l-type ca2+ channels play a pivotal role in regulating cellular excitability and contractility in cardiac tissue. Ca2+ influx through these channels [L-type Ca2+ current (ICa,L)] contributes to the plateau phase of the ventricular action potential and initiates excitation-contraction coupling by activating Ca2+-release channels or ryanodine receptors in the sarcoplasmic reticulum (1). The resulting Ca2+-induced Ca2+ release leads to the intracellular Ca2+ transient, which activates the myofilaments. Control of this critical intracellular Ca2+ transient is afforded, in part, by the precise regulation of the triggering ICa,L by a variety of neurohormonal and second-messenger pathways, including prominently the β-adrenergic receptor (β-AR)/protein kinase A (PKA) signaling pathway (5).
L-type Ca2+ channels can also be the culprit in electrical instability and arrhythmogenesis. Studies using sheep and canine Purkinje fibers, in which the L-type Ca2+ channel activator Bay K 8644 was applied, demonstrated the importance of ICa,L in the initiation of early afterdepolarizations (EADs), particularly at slow pacing frequencies and with longer action potential durations (APD) (4). Reactivation of ICa,L has been implicated in EAD formation that can trigger the life-threatening ventricular arrhythmia torsades de pointes in congenital long QT (LQT) syndromes due to mutations in a variety of genes encoding ion channels and associated proteins, as well as in acquired LQT syndrome (7). Furthermore, increased activity of L-type Ca2+ channels may lead to Ca2+ overload in cardiac muscle, which results in another form of triggered activity from delayed afterdepolarizations (DADs). For example, in canine wedge preparations, pharmacological activation of L-type Ca2+ channels again using Bay K 8644 resulted in DADs and triggered arrhythmias (8).
The recent description of Timothy syndrome (TS), a multisystem disorder characterized by prolonged QT interval on ECG, ventricular arrhythmias, syndactly, and autism, has again implicated L-type Ca2+ channels in arrhythmogenesis. TS (LQT8) arises from autosomal dominant mutations within the L-type Ca2+ channel pore forming Cav1.2 subunit that lead to a marked loss of voltage-dependent inactivation (9, 10). Action potential modeling studies intuitively predicted that the maintained ICa,L would result in APD prolongation, consistent with the clinically observed QT prolongation (10). Furthermore, the increase in ICa,L was predicted to lead to DADs (9). However, the clinically observed arrhythmias typically occur in the setting of increased sympathetic tone, such as crying, and so the impact of β-AR signaling on the TS cardiac phenotype is of importance.
In this issue of the American Journal of Physiology-Heart and Circulatory Physiology, Sung and colleagues employ state-of-the-art computational modeling of the cardiac action potential in the presence of the Cav1.2 G406R TS mutation to evaluate the impact of β-AR stimulation on the induction of arrhythmias (11). The study uses a dynamic Luo-Rudy ventricular myocyte model, incorporating detailed properties of Cav1.2 channel gating and intracellular Ca2+ cycling, along with the one-dimensional multicellular strand, which accounts for the transmural heterogeneity of cardiac action potentials (3, 6). Because the equivalent G406R TS mutation is expressed in alternative exons 8A and 8 for TS1 and TS2 patients, respectively, which are differentially expressed (23% exon 8A and 77% exon 8 in heart), the authors tested different percentages of TS mutant Cav1.2 channels in their model. In agreement with the initial modeling studies, the authors demonstrated that, with increasing percentage of Cav1.2 TS mutant channels, APD and QT intervals were prolonged (3, 9, 10). In addition, as the percentage of TS channels was increased, Sung and colleagues began to observe the generation of DADs and DAD-triggered activity, consistent with some previous modeling and experimental studies (8, 9, 12). β-AR stimulation was shown to facilitate the generation of DADs, which occurred at lower percentages of TS Cav1.2 mutant channels and were detected more broadly in all cell types in the strand, endocardial, midmyocardial, and epicardial. The impact of β-AR stimulation on intracellular Ca2+ cycling was found to be particularly important in increasing the propensity to generate DADs. In addition, in this model, the authors also observed phase 2 EADs in midmyocardial cells, starting at 11.5% G406R Cav1.2 channels. This finding, along with increased transmural dispersion of repolarization, is consistent with the occurrence of torsades de pointes in affected patients. At 77% G406R Cav1.2 channels, pacing-induced phase 3 EADs led to triggered activity. However, the authors' choice to study 77% G406R TS Cav1.2 channels may have little clinical relevance, as TS arises from de novo mutations in Cav1.2 and is autosomal dominant in nature, so patients are heteryozygous for the mutant allele, and, in the case of TS2, this would mean one-half of 77% or 38.5% mutant Cav1.2 channels (9, 10). Whether 77% TS G406R Cav1.2 channels would be compatible with life is unclear, and perhaps the modeling data argue against it, given the dramatically prolonged APDs and supraphysiological Ca2+ transients predicted. Nevertheless, the modeling data provide novel information with relevant levels of TS Cav1.2 channels highlighting the importance of β-AR signaling in stimulating intracellular Ca2+ cycling for the genesis of DADs and DAD-triggered arrhythmias.
