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Cardiovasc Res. 2011 March 1; 89(4): 734–743.
Published online 2010 October 12. doi:  10.1093/cvr/cvq324
PMCID: PMC3039246

The ryanodine receptor channel as a molecular motif in atrial fibrillation: pathophysiological and therapeutic implications


Atrial fibrillation (AF) is the most common cardiac arrhythmia and is associated with substantial morbidity and mortality. It causes profound changes in sarcoplasmic reticulum (SR) Ca2+ homeostasis, including ryanodine receptor channel dysfunction and diastolic SR Ca2+ leak, which might contribute to both decreased contractile function and increased propensity to atrial arrhythmias. In this review, we will focus on the molecular basis of ryanodine receptor channel dysfunction and enhanced diastolic SR Ca2+ leak in AF. The potential relevance of increased incidence of spontaneous SR Ca2+ release for both AF induction and/or maintenance and the development of novel mechanism-based therapeutic approaches will be discussed.

Keywords: Calcium handling, Atrial fibrillation, Remodelling, Ryanodine

1. Introduction

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia and is associated with increased cardiovascular morbidity and mortality.1 Current therapeutic approaches have major limitations, including low efficacy and enhanced risk of proarrhythmic events in the ventricle.2 The major cause of increased morbidity in AF patients is stroke due to thromboembolism, which arises from blood stasis in atria as a consequence of decreased atrial contractility. Impaired intracellular Ca2+ handling is believed to play a critical role in atrial mechanical dysfunction.36 The smaller increase of intracellular Ca2+ during systole (i.e. the systolic Ca2+ transient) is a major cellular determinant of contractile dysfunction in AF.5,6 Moreover, susceptibility to spontaneous diastolic sarcoplasmic reticulum (SR) Ca2+ release through ryanodine receptor channels (RyR2) appears higher in AF and might contribute to AF-associated arrhythmogenesis by promoting both triggered activity and/or reentry.7 This review focuses on the molecular basis of altered RyR2 function in AF, particularly on defective phosphorylation that causes SR Ca2+ leak and discusses possible consequences of increased SR Ca2+ leak for AF promotion and/or maintenance. For discussions of other important aspects of RyR2 regulation (e.g. competitive binding of Mg2+ or stretch) or modelling of RyR2 function, the readers are referred to recent pertinent articles.811

2. Fundamental mechanisms of AF: reentry vs. triggered activity

Two major mechanisms may cause AF at the organ level: reentry and ectopic activity (Figure 1). The sources underlying these mechanisms are often localized in one of the pulmonary veins or in the left-atrial posterior wall.12 Risk factors (i.e. age or gene mutations) and co-morbidities (i.e. heart failure or hypertension) predispose to AF by causing specific electrical and structural changes (remodelling) that produce diverse arrhythmogenic substrates, promoting arrhythmia maintenance. The homeostatic responses to the high-atrial rate rapidly cause remodelling of atrial repolarization, promoting reentry, and AF perpetuation.13 Concomitant cardiovascular diseases can also provide the trigger for AF initiation (e.g. acute atrial dilatation). Thus, the substrate caused by risk factors and concomitant disease conditions preceding AF is the key for AF perpetuation.14,15

Figure 1
Fundamental mechanisms of AF induction and persistence. AF is maintained by either reentry or ectopic activity. Reentry formation requires a trigger that acts on an arrhythmogenic substrate. Atrial remodelling creates a substrate for reentrant AF and ...

In addition to a susceptible substrate, reentry requires a trigger, usually provided by an ectopic beat (Figure 1). Ectopic activity may contribute to AF initiation by acting as a trigger of reentry, but persistence of ectopic activity may also sustain AF by facilitating reentry.7 Ectopic activity is caused by abnormal spontaneous discharges that can result from oscillations of the myocyte membrane potential (afterdepolarizations). Afterdepolarizations may occur during repolarization [early afterdepolarizations (EADs)] or after completion of the action potential [delayed afterdepolarizations (DADs)]. Left-atrial sources of ectopic activity appear to be of particular importance in a subset of patients with paroxysmal AF.16 EADs occur during excessive action potential (AP) prolongations and are commonly associated with bradycardia or pauses.17 DADs typically result from SR Ca2+ leak due to spontaneous (non-synchronized) diastolic SR Ca2+ release, which is caused by either SR Ca2+ overload, RyR2 dysfunction, or a combination of both. With confocal microscopy, SR Ca2+ leak has been visualized as Ca2+ sparks, but invisible non-spark release events may also occur.18 Diastolic SR Ca2+ release manifest as either individual or series of small-amplitude membrane-voltage oscillations that could also lead to triggered APs. In diseased hearts, spontaneous SR Ca2+ release through RyR2 activates a transient inward current (Iti), which is largely carried by Na+/Ca2+ exchanger (NCX)19,20 and is the dominant contributor to induction of cellular DADs.21,22 Potentially arrhythmogenic DADs have been demonstrated in isolated diseased human atrial appendages.23 AF-related remodelling increases the likelihood of atrial cellular DADs, which is described subsequently.

3. Ca2+-signalling properties in normal atria

As in ventricular myocytes, excitation-contraction coupling in atrial myocytes starts with Ca2+ entry via voltage-gated L-type Ca2+-channels (ICa,L) that triggers a greater SR Ca2+ release via RyR2, a process known as Ca2+-induced Ca2+ release.7 However, in the absence of a fully developed T-tubule system in atria,24 Ca2+ influx triggers a sequential (non-synchronous) increase in [Ca2+]i, whereby Ca2+ waves start in the atrial-myocyte periphery (junctional SR) and then propagate to the myocyte centre, recruiting additional Ca2+-releasing sites.7 The size of the systolic Ca2+ transient depends on both open probability of RyR2 and SR Ca2+ content, which is indirectly a function of Ca2+ reuptake through SR Ca2+-ATPase (SERCA2a). Removal of cytosolic Ca2+ during diastole occurs primarily by SERCA2a-mediated reuptake into SR and extrusion by NCX. Under physiological steady-state conditions, Ca2+ influx equals Ca2+ efflux and there is little change in SR Ca2+ content.25

There are important differences in intracellular Ca2+ cycling between atrial and ventricular cardiomyocytes. Compared with ventricular myocytes, SR Ca2+ reuptake is higher in atrial myocytes, likely due to greater expression of SERCA2a and lower levels of SERCA2a-inhibitory phospholamban (PLB).26,27 Atrial myocytes exhibit higher SR Ca2+ content and cellular Ca2+-buffering capacity27 than ventricular myocytes, which is consistent with an enhanced SR Ca2+ reuptake via SERCA2a. The stronger Ca2+-buffering power of atrial myocytes may also result from altered Ca2+-binding kinetics to myofilaments.28 Atrial NCX currents are smaller compared with those of ventricular cells.27 However, atrial myocytes are also smaller than ventricular myocytes, and correcting NCX current amplitude for cell size showed that atrial myocytes have larger NCX current density compared with ventricular cells.27

RyR2 is the major SR Ca2+-release channel in the heart. The sensitivity of RyR2 to cytosolic and luminal (intra-SR) Ca2+ and thus its open probability are modulated by accessory binding proteins (e.g. calsequestrin, junctin, triadin, FK506-binding protein 12.6 kDa [FKBP12.6]) and posttranslational modifications (e.g. phosphorylation) (Figure 2A).29,30 Calsequestrin (CSQ), the major SR Ca2+ buffer and sensor, is linked to RyR2 via junctin and triadin.31 Changes in the relative amounts of junctin and triadin were shown to modulate RyR2 Ca2+ release and cause cardiac arrhythmias.32,33 Protein kinase A (PKA) and Ca2+/calmodulin-dependent protein kinase II (CaMKII) bind to RyR2 macromolecular complex, which enables these enzymes to dynamically phosphorylate RyR2.34 Conversely, RyR2-bound type-1 (PP1) and type-2A (PP2A) protein phosphatases can dephosphorylate RyR2 depending on relative kinase-phosphatase activity balance.35 The relative level of RyR2 phosphorylation determines RyR2 sensitivity to cytosolic Ca2+ and thus open probability of RyR2 and the amount of SR Ca2+ release during diastole and systole.36,37

Figure 2
Composition of the atrial RyR2 macromolecular complex and Ca2+ spark incidence in AF. (A) Schematic representation of one out of four RyR2 monomers of the RyR2 macromolecular complex, each associating with various subunits indicated. PKA, protein kinase ...

