In heart failure (HF) augmented late sodium current (I_NaL) and increased activity of Ca²⁺/Calmodulin dependent protein-kinase II (CaMKII) have detrimental effects on cellular electrophysiology involving diastolic Ca²⁺-leak from the sarcoplasmic reticulum (SR) and the occurrence of early- (EADs) and delayed afterdepolarizations (DADs). I_NaL and CaMKII are linked to each other, as they maintain a proarrhythmic vicious cycle where both can activate each other. Lately, we found that the neuronal sodium channel isoform Na_V1.8 contributes to I_NaL in heart failure. Here, we elucidated the interaction of Na_V1.8 and CaMKII and its effects on cellular proarrhythmia.

We performed Co-Immunoprecipitation experiments and found a Co-expression of Na_V1.8 and CaMKII in myocardium of healthy and HF patients. Further, we could confirm a co-localization in isolated cardiomyocytes from failing human hearts by immune-cytochemical staining. For further investigation of this interaction, we crossbred Na_V1.8 knock-out mice (SCN10A^-/-) with CaMKII overexpressing mice (CaMKII^+/T) exhibiting a proarrhythmogenic heart failure phenotype. Patch clamp experiments in cardiomyocytes of human failing hearts and CaMKII^+/T mice revealed a significant contribution of both Na_V1.8 and CaMKII to I_NaL augmentation in these models. I_NaL integral was significantly reduced after application of the CaMKII-inhibitor AIP (1 µmol/L) or the Na_V1.8-blocker PF-01247324 (1 µmol/L). Moreover, I_NaL was significantly lower in CaMKII^+/T mice lacking Na_V1.8 (SCN10A^-/-/ CaMKII^+/T) compared to CaMKII^+/T alone. Using confocal microscopy with the dye Fluo4-AM we could further detect a significant reduction in diastolic Ca²⁺-spark frequency in human HF and CaMKII^+/T mouse cardiomyocytes after inhibition of either Na_V1.8 or CaMKII. Application of AIP and PF-01247324 together did not result in additive effects on I_NaL and SR-Ca²⁺-leak, suggesting that CaMKII-inhibition already abolishes the Na_V1.8-driven I_NaL. 

Interestingly, survival was significantly improved in SCN10A^-/-/ CaMKII^+/T mice compared to CaMKII^+/T (98 vs 77.5 days median survival). CaMKII^+/T mice exhibited significantly increased heart weight to tibia length ratio, reduced ejection fraction and elevated left ventricular end-diastolic diameter compared to Wild-Type or SCN10A^-/- mice. However, there were no significant differences between SCN10A^-/-/ CaMKII^+/T and CaMKII^+/T regarding these parameters. In the line of these findings, significantly less proarrhythmic triggers like afterdepolarizations or diastolic Ca²⁺-waves occurred in SCN10A^-/-/ CaMKII^+/T compared to CaMKII^+/T as the most likely explanation for improved survival in the CaMKII^+/T mouse model. To further investigate the antiarrhythmic potential of SCN10A Knock-Out in vivo, we perform electrophysiology experiments in CaMKII^+/Tand SCN10A^-/-/ CaMKII^+/T mice to provoke ventricular arrhythmias. However, by now the numbers are too low, but final results are expected soon.

The results of our study indicate a detrimental proarrhythmic interaction of Na_V1.8 and CaMKII, as well as protective effects of Na_V1.8 knock-out in a heart failure mouse model of CaMKII overexpression. While no effects by Na_V1.8 knock-out on the progression of heart failure in these mice could be observed, a significant reduction of proarrhythmogenic events on the cellular level may underlay the improved survival.