Importantly, Sung and colleagues used the model to make predictions on specific components that may be potential therapeutic targets for TS by testing the impact of altering a variety of currents. To suppress the inducibility of DAD-triggered activity in the presence of β-AR stimulation, the authors found that the most sensitive intervention was a reduction in ICa,L, followed by reduction in IUP, which represents SERCA-mediated uptake of Ca2+ into the sarcoplasmic reticulum. A reduction in IUP by 20% could totally suppress DAD inducibility, but it took a 40% or more reduction in Na/Ca exchanger current to have a similar effect and even a greater reduction in Ca2+ release current of the sarcoplasmic reticulum. Thus the authors conclude that, in addition to Ca2+ channel blockers and β-AR blockers, an intervention to reduce IUP could also have therapeutic utility, but no such intervention is currently available clinically.
Although computational models of cardiac electrophysiology continue to improve, many key limitations remain. For example, comprehensive descriptions of gating of a given ion channel may not be available, especially in the case of mutant channels. For example, the precise impact of TS mutations on the gating of Cav1.2 channels has not been resolved, with a recent study suggesting that, in addition to a loss of open-state, voltage-dependent inactivation, as utilized by the Sung et al. model, a pronounced slowing of deactivation occurs, which significantly increases Ca2+ influx during repolarization (13). Like all existing computational models for cardiac electrophysiology, the Sung et al. model does not recapitulate the three-dimensional nature of the heart, and rather it is limited to single cardiomyocytes or a one-dimensional strand. Thus questions remain regarding the relative importance of EADs, DADs, transmural dispersion of refractoriness, and other factors in their contribution to clinically relevant whole heart arrhythmias. Finally, the results and predictions are limited by the fact that this model is based on the properties of guinea pig ventricular myocytes rather than human cardiomyocytes, in part, because of limited accessibility of human cells for experimentally determining ionic conductances. However, the recent emergence of abundant sources of human cardiomyocytes from embryonic stem cells and induced pluripotent stem cells may provide critical human-specific data to populate computational models. Furthermore, induced pluripotent stem cells derived from patients with genetic diseases, including inherited arrhythmias, may help close the gap between computational models and human disease biology.
Perhaps the most important feature of the study by Sung et al. is the effort to integrate cellular signaling pathways into models of cardiac electrophysiology. Although the rationale is straightforward, the implementation is challenging, given the pleiotropic and sometimes undefined effects that activation of signaling pathways can have on multiple channels and transporters. Sung and colleagues made progress integrating the β-AR/PKA signaling cascade into their model, but even this valiant attempt does not address some of the complexities of the signaling pathways, such as the differential contribution of β-AR receptor subtypes, e.g., of β1-AR and β2-AR, to the observed behavior. This work also ignores the contribution of Ca2+/calmodulin-dependent kinase II signaling, which has been suggested to be essential in models of TS (2, 12). Certainly, other signaling pathways and second messengers will play a role in the ultimate disease phenotype. Nevertheless, this work sets the stage for more comprehensive models integrating cellular signaling with cardiac electrophysiology, which can lead to advances in the understanding of β-AR stimulation in other inherited arrhythmia syndromes and in arrhythmias in more complex diseases, such as heart failure.
Support for T. J. Kamp was from National Institutes of Health (NIH) Grant R01 HL078878, and for J. M. Best from NIH Grant T32GM008688.
No conflicts of interest are declared by the author(s).
- Copyright © 2010 the American Physiological Society