4. Defective SR Ca2+ release through RyR2 channels in AF

Multiple studies have shown that abnormal SR Ca2+ handling plays a central role in initiation and/or maintenance of chronic AF (cAF) in humans.35,3843 Defective Ca2+ handling was shown to predispose to this spontaneous SR Ca2+ release in atrial myocytes from cAF patients (Figure 2B).38,40,41 SR Ca2+ load was not increased in cAF patients,38,41,43 suggesting that this spontaneous SR Ca2+ release most likely occurred because of changes in RyR2. Altered RyR2 function in cAF is accompanied by an increase in NCX expression and function,4,41,4345 suggesting that diastolic SR Ca2+ leak can be amplified by NCX, triggering ectopic focal discharges or facilitating microreentry circuits promoting AF maintenance.

Expression levels of RyR2 were unaltered6,46,47 or reduced in dogs, goats, and cAF patients, respectively.41,48,49 However, and perhaps more importantly, the binding levels of accessory subunits and posttranslational modification were altered in cAF,35,41,45,50,51 leading to increased open probability of RyR2 (Figure 3A–C).35 Protein levels of CSQ2 appear normal in cAF patients,45 whereas atrial CSQ2 levels are reduced in dogs with heart failure, likely contributing to SR dysfunction in heart failure-based AF.52 The level of the RyR2-stabilizing subunit FKBP12.6 (also known as calstabin2) was 50% lower in cAF patients (Figure 3A), which could explain why RyR2 channels fail to remain closed during diastole.35 Consistent with this idea, FKBP12.6-deficient mice exhibit an increased vulnerability to pacing-induced AF and enhanced spontaneous SR Ca2+ leak.53 It is very likely that enhanced RyR2 activity plays a role in AF pathogenesis, as mice with a gain-of-function mutation in RyR2 exhibit an increased susceptibility to pacing-induced AF.51 Using these knock-in mice, we demonstrated that increased SR Ca2+ leak in atrial myocytes can promote triggered activity and atrial arrhythmias.

Figure 3
Increased phosphorylation and open probability of RyR2 in AF patients. (A) Compared with sinus rhythm (SR) patients, AF patients show increased Ser2809 phosphorylation of RyR2. Calstabin2 (FKBP12.6) levels bound to RyR2 are decreased in AF. Reproduced ...

Changes in the phosphorylation level of RyR2 have been reported consistently in cAF (Figure 3A and B). Phosphorylation of Ser2808 (Ser2809 depending on species) on RyR2 is higher in dogs with pacing-induced cAF and cAF patients.35 PKA-overexpressing mice have hyperphosphorylated RyR2 at Ser2808 (and hyperphosphorylated PLB at Ser16) and develop AF,54 suggesting potential importance of Ser2808 phosphorylation for AF promotion. One additional PKA site (Ser2030) was also suggested to affect open probability of RyR2,55 but the functional importance of Ser2030 phosphorylation in the heart is still unknown. In atria of cAF patients, RyR2 is also hyperphosphorylated by CaMKII at Ser2814 (Ser2815 depending on species).41,51 This might result from increased expression and autophosphorylation at Thr287 (and thus activity) of CaMKII (Figure 3B).51,56,57 Goats with sustained AF show enhanced autophosphorylation and thus increased activity of CaMKII along with higher CaMKII-dependent RyR2 phosphorylation at Ser2815,50 clearly suggesting that the high-atrial rate is sufficient to cause these alterations. Notably, goats with atrial dilatation, but without sustained AF, also exhibit increased CaMKII activity and RyR2 hyperphosphorylation at Ser2815,50 pointing to the possibility that structural atrial diseases may predispose to AF by producing changes in cellular Ca2+ signalling similar to those observed in patients being in cAF. Consistent with the critical role of enhanced CaMKII activity for AF pathophysiology, mice with a genetic gain-of-function defect in RyR2 (RyR2R176Q/+ mice) show enhanced SR Ca2+ leak and increased vulnerability to rapid atrial pacing-induced AF compared with wild-type mice.51 Both genetic CaMKII inhibition or blockade of CaMKII with KN-93 prevented pacing-induced AF in these knock-in mice, clearly suggesting that inhibition of CaMKII effectively control AF inducibility in mice.51 Moreover, the absence of carbachol-induced AF in RyR2-S2814A knock-in mice, in which CaMKII phosphorylation of RyR2 was genetically inhibited, confirmed the importance of this single phosphorylation event in the pathogenesis of atrial arrhythmias.51

The molecular mechanisms of enhanced steady-state RyR2 phosphorylation at Ser2808 and Ser2814 in cAF are poorly understood. Although total protein levels and activities of PP1 and PP2A are increased in atria of cAF patients,45,58 the actual PP1 and PP2A activities within the RyR2 macromolecular complex are unknown. In addition, PP1 is regulated by inhibitor-1 and inhibitor-2.59 Inhibitor-1 levels were not altered in cAF patients, but Thr35 phosphorylation of inhibitor-1, which controls PP1 function exclusively at the SR, specifically targeting phosphorylation of PLB and RyR2 at Ser16 (PKA-site) and Ser2814 (CaMKII-site), respectively,60,61 was increased. This should lead to a strong PP1 inhibition at specific phosphorylation sites within the SR compartment, possibly contributing to enhanced RyR2 phosphorylation at Ser2814.60 The mechanism of enhanced RyR2 phosphorylation at Ser2808 in cAF is less clear. In atria of goats with sustained AF basal and cAMP-induced PKA activities were 50% lower, rendering an increase in PKA activity unlikely. As CaMKII phosphorylate Ser2808/2809,62 it could be speculated that within the SR compartment, the AF-related increase in CaMKII activity overcomes the enhanced PP1 activity, causing greater steady-state Ser2808 phosphorylation. Indeed, application of tetracaine, which decreases Ca2+ sensitivity of RyR2, in both quiescent and voltage-clamped (at –80 mV) human atrial myocytes in the absence of Na+ and Ca2+ in bath solution (to prevent transsarcolemmal fluxes) caused a stronger decrease in diastolic [Ca2+]i in cAF patients, unmasking a larger SR Ca2+ leak in cAF vs. sinus-rhythm patients.41,42 Moreover, inhibition of CaMKII abolished Ca2+ sparking and normalized SR Ca2+ leak in cAF to levels seen in sinus-rhythm patients,41,42 pointing to the possibility that the AF-associated increase in SR Ca2+ leak is mediated exclusively by CaMKII.

