CaMKII delta interaction with neuronal sodium channel Nav1.8 contributes to arrhythmogenic triggers in failing human and mouse cardiomyocytes

 

In heart failure, enhanced persistent current through neuronal sodium channel NaV1.8 (INaL) may induce influx of Na+ into cardiomyocytes. This may cause Ca2+ influx via the Na+/Ca2+ exchanger leading to increased proarrhythmogenic diastolic sarcoplasmic reticulum (SR) Ca2+ leak. This Ca2+ may activate Ca2+/calmodulin-dependent protein kinase IIδ (CaMKIIδ) which can induce INaL augmentation by phosphorylating NaV1.5 channels leading to a vicious cycle between INaL and CaMKIIδ.

Here, we examined whether CaMKIIδ associates with NaV1.8 in human and mouse cardiomyocytes thereby regulating its function. Interaction and co-localisation of CaMKIIδ and NaV1.8 were confirmed by co-immunoprecipitation and immunocytochemistry. Whole-cell patch clamp showed a potent reduction of INaL after addition of novel specific Nav1.8 blockers, either A-803467 (30 nmol/L) or PF-01247324 (1 μmol/L) in failing mouse cardiomyocytes overexpressing CaMKIIδc (CaMKIIδc+/T: −109.4±10.6 vs A-803467: −56.9±11.7 and PF-01247324:−-69.9±8.6 A*ms*F-1). In failing human cardiomyocytes inhibition of either NaV1.8 or CaMKIIδ using AIP (1 μmol/L) or AIP and PF-01247324 together led to a significant and comparable decrease of INaL (control: −93.7±7.1 vs PF-01247324: −56.8±6.6; AIP: −44.2±6.6; AIP+PF-01247324: −39.8±5.4 A*ms*F-1). Furthermore, to confirm whether observed alterations in INaL after inhibition of NaV1.8 are not due to an overall reduction in peak sodium current (INa) we measured INa properties in mouse cardiomyocytes. Importantly, we observed no difference neither in the peak nor in inactivation between wild type (WT), WT with PF-01247324 and in mice lacking NaV1.8. Using confocal microscopy we investigated whether inhibition of the NaV1.8-mediated INaL could attenuate the increase of proarrhythmogenic SR Ca2+ spark frequency (CaSpF) caused by overexpression of CaMKIIδ in mice. We observed a significant reduction of CaSpF in both NaV1.8 inhibitor groups (PF-01247324: 0.51±0.08 and A-803467: 0.57±0.08 μm–1 s–1) compared to control (1.00±0.13 μm–1 s–1). Incubation of human failing cardiomyocytes with either AIP (0.35±0.06 μm–1 s–1) or PF-01247324 (0.44±0.11 μm–1 s–1), or blocking CaMKIIδ and NaV1.8 together (0.30±0.08 μm–1 s–1) resulted in significant decrease of CaSpF compared to control (0.89±0.13 μm–1 s–1).

In conclusion, we show for the first time subcellular localisation of the neuronal sodium channel NaV1.8 and its interaction with CaMKIIδ in both human and mouse ventricular cardiomyocytes. Moreover, pharmacological inhibition of NaV1.8 caused a reduction of the augmented INaL and spontaneous diastolic SR-Ca2+ release in both failing human and mouse cardiomyocytes. NaV1.8 and CaMKIIδ interaction seem to play a relevant role for the generation of arrhythmogenic triggers (INaL & spontaneous diastolic SR-Ca2+ release) in both human and mouse cardiomyocytes from failing hearts.

Funding Acknowledgement

Type of funding source: None

SCN10A-knock-out improves survival and proarrhythmia in a transgenic heart failure mouse model

Background

In heart failure (HF) both Ca2+/Calmodulin-dependent protein-kinase II (CaMKII) and late sodium current (INaL) are known to contribute to arrhythmogenesis as they contribute to action-potential (AP) prolongation and the occurrence of early- (EADs) and delayed afterdepolarizations (DADs). Further, augmented CaMKII and INaL maintain a vicious cycle as they both can activate each other. We recently found that the sodium channel isoform NaV1.8 is upregulated in HF and hypertrophy and that it is involved in INaL-generation. In the current study we investigated the effects of NaV1.8-knock-out (KO) on HF-progression and arrhythmogenesis in a CaMKII-overexpressing HF mouse model.

