Increased vulnerability to inducible atrial fibrillation caused by partial cellular uncoupling with heptanol

Heterogeneities in Ventricular Conduction Following Treatment with Heptanol: A Multi-Electrode Array Study in Langendorff-Perfused Mouse Hearts

Background: Previous studies have associated slowed ventricular conduction with the arrhythmogenesis mediated by the gap junction and sodium channel inhibitor heptanol in mouse hearts. However, they did not study the propagation patterns that might contribute to the arrhythmic substrate. This study used a multi-electrode array mapping technique to further investigate different conduction abnormalities in Langendorff-perfused mouse hearts exposed to 0.1 or 2 mM heptanol. Methods: Recordings were made from the left ventricular epicardium using multi-electrode arrays in spontaneously beating hearts during right ventricular 8 Hz pacing or S1S2 pacing. Results: In spontaneously beating hearts, heptanol at 0.1 and 2 mM significantly reduced the heart rate from 314 ± 25 to 189 ± 24 and 157 ± 7 bpm, respectively (ANOVA, p < 0.05 and p < 0.001). During regular 8 Hz pacing, the mean LATs were increased by 0.1 and 2 mM heptanol from 7.1 ± 2.2 ms to 19.9 ± 5.0 ms (p < 0.05) and 18.4 ± 5.7 ms (p < 0.05). The standard deviation of the mean LATs was increased from 2.5 ± 0.8 ms to 10.3 ± 4.0 ms and 8.0 ± 2.5 ms (p < 0.05), and the median of phase differences was increased from 1.7 ± 1.1 ms to 13.9 ± 7.8 ms and 12.1 ± 5.0 ms by 0.1 and 2 mM heptanol (p < 0.05). P5 took a value of 0.2 ± 0.1 ms and was not significantly altered by heptanol at 0.1 or 2 mM (1.1 ± 0.9 ms and 0.9 ± 0.5 ms, p > 0.05). P50 was increased from 7.3 ± 2.7 ms to 24.0 ± 12.0 ms by 0.1 mM heptanol and then to 22.5 ± 7.5 ms by 2 mM heptanol (p < 0.05). P95 was increased from 1.7 ± 1.1 ms to 13.9 ± 7.8 ms by 0.1 mM heptanol and to 12.1 ± 5.0 ms by 2 mM heptanol (p < 0.05). These changes led to increases in the absolute inhomogeneity in conduction (P5−95) from 7.1 ± 2.6 ms to 31.4 ± 11.3 ms, 2 mM: 21.6 ± 7.2 ms, respectively (p < 0.05). The inhomogeneity index (P5−95/P50) was significantly reduced from 3.7 ± 1.2 to 3.1 ± 0.8 by 0.1 mM and then to 3.3 ± 0.9 by 2 mM heptanol (p < 0.05). Conclusion: Increased activation latencies, reduced CVs, and the increased inhomogeneity index of conduction were associated with both spontaneous and induced ventricular arrhythmias.

Connexins in the Heart: Regulation, Function and Involvement in Cardiac Disease

Connexins are a family of transmembrane proteins that play a key role in cardiac physiology. Gap junctional channels put into contact the cytoplasms of connected cardiomyocytes, allowing the existence of electrical coupling. However, in addition to this fundamental role, connexins are also involved in cardiomyocyte death and survival. Thus, chemical coupling through gap junctions plays a key role in the spreading of injury between connected cells. Moreover, in addition to their involvement in cell-to-cell communication, mounting evidence indicates that connexins have additional gap junction-independent functions. Opening of unopposed hemichannels, located at the lateral surface of cardiomyocytes, may compromise cell homeostasis and may be involved in ischemia/reperfusion injury. In addition, connexins located at non-canonical cell structures, including mitochondria and the nucleus, have been demonstrated to be involved in cardioprotection and in regulation of cell growth and differentiation. In this review, we will provide, first, an overview on connexin biology, including their synthesis and degradation, their regulation and their interactions. Then, we will conduct an in-depth examination of the role of connexins in cardiac pathophysiology, including new findings regarding their involvement in myocardial ischemia/reperfusion injury, cardiac fibrosis, gene transcription or signaling regulation.

Connexins and Disease

Inherited or acquired alterations in the structure and function of connexin proteins have long been associated with disease. In the present work, we review current knowledge on the role of connexins in diseases associated with the heart, nervous system, cochlea, and skin, as well as cancer and pleiotropic syndromes such as oculodentodigital dysplasia (ODDD). Although incomplete by virtue of space and the extent of the topic, this review emphasizes the fact that connexin function is not only associated with gap junction channel formation. As such, both canonical and noncanonical functions of connexins are fundamental components in the pathophysiology of multiple connexin related disorders, many of them highly debilitating and life threatening. Improved understanding of connexin biology has the potential to advance our understanding of mechanisms, diagnosis, and treatment of disease.

Sublimation-like Behavior of Cardiac Dynamics in Heart Failure: A Malignant Phase Transition?

An abrupt transition from sinus cardiac rhythm to atrial fibrillation (AF) is common in patients with chronic heart failure (CHF). We propose a conceptual framework for viewing this malignant transition in terms of a type of sublimation marked by the switch from highly periodic sinus interbeat interval dynamics characteristic of CHF to a state of random disorganization with AF. Sublimation of physical substances involves an increase in entropy via heat transfer. In contrast, the disease-related sublimation-like behavior involves a loss of information content, associated decreases in cardiac bioenergetic capacity and in multiscale entropy.