In the absence of alterations in other Ca2+-handling proteins or atrial remodelling, increased open probability of RyR2 alone (Figure 3C) is probably not sufficient to trigger spontaneous Ca2+ waves causing DADs/triggered activity.51,63 It is possible that diastolic SR Ca2+ leak via RyR2 can only be sustained if a certain threshold of SR Ca2+ load can be maintained to ensure sufficient RyR2 sensitization to luminal (intra-SR) Ca2+.63 SR Ca2+ load not only determines the size of the Ca2+-induced Ca2+ release by the law of mass action, but also sensitizes RyR2 to luminal Ca2+ by an allosteric interaction that changes RyR2 gating properties.64 In dogs with tachycardiomyopathic heart failure, the luminal SR Ca2+ concentration that causes half-maximal RyR2 activation is about 100 times lower than that in control animals65 and similar increases in sensitivity to luminal Ca2+ were reported for some RyR2 mutations related to CPVT.66 These studies clearly demonstrate that disease-related changes in the ability of RyR2 to sense luminal Ca2+ may promote diastolic SR Ca2+ leak in the presence of both normal and reduced SR Ca2+ load. However, it remains to be determined experimentally whether the sensitivity of RyR2 to luminal Ca2+ is changed in AF. Thus, the specific consequences of changes in SR Ca2+ load are not as clear, because SR Ca2+ load modulates sensitivity of RyR2 to luminal Ca2+ and this may have important implications for subsequent phosphorylation-dependent regulation of RyR2 sensitivity to cytosolic Ca2+. Although SR Ca2+ load appears normal in cAF (Figure 4A and B),4,41,43 which may play a permissive role for RyR2 dysfunction, the mechanisms that help to maintain sufficient SR Ca2+ content are poorly understood. In cAF patients, PLB is hyperphosphorylated at both Ser16 (PKA-site) and Thr17 (CaMKII-site), respectively, which may prevent SR Ca2+ depletion during AF, potentially contributing to preserved SR Ca2+ content.41,43,45 Again, elevated PLB phosphorylation occurs in the face of globally increased activity levels of PP1 and PP2A,45,58 which highlights the importance of local differences in protein phosphatase activity and/or targeting within discrete microdomains in atrial myocytes. In addition, it has been demonstrated that the expression levels of sarcolipin (SLN), an SERCA2a inhibitor that like PLB loses its SERCA2a-inhibitory properties when phosphorylated by CaMKII at Thr5,67 is decreased in cAF patients.68 Reduced SLN binding to SERCA2a together with altered PLB regulation could theoretically enhance SR Ca2+ reuptake, offsetting the Ca2+ loss due to increased SR Ca2+ leak.

Figure 4
Increased NCX function in patients with cAF. (A) Representative examples of caffeine-evoked Ca2+ transients (CaT) in voltage-clamped (at –80 mV) atrial myocytes from patients in sinus rhythm (SR) or cAF. (B) Bar graphs showing no significant differences ...

In addition, amplitude of ICa,L is ~50–70% lower in cAF patients,6971 and reduced SR Ca2+ release due to the decreased trigger ICa,L might be the major cause for the ~50% smaller Ca2+-transient amplitude in cAF patients (Figure 5) because SR Ca2+ content is unchanged.38,41,43 ICa,L is subject to Ca2+-dependent inactivation, and SR Ca2+ release contributes importantly to Ca2+-dependent ICa,L inactivation creating a negative feedback on Ca2+ influx. In fact, the smaller Ca2+-transient amplitude in cAF patients allows larger time-dependent Ca2+ influx and the integrated Ca2+ influx through ICa,L is only ~25% lower in cAF vs. sinus-rhythm patients (N. Voigt et al., unpublished observations). In addition, as under steady-state conditions, Ca2+ influx equals Ca2+ efflux with little change in SR Ca2+ content,25 the lower ICa,L in cAF will trigger smaller release from the SR (Figure 5)43 and accordingly less Ca2+ is pumped out of the cell by NCX with each beat, which may possibly explain why there is little change in SR-Ca2+ load in cAF patients. Thus, in cAF patients, normal SR Ca2+ load might be maintained through multiple mechanisms. Interestingly, dogs with experimental heart failure show increased SR Ca2+ load, along with enhanced CaMKII phosphorylation of PLB and increased frequency of DADs in atrial cells,52 suggesting that DADs and triggered activity might occur particularly in heart failure-induced AF.

Figure 5
Reduced L-type Ca2+ current (ICa,L) triggers smaller [Ca2+]i transients in patients with cAF. (Left) Representative recordings of ICa,L on depolarization from the prepulse potential of −40 to +10 mV for 100 ms and corresponding triggering of [Ca ...

5. Upregulation of NCX may amplify the consequences of increased diastolic SR Ca2+ leak in AF

It is believed that NCX can contribute to arrhythmogenesis in AF by its ability to generate a transient inward current (Iti) following forward-mode activation triggered by SR Ca2+ release.20 By removing excess cytosolic Ca2+ during diastole, NCX promotes Na+ entry which could initiate abnormal APs and triggered activity, thereby contributing to atrial arrhythmogenesis. Moreover, it is believed that Iti in atrial myocytes is largely mediated by INCX without significant contributions by Ca2+-dependent chloride or non-selective cation currents.27 The expression levels of NCX1 are upregulated in patients and sheep with cAF, consistent with a potential role for NCX in the generation of DADs.4,41,4345

The molecular mechanisms underlying increased NCX activity in AF is unknown. NCX is organized in a macromolecular complex that contains kinases (protein kinases A and C), phosphatases (PP1 and PP2A), and multiple regulatory proteins (ankyrin-B, β-spectrin, actin) that are proposed to exert tonic NCX modulation.72 Thus, regulatory proteins and posttranslational modifications could also contribute to enhanced NCX activity, but the impact of impaired regulation on altered NCX function requires further investigation. The larger amount of NCX1 protein in AF is probably due to the increase in NCX1 mRNA levels, which are reversible after restoration of sinus rhythm in cAF patients.73 This points to the possibility that increased NCX1 expression is the consequence of AF itself, contributing to maintenance rather than induction of AF. Regardless of the underlying molecular mechanism, the enhanced function of NCX in response to a given SR Ca2+ release (Figure 4D and F) in combination with the increased diastolic SR Ca2+ leak41,42 may cause DADs and triggered activity that may contribute to AF maintenance.

6. Potential relevance of spontaneous RyR2 Ca2+ release for AF pathophysiology

It is well recognized that AF is a progressive condition, with AF-induced atrial remodelling increasing the susceptibility to and stability of AF.74 Multiple factors underlie atrial remodelling14 and there is now emerging evidence that altered atrial subcellular Ca2+ signalling may cause DADs/triggered activity that may promote the induction and/or maintenance of AF.7 SR Ca2+ leak in AF appears to result from increased CaMKII activity with subsequent RyR2 hyperphosphorylation (Figure 3B).41,42,51 Spontaneous Ca2+ waves in ventricular myocytes from failing rabbit hearts depend on CaMKII,75 suggesting that CaMKII-mediated SR Ca2+ leak in AF might cause potentially arrhythmogenic spontaneous Ca2+ waves. It is generally assumed that SR Ca2+ leak and waves of spontaneous Ca2+ release result from similar mechanisms, but sustained SR Ca2+ leak could produce a background INCX rather than a triggering current. Furthermore, spontaneous Ca2+ waves are slow and dyssynchronized and it is therefore unlikely that under resting conditions diastolic Ca2+ waves will cause DADs that are sufficiently large for generating triggered APs.76 The latter can only occur if the physiological safety margin decreases,19 for instance, due to remodelling-induced changes in [Ca2+]i–membrane voltage coupling gain. Indeed, INCX increases linearly with the rise of [Ca2+]i, but a given [Ca2+]i generates a larger INCX in cAF (Figure 4D and E),4,43 suggesting an increased coupling gain between [Ca2+]i and INCX in cAF. However, the degree of subsequent membrane depolarization is nonlinear, because the stability of the resting membrane potential (RMP) depends on multiple factors including activity of the inward-rectifier IK1 and possibly other background currents.20 As the amplitude of inward-rectifier currents, which are the major determinants of RMP, is larger and is associated with a slightly more negative RMP in cAF patients,7782 cellular DADs and triggered AP can occur only if there is an AF-associated change in the diastolic [Ca2+]i–membrane voltage coupling gain19,83.

It is very likely that altered RyR2 and NCX functions contribute to atrial arrhythmogenesis in AF by providing an arrhythmogenic substrate, especially in vivo whereby the high-atrial rate (5–8 Hz) and the neurohumoral influences should amplify the consequences of altered cellular Ca2+ signalling potentially serving as triggers (Figure 6). However, there are still important gaps in our knowledge about impaired atrial Ca2+ handling. For instance, persistence of diastolic SR Ca2+ leak requires maintained SR Ca2+ load.84 Although recent results in AF patients point to preserved global SR Ca2+ load and enhanced functional NCX,41,43 it remains to be determined whether increased SR Ca2+ leak and sufficient SR Ca2+ load occur at the sub-sarcolemmal SR compartment. Although SR Ca2+ leak is higher in cAF vs. sinus-rhythm patients,41,42 the quantitative relation between SR Ca2+ leak, probability of occurrence of diastolic Ca2+ waves, and amplitude of NCX is currently unknown. Nevertheless, the size of the NCX current associated with spontaneous SR Ca2+ release appears sufficient to depolarize the membrane to the threshold needed for triggering an AP because preliminary results show that incidence of spontaneous (non-stimulated) Ca2+-release events accompanied by corresponding inward INCX currents is higher in cAF patients, and cellular DADs/triggered APs occur more often in patients with cAF than those in sinus rhythm.85 This makes an increase in coupling gain between alterations in diastolic [Ca2+]i and changes in membrane voltage in AF patient very likely, but this remains to be determined.