Methods/Results

CaMKII overexpressing mice (CaMKII+/T) were crossbred with NaV1.8-KO mice (SCN10A−/−). To our surprise knock-out of NaV1.8 in CaMKII+/T mice (SCN10A−/−/CaMKII+/T) significantly improved survival (median survival 103 days vs 74.5 CaMKII+/T, p<0.01). CaMKII+/T mice exhibited a strong HF phenotype compared to WT with increased heart-weight to tibia length ratio as well as reduced ejection fraction and left-ventricular end-diastolic diameter obtained by echocardiography. However, these structural parameters did not differ between SCN10A−/−/CaMKII+/T and CaMKII+/T. Therefore, cellular electrophysiology experiments were performed in isolated cardiomyocytes for a better understanding of the observed improvement in survival. INaL, measured by patch-clamp technique, was significantly augmented in CaMKII+/T vs WT and SCN10A−/−, while SCN10A−/−/CaMKII+/T showed significantly less INaL than CaMKII+/T alone. Further, AP-duration (APD) was significantly reduced in SCN10A−/−/CaMKII+/T vs CaMKII+/T while AP-amplitude, resting membrane-potential and upstroke velocity (dv/dtmax) remained unchanged. In addition, the occurrence of afterdepolarizations was significantly lower in SCN10A−/−/ CaMKII+/T vs CaMKII+/T. Confocal microscopy using the dye Fluo-4AM was performed and significantly less diastolic Ca2+-waves occurred in SCN10A−/−/CaMKII+/T compared to CaMKII+/T. In order to analyze an organ-specific SCN10A-KO, we generated homozygous SCN10A-KO lines of induced pluripotent stem cells by using CRISPR/Cas9 technology. 2-month old iPSC-cardiomyocytes lacking NaV1.8 were treated with low dose isoprenaline and showed significantly less INaL, thereby serving as a final proof of the relevant role of this Na+-channel on INaL-generation in the cardiomyocyte.

Conclusion

We found a survival benefit by selective knock-out of the neuronal sodium channel isoform NaV1.8 in a proarrhythmic HF mouse model with augmented CaMKII expression. However, in our model NaV1.8-knock-out showed no effects on HF progression, while cellular proarrhythmic triggers were attenuated. Taken together with our findings in IPS-cardiomyocytes treated with the CRSIPR/Cas9 technology NaV1.8 plays a significant role for the generation of INaL and cellular arrhythmogenic triggers in the cardiomyocyte.

Funding Acknowledgement

Type of funding source: Foundation. Main funding source(s): Deutsche Stiftung für Herzforschung

Sacubitrilat reduces pro-arrhythmogenic sarcoplasmic reticulum Ca2+ leak in human ventricular cardiomyocytes of patients with end-stage heart failure

Aims

Inhibition of neprilysin and angiotensin II receptor by sacubitril/valsartan (Val) (LCZ696) reduces mortality in heart failure (HF) patients compared with sole inhibition of renin–angiotensin system. Beneficial effects of increased natriuretic peptide levels upon neprilysin inhibition have been proposed, whereas direct effects of sacubitrilat (Sac) (LBQ657) on myocardial Ca²⁺ cycling remain elusive.