Gap junction modifiers regulate electrical activities of the sinoatrial node and pulmonary vein: Therapeutic implications in atrial arrhythmogenesis

BACKGROUND

Gap junction (GJ) dysfunctions predispose cardiac tissues to various arrhythmias. Sinoatrial node (SAN) and pulmonary veins (PVs) are closely related atrial dysrhythmia. This study evaluated whether GJ modifications modulate SAN and PVs electrical activities.

METHODS

Conventional microelectrodes were used to record action potentials in isolated rabbit SAN, PVs, and connected PV-SAN tissue preparations before and after heptanol (GJ inhibitor) and PQ1 (GJ enhancer) administration with and without isoproterenol. A whole-cell patch clamp was used to record the electrical activities before and after heptanol in single SAN and PV cardiomyocytes.

RESULTS

Heptanol (1, 3, and 10μM) reduced the spontaneous beating rates of isolated SAN preparations but not PVs. Heptanol (10μM) decelerated the SAN leading rhythm in the PV-SAN preparations and induced PV burst firings without (3 of 6, 50%) and with (6 of 6, 100%) isoproterenol (1μM). Heptanol (10μM) also reduced the spontaneous beating rates in single SAN cardiomyocyte, but not PV cardiomyocyte, with a decreased pacemaker current. PQ1 (50 and 500nM) treatment did not change the spontaneous beating rates in isolated SAN and PV preparations. In the connected PV-SAN preparations, PQ1 (500nM) did not induce any PV firing even having additional isoproterenol treatment (1μM). Moreover, PQ1 (500nM) prevented heptanol-induced electrical changes in SAN and PVs preparations.

CONCLUSION

GJ dysfunction modulates SAN and PV electrical activity, which may contribute to atrial arrhythmogenesis. GJ enhancer has a therapeutic potential in SAN dysfunction and atrial arrhythmogenesis.

Atrial Anti-Arrhythmic Effects of Heptanol in Langendorff-Perfused Mouse Hearts

Acute effects of heptanol (0.1 to 2 mM) on atrial electrophysiology were explored in Langendorff-perfused mouse hearts. Left atrial bipolar electrogram or monophasic action potential recordings were obtained during right atrial stimulation. Regular pacing at 8 Hz elicited atrial activity in 11 out of 11 hearts without inducing atrial arrhythmias. Programmed electrical stimulation using a S1S2 protocol provoked atrial tachy-arrhythmias in 9 of 17 hearts. In the initially arrhythmic group, 2 mM heptanol exerted anti-arrhythmic effects (Fisher’s exact test, P < 0.05) and increased atrial effective refractory period (ERP) from 26.0 ± 1.9 to 57.1 ± 2.5 ms (ANOVA, P < 0.001) despite increasing activation latency from 18.7 ± 1.1 to 28.9 ± 2.1 ms (P < 0.001) and leaving action potential duration at 90% repolarization (APD₉₀) unaltered (25.6 ± 1.2 vs. 27.2 ± 1.2 ms; P > 0.05), which led to increases in ERP/latency ratio from 1.4 ± 0.1 to 2.1 ± 0.2 and ERP/APD₉₀ ratio from 1.0 ± 0.1 to 2.1 ± 0.2 (P < 0.001). In contrast, in the initially non-arrhythmic group, heptanol did not alter arrhythmogenicity, increased AERP from 47.3 ± 5.3 to 54.5 ± 3.1 ms (P < 0.05) and activation latency from 23.7 ± 2.2 to 31.3 ± 2.5 ms and did not alter APD₉₀ (24.1 ± 1.2 vs. 25.0 ± 2.3 ms; P > 0.05), leaving both AERP/latency ratio (2.1 ± 0.3 vs. 1.9 ± 0.2; P > 0.05) and ERP/APD₉₀ ratio (2.0 ± 0.2 vs. 2.1 ± 0.1; P > 0.05) unaltered. Lower heptanol concentrations (0.1, 0.5 and 1 mM) did not alter arrhythmogenicity or the above parameters. The present findings contrast with known ventricular pro-arrhythmic effects of heptanol associated with decreased ERP/latency ratio, despite increased ERP/APD ratio observed in both the atria and ventricles.

Heptanol decreases the incidence of ischemia-induced ventricular arrhythmias through altering electrophysiological properties and connexin 43 in rat hearts

Heptanol is a type of gap junction inhibitor that decreases electrical conduction velocity. However, little is known regarding the effects of heptanol on the arrhythmias induced by regional myocardial ischemia. This study aimed to investigate the effects of heptanol on ventricular arrhythmias and the underlying mechanisms. On the Langendorff apparatus, isolated hearts of Sprague-Dawley rats underwent 30 min of ischemia, with or without pretreatment with heptanol (0.1, 0.3 or 0.5 mM), 15 min prior to the induction of regional ischemia through ligation of the left anterior descending coronary artery. The incidence of ventricular tachycardia (VT) and ventricular fibrillation (VF) were recorded after ligation. Heptanol decreased the incidence of ventricular arrhythmias (45% in the control group vs. 10% in the 0.1 mM group, 0% in the 0.3 mM group and 0% in the 0.5 mM group, P<0.05), whereas it prolonged the PR interval, QT interval and monophasic action potential duration at 90% repolarization (MAPD90). As evaluated with immunofluorescence microscopy, heptanol was able to partly reverse the downregulation of connexin 43 (Cx43) induced by ischemia. The results of the reverse transcription-polymerase chain reaction were consistent with those of immunofluorescence. In conclusion, heptanol significantly decreased the incidence of VT and VF induced by regional ischemia and prolonged the PR interval, QT interval and MAPD90. Heptanol also partly reversed the downregulation of Cx43 induced by ischemia.