Figure 6
Increased RyR2 Ca2+ leak and enhanced functional NCX might produce DADs and triggered activity during AF. During AF, faster atrial rates, increased oxidative stress, and enhanced sympathetic drive can strongly activate CaMKII, thereby increasing phosphorylation ...

In addition to DADs and triggered activity, spontaneous Ca2+ waves may also cause subcellular Ca2+ alternans and sudden repolarization changes that may increase dispersion of refractoriness,86 promoting reentry, and arrhythmia maintenance (Figure 6). Finally, although Ca2+-dependent sources are suggested to contribute to maintenance of clinical AF,87 direct experimental evidence of the causal relationship between Ca2+-related cellular proarrhythmic events and focal sources and/or continuous conduction in fibrillating human atria is still lacking. Thus, additional work will be needed to definitely prove the clinical impact of altered atrial Ca2+ signalling for AF promotion and maintenance.

7. Potential therapeutic opportunities to target abnormal RyR2 function in AF

Traditionally, AF has been treated with drugs that block voltage-gated ion channels. However, current drug therapy is also associated with serious proarrhythmic effects, which strongly limits its clinical applicability.2 There is hope that a better understanding of molecular pathways involved in atrial arrhythmogenesis will lead to the development of safer and more effective therapeutic approaches for AF treatment.2,14

New therapeutic modalities may include compounds that inhibit SR Ca2+ leak by normalizing the function of the RyR2 macromolecular complex.21 The 1,4-benzothiazepine analogue JTV519 (K201) prevents AF in a canine model of sterile pericarditis88 and reduces the RyR2-mediated SR Ca2+ leak in mice by reversing disease-associated reduction in FKBP12.6 binding to RyR2.35,89,90 In addition, JTV519 reduces the incidence of spontaneous SR Ca2+ release and DADs that arise as a consequence of SR Ca2+ leak in mouse ventricular myocytes.22 It also reduces firing rates in rabbit pulmonary vein cardiomyocytes, decreases amplitude of DADs, prolongs AP duration, and decreases incidence of pacing-induced AF.91 In addition to RyR2, JTV519 also affects several atrial92 and pulmonary vein ion channels.91 JTV519 appears a suitable lead structure for development of drugs that specifically target RyR2 function and the RyR2/FKBP12.6 interaction. A more specific JTV519 analogue (S107)93 is currently under investigation, but efficacy and safety of S107 remains to be determined in experimental AF paradigms and AF patients.

RyR2 blocking agents may also be of potential therapeutic value. For example, the local anaesthetic drug tetracaine completely suppresses Ca2+ sparks in atrial myocytes from patients in cAF.40,41 Flecainide, a class 1C anti-arrhythmic drug blocking voltage-gated Na+ channels and K+ channels known to promote AF maintenance,94 effectively inhibits arrhythmias due to SR Ca2+ leak in mouse ventricular myocytes.95 It also inhibits intracellular Ca2+ waves, probably due to a combined effect on RyR2 gating and voltage-gated Na+ channels.96,97 Interestingly, in contrast to flecainide, tetracaine does not inhibit Ca2+-wave propagation and this likely results from the different mechanism of action of tetracaine on RyR2 gating.96 Tetracaine stabilizes RyR2 channels in their closed state reducing RyR2 open probability and increasing SR Ca2+ content, whereas flecainide is an open-channel blocker that decreases RyR2 mean-open time, thereby reducing Ca2+-spark mass, although Ca2+-spark frequency was increased, which could explain why flecainide does not increase SR Ca2+ content. The smaller Ca2+-spark mass could prevent Ca2+ waves, because it makes saltatory Ca2+-wave propagation which may induce DADs less likely. Although flecainide is ideally suited to suppress arrhythmogenic Ca2+ waves without causing compensatory increases of SR Ca2+ content,96 the primary action of flecainide in vivo is to inhibit Na+ channels which may cause malignant ventricular arrhythmias, especially if applied chronically in patients with severe coronary artery disease. Analogue substances causing open-channel block of RyR2 without effects on Na+ channels may have anti-AF efficacy without collateral effects at the ventricular level.

In addition to direct inhibition of RyR2, SR Ca2+ leak may be reduced by suppressing the activity of CaMKII in the atrium. Local targeting of kinase and phosphatase functions (e.g. targeting of inhibitor-1 of PP1) is a possible, but yet unproven, therapeutic strategy. Nevertheless, we demonstrated that pharmacological inhibition of CaMKII could inhibit the induction of AF in mice with mutant RyR2 channels by reducing SR Ca2+ leak.51 In addition, inhibition of calmodulin should prevent activation of CaMKII and could rescue the down-regulation of ICa,L, a major determinant of the reentry-promoting refractoriness abbreviation, by suppressing activity of the Ca2+-dependent protein phosphatase calcineurin.98 However, general targeting of CaMKII might exert adverse effects on memory and fertility99 and can cause negative-inotropic effects,37,100 and targeting the mechanisms of increased cardiac CaMKII activity could be one alternative approach. Oxidative stress-induced afterdepolarizations are linked to CaMKII signalling86 and it is known that abnormal CaMKII activity might also result from oxidative stress and increased angiotensin-II levels that cause CaMKII oxidation at Met281/282, leading to sustained CaMKII activation.101 Although it is unknown whether atrial CaMKII is hyperoxidized in AF, oxidative stress and inflammation are the hallmarks of AF.102,103 Therefore, it is possible that at least part of the efficacy of current anti-oxidative/anti-inflammatory therapeutic anti-AF options using statins, ACE-inhibitors, and AT1-receptor blockers result from inhibition of CaMKII activation. Thus, use of statins and ACE-inhibitors/AT1-receptor blockers to suppress abnormal CaMKII activation and subsequent RyR2 hyperphosphorylation in combination with open-channel RyR2 blockers and/or stabilizer of RyR2-FKBP12.6 binding may prove to be a useful principle in future anti-AF drug therapy targeting defective RyR2 function.

8. Conclusions and perspectives

There is emerging evidence that increased diastolic SR Ca2+ leak along with enhanced NCX function may cause DADs and triggered activity that contribute to AF maintenance. Recent work in genetically modified mice51,53 have provided important insights into the causal relationships between molecular alterations of RyR2 and AF susceptibility, clearly validating the importance of these specific Ca2+-handling alterations for AF pathophysiology. However, it remains to be established whether these cellular Ca2+-related proarrhythmic events contribute to atrial arrhythmogenic foci in AF patients in vivo. Nevertheless, the development of new drugs specifically targeting arrhythmogenic diastolic SR Ca2+ leak might offer unique therapeutic opportunities to reduce atrial arrhythmogenesis by normalizing SR Ca2+ handling.

Conflict of interest: none declared.


D.D. was supported by the German Federal Ministry of Education and Research through the Atrial Fibrillation Competence Network (grant 01Gi0204; projects C3–C5), the Deutsche Forschungsgemeinschaft (Do 769/1-3), and an European Union FP7 programme, large-scale collaborative project (grant 261057, The European Network for Translational Research in Atrial Fibrillation). X.H.T.W. is a W.M. Keck Foundation Distinguished Young Scholar in Medical Research and was supported by NIH/NHLBI grants R01-HL089598 and R01-HL091947. The authors are also supported by the Fondation Leducq (07CVD03, European North American Atrial Fibrillation Research Alliance, to D.D.; and 08CVD01, Alliance for Calmodulin Kinase Signaling in Heart Disease, to X.H.T.W.).


