Proarrhythmia in a non-failing murine model of cardiac-specific Na+/Ca2+ exchanger overexpression: whole heart and cellular mechanisms

Abstract

The cardiac Na+/Ca2+ exchanger (NCX) generates an inward electrical current during SR-Ca2+ release, thus possibly promoting afterdepolarizations of the action potential (AP). We used transgenic mice 12.5 weeks or younger with cardiomyocyte-directed overexpression of NCX (NCX-Tg) to study the proarrhythmic potential and mechanisms of enhanced NCX activity. NCX-Tg exhibited normal echocardiographic left ventricular function and heart/body weight ratio, while the QT interval was prolonged in surface ECG recordings. Langendorff-perfused NCX-Tg, but not wild-type (WT) hearts, developed ventricular tachycardia. APs and ionic currents were measured in isolated cardiomyocytes. Cell capacitance was unaltered between groups. APs were prolonged in NCX-Tg versus WT myocytes along with voltage-activated K+ currents (Kv) not being reduced but even increased in amplitude. During abrupt changes in pacing cycle length, early afterdepolarizations (EADs) were frequently recorded in NCX-Tg but not in WT myocytes. Next to EADs, delayed afterdepolarizations (DAD) triggering spontaneous APs (sAPs) occurred in NCX-Tg but not in WT myocytes. To test whether sAPs were associated with spontaneous Ca2+ release (sCR), Ca2+ transients were recorded. Despite the absence of sAPs in WT, sCR was observed in myocytes of both genotypes suggesting a facilitated translation of sCR into DADs in NCX-Tg. Moreover, sCR was more frequent in NCX-Tg as compared to WT. Myocardial protein levels of Ca2+-handling proteins were not different between groups except the ryanodine receptor (RyR), which was increased in NCX-Tg versus WT. We conclude that NCX overexpression is proarrhythmic in a non-failing environment even in the absence of reduced KV. The underlying mechanisms are: (1) occurrence of EADs due to delayed repolarization; (2) facilitated translation from sCR into DADs; (3) proneness to sCR possibly caused by altered Ca2+ handling and/or increased RyR expression.

Suppression of Early and Late Afterdepolarizations by Heterozygous Knockout of the Na + /Ca 2+ Exchanger in a Murine Model

“…Indeed, this has been observed in homozygous knockout mice 16,25 and-conversely-a slowed inactivation was observed in homozygous overexpressor mice. 5,17 In our study, we also observed a trend toward accelerated I Ca inactivation in heterozygous KO mice, however, other than in homozygous KO mice, this trend did not reach statistical significance. One explanation may be that in homozygous KO mice, there is a complete NCX ablation, whereas in the heterozygous KO mice used in this study, NCX is only reduced to about half the normal expression level, thus resulting in a milder effect of NCX suppression on I Ca inactivation.…”

“…At the same time, if the amount of spontaneously released SR Ca 2+ is similar, NCX inactivation will be slower, that is, the period of NCX activity will be longer to extrude the same amount of Ca 2+ , a finding that has been demonstrated in previous studies. 5,[25][26][27] This would result in a DAD of smaller amplitude but longer duration as observed in this study ( Figure 5). Because the triggering of a spontaneous AP is an all or nothing response, it is the peak amplitude of a DAD and not the absolute number of DADs or the duration of the individual DAD that is crucial for the generation of sAPs.…”

“…Also, as with other genetically modified murine models of arrhythmia, direct comparisons between different murine models may not be always be possible because different pacing protocols and different genetic backgrounds are used. 5,29,35 Conclusions This is the first work to investigate the effect of genetic NCX knockout on arrhythmogenesis and on the underlying molecular and electrophysiological mechanisms. This study demonstrates that chronic and specific NCX knockout directly reduces cellular proarrhythmia mediated by both late and early afterdepolarizations, although the underlying mechanisms downstream of NCX inhibition may differ.…”

“…2,3 However, others have suggested that DADs and EADs share a common mechanism that may be NCX dependent. 4 To date, the role of chronically altered NCX expression in the generation of arrhythmia has been limited to models with increased NCX expression 5 and to animal models of heart failure or hypertrophy 1,[6][7][8][9][10] in which NCX is known to be overexpressed. Yet, in heart failure, a multitude of proarrhythmic alterations other than the upregulation of NCX has been identified, 11 most importantly a reduction of K + currents 12,13 which promote a prolongation of AP duration and the occurrence of arrhythmia.…”

Nonequilibrium Reactivation of Na + Current Drives Early Afterdepolarizations in Mouse Ventricle

“…These characteristics sharply limit the potential for I Ca L reactivation, yet numerous studies have observed EADs in the mouse 9-13 , and most have implicated I Ca L 11-14 . This presents a paradox regarding the electrophysiologic conditions required to elicit EADs in the mouse ventricle, which has important implications for interpreting arrhythmia mechanisms in a large number of disease models.…”