Methods and results

Confocal microscopy (Fluo‐4 AM) was used to investigate pro‐arrhythmogenic sarcoplasmic reticulum (SR) Ca²⁺ leak in freshly isolated murine and human ventricular cardiomyocytes (CMs) upon Sac (40 μmol/L)/Val (13 μmol/L) treatment. The concentrations of Sac and Val equalled plasma concentrations of LCZ696 treatment used in PARADIGM‐HF trial. Epifluorescence microscopy measurements (Fura‐2 AM) were performed to investigate effects on systolic Ca²⁺ release, SR Ca²⁺ load, and Ca²⁺‐transient kinetics in freshly isolated murine ventricular CMs. The impact of Sac on myocardial contractility was evaluated using in toto‐isolated, isometrically twitching ventricular trabeculae from human hearts with end‐stage HF. Under basal conditions, the combination of Sac/Val did not influence diastolic Ca²⁺‐spark frequency (CaSpF) nor pro‐arrhythmogenic SR Ca² leak in isolated murine ventricular CMs (n CMs/hearts = 80/7 vs. 100/7, P = 0.91/0.99). In contrast, Sac/Val treatment reduced CaSpF by 35 ± 9% and SR Ca²⁺ leak by 45 ± 9% in CMs put under catecholaminergic stress (isoproterenol 30 nmol/L, n = 81/7 vs. 62/7, P < 0.001 each). This could be attributed to Sac, as sole Sac treatment also reduced both parameters by similar degrees (reduction of CaSpF by 57 ± 7% and SR Ca²⁺ leak by 76 ± 5%; n = 101/4 vs. 108/4, P < 0.01 each), whereas sole Val treatment did not. Systolic Ca²⁺ release, SR Ca²⁺ load, and Ca²⁺‐transient kinetics including SERCA activity (k_SERCA) were not compromised by Sac in isolated murine CMs (n = 41/6 vs. 39/6). Importantly, the combination of Sac/Val and Sac alone also reduced diastolic CaSpF and SR Ca²⁺ leak (reduction by 74 ± 7%) in human left ventricular CMs from patients with end‐stage HF (n = 71/8 vs. 78/8, P < 0.05 each). Myocardial contractility of human ventricular trabeculae was not acutely affected by Sac treatment as the developed force remained unchanged over a time course of 30 min (n trabeculae/hearts = 3/3 vs. 4/3).

Conclusion

This study demonstrates that neprilysin inhibitor Sac directly improves Ca²⁺ homeostasis in human end‐stage HF by reducing pro‐arrhythmogenic SR Ca²⁺ leak without acutely affecting systolic Ca²⁺ release and inotropy. These effects might contribute to the mortality benefits observed in the PARADIGM‐HF trial.

Inhibition of NaV1.8 prevents atrial arrhythmogenesis in human and mice

Pharmacologic approaches for the treatment of atrial arrhythmias are limited due to side effects and low efficacy. Thus, the identification of new antiarrhythmic targets is of clinical interest. Recent genome studies suggested an involvement of SCN10A sodium channels (Na_V1.8) in atrial electrophysiology. This study investigated the role and involvement of Na_V1.8 (SCN10A) in arrhythmia generation in the human atria and in mice lacking Na_V1.8. Na_V1.8 mRNA and protein were detected in human atrial myocardium at a significant higher level compared to ventricular myocardium. Expression of Na_V1.8 and Na_V1.5 did not differ between myocardium from patients with atrial fibrillation and sinus rhythm. To determine the electrophysiological role of Na_V1.8, we investigated isolated human atrial cardiomyocytes from patients with sinus rhythm stimulated with isoproterenol. Inhibition of Na_V1.8 by A-803467 or PF-01247324 showed no effects on the human atrial action potential. However, we found that Na_V1.8 significantly contributes to late Na⁺ current and consequently to an increased proarrhythmogenic diastolic sarcoplasmic reticulum Ca²⁺ leak in human atrial cardiomyocytes. Selective pharmacological inhibition of Na_V1.8 potently reduced late Na⁺ current, proarrhythmic diastolic Ca²⁺ release, delayed afterdepolarizations as well as spontaneous action potentials. These findings could be confirmed in murine atrial cardiomyocytes from wild-type mice and also compared to SCN10A^-/- mice (genetic ablation of Na_V1.8). Pharmacological Na_V1.8 inhibition showed no effects in SCN10A^-/- mice. Importantly, in vivo experiments in SCN10A^-/- mice showed that genetic ablation of Na_V1.8 protects against atrial fibrillation induction. This study demonstrates that Na_V1.8 is expressed in the murine and human atria and contributes to late Na⁺ current generation and cellular arrhythmogenesis. Blocking Na_V1.8 selectively counteracts this pathomechanism and protects against atrial arrhythmias. Thus, our translational study reveals a new selective therapeutic target for treating atrial arrhythmias.

Differential regulation of sodium channels as a novel proarrhythmic mechanism in the human failing heart

Aims

In heart failure (HF), enhanced persistent Na+ current (INaL) exerts detrimental effects on cellular electrophysiology and can induce arrhythmias. However, the underlying regulatory mechanisms remain unclear. Our aim was to potentially investigate the regulation and electrophysiological contribution of neuronal sodium channel NaV1.8 in failing human heart and eventually to reveal a novel anti-arrhythmic therapy.