1. Lloyd-Jones DM, Wang TJ, Leip EP, Larson MG, Levy D, Vasan RS, et al. Lifetime risk for development of atrial fibrillation: the Framingham Heart Study. Circulation. 2004;110:1042–1046. doi:10.1161/01.CIR.0000140263.20897.42. [PubMed]
2. Dobrev D, Nattel S. New antiarrhythmic drugs for treatment of atrial fibrillation. Lancet. 2010;375:1212–1223. doi:10.1016/S0140-6736(10)60096-7. [PubMed]
3. Schotten U, Ausma J, Stellbrink C, Sabatschus I, Vogel M, Frechen D, et al. Cellular mechanisms of depressed atrial contractility in patients with chronic atrial fibrillation. Circulation. 2001;103:691–698. [PubMed]
4. Lenaerts I, Bito V, Heinzel FR, Driesen RB, Holemans P, D'Hooge J, et al. Ultrastructural and functional remodeling of the coupling between Ca2+ influx and sarcoplasmic reticulum Ca2+ release in right atrial myocytes from experimental persistent atrial fibrillation. Circ Res. 2009;105:876–885. doi:10.1161/CIRCRESAHA.109.206276. [PubMed]
5. Sun H, Gaspo R, Leblanc N, Nattel S. Cellular mechanisms of atrial contractile dysfunction caused by sustained atrial tachycardia. Circulation. 1998;98:719–727. [PubMed]
6. Wakili R, Yeh HS, Qi XY, Greiser M, Chartier D, Nishida K, et al. Multiple potential molecular contributors to atrial hypocontractility caused by atrial tachycardia remodeling in dogs. Circ Arrhythm Electrophysiol. 2010;3:530–541. [PubMed]
7. Dobrev D, Nattel S. Calcium handling abnormalities in atrial fibrillation as a target for innovative therapeutics. J Cardiovasc Pharmacol. 2008;52:293–299. doi:10.1097/FJC.0b013e318171924d. [PubMed]
8. Gusev K, Niggli E. Modulation of the local SR Ca2+ release by intracellular Mg2+ in cardiac myocytes. J Gen Physiol. 2008;132:721–730. doi:10.1085/jgp.200810119. [PMC free article] [PubMed]
9. Prosser BL, Ward CW, Lederer WJ. Subcellular Ca2+ signaling in the heart: the role of ryanodine receptor sensitivity. J Gen Physiol. 2010;136:135–142. doi:10.1085/jgp.201010406. [PMC free article] [PubMed]
10. Sobie EA, Dilly KW, dos Santos Cruz J, Lederer WJ, Jafri MS. Termination of cardiac Ca2+ sparks: an investigative mathematical model of calcium-induced calcium release. Biophys J. 2002;83:59–78. doi:10.1016/S0006-3495(02)75149-7. [PubMed]
11. Stern MD, Song LS, Cheng H, Sham JS, Yang HT, Boheler KR, et al. Local control models of cardiac excitation-contraction coupling: a possible role for allosteric interactions between ryanodine receptors. J Gen Physiol. 1999;113:469–489. doi:10.1085/jgp.113.3.469. [PMC free article] [PubMed]
12. Nattel S. Basic electrophysiology of the pulmonary veins and their role in atrial fibrillation: precipitators, perpetuators, and perplexers. J Cardiovasc Electrophysiol. 2003;14:1372–1375. doi:10.1046/j.1540-8167.2003.03445.x. [PubMed]
13. Michael G, Xiao L, Qi XY, Dobrev D, Nattel S. Remodelling of cardiac repolarization: how homeostatic responses can lead to arrhythmogenesis. Cardiovasc Res. 2009;81:491–499. doi:10.1093/cvr/cvn266. [PubMed]
14. Nattel S, Burstein B, Dobrev D. Atrial remodeling and atrial fibrillation: mechanisms and implications. Circ Arrhythm Electrophysiol. 2008;1:62–73. doi:10.1161/CIRCEP.107.754564. [PubMed]
15. Dobrev D. Atrial Ca2+ signaling in atrial fibrillation as an antiarrhythmic drug target. Naunyn Schmiedebergs Arch Pharmacol. 2010;381:195–206. doi:10.1007/s00210-009-0457-1. [PubMed]
16. Haissaguerre M, Jais P, Shah DC, Takahashi A, Hocini M, Quiniou G, et al. Spontaneous initiation of atrial fibrillation by ectopic beats originating in the pulmonary veins. N Engl J Med. 1998;339:659–666. doi:10.1056/NEJM199809033391003. [PubMed]
17. Burashnikov A, Antzelevitch C. Late-phase 3 EAD: a unique mechanism contributing to initiation of atrial fibrillation. Pacing Clin Electrophysiol. 2006;29:290–295. doi:10.1111/j.1540-8159.2006.00336.x. [PMC free article] [PubMed]
18. Santiago DJ, Curran JW, Bers DM, Lederer WJ, Stern MD, Rios E, et al. Ca sparks do not explain all ryanodine receptor-mediated SR Ca leak in mouse ventricular myocytes. Biophys J. 98:2111–2120. doi:10.1016/j.bpj.2010.01.042. [PubMed]
19. Schlotthauer K, Bers DM. Sarcoplasmic reticulum Ca2+ release causes myocyte depolarization: underlying mechanism and threshold for triggered action potentials. Circ Res. 2000;87:774–780. [PubMed]
20. Pogwizd SM, Schlotthauer K, Li L, Yuan W, Bers DM. Arrhythmogenesis and contractile dysfunction in heart failure: roles of sodium–calcium exchange, inward rectifier potassium current, and residual β-adrenergic responsiveness. Circ Res. 2001;88:1159–1167. doi:10.1161/hh1101.091193. [PubMed]
21. Santonastasi M, Wehrens XH. Ryanodine receptors as pharmacological targets for heart disease. Acta Pharmacol Sin. 2007;28:937–944. doi:10.1111/j.1745-7254.2007.00582.x. [PubMed]
22. Lehnart SE, Terrenoire C, Reiken S, Wehrens XH, Song LS, Tillman EJ, et al. Stabilization of cardiac ryanodine receptor prevents intracellular calcium leak and arrhythmias. Proc Natl Acad Sci USA. 2006;103:7906–7910. doi:10.1073/pnas.0602133103. [PubMed]
23. Benardeau A, Hatem SN, Rucker-Martin C, Tessier S, Dinanian S, Samuel JL, et al. Primary culture of human atrial myocytes is associated with the appearance of structural and functional characteristics of immature myocardium. J Mol Cell Cardiol. 1997;29:1307–1320. doi:10.1006/jmcc.1996.0366. [PubMed]
24. Dobrev D, Teos LY, Lederer WJ. Unique atrial myocyte Ca2+ signaling. J Mol Cell Cardiol. 2009;46:448–451. doi:10.1016/j.yjmcc.2008.12.004. [PMC free article] [PubMed]
25. Trafford AW, Diaz ME, Eisner DA. Coordinated control of cell Ca2+ loading and triggered release from the sarcoplasmic reticulum underlies the rapid inotropic response to increased L-type Ca2+ current. Circ Res. 2001;88:195–201. [PubMed]
26. Koss KL, Grupp IL, Kranias EG. The relative phospholamban and SERCA2 ratio: a critical determinant of myocardial contractility. Bas Res Cardiol. 1997;92(Suppl. 1):17–24. doi:10.1007/BF00794064. [PubMed]
27. Walden AP, Dibb KM, Trafford AW. Differences in intracellular calcium homeostasis between atrial and ventricular myocytes. J Mol Cell Cardiol. 2009;46:463–473. doi:10.1016/j.yjmcc.2008.11.003. [PubMed]
28. Forbes MS, Van Niel EE, Purdy-Ramos SI. The atrial myocardial cells of mouse heart: a structural and stereological study. J Struct Biol. 1990;103:266–279. doi:10.1016/1047-8477(90)90045-E. [PubMed]
