Repolarization gradients and arrhythmogenicity in the murine heart

The long-QT syndromes (LQTS) represent important causes of ventricular arrhythmia and sudden cardiac death in humans. In recent years, the feasibility of genetic manipulation has made the murine heart an invaluable asset in the quest to understand the electrophysiological events underlying arrhythmia in congenital LQTS. A recent paper published in The Journal of Physiology by London et al. (2007) sheds new light on such arrhythmogenicity.

Circus-type re-entry, where an action potential waveform circulates around an obstacle most often arising as a result of a region of refractoriness brought about by an extrasystole, is generally felt to underlie arrhythmogenicity in LQTS. In order to prevent this circulating wavefront from self-extinguishing it is vital that the wave only be able to propagate in one direction. The paper by London et al. (2007) describes optical mapping and patch-clamp studies investigating an association between increases in spatial dispersions of repolarization and refractoriness, two key parameters used to represent such unidirectional block, with arrhythmogenicity in transgenic mice lacking repolarizing K⁺ currents.

Altered transmural dispersion of repolarization has been associated with arrhythmogenicity in a range of situations. In the murine heart, decreased repolarization currents under conditions of hypokalaemia have been associated with such changes (Killeen et al. 2007). London et al. (2007) now go on to demonstrate similar phenomena in genetically manipulated hearts.

The paper by London et al. (2007) uses three different genetically modified mouse models which lack either I_to,f, I_to,s or a combination of both currents, to increase our understanding of arrhythmia induction and propagation. In the present study, the authors used an optical mapping technique where Langendorff-perfused hearts were stained with the voltage-sensitive dye di-4-ANEPPS and fluorescence captured from the epicardial surface and monitored in real time. Initial experiments measuring action potential durations (APDs) in the three mutant hearts showed significant increases in APD in mutant as compared with control hearts. Experiments then went on to measure the epicardial apex–base dispersion of repolarization by subtracting apical from basal APD. In control hearts, APDs were shorter at the apex as compared with the base, with a dispersion of 8.0 ± 2.3 ms. Mice lacking I_to,f demonstrated reduced dispersions, whereas mice lacking I_to,s alone, or both I_to,f and I_to,s, showed significantly increased dispersions. These findings were closely correlated with corresponding increases in the dispersion of refractory period between apex and base in mice lacking I_to,s alone, or both I_to,f and I_to,s, and reduced dispersion of refractory period in mice lacking I_to,f alone.

Susceptibility to arrhythmia was assessed by applying a single premature electrical stimulus to the apex or base close to the refractory period. Arrhythmia was never induced in mice lacking I_to,f, but could be readily induced in mice lacking I_to,s when stimuli were delivered to the apex. As expression of I_to,s is limited to the ventricular septum, reduced I_to,s would be expected to prolong the septal refractory period, producing a region of functional block for an impulse originating in the apex. Mice lacking both I_to,f and I_to,s demonstrated the largest degree of arrhythmia susceptibility when paced either from the apex or the base. Premature impulses from the base and apex encountered refractory tissue, giving rise to lines of functional block which provided the necessary unidirectional block for the initiation and maintenance of re-entrant arrhythmia.

Arrhythmia was also provoked using a burst-pacing protocol that comprised a brief period of pacing at a short basic cycle length. The durations of arrhythmia were significantly shorter in hearts lacking I_to,f as compared with control hearts, correlating with reduced dispersions of repolarization and refractoriness in these hearts as compared with controls. Arrhythmia duration progressively increased in hearts lacking I_to,s alone or both I_to,f and I_to,s, findings fully in keeping with the increased dispersions of repolarization and refractoriness observed in these hearts.

Finally, the study by London et al. (2007) used patch-clamp measurements to quantify I_to,f densities in myocytes isolated from the apices and bases of control hearts. I_to,f was significantly greater in cells isolated from the apex compared with the base. Mice lacking I_to,f demonstrated significantly reduced dispersions of repolarization and refractoriness, in keeping with these single-cell measurements. These data suggest that I_to,f gradients in density along the surface of the epicardium from apex to base are primarily responsible for establishing these dispersions and consequently in part responsible for the observed arrhythmogenesis.

The induction of arrhythmia is thought to involve the interplay of triggering extrasystoles, often initiated by early afterdepolarizations (EADs), and altered dispersions of repolarization providing unidirectional block. Although EADs were not observed in the present study, premature stimuli acting as ‘surrogate EADs’ readily induced arrhythmia. Indeed, arrhythmias were more prevalent and were maintained over longer durations in hearts exhibiting increased dispersions of repolarization and refractoriness. This is in full agreement with previous studies which have associated alterations in transmural dispersions of repolarization with arrhythmogenicity in both congenital LQTS (Shimizu & Antzelevitch, 1999) and in the setting of hypokalaemia (Killeen et al. 2007). Notably the study by London et al. (2007) makes no comment on relationships between repolarization and recovery from refractoriness, previously established as important determinants of arrhythmogenicity in hypokalaemic murine hearts which demonstrated decreased K⁺ currents (Sabir et al. 2007).

From a mechanistic view point, this study provides important, useful information regarding the roles of dispersion of repolarization and refractoriness between apex and base in arrhythmia induction and propagation in the mammalian heart. However, it is important to bear in mind that these K⁺ channel currents do not play as large a role in human cardiac repolarization as they do in repolarization in the murine heart (London, 2001), and as such caution is required in applying these data in the human case. Nevertheless, the study by London et al. (2007) clarifies our understanding of the mechanisms underlying arrhythmia induction in the setting of impaired cardiac repolarization using the genetically amenable mouse heart. Further experiments could identify whether similar arrhythmogenic mechanisms play a part in arrhythmogenesis in genetically modified murine hearts which model human arrhythmia disorders, such as congenital LQTS and the Brugada syndrome (Papadatos et al. 2002).