Methods and results

By western blot, we found that NaV1.8 protein expression is significantly up-regulated, while of the predominant cardiac isoform NaV1.5 is inversely reduced in human HF. Furthermore, to investigate the relation of NaV1.8 regulation with the cellular proarrhythmic events, we performed comprehensive electrophysiology recordings and explore the effect of NaV1.8 on INaL, action potential duration (APD), Ca2+ spark frequency, and arrhythmia induction in human failing cardiomyocytes. NaV1.8 inhibition with the specific blockers A-803467 and PF-01247324 decreased INaL, abbreviated APD and reduced cellular-spontaneous Ca2+-release and proarrhythmic events in human failing cardiomyocytes. Consistently, in mouse cardiomyocytes stressed with isoproterenol, pharmacologic inhibition and genetically knockout of NaV1.8 (SCN10A-/-), were associated with reduced INaL and abbreviated APD.

Conclusion

We provide first evidence of differential regulation of NaV1.8 and NaV1.5 in the failing human myocardium and their contribution to arrhythmogenesis due to generation of INaL. We propose inhibition of NaV1.8 thus constitutes a promising novel approach for selective anti-arrhythmic therapy in HF.

P1596Interaction of CaMKII and NaV1.8 modulates cardiac electrophysiology in human heart failure

Introduction

In human heart failure, electrical remodeling contributes to the risk of arrhythmia generation. Increased expression of Ca/Calmodulin-dependent protein kinase IIδ (CaMKIIδ) and an enhanced persistent Na current (INaL) have been linked to arrhythmogenesis. CaMKIIδ increases INaL via regulation of sodium channels thereby contributing to arrhythmias through early- and delayed-afterdepolarizations (EADs and DADs). Genome-wide association studies (GWAS) have described the implication of the neuronal sodium channel isoform NaV1.8 (SCN10A) in cardiac electrophysiology showing modulation in cardiac conduction. We showed that the expression of the isoform Nav1.8 is significantly increased in human failing cardiomyocytes and contributes substantially to the enhanced INaL.

Purpose

We investigated a potential interaction of CaMKIIδ and NaV1.8 and thereby its role in arrhythmia generation and electrophysiology in human and murine failing hearts.

Methods

Cardiomyocytes were isolated from explanted failing hearts and CaMKIIδ transgenic (TG) mice. We performed immunostainings and co-immunoprecipitation (Co-IP) to show interactions of CaMKIIδ and Nav1.8 in isolated cardiomyocytes and homogenates. Whole-cell patch clamp experiments were conducted in isolated human and murine ventricular cardiomyocytes. Additionally, Ca2+ transients were measured using epifluorescence microscopy with the Ca2+ dye fura-2 (10μmol/L) whereas Ca2+ sparks measurements were performed by using confocal microscopy with the Ca2+ dye fluo-4 (10μmol/L). PF-01247324 is a novel specific NaV1.8 inhibitor (orally bioavailable; 1 μmol/L) and autocamtide inhibitory peptide (AIP, 1 μmol/L) was used to inhibit CaMKIIδ.

Results

Co-immunoprecipitation experiments revealed an association of CaMKIIδ and Nav1.8 in human homogenates compared to healthy controls. Furthermore, immunohistochemistry stainings in isolated human cardiomyocytes showed a co-localization of CaMKIIδ and NaV1.8 at the intercalated disc and t-tubules. We observed a significant reduction of INaL integral and proarrhythmic SR-Ca2+ spark frequency (CaSpF) after addition of either PF-01247324 or the CaMKIIδ inhibitor AIP in failing human and murine ventricular cardiomyocytes. When PF-01247324 and AIP were added together, the decrease in INaL integral and CaSpF was comparable to PF-01247324 alone in human failing cardiomyocytes. Inhibition of NaV1.8 did not show an effect on Ca2+ transient amplitude or Ca2+ transient decay at different stimulation frequencies in CaMKIIδ TG cardiomyocytes.

Conclusion

Our results demonstrate the significance of both CaMKIIδ and NaV1.8 in INaL generation and their detrimental interaction. This data suggest that increased CaMKIIδ activity plays a substantial role for the activation of NaV1.8-mediated late sodium current and SR-Ca2+ leak.