29. Wehrens XH, Lehnart SE, Marks AR. Intracellular calcium release and cardiac disease. Annu Rev Physiol. 2005;67:69–98. doi:10.1146/annurev.physiol.67.040403.114521. [PubMed]
30. Chelu MG, Wehrens XH. Sarcoplasmic reticulum calcium leak and cardiac arrhythmias. Biochem Soc Trans. 2007;35:952–956. doi:10.1042/BST0350952. [PubMed]
31. Zhang L, Kelley J, Schmeisser G, Kobayashi YM, Jones LR. Complex formation between junctin, triadin, calsequestrin, and the ryanodine receptor: proteins of the cardiac junctional sarcoplasmic reticulum membrane. J Biol Chem. 1997;272:23389–23397. doi:10.1074/jbc.272.37.23389. [PubMed]
32. Chopra N, Yang T, Asghari P, Moore ED, Huke S, Akin B, et al. Ablation of triadin causes loss of cardiac Ca2+ release units, impaired excitation-contraction coupling, and cardiac arrhythmias. Proc Natl Acad Sci USA. 2009;106:7636–7641. doi:10.1073/pnas.0902919106. [PubMed]
33. Yuan Q, Fan GC, Dong M, Altschafl B, Diwan A, Ren X, et al. Sarcoplasmic reticulum calcium overloading in junctin deficiency enhances cardiac contractility but increases ventricular automaticity. Circulation. 2007;115:300–309. doi:10.1161/CIRCULATIONAHA.106.654699. [PubMed]
34. Marx SO, Reiken S, Hisamatsu Y, Gaburjakova M, Gaburjakova J, Yang YM, et al. Phosphorylation-dependent regulation of ryanodine receptors: a novel role for leucine/isoleucine zippers. J Cell Biol. 2001;153:699–708. doi:10.1083/jcb.153.4.699. [PMC free article] [PubMed]
35. Vest JA, Wehrens XH, Reiken SR, Lehnart SE, Dobrev D, Chandra P, et al. Defective cardiac ryanodine receptor regulation during atrial fibrillation. Circulation. 2005;111:2025–2032. doi:10.1161/01.CIR.0000162461.67140.4C. [PubMed]
36. Marx SO, Reiken S, Hisamatsu Y, Jayaraman T, Burkhoff D, Rosemblit N, et al. PKA phosphorylation dissociates FKBP12.6 from the calcium release channel (ryanodine receptor): defective regulation in failing hearts. Cell. 2000;101:365–376. doi:10.1016/S0092-8674(00)80847-8. [PubMed]
37. Wehrens XH, Lehnart SE, Reiken SR, Marks AR. Ca2+/calmodulin-dependent protein kinase II phosphorylation regulates the cardiac ryanodine receptor. Circ Res. 2004;94:e61–e70. doi:10.1161/01.RES.0000125626.33738.E2. [PubMed]
38. Hove-Madsen L, Llach A, Bayes-Genis A, Roura S, Rodriguez Font E, Aris A, et al. Atrial fibrillation is associated with increased spontaneous calcium release from the sarcoplasmic reticulum in human atrial myocytes. Circulation. 2004;110:1358–1363. doi:10.1161/01.CIR.0000141296.59876.87. [PubMed]
39. Dobrev D. Electrical remodeling in atrial fibrillation. Herz. 2006;31:108–112. doi:10.1007/s00059-006-2787-9. [PubMed]
40. Liang X, Xie H, Zhu PH, Hu J, Zhao Q, Wang CS, et al. Ryanodine receptor-mediated Ca2+ events in atrial myocytes of patients with atrial fibrillation. Cardiology. 2008;111:102–110. doi:10.1159/000119697. [PubMed]
41. Neef S, Dybkova N, Sossalla S, Ort KR, Fluschnik N, Neumann K, et al. CaMKII-dependent diastolic SR Ca2+ leak and elevated diastolic Ca2+ levels in right atrial myocardium of patients with atrial fibrillation. Circ Res. 2010;106:1134–1144. doi:10.1161/CIRCRESAHA.109.203836. [PubMed]
42. Voigt N, Traffords AW, Ravens U, Dobrev D. Increased diastolic sarcoplasmic reticulum calcium leak may contribute to arrhythmogenesis in patients with atrial fibrillation. Cardiovasc Res. 2010;87:S53. (abstract)
43. Voigt N, Trafford AW, Ravens U, Dobrev D. Cellular and molecular determinants of altered atrial Ca2+ signaling in patients with chronic atrial fibrillation. Circulation. 2009;120:S667–S668. (abstract)
44. Schotten U, Greiser M, Benke D, Buerkel K, Ehrenteidt B, Stellbrink C, et al. Atrial fibrillation-induced atrial contractile dysfunction: a tachycardiomyopathy of a different sort. Cardiovasc Res. 2002;53:192–201. doi:10.1016/S0008-6363(01)00453-9. [PubMed]
45. El-Armouche A, Boknik P, Eschenhagen T, Carrier L, Knaut M, Ravens U, et al. Molecular determinants of altered Ca2+ handling in human chronic atrial fibrillation. Circulation. 2006;114:670–680. doi:10.1161/CIRCULATIONAHA.106.636845. [PubMed]
46. Brundel BJ, van Gelder IC, Henning RH, Tuinenburg AE, Deelman LE, Tieleman RG, et al. Gene expression of proteins influencing the calcium homeostasis in patients with persistent and paroxysmal atrial fibrillation. Cardiovasc Res. 1999;42:443–454. doi:10.1016/S0008-6363(99)00045-0. [PubMed]
47. Lai LP, Su MJ, Lin JL, Lin FY, Tsai CH, Chen YS, et al. Down-regulation of L-type calcium channel and sarcoplasmic reticular Ca2+-ATPase mRNA in human atrial fibrillation without significant change in the mRNA of ryanodine receptor, calsequestrin and phospholamban: an insight into the mechanism of atrial electrical remodeling. J Am Coll Cardiol. 1999;33:1231–1237. doi:10.1016/S0735-1097(99)00008-X. [PubMed]
48. Ohkusa T, Ueyama T, Yamada J, Yano M, Fujumura Y, Esato K, et al. Alterations in cardiac sarcoplasmic reticulum Ca2+ regulatory proteins in the atrial tissue of patients with chronic atrial fibrillation. J Am Coll Cardiol. 1999;34:255–263. doi:10.1016/S0735-1097(99)00169-2. [PubMed]
49. Zhao ZH, Zhang HC, Xu Y, Zhang P, Li XB, Liu YS, et al. Inositol-1,4,5-trisphosphate and ryanodine-dependent Ca2+ signaling in a chronic dog model of atrial fibrillation. Cardiology. 2007;107:269–276. doi:10.1159/000095517. [PubMed]
50. Greiser M, Neuberger HR, Harks E, El-Armouche A, Boknik P, de Haan S, et al. Distinct contractile and molecular differences between two goat models of atrial dysfunction: AV block-induced atrial dilatation and atrial fibrillation. J Mol Cell Cardiol. 2009;46:385–394. doi:10.1016/j.yjmcc.2008.11.012. [PubMed]
51. Chelu MG, Sarma S, Sood S, Wang S, van Oort RJ, Skapura DG, et al. Calmodulin kinase II-mediated sarcoplasmic reticulum Ca2+ leak promotes atrial fibrillation in mice. J Clin Invest. 2009;119:1940–1951. [PMC free article] [PubMed]
52. Yeh YH, Wakili R, Qi XY, Chartier D, Boknik P, Kaab S, et al. Calcium-handling abnormalities underlying atrial arrhythmogenesis and contractile dysfunction in dogs with congestive heart failure. Circ Arrhythm Electrophysiol. 2008;1:93–102. doi:10.1161/CIRCEP.107.754788. [PubMed]
53. Sood S, Chelu MG, van Oort RJ, Skapura D, Santonastasi M, Dobrev D, et al. Intracellular calcium leak due to FKBP12.6 deficiency in mice facilitates the inducibility of atrial fibrillation. Heart Rhythm. 2008;5:1047–1054. doi:10.1016/j.hrthm.2008.03.030. [PMC free article] [PubMed]