Proarrhythmic effect of blocking the small conductance calcium activated potassium channel in isolated canine left atrium

BACKGROUND

Small conductance calcium activated potassium (SKCa) channels are voltage insensitive and are activated by intracellular calcium. Genome-wide association studies revealed that a variant of SKCa is associated with lone atrial fibrillation in humans. Roles of SKCa in atrial arrhythmias remain unclear.

OBJECTIVE

To determine roles of SKCa in atrial arrhythmias.

METHODS

Optical mapping using the isolated canine left atrium was performed. The optical action potential duration (APD) and induction of arrhythmia were evaluated before and after the addition of specific SKCa blockers-apamin or UCL-1684.

RESULTS

SKCa blockade significantly increased APD₈₀ (188 ± 19 ms vs 147 ± 11 ms; P<.001). The pacing cycle length thresholds to induce 2:2 alternans, and wave breaks were prolonged by SKCa blockade. Increased APD heterogeneity was observed after the SKCa blockade, as measured by the difference between the maximum and the minimum APD (39 ± 4 ms vs 26 ± 5 ms; P<.05), by standard deviation (12.43 ± 2.36 ms vs 7.49 ± 1.47 ms; P<.001), or by coefficient of variation (6.68% ± 0.97% vs 4.90% ± 0.84%; P<.05). No arrhythmia was induced at baseline by an S1-S2 protocol. After SKCa blockade, 4 of 6 atria developed arrhythmia.

CONCLUSIONS

SKCa blockade promotes arrhythmia and prolongs the pacing cycle length threshold of 2:2 alternans and wave breaks in the canine left atrium. The proarrhythmic effect could be attributed to increased APD heterogeneity in the canine left atrium. This study provides supportive evidence of genome-wide association studies showing association of KCNN3 and lone atrial fibrillation.

Cardiac cellular coupling and the spread of early instabilities in intracellular Ca2+

Recent experimental and modeling studies demonstrate the fine spatial scale, complex nature, and independent contribution of Ca(2+) dynamics as a proarrhythmic factor in the heart. The mechanism of progression of cell-level Ca(2+) instabilities, known as alternans, to tissue-level arrhythmias is not well understood. Because gap junction coupling dictates cardiac syncytial properties, we set out to elucidate its role in the spatiotemporal evolution of Ca(2+) instabilities. We experimentally perturbed cellular coupling in cardiac syncytium in vitro. Coupling was quantified by fluorescence recovery after photobleaching, and related to function, including subtle fine-scale Ca(2+) alternans, captured by optical mapping. Conduction velocity and threshold for alternans monotonically increased with coupling. Lower coupling enhanced Ca(2+) alternans amplitude, but the spatial spread of early (<2 Hz) alternation was the greatest under intermediate (not low) coupling. This nonmonotonic relationship was closely matched by the percent of samples exhibiting large-scale alternans at higher pacing rates. Computer modeling corroborated these experimental findings for strong but not weak electromechanical (voltage-Ca(2+)) coupling, and offered mechanistic insight. In conclusion, using experimental and modeling approaches, we reveal a general mechanism for the spatial spread of subtle cellular Ca(2+) alternans that relies on a combination of gap-junctional and voltage-Ca(2+) coupling.

Repolarization and activation restitution near human pulmonary veins and atrial fibrillation initiation: a mechanism for the initiation of atrial fibrillation by premature beats

OBJECTIVES

The authors sought to study mechanisms to explain why single premature atrial complexes (PACs) from the pulmonary veins (PVs) may initiate human atrial fibrillation (AF).

BACKGROUND

Theoretically, single PACs may initiate AF if the rate response of action potential duration (APD) restitution has a slope >1. However, human left atrial APD restitution and its relationship to AF have not been studied. We hypothesized that an APD restitution slope >1 near PVs explains the initiation of clinical AF.

METHODS

We studied 27 patients with paroxysmal and persistent (n = 13) AF. We advanced monophasic action potential catheters transseptally to superior PVs. Restitution was plotted as APD of progressively early PACs against their diastolic interval (DI) from prior beats. Activation time restitution was measured using the time from the pacing artifact to each PAC.

RESULTS

Compared with paroxysmal AF, patients with persistent AF had shorter left atrial APD and effective refractory period (p = 0.01). In paroxysmal AF, maximum left atrial APD restitution slope was 1.5 +/- 0.4; and 12 of 13 patients had slope >1 (p < 0.001). In persistent AF, PACs encountered prolonged activation for a wider range of beats than in paroxysmal AF (p = 0.01), which prolonged DI and flattened APD restitution (slope 0.7 +/- 0.2; p < 0.001); no patient had APD restitution slope >1. A single PAC produced AF in 5 patients; in all, an APD restitution slope >1 caused extreme APD oscillations after the PAC, then AF.

