While previous work has indicated that flecainide may inhibit RyRmediated Ca release (at low concentrations) and block INa (at higher concentrations), the data presented here indicate that the RyR-mediated Ca release may directly modulate INa in the intact physiological environment. The authors show that low-dose flecainide leads to a significant increase in INa in intact atria isolated from the RyR2 model, paradoxical to the expected effect of flecainide on the Na channel. The authors suggest that RyR2-mediated Ca leak may somehow inhibit INa at baseline, an effect unmasked by flecainide application. Importantly, the authors support this hypothesis by showing that when the RyR-specific blocker, dantrolene, is applied to intact RyR2 atria, a similar increase in INa is observed. This experiment limits the possibility that a non-RyR-mediated flecainide-dependent effect underlies the observed increase on INa, and strongly suggests that SR Ca leak into the cytosol can directly modulate INa in the intact physiological environment. The exact biophysical reason for this Ca-dependent inhibition of INa was not established in this study, and further work is needed to elucidate this potentially exciting mechanism. However, two lines of reasoning have emerged indicating that the interplay between local levels of Na and Ca in the tight confines of the dyadic cleft may play a role. ‘Leaked’ SR Ca is removed from the dyadic cleft by nearby Na/Ca exchangers (NCX, Biesmans et al., PLoSone 2011), resulting in Na influx. Assuming close proximity between the NCX and Na channels (Lines et al., Biophys J, 2006), a leak-dependent local elevation of [Na]i could reduce the driving force for Na + entry through the channel. Indeed, elevation of intracellular Ca has been previously shown to inhibit INa in cardiomyocytes, although it was suggested that this effect might result from direct modulation of Na channel activity by Ca, or the involvement of Ca + -activated proteins (Casini et al., Cardiovasc Res, 2008). Interestingly, Sikkel et al. (2013) also proposed that the NCX may mediate the interplay between INa and RyR activity during flecainide treatment, but in the opposite direction. Instead of the effect being initiated