4966Targeting INaL by a neuronal sodium channel isoform improves survival in a CaMKII-transgenic heart failure mouse model

Background

Cardiac pathologies like hypertrophy and heart failure are known to be associated with proarrhythmogenic triggers like early- (EADs) and delayed afterdepolarizations (DADs) that can be partly attributed to an augmentation of late sodium current (INaL). Enhanced INaL is closely connected with increased activity of Ca2+/calmodulin dependent-kinase II (CaMKII) in pathology as it is enhanced by CaMKII on the one hand but can also indirectly increase CaMKII-activity on the other. We recently found neuronal sodium channel NaV1.8 to be involved in INaL-augmentation in heart failure and cardiac hypertrophy. Here, we studied possible antiarrhythmic effects of NaV1.8-inhibition in a transgenic mouse model with enhanced CaMKII-expression by selectively knocking out NaV1.8.

Methods/Results

To investigate antiarrhythmic effects of NaV1.8-depletion in-vivo and in-vitro we crossbred CaMKII-transgenic mice (CaMKII+/T) with NaV1.8-knock-out mice (SCN10A−/−). Surprisingly, CaMKII+/T-mice lacking NaV1.8 (CaMKII+/T & SCN10A−/−) showed a significantly improved survival compared to CaMKII+/T alone (97.5 vs 72.0 days, p<0.05). Heart weight to tibia length ratio was significantly increased in CaMKII+/T-mice compared to wild-type, without any differences between CaMKII+/T and CaMKII+/T & SCN10A−/−. To investigate the underlying mechanisms out of this observation we isolated single cardiomyocytes and performed patch-clamp experiments as well as confocal microscopy to measure Ca2+-transients and diastolic Ca2+-waves. INaL-integral was significantly smaller in cardiomyocytes from CaMKII+/T & SCN10A−/−-mice compared to CaMKII+/T alone. During action potential recordings, significantly less afterdepolarizations occurred in CaMKII+/T & SCN10A−/− compared to cardiomyocytes from CaMKII+/T -mice (16.7/min vs 34.9/min, p<0.05). There was a trend of less cells exhibiting diastolic Ca2+-waves in Ca2+-measurements from CaMKII+/T & SCN10A−/− compared to CaMKII+/T (15% vs 25%, p=0.09). As some cells showed more than one event, we calculated the frequency of Ca2+-waves and found a significant reduction of Ca2+-waves in CaMKII+/T & SCN10A−/− vs. CaMKII+/T (22.8/min vs 43.0/min, p<0.05). Moreover, the time to the first event was significantly longer in CaMKII+/T & SCN10A−/−. Ca2+-transient amplitude (F/F0) was significantly lower in CaMKII+/T compared to CaMKII+/T & SCN10A−/− (4.6 vs. 5.3, p=0.05). Further, Ca2+-extrusion from the cytosol was significantly faster in CaMKII+/T & SCN10A−/−.

Conclusion

Our data demonstrates, that inhibition of INaL by targeting NaV1.8 has a potent antiarrhythmic potential as we found a reduction of EADs, DADs and diastolic Ca2+-waves in CaMKII+/T & SCN10A−/−-cardiomyocytes. This antiarrhythmic potential appears to be potent enough to improve survival and to rescue the proarrhythmogenic phenotype of CaMKII-overexpression. However, further in-vivo experiments are necessary to investigate NaV1.8-inhibition for a possible therapeutic approach.

The functional consequences of sodium channel NaV 1.8 in human left ventricular hypertrophy

Aims

In hypertrophy and heart failure, the proarrhythmic persistent Na⁺ current (I_NaL) is enhanced. We aimed to investigate the electrophysiological role of neuronal sodium channel Na_V1.8 in human hypertrophied myocardium.

Methods and results

Myocardial tissue of 24 patients suffering from symptomatic severe aortic stenosis and concomitant significant afterload‐induced hypertrophy with preserved ejection fraction was used and compared with 12 healthy controls. We performed quantitative real‐time PCR and western blot and detected a significant up‐regulation of Na_V1.8 mRNA (2.34‐fold) and protein expression (1.96‐fold) in human hypertrophied myocardium compared with healthy hearts. Interestingly, Na_V1.5 protein expression was significantly reduced in parallel (0.60‐fold). Using whole‐cell patch‐clamp technique, we found that the prominent I_NaL was significantly reduced after addition of novel Na_V1.8‐specific blockers either A‐803467 (30 nM) or PF‐01247324 (1 μM) in human hypertrophic cardiomyocytes. This clearly demonstrates the relevant contribution of Na_V1.8 to this proarrhythmic current. We observed a significant action potential duration shortening and performed confocal microscopy, demonstrating a 50% decrease in proarrhythmic diastolic sarcoplasmic reticulum (SR)‐Ca²⁺ leak and SR‐Ca²⁺ spark frequency after exposure to both Na_V1.8 inhibitors.

