A leap(frog) forward in understanding focal arrhythmia

Arrhythmogenic triggered activity has been a subject of intense investigation and speculation since the early reports of Segers in the 1940s. Segers observed bursts of non-driven activity in preparations from frog, turtle and rabbit hearts and that this activity was potentiated by Ba2+ or elevated [Ca2+]o. Segers documented what would later be referred to as early and delayed afterdepolarizations, which he termed ‘potentiels tardifs’ (late potentials) (Segers, 1941) and described the phenomenon we call triggered activity as ‘battement auto-entretenu de coeur’ (self-sustained beating of the heart) (Segers, 1947). In the decades since these seminal observations, investigators have painstakingly dissected the molecular and ionic mechanisms by which afterdepolarizations occur and the pathological conditions that predispose cardiac myocytes to triggered activity. In the case of delayed afterdepolarizations (DADs), spontaneous Ca2+ release from the sarcoplasmic reticulum (SR) elevates [Ca2+]i, which in turn, activates a transient inward current (Iti). If sufficiently large, Iti can depolarize the membrane potential to threshold, triggering an action potential. Both inherited and acquired pathologies can increase the likelihood of DADs and triggered activity via an increased propensity for spontaneous SR Ca2+ release, as is the case with mutations causing catecholaminergic polymorphic ventricular tachycardia (CPVT). Despite unprecedented knowledge into the mechanisms of DADs and how various pathologies contribute to this phenomenon, a major outstanding question remains: How is SR Ca2+ release synchronized (within and between cells; in both space and time) to sufficiently depolarize a critical mass of myocardium to overcome the source–sink mismatch and produce a propagating action potential? Indeed, recent computational studies suggest that, at minimum, several thousand cells must all simultaneously exhibit a DAD to overcome the source–sink mismatch in normal myocardium (Xie et al. 2010). Our group recently confirmed these findings experimentally and showed that locally heterogeneous β-adrenergic receptor stimulation (e.g. due to cardiac nerve remodelling) may be one possible means of external spatio-temporal synchronization of SR Ca2+ release (Myles et al. 2012). However, in the absence of an external synchronizing cue, is there an intrinsic synchronization mechanism that promotes triggered activity? In this issue of The Journal of Physiology, Lou et al. elegantly and comprehensively demonstrate a novel, and probably extremely relevant mechanism for spatio-temporal synchronization of SR Ca2+ release and triggered activity: the ‘leapfrog’ mechanism (Lou et al. 2015). Using a multi-scale imaging approach, from Ca2+ imaging in isolated myocytes, to confocal imaging in intact tissue, to macroscopic optical mapping, the authors demonstrated intrinsic synchronization of SR Ca2+ release in the atria of mice with CPVT (Casq2−/−) and attributed focal activity to alternating interplay between transmembrane potential (Vm) and SR Ca2+ release. The authors noted that in the presence of elevated [Ca2+]o, spatio-temporally asynchronous diastolic Ca2+ release occurred in Casq2−/− myocytes. Because these Ca2+ release events were out of phase with one another, they did not trigger an action potential. Following a single paced stimulus, however, self-sustained triggered activity occurred. The authors concluded that the single action potential temporally aligned SR Ca2+ release by aligning in time the refractory periods of ryanodine receptors (RyRs). Thus, as RyRs collectively recover from refractoriness, SR Ca2+ release becomes temporally synchronized and results in sufficient Iti to produce a propagating action potential. That triggered action potential, in turn, synchronizes recovery of RyRs making another triggered action potential more likely. Thus, a classical vicious cycle occurs where each reiteration of the event reinforces the next. Or, as the authors describe, a game of leapfrog between Vm and Ca2+ begins. Notably, just as a single action potential can initiate this vicious cycle, a single failure of a triggered action potential can also break the cycle by desynchronizing SR Ca2+ release. Of course, initiation of sustained triggered activity following a single stimulus has been well documented and was also an early and consistent observation of Segers and others (Segers, 1947; Cranefield, 1977). In fact, it is for this reason that Cranefield termed non-driven impulses as ‘triggered,’ because they require one or more stimuli (action potentials) as an initiator and do not arise de novo (Cranefield, 1977). In addition to initiating triggered activity, Cranefield also noted that a single premature pacing stimulus was often sufficient to terminate sustained triggered activity. In view of the present findings, it may be that slow conduction produced by a premature stimulus acts to spatially and temporally desynchronize RyR recovery to the point that SR Ca2+ release fails to produce an action potential, halting the vicious cycle. Lou and colleagues did not attempt to terminate triggered activity in their preparations with premature stimuli, but indeed postulate (albeit cautiously) that factors that slow conduction may be anti-arrhythmic in this sense. Despite the novel insight and elegant mechanism described by Lou et al., many important questions remain. Although the leapfrog mechanism provides insight into the temporal synchronization of SR Ca2+ release, it is unclear what ultimately dictates spatial synchronization and where focal activity will arise. Is there simply enough homogeneity among RyR recovery in neighbouring cells to provide sufficient spatial synchronization? We normally think of myocardial heterogeneity as a pro-arrhythmic factor, but in light of these findings, a certain amount of cell-to-cell heterogeneity may, in fact, be protective and act to prevent triggered activity. Lou and colleagues studied triggered activity in quiescent atrial cells and tissues where a single action potential was often sufficient to initiate leapfrog-triggered activity. In the normally beating heart, every action potential might then serve as an initiator of triggered activity. Thus, abbreviated RyR refractoriness (as occurs in CPVT) may be a prerequisite for leapfrog-type activity to occur in the beating heart by allowing sufficient SR Ca2+ release to occur before excitation via the next sinus beat. Indeed, the frequency of leapfrog-triggered activity was increased following caffeine administration (which abbreviates RyR refractoriness). Finally, only atrial cells and tissues were examined in the present study. Thus, it remains to be determined if ventricular focal activity in CPVT or other pathologies also exhibits this complex Vm/Ca2+ interplay. Thus, just as one player in the game of leapfrog sets up the next, the exciting findings of Lou and colleagues set up the next series of intriguing questions that remain to be answered.