CONCLUSIONS

In patients with paroxysmal AF, maximum APD restitution slope >1 near the PVs enables single PACs to initiate AF. However, patients with persistent AF show marked dynamic activation delay near PVs that flattens APD restitution. Studies should determine how regional APD and conduction dynamics contribute to the substrates of persistent AF.

Pharmacological modulation of gap junction function with the novel compound rotigaptide: a promising new principle for prevention of arrhythmias

Existing anti-arrhythmic therapy is hampered by lack of efficacy and unacceptable side effects. Thus, ventricular tachycardia and fibrillation remains the strongest predictor of in-hospital mortality in patients with myocardial infarction. In atrial fibrillation, rhythm control with conventional ion channel blockers provide no therapeutic benefit relative to rate control. Several lines of research indicate that impaired gap junctional cell-to-cell coupling between neighbouring cardiomyocytes is critical for the development of cardiac re-entry arrhythmias. Rotigaptide is the first drug that has been developed to prevent arrhythmias by re-establishing gap junctional intercellular communication. During conditions with acute cardiac ischaemia, rotigaptide effectively prevents induction of both ventricular and atrial tachyarrhythmia. Moreover, rotigaptide effectively prevents ischaemia reperfusion arrhythmias. At the cellular level, rotigaptide inhibits ischaemia-induced dephosphorylation of Ser297 and Ser368, which is considered important for the gating of connexin43 gap junction channels. No drug-related toxicity has been demonstrated at plasma concentrations 77,000 times above therapeutic concentrations. In rats and dogs, rotigaptide reduces infarct size following myocardial infarction. A series of phase I trials has been completed in which rotigaptide has been administered intravenously to ~200 healthy persons. No drug-related side effects have been demonstrated in healthy human beings. Clinical safety, tolerability and efficacy in patients with heart disease are being evaluated in ongoing clinical trials. Rotigaptide represents a pioneering pharmacological principle with a highly favourable preclinical and clinical safety profile, which makes this molecule a promising drug candidate for the prevention of cardiac arrhythmias.

The Mechanisms of Atrial Fibrillation

In this article we have reviewed the mechanisms of atrial fibrillation (AF) with special emphasis on the thoracic veins. Based on a number of features, the thoracic veins are highly arrhythmogenic. The pulmonary vein (PV)-left atrial (LA) junction has discontinuous myocardial fibers separated by fibrotic tissues. The PV muscle sleeve is highly anisotropic. The vein of Marshall (VOM) in humans has multiple small muscle bundles separated by fibrosis and fat. Insulated muscle fibers can promote reentrant excitation, automaticity, and triggered activity. The PV muscle sleeves contain periodic acid-Schiff (PAS)-positive large pale cells that are morphologically reminiscent of Purkinje cells. These special cells could be the sources of focal discharge. Antiarrhythmic drugs have significant effects on PV muscle sleeves both at baseline and during AF. Both class I and III drugs have effects on wavefront traveling from PV to LA and from LA to PV. Separating the thoracic veins and the LA with ablation techniques also prevents PV-LA interaction. By reducing PV-LA interaction, pharmacological therapy and PV isolation reduce the activation rate in PV, intracellular calcium accumulation, and triggered activity. Therefore, thoracic vein isolation is an important technique in AF control. We conclude that thoracic veins are important in the generation and maintenance of AF.

Spontaneous atrial fibrillation initiated by triggered activity near the pulmonary veins in aged rats subjected to glycolytic inhibition

Aging and glycolytic inhibition (GI) are known to alter intracellular calcium ion (Ca(i)(2+)) handling in cardiac myocytes, causing early afterpotentials (EADs) and delayed afterpotentials. We hypothesized that aging and GI interact synergistically in intact hearts to generate EADs and triggered activity leading to atrial fibrillation (AF). We studied isolated and Langendorff-perfused hearts of young (age 3-5 mo, N = 8) and old (age 27-29 mo, N = 14) rats subjected to GI (0 glucose + 10 mmol/l pyruvate). Epicardial atrial activation maps were constructed using optical action potentials, while simultaneously monitoring Ca(i)(2+) by means of dual-voltage and calcium-sensitive fluorescent dyes. During GI, spontaneous AF occurred in 13 of 14 old but in no young rats. AF was initiated by EAD-induced triggered activity at the left atrial pulmonary vein junction (LA-PVJ). The triggered activity initially propagated as single wave front, but within 1 s degenerated into multiple wavelets. The EADs and triggered activity in the old atria were associated with significantly elevated diastolic Ca(i)(2+) levels at the LA-PVJ, where the time constant tau of the Ca(i)(2+) transient decline and action potential duration were significantly (P < 0.01) prolonged compared with atrial sites 5 mm away from LA-PVJ. During GI and rapid atrial pacing, spatially discordant APD and Ca(i)(2+) transient alternans developed in the old but not young atria, leading to AF. Atria in old rats had significantly more fibrotic tissue than atria in young rats. We conclude that GI interacts with the aged and fibrotic atria to amplify Ca(i)(2+) handling abnormalities that facilitate EAD-mediated triggered activity and AF.