Conclusions

We show for the first time that the neuronal sodium channel Na_V1.8 is up‐regulated on mRNA and protein level in the human hypertrophied myocardium. Furthermore, inhibition of Na_V1.8 reduced augmented I_NaL, abbreviated the action potential duration, and decreased the SR‐Ca²⁺ leak. The findings of our study suggest that Na_V1.8 could be a promising antiarrhythmic therapeutic target and merits further investigation.

Empagliflozin directly improves diastolic function in human heart failure

AIMS

Empagliflozin, a clinically used oral antidiabetic drug that inhibits the sodium-dependent glucose co-transporter 2, has recently been evaluated for its cardiovascular safety. Surprisingly, empagliflozin reduced mortality and hospitalization for heart failure (HF) compared to placebo. However, the underlying mechanisms remain unclear. Therefore, our study aims to investigate whether empagliflozin may cause direct pleiotropic effects on the myocardium.

METHODS AND RESULTS

In order to assess possible direct myocardial effects of empagliflozin, we performed contractility experiments with in toto-isolated human systolic end-stage HF ventricular trabeculae. Empagliflozin significantly reduced diastolic tension, whereas systolic force was not changed. These results were confirmed in murine myocardium from diabetic and non-diabetic mice, suggesting independent effects from diabetic conditions. In human HF cardiomyocytes, empagliflozin did not influence calcium transient amplitude or diastolic calcium level. The mechanisms underlying the improved diastolic function were further elucidated by studying myocardial fibres from patients and rats with diastolic HF (HF with preserved ejection fraction, HFpEF). Empagliflozin beneficially reduced myofilament passive stiffness by enhancing phosphorylation levels of myofilament regulatory proteins. Intravenous injection of empagliflozin in anaesthetized HFpEF rats significantly improved diastolic function measured by echocardiography, while systolic contractility was unaffected.

CONCLUSION

Empagliflozin causes direct pleiotropic effects on the myocardium by improving diastolic stiffness and hence diastolic function. These effects were independent of diabetic conditions. Since pharmacological therapy of diastolic dysfunction and HF is an unmet need, our results provide a rationale for new translational studies and might also contribute to the understanding of the EMPA-REG OUTCOME trial.

Sarcoplasmic reticulum calcium leak contributes to arrhythmia but not to heart failure progression

Increased sarcoplasmic reticulum (SR) Ca2+ leak via the cardiac ryanodine receptor (RyR2) has been suggested to play a mechanistic role in the development of heart failure (HF) and cardiac arrhythmia. Mice treated with a selective RyR2 stabilizer, rycal S36, showed normalization of SR Ca2+ leak and improved survival in pressure overload (PO) and myocardial infarction (MI) models. The development of HF, measured by echocardiography and molecular markers, showed no difference in rycal S36– versus placebo-treated mice. Reduction of SR Ca2+ leak in the PO model by the rycal-unrelated RyR2 stabilizer dantrolene did not mitigate HF progression. Development of HF was not aggravated by increased SR Ca2+ leak due to RyR2 mutation (R2474S) in volume overload, an SR Ca2+ leak–independent HF model. Arrhythmia episodes were reduced by rycal S36 treatment in PO and MI mice in vivo and ex vivo in Langendorff-perfused hearts. Isolated cardiomyocytes from murine failing hearts and human ventricular failing and atrial nonfailing myocardium showed reductions in delayed afterdepolarizations, in spontaneous and induced Ca2+ waves, and in triggered activity in rycal S36 versus placebo cells, whereas the Ca2+ transient, SR Ca2+ load, SR Ca2+ adenosine triphosphatase function, and action potential duration were not affected. Rycal S36 treatment of human induced pluripotent stem cells isolated from a patient with catecholaminergic polymorphic ventricular tachycardia could rescue the leaky RyR2 receptor. These results suggest that SR Ca2+ leak does not primarily influence contractile HF progression, whereas rycal S36 treatment markedly reduces ventricular arrhythmias, thereby improving survival in mice.