54. Antos CL, Frey N, Marx SO, Reiken S, Gaburjakova M, Richardson JA, et al. Dilated cardiomyopathy and sudden death resulting from constitutive activation of protein kinase A. Circ Res. 2001;89:997–1004. doi:10.1161/hh2301.100003. [PubMed]
55. Xiao B, Jiang MT, Zhao M, Yang D, Sutherland C, Lai FA, et al. Characterization of a novel PKA phosphorylation site, serine-2030, reveals no PKA hyperphosphorylation of the cardiac ryanodine receptor in canine heart failure. Circ Res. 2005;96:847–855. doi:10.1161/01.RES.0000163276.26083.e8. [PubMed]
56. Tessier S, Karczewski P, Krause EG, Pansard Y, Acar C, Lang-Lazdunski M, et al. Regulation of the transient outward K+ current by Ca2+/calmodulin-dependent protein kinases II in human atrial myocytes. Circ Res. 1999;85:810–819. [PubMed]
57. Dobrev D, Wehrens XH. Calmodulin kinase II, sarcoplasmic reticulum Ca2+ leak, and atrial fibrillation. Trends Cardiovasc Med. 2010;20:30–34. doi:10.1016/j.tcm.2010.03.004. [PMC free article] [PubMed]
58. Greiser M, Halaszovich CR, Frechen D, Boknik P, Ravens U, Dobrev D, et al. Pharmacological evidence for altered src kinase regulation of I(Ca,L) in patients with chronic atrial fibrillation. Naunyn Schmiedebergs Arch Pharmacol. 2007;375:383–392. doi:10.1007/s00210-007-0174-6. [PubMed]
59. Herzig S, Neumann J. Effects of serine/threonine protein phosphatases on ion channels in excitable membranes. Physiol Rev. 2000;80:173–210. [PubMed]
60. El-Armouche A, Wittkopper K, Degenhardt F, Weinberger F, Didie M, Melnychenko I, et al. Phosphatase inhibitor-1-deficient mice are protected from catecholamine-induced arrhythmias and myocardial hypertrophy. Cardiovasc Res. 2008;80:396–406. doi:10.1093/cvr/cvn208. [PubMed]
61. Wittkopper K, Fabritz L, Neef S, Ort KR, Grefe C, Unsold B, et al. Constitutively active phosphatase inhibitor-1 improves cardiac contractility in young mice but is deleterious after catecholaminergic stress and with aging. J Clin Invest. 2010;120:617–626. doi:10.1172/JCI40545. [PMC free article] [PubMed]
62. Witcher DR, Kovacs RJ, Schulman H, Cefali DC, Jones LR. Unique phosphorylation site on the cardiac ryanodine receptor regulates calcium channel activity. J Biol Chem. 1991;266:11144–11152. [PubMed]
63. Venetucci LA, Trafford AW, Eisner DA. Increasing ryanodine receptor open probability alone does not produce arrhythmogenic calcium waves: threshold sarcoplasmic reticulum calcium content is required. Circ Res. 2007;100:105–111. doi:10.1161/01.RES.0000252828.17939.00. [PubMed]
64. Xiao B, Tian X, Xie W, Jones PP, Cai S, Wang X, et al. Functional consequence of protein kinase A-dependent phosphorylation of the cardiac ryanodine receptor: sensitization of store overload-induced Ca2+ release. J Biol Chem. 2007;282:30256–30264. doi:10.1074/jbc.M703510200. [PubMed]
65. Kubalova Z, Terentyev D, Viatchenko-Karpinski S, Nishijima Y, Gyorke I, Terentyeva R, et al. Abnormal intrastore calcium signaling in chronic heart failure. Proc Natl Acad Sci USA. 2005;102:14104–14109. doi:10.1073/pnas.0504298102. [PubMed]
66. Jiang D, Xiao B, Yang D, Wang R, Choi P, Zhang L, et al. RyR2 mutations linked to ventricular tachycardia and sudden death reduce the threshold for store-overload-induced Ca2+ release (SOICR) Proc Natl Acad Sci USA. 2004;101:13062–13067. doi:10.1073/pnas.0402388101. [PubMed]
67. Bhupathy P, Babu GJ, Ito M, Periasamy M. Threonine-5 at the N-terminus can modulate sarcolipin function in cardiac myocytes. J Mol Cell Cardiol. 2009;47:723–729. doi:10.1016/j.yjmcc.2009.07.014. [PMC free article] [PubMed]
68. Uemura N, Ohkusa T, Hamano K, Nakagome M, Hori H, Shimizu M, et al. Down-regulation of sarcolipin mRNA expression in chronic atrial fibrillation. Eur J Clin Invest. 2004;34:723–730. doi:10.1111/j.1365-2362.2004.01422.x. [PubMed]
69. Bosch RF, Zeng X, Grammer JB, Popovic K, Mewis C, Kuhlkamp V. Ionic mechanisms of electrical remodeling in human atrial fibrillation. Cardiovasc Res. 1999;44:121–131. doi:10.1016/S0008-6363(99)00178-9. [PubMed]
70. Van Wagoner DR, Pond AL, Lamorgese M, Rossie SS, McCarthy PM, Nerbonne JM. Atrial L-type Ca2+ currents and human atrial fibrillation. Circ Res. 1999;85:428–436. [PubMed]
71. Christ T, Boknik P, Wohrl S, Wettwer E, Graf EM, Bosch RF, et al. L-type Ca2+ current downregulation in chronic human atrial fibrillation is associated with increased activity of protein phosphatases. Circulation. 2004;110:2651–2657. doi:10.1161/01.CIR.0000145659.80212.6A. [PubMed]
72. Schulze DH, Muqhal M, Lederer WJ, Ruknudin AM. Sodium/calcium exchanger (NCX1) macromolecular complex. J Biol Chem. 2003;278:28849–28855. doi:10.1074/jbc.M300754200. [PubMed]
73. Gaborit N, Steenman M, Lamirault G, Le Meur N, Le Bouter S, Lande G, et al. Human atrial ion channel and transporter subunit gene-expression remodeling associated with valvular heart disease and atrial fibrillation. Circulation. 2005;112:471–481. doi:10.1161/CIRCULATIONAHA.104.506857. [PubMed]
74. Wijffels MC, Kirchhof CJ, Dorland R, Allessie MA. Atrial fibrillation begets atrial fibrillation: a study in awake chronically instrumented goats. Circulation. 1995;92:1954–1968. [PubMed]
75. Curran J, Brown KH, Santiago DJ, Pogwizd S, Bers DM, Shannon TR. Spontaneous Ca waves in ventricular myocytes from failing hearts depend on Ca2+calmodulin-dependent protein kinase II. J Mol Cell Cardiol. 2010;49:25–32. doi:10.1016/j.yjmcc.2010.03.013. [PMC free article] [PubMed]
76. Fujiwara K, Tanaka H, Mani H, Nakagami T, Takamatsu T. Burst emergence of intracellular Ca2+ waves evokes arrhythmogenic oscillatory depolarization via the Na+–Ca2+ exchanger: simultaneous confocal recording of membrane potential and intracellular Ca2+ in the heart. Circ Res. 2008;103:509–518. doi:10.1161/CIRCRESAHA.108.176677. [PubMed]
77. Dobrev D, Friedrich A, Voigt N, Jost N, Wettwer E, Christ T, et al. The G protein-gated potassium current I(K,ACh) is constitutively active in patients with chronic atrial fibrillation. Circulation. 2005;112:3697–3706. doi:10.1161/CIRCULATIONAHA.105.575332. [PubMed]
78. Dobrev D, Graf E, Wettwer E, Himmel HM, Hala O, Doerfel C, et al. Molecular basis of downregulation of G-protein-coupled inward rectifying K+ current (I(K,ACh) in chronic human atrial fibrillation: decrease in GIRK4 mRNA correlates with reduced I(K,ACh) and muscarinic receptor-mediated shortening of action potentials. Circulation. 2001;104:2551–2557. doi:10.1161/hc4601.099466. [PubMed]