The Role of the Hyperpolarization-Activated Inward Current$I_rm f$in Arrhythmogenesis: A Computer Model Study

Atrial fibrillation is the most common cardiac arrhythmia. Structural cardiac defects such as fibrosis and gap junction remodeling lead to a reduced cellular electrical coupling and are known to promote atrial fibrillation. It has been observed that the expression of the hyperpolarization-activated current If is increased under pathological conditions. Recent experimental data indicate a possible contribution of If to arrhythmogenesis. In this paper, the role of If in action potential propagation in normal and in pathological tissue is investigated by means of computer simulations. The effect of diffuse fibrosis and gap junction remodeling is simulated by reducing cellular coupling nonuniformly. As expected, the conduction velocity decreases when cellular coupling is reduced. In the presence of If the conduction velocity increases both in normal and in pathological tissue. In our simulations, ectopic activity is present in regions with high expression of If and is facilitated by cellular uncoupling. We conclude that an increased If may facilitate propagation of the action potential. Hence, If may prevent conduction slowing and block. Overexpression of If may lead to ectopic activity, especially when cellular coupling is reduced under pathological conditions.

Heterogeneous pulmonary vein myocardial cell repolarization implications for reentry and triggered activity

BACKGROUND

Myocardial cells in the pulmonary veins (PVs) are thought to play a major role in the initiation and maintenance of atrial arrhythmias, including atrial fibrillation. However, systematic single-cell microelectrode recordings from different regions in intact PV-atrial tissues are lacking.

OBJECTIVES

The purpose of this study was to determine the transmembrane action potential properties of myocardial cells in different regions of the PV and the left atrium (LA) and assess their arrhythmogenic potential during perfusion with isoproterenol (ISO) and rapid atrial pacing.

METHODS

Glass microelectrode recordings of action potentials were made from the left PV and the LA in Langendorff-perfused young (3-4 month) male rats (Fisher344) (n = 9).

RESULTS

Action potential duration (APD) of atrial and PV cells had similar duration at a pacing cycle length (CL) of 200 ms. However, shortening of the pacing CL to 100 ms led to heterogeneous repolarization of PV cells. Mid-PV cells had a significantly higher maximum slope of APD restitution than atrial or other PV sites. Intra-PV conduction block developed at rates when LA and proximal PV cells manifested 1:1 capture. Perfusion of ISO and rapid atrial pacing promoted the emergence of early afterdepolarization (EAD) and triggered beats in two out of nine tissues, causing premature atrial activation. No difference in resting potential or AP amplitude could be detected among the PV and LA cells.

CONCLUSIONS

PV myocardial cells develop marked heterogeneity in repolarization, and there is a slight ease of developing EAD and triggered activity in response to rapid pacing and ISO infusion.

Role of repolarization restitution in the development of coarse and fine atrial fibrillation in the isolated canine right atria

INTRODUCTION

Although the role of action potential duration restitution (APD-R) in the initiation and maintenance of ventricular fibrillation (VF) has been the subject of numerous investigations, its role in the generation of atrial fibrillation (AF) is less well studied. The cellular and ionic basis for coarse versus fine AF is not well delineated.

METHODS AND RESULTS

We measured APD-R during acetylcholine-mediated AF as well as during pacing (standard and dynamic protocols) in crista teriminalis, pectinate muscle, superior vena cava, and appendage of isolated canine arterially perfused right atria (n = 15). Transmembrane action potential (TAP), pseudo-ECG, and isometric tension development were simultaneously recorded. Acetylcholine flattened APD-R measured by both standard and dynamic protocols, but promoted induction of AF. AF was initially coarse, converting to fine within 3-15 minutes of AF. Coarse, but not fine AF was associated with dramatic fluctuations in tension development, reflecting wide variations in intracellular calcium activity ([Ca(2+)](i)). During coarse AF, APD-R data displayed a cloud-like distribution pattern, with a wide range of maximum APD-R slope (from 1.21 to 0.35). A maximum APD-R slope >1 was observed only in crista terminalis (3/10). The APD-R relationship was relatively linear and flat during fine AF. Reduction of [Ca(2+)](i) was associated with fine AF whereas augmentation of [Ca(2+)](i) with coarse AF.

CONCLUSIONS

Our data indicate that while APD-R may have a limited role in the maintenance of coarse AF, it is unlikely to contribute to the maintenance of fine AF and that [Ca(2+)](i) dynamics determine the degree to which AF is coarse or fine.

Cardiac-specific loss of N-cadherin leads to alteration in connexins with conduction slowing and arrhythmogenesis

The remodeling of ventricular gap junctions, as defined by changes in size, distribution, or function, is a prominent feature of diseased myocardium. However, the regulation of assembly and maintenance of gap junctions remains poorly understood. To investigate N-cadherin function in the adult myocardium, we used a floxed N-cadherin gene in conjunction with a cardiac-specific tamoxifen-inducible Cre transgene. The mutant animals appeared active and healthy until their sudden death approximately 2 months after deleting N-cadherin from the heart. Electrophysiologic analysis revealed abnormal conduction in the ventricles of mutant animals, including diminished QRS complex amplitude consistent with loss of electrical coupling in the myocardium. A significant decrease in the gap junction proteins, connexin-43 and connexin-40, was observed in N-cadherin-depleted myocytes. Perturbation of connexin function resulted in decreased ventricular conduction velocity, as determined by optical mapping. Our data suggest that perturbation of the N-cadherin/catenin complex in heart disease may be an underlying cause, leading to the establishment of the arrythmogenic substrate by destabilizing gap junctions at the cell surface.