79. Dobrev D, Wettwer E, Kortner A, Knaut M, Schuler S, Ravens U. Human inward rectifier potassium channels in chronic and postoperative atrial fibrillation. Cardiovasc Res. 2002;54:397–404. doi:10.1016/S0008-6363(01)00555-7. [PubMed]
80. Voigt N, Trausch A, Knaut M, Matschke K, Varro A, Van Wagoner DR, et al. Left-to-right atrial inward-rectifier potassium current gradients in patients with paroxysmal versus chronic atrial fibrillation. Circ Arrhythm Electrophysiol. 2010;3:472–480. [PubMed]
81. Voigt N, Friedrich A, Bock M, Wettwer E, Christ T, Knaut M, et al. Differential phosphorylation-dependent regulation of constitutively active and muscarinic receptor-activated IK,ACh channels in patients with chronic atrial fibrillation. Cardiovasc Res. 2007;74:426–437. doi:10.1016/j.cardiores.2007.02.009. [PubMed]
82. Voigt N, Maguy A, Yeh YH, Qi X, Ravens U, Dobrev D, et al. Changes in IK,ACh single-channel activity with atrial tachycardia remodelling in canine atrial cardiomyocytes. Cardiovasc Res. 2008;77:35–43. doi:10.1093/cvr/cvm051. [PubMed]
83. Maruyama M, Joung B, Tang L, Shinohara T, On YK, Han S, et al. Diastolic intracellular calcium-membrane voltage coupling gain and postshock arrhythmias: role of Purkinje fibers and triggered activity. Circ Res. 2010;106:399–408. doi:10.1161/CIRCRESAHA.109.211292. [PMC free article] [PubMed]
84. Venetucci LA, Trafford AW, O'Neill SC, Eisner DA. The sarcoplasmic reticulum and arrhythmogenic calcium release. Cardiovasc Res. 2008;77:285–292. doi:10.1093/cvr/cvm009. [PubMed]
85. Voigt N, Trafford AW, Qiongling W, Wehrens XH, Ravens U, Dobrev D. Sarcoplasmic reticulum Ca2+ leak and enhanced NCX increase occurrence delayed afterdepolarisations in atrial myocytes from patients with chronic atrial fibrillation. Circulation. in press (abstract) [PubMed]
86. Xie LH, Chen F, Karagueuzian HS, Weiss JN. Oxidative-stress-induced afterdepolarizations and calmodulin kinase II signaling. Circ Res. 2009;104:79–86. doi:10.1161/CIRCRESAHA.108.183475. [PMC free article] [PubMed]
87. Patterson E, Jackman WM, Beckman KJ, Lazzara R, Lockwood D, Scherlag BJ, et al. Spontaneous pulmonary vein firing in man: relationship to tachycardia-pause early afterdepolarizations and triggered arrhythmia in canine pulmonary veins in vitro. J Cardiovasc Electrophysiol. 2007;18:1067–1075. doi:10.1111/j.1540-8167.2007.00909.x. [PubMed]
88. Kumagai K, Nakashima H, Gondo N, Saku K. Antiarrhythmic effects of JTV-519, a novel cardioprotective drug, on atrial fibrillation/flutter in a canine sterile pericarditis model. J Cardiovasc Electrophysiol. 2003;14:880–884. doi:10.1046/j.1540-8167.2003.03050.x. [PubMed]
89. Wehrens XH, Marks AR. Novel therapeutic approaches for heart failure by normalizing calcium cycling. Nat Rev Drug Discov. 2004;3:565–573. doi:10.1038/nrd1440. [PubMed]
90. Wehrens XH, Lehnart SE, Reiken SR, Deng SX, Vest JA, Cervantes D, et al. Protection from cardiac arrhythmia through ryanodine receptor-stabilizing protein calstabin2. Science. 2004;304:292–296. doi:10.1126/science.1094301. [PubMed]
91. Chen YJ, Chen YC, Wongcharoen W, Lin CI, Chen SA. Effect of K201, a novel antiarrhythmic drug on calcium handling and arrhythmogenic activity of pulmonary vein cardiomyocytes. Br J Pharmacol. 2008;153:915–925. doi:10.1038/sj.bjp.0707564. [PMC free article] [PubMed]
92. Nakaya H, Furusawa Y, Ogura T, Tamagawa M, Uemura H. Inhibitory effects of JTV-519, a novel cardioprotective drug, on potassium currents and experimental atrial fibrillation in guinea-pig hearts. Br J Pharmacol. 2000;131:1363–1372. doi:10.1038/sj.bjp.0703713. [PMC free article] [PubMed]
93. Lehnart SE, Mongillo M, Bellinger A, Lindegger N, Chen BX, Hsueh W, et al. Leaky Ca2+ release channel/ryanodine receptor 2 causes seizures and sudden cardiac death in mice. J Clin Invest. 2008;118:2230–2245. [PMC free article] [PubMed]
94. Voigt N, Rozmaritsa N, Trausch A, Zimniak T, Christ T, Wettwer E, et al. Inhibition of I(K,ACh) current may contribute to clinical efficacy of class I and class III antiarrhythmic drugs in patients with atrial fibrillation. Naunyn Schmiedebergs Arch Pharmacol. 2010;381:251–259. doi:10.1007/s00210-009-0452-6. [PubMed]
95. Watanabe H, Chopra N, Laver D, Hwang HS, Davies SS, Roach DE, et al. Flecainide prevents catecholaminergic polymorphic ventricular tachycardia in mice and humans. Nat Med. 2009;15:380–383. doi:10.1038/nm.1942. [PMC free article] [PubMed]
96. Hilliard FA, Steele DS, Laver D, Yang Z, Le Marchand SJ, Chopra N, et al. Flecainide inhibits arrhythmogenic Ca2+ waves by open state block of ryanodine receptor Ca2+ release channels and reduction of Ca2+ spark mass. J Mol Cell Cardiol. 2010;48:293–301. doi:10.1016/j.yjmcc.2009.10.005. [PMC free article] [PubMed]
97. Lehnart SE, Lederer WJ. An antidote for calcium leak: targeting molecular arrhythmia mechanisms. J Mol Cell Cardiol. 2010;48:279–282. doi:10.1016/j.yjmcc.2009.11.005. [PubMed]
98. Qi XY, Yeh YH, Xiao L, Burstein B, Maguy A, Chartier D, et al. Cellular signaling underlying atrial tachycardia remodeling of L-type calcium current. Circ Res. 2008;103:845–854. doi:10.1161/CIRCRESAHA.108.175463. [PubMed]
99. Backs J, Stein P, Backs T, Duncan FE, Grueter CE, McAnally J, et al. The gamma isoform of CaM kinase II controls mouse egg activation by regulating cell cycle resumption. Proc Natl Acad Sci USA. 107:81–86. doi:10.1073/pnas.0912658106. [PubMed]
100. Kushnir A, Shan J, Betzenhauser MJ, Reiken S, Marks AR. Role of CaMKIIdelta phosphorylation of the cardiac ryanodine receptor in the force frequency relationship and heart failure. Proc Natl Acad Sci USA. 2010;107:10274–10279. doi:10.1073/pnas.1005843107. [PubMed]
101. Erickson JR, Joiner ML, Guan X, Kutschke W, Yang J, Oddis CV, et al. A dynamic pathway for calcium-independent activation of CaMKII by methionine oxidation. Cell. 2008;133:462–474. doi:10.1016/j.cell.2008.02.048. [PMC free article] [PubMed]
102. Van Wagoner DR. Oxidative stress and inflammation in atrial fibrillation: role in pathogenesis and potential as a therapeutic target. J Cardiovasc Pharmacol. 2008;52:306–313. doi:10.1097/FJC.0b013e31817f9398. [PubMed]
103. Goette A, Bukowska A, Dobrev D, Pfeiffenberger J, Morawietz H, Strugala D, et al. Acute atrial tachyarrhythmia induces angiotensin II type 1 receptor-mediated oxidative stress and microvascular flow abnormalities in the ventricles. Eur Heart J. 2009;30:1411–1420. doi:10.1093/eurheartj/ehp046. [PMC free article] [PubMed]

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