Critical mass hypothesis revisited: role of dynamical wave stability in spontaneous termination of cardiac fibrillation

The tendency of atrial or ventricular fibrillation to terminate spontaneously in finite-sized tissue is known as the critical mass hypothesis. Previous studies have shown that dynamical instabilities play an important role in creating new wave breaks that maintain cardiac fibrillation, but its role in self-termination, in relation to tissue size and geometry, is not well understood. This study used computer simulations of two- and three-dimensional tissue models to investigate qualitatively how, in relation to tissue size and geometry, dynamical instability affects the spontaneous termination of cardiac fibrillation. The major findings are as follows: 1) Dynamical instability promotes wave breaks, maintaining fibrillation, but it also causes the waves to extinguish, facilitating spontaneous termination of fibrillation. The latter effect predominates as dynamical instability increases, so that fibrillation is more likely to self-terminate in a finite-sized tissue. 2) In two-dimensional tissue, the average duration of fibrillation increases exponentially as tissue area increases. In three-dimensional tissue, the average duration of fibrillation decreases initially as tissue thickness increases as a result of thickness-induced instability but then increases after a critical thickness is reached. Therefore, in addition to tissue mass and geometry, dynamical instability is an important factor influencing the maintenance of cardiac fibrillation.

Relationship between pattern of occurrence of atrial fibrillation and surface electrocardiographic fibrillatory wave characteristics

OBJECTIVES

The purpose of this study was to assess whether surface ECG fibrillatory (f)-wave characteristics reflect clinical variables, especially pattern of occurrence.

BACKGROUND

In clinically stable patients, f waves have fairly constant quantitative characteristics. Both electrophysiologic and structural remodeling might modify f waves.

METHODS

We analyzed f waves from 238 patients (120 men and 118 women; age range 30-97 years, mean 77 +/- 12) with atrial fibrillation identified by retrospective chart review as paroxysmal, persistent, or permanent fibrillation. Analysis was performed in the time and frequency domains on ECGs after QRS-T cancellation. Student's t-test and multivariate analysis were used for comparison.

RESULTS

The f waves of 12 patients taking rhythm control drugs had lower frequency ("slower" fibrillation) than the f waves of patients not taking such drugs (5.3 +/- 0.6 vs 6.0 +/- 0.7 Hz, P < .001). Of the 226 remaining patients, 59 were paroxysmal, 30 were persistent, and 72 were permanent; 65 had an unknown pattern. Paroxysmal and persistent patients were younger than permanent (74 +/- 12 and 72 +/- 15 vs 80 +/- 9 years, P < .002 for both). Paroxysmal, persistent, and permanent patients had different f-wave frequencies of 5.7 +/- 0.7, 6.1 +/- 0.8, and 6.2 +/- 0.6 Hz, respectively (P = .01 for paroxysmal vs persistent and P < .001 for paroxysmal vs permanent). Patients older than 77 years (mean age) had lower f wave frequency than those younger 77 years (6.0 +/- 0.7 vs 6.2 +/- 0.7 Hz, P = .01). Using multivariate analysis, the overall pattern-frequency relationship was significant (p = .014). There was a statistically significant inverse correlation between frequency and age (R = .27, slope = -0.017 Hz/year, P < .001).

CONCLUSIONS

ECG f-wave frequency reflects specific clinical variables, with higher frequency in permanent than paroxysmal fibrillation but lower frequency in older than younger patients. These findings are consistent with the idea that fibrillatory waves are modified by both electrophysiologic and structural remodeling.

Chronic nicotine in hearts with healed ventricular myocardial infarction promotes atrial flutter that resembles typical human atrial flutter

The potential of chronic nicotine exposure for atrial fibrillation (AF) and atrial flutter (AFL) in hearts with and without chronic myocardial infarction (MI) remains poorly explored. MI was created in dogs by permanent occlusion of the left anterior descending coronary artery, and dogs were administered nicotine (5 mg·kg−1·day−1 sc) for 1 mo using osmotic minipumps. High-resolution epicardial (1,792 bipolar electrodes) and endocardial Halo catheters were used to map activation during induced atrial rhythms. Nicotine promoted inducible sustained AFL at a mean cycle length of 134 ± 10 ms in all MI dogs ( n = 6) requiring pacing and electrical shocks for termination. No AFL could be induced in MI dogs ( n = 6), control (non-MI) dogs ( n = 3) not exposed to nicotine, and dogs with no MI and exposed to nicotine ( n = 3). Activation maps during AFL showed a single reentrant wavefront in the right atrium that rotated either clockwise (60%) or counterclockwise (40%) around the crista terminalis and through the isthmus. Ablation of the isthmus prevented the induction of AFL. Nicotine caused a significant ( P < 0.01) but highly heterogeneous increase in atrial interstitial fibrosis (2- to 10-fold increase in left and right atria, respectively) in the MI group but only a 2-fold increase in the right atrium in the non-MI group. Nicotine also flattened ( P < 0.05) the slope of the epicardial monophasic action potential duration (electrical restitution) curve of both atria in the MI but not in non-MI dogs. Two-dimensional simulation in an excitable matrix containing an isthmus and nicotine's restitutional and reduced gap junctional coupling (fibrosis) parameters replicated the experiments. Chronic nicotine in hearts with MI promotes AFL that closely resembles typical human AFL. Increased atrial interstitial fibrosis and flattened electrical restitution are important substrates for the AFL.

Focal gap junction uncoupling and spontaneous ventricular ectopy

Genetic studies in the mouse have demonstrated that conditional cardiac-restricted loss of connexin43 (Cx43), the major ventricular gap junction protein, is highly arrhythmogenic. However, whether more focal gap junction remodeling, as is commonly seen in acquired cardiomyopathies, influences the propensity for arrhythmogenesis is not known. We examined electrophysiological properties and the frequency of spontaneous and inducible arrhythmias in genetically engineered chimeric mice derived from injection of Cx43-deficient embryonic stem cells into wild-type recipient blastocysts. Chimeric mice had numerous well-circumscribed microscopic Cx43-negative foci in their hearts, comprising approximately 15% of the total surface area as determined by immunohistochemical analysis. Systolic function in the chimeric mice was significantly depressed as measured echocardiographically (19.0% decline in fractional shortening compared with controls, P < 0.05) and by invasive hemodynamics (17.6% reduction in change of pressure over time, P < 0.01). Chimeras had significantly more spontaneous arrhythmic events than controls (P < 0.01), including frequent runs of nonsustained ventricular tachycardia in some of the chimeric mice. However, in contrast to mice with conditional cardiac-resricted loss of Cx43 in the heart, no sustained ventricular tachyarrhythmias were observed. We conclude that focal areas of uncoupling in the myocardium increase the likelihood of arrhythmic triggers, but more widespread uncoupling is required to support sustained arrhythmias.

The Antiarrhythmic Peptide Analog ZP123 Prevents Atrial Conduction Slowing During Metabolic Stress

OBJECTIVE

As atrial conduction slowing is important in the pathogenesis of atrial reentry arrhythmias, a drug that increases atrial conduction or prevents atrial conduction slowing could serve to prevent atrial reentry arrhythmias. In this study, we investigated whether the novel stable antiarrhythmic peptide analog, ZP123, was able to prevent atrial conduction slowing.

METHODS

We examined the effect of ZP123 on metabolic stress-induced changes in conduction velocity (CV) and on dynamic CV restitution in isolated left atria from male Sprague-Dawley rats. We performed binding of ZP123 to a broad panel of 80 different cardiac and noncardiac ion channels and receptors and examined the effect of ZP123 on HERG channel conductance.

RESULTS

ZP123 dose-dependently prevented metabolic stress-induced atrial CV slowing at doses ranging from 1 nM to 10 microM. ZP123 did not affect CV during physiological conditions nor did it affect dynamic CV restitution. ZP123 had no effect on atrial contractility. ZP123 showed no or low affinity binding to all ion channels and receptors examined. ZP123 had no effects on HERG channel activity in concentrations that affected atrial conduction. The concentration of ZP123 giving maximal effect on atrial conduction (100 nM) inhibited the outward K(+)-current by 2.7 +/- 0.1%.

CONCLUSION

ZP123 has no effects on atrial conduction during physiological conditions, but it selectively prevents atrial conduction slowing during metabolic stress.

Modulation of cardiac gap junction expression and arrhythmic susceptibility

Connexin43 (Cx43), the predominant ventricular gap junction protein, is critical for maintaining normal cardiac electrical conduction, and its absence in the mouse heart results in sudden arrhythmic death. The mechanisms linking reduced Cx43 abundance in the heart and inducibility of malignant ventricular arrhythmias have yet to be established. In this report, we investigate arrhythmic susceptibility in a murine model genetically engineered to express progressively decreasing levels of Cx43. Progressively older cardiac-restricted Cx43 conditional knockout (CKO) mice were selectively bred to produce a heart-specific Cx43-deficient subline ("O-CKO" mice) in which the loss of Cx43 in the heart occurs more gradually. O-CKO mice lived significantly longer than the initial series of CKO mice but still died suddenly and prematurely. At 25 days of age, cardiac Cx43 protein levels decreased to 59% of control values (P<0.01), but conduction velocity was not significantly decreased and no O-CKO mice were inducible into sustained ventricular tachyarrhythmias. By 45 days of age, cardiac Cx43 abundance had decreased in a heterogeneous fashion to 18% of control levels, conduction velocity had slowed to half of that observed in control hearts, and 80% of O-CKO mice were inducible into lethal tachyarrhythmias. Enhanced susceptibility to induced arrhythmias was not associated with altered invasive hemodynamic measurements or changes in ventricular effective refractory period. Thus, moderately severe reductions in Cx43 abundance are associated with slowing of impulse propagation and a dramatic increase in the susceptibility to inducible ventricular arrhythmias.

Dynamical effects of diffusive cell coupling on cardiac excitation and propagation: a simulation study

Cell coupling is considered to be important for cardiac action potential propagation and arrhythmogenesis. We carried out computer simulations to investigate the effects of stimulation strength and cell-to-cell coupling on action potential duration (APD) restitution, APD alternans, and stability of reentry in models of isolated cell, one-dimensional cable, and two-dimensional tissue. Phase I formulation of the Luo and Rudy action potential model was used. We found that stronger stimulation resulted in a shallower APD restitution curve and onset of APD alternans at a faster pacing rate. Reducing diffusive coupling between cells prolonged APD. Weaker diffusive currents along the direction of propagation steepened APD restitution and caused APD alternans to occur at a slower pacing rate in tissue. Diffusive current due to curvature changed APD but had little effect on APD restitution slope and onset of instability. Heterogeneous cell coupling caused APD inhomogeneities in space. Reduction in coupling strength either uniformly or randomly had little effect on the rotation period and stability of a reentry, but random cell decoupling slowed the rotation period and, thus, stabilized the reentry, preventing it from breaking up into multiple waves. Therefore, in addition to its effects on action potential conduction velocity, diffusive cell coupling also affects APD in a rate-dependent manner, causes electrophysiological heterogeneities, and thus modulates the dynamics of cardiac excitation. These effects are brought about by the modulation of ionic current activation and inactivation.

Atrial Fibrillation in Rats Induced by Rapid Transesophageal Atrial Pacing during Brief Episodes of Asphyxia: A New In Vivo Model

Non-pharmacological in vivo models of atrial fibrillation (AF) have been developed in large animals only. We aimed to develop and characterize a new small animal non-pharmacological in vivo model of AF. AF was induced by transesophageal atrial burst pacing during 35 seconds periods of asphyxia in anesthetized male Sprague-Dawley rats. AF was reproducibly induced in 81% of the rats. The presence of AF was associated with an increased heart rate, and a decreased blood pressure. Treatment with amiodarone, D,L-sotalol, flecainide, and propranolol all reduced duration of AF, whereas verapamil treatment was associated with a marked profibrillatory effect. Increasing gap junction intracellular communication using the antiarrhythmic peptide analogue AAP10 did not affect AF duration. Basal plasma level of epinephrine and norepinephrine were increased 5- to 20-fold relative to values reported by others, but unchanged following 35 seconds of asphyxia. The results from our study demonstrate that the rat model shares several clinical key characteristics with human AF: (1) hemodynamic response to AF; (2) increased autonomic tone; (3) antiarrhythmic effects of clinically used drugs; (4) profibrillatory effect of verapamil. Relative to existing models of AF in larger animals, this model offers rapid, predictive, and inexpensive testing of antiarrhythmic/profibrillatory effects of new drugs.

Electrical current-induced atrial and pulmonary vein action potential duration shortening and repetitive activity

The influence of threshold electrical currents (EC) during regular drive on pulmonary vein (PV) and atrial myocardial cell action potential (AP) duration (APD) is unknown. We determined the effects of EC on cellular APD of PV, atria, and ventricles in isolated perfused and superfused male rat hearts (Fisher-344 strain, 3–4 mo old) at 37°C ( n = 14). We determined APD changes caused by subthreshold and threshold EC synchronized with a distant pacing electrode and delivered nearby cells from which transmembrane APs were recorded with a glass microelectrode. Progressive APD shortening ( P < 0.001) and membrane hyperpolarization ( P < 0.05) developed over a 20-s interval in the PV and atrial cells when the EC was delivered at <2 mm but not at >4 mm from the microelectrode. No such effects were seen in ventricular muscle cells. APD fully recovered 25 s after the cessation of EC application. Premature stimuli applied during EC-induced shortening of the APD caused rapid repetitive PV and atrial activity lasting two to five beats. Atropine (2 μM, n = 10) prevented, whereas propranolol (2 μM, n = 5) had no effect, on EC-induced APD shortening or repetitive activity. We conclude that EC shortens the APD and hyperpolarizes the membrane by local release of acetylcholine and causes the steep repolarization gradient in the vicinity of the current source leading to repetitive activity in atrial and PV cells during premature stimulation.

Effects of Na+ channel and cell coupling abnormalities on vulnerability to reentry: a simulation study

The role of dynamic instabilities in the initiation of reentry in diseased (remodeled) hearts remains poorly explored. Using computer simulations, we studied the effects of altered Na+ channel and cell coupling properties on the vulnerable window (VW) for reentry in simulated two-dimensional cardiac tissue with and without dynamic instabilities. We related the VW for reentry to effects on conduction velocity, action potential duration (APD), effective refractory period dispersion and restitution, and concordant and discordant APD alternans. We found the following: 1) reduced Na+ current density and slowed recovery promoted postrepolarization refractoriness and enhanced concordant and discordant APD alternans, which increased the VW for reentry; 2) uniformly reduced cell coupling had little effect on cellular electrophysiological properties and the VW for reentry. However, randomly reduced cell coupling combined with decoupling promoted APD dispersion and alternans, which also increased the VW for reentry; 3) the combination of decreased Na+ channel conductance, slowed Na+ channel recovery, and cellular uncoupling synergistically increased the VW for reentry; and 4) the VW for reentry was greater when APD restitution slope was steep than when it was flat. In summary, altered Na+ channel and cellular coupling properties increase vulnerability to reentrant arrhythmias. In remodeled hearts with altered Na+ channel properties and cellular uncoupling, dynamic instabilities arising from electrical restitution exert important influences on the VW for reentry.

Catheter ablation therapy for atrial fibrillation

The past several years have witnessed a significant growth in the number of institutions offering catheter ablation for atrial fibrillation (AF). This growth has been a result of a better understanding of pathophysiology of AF and use of instruments and approaches that made catheter ablation of AF a safe and an effective alternative to the drug therapy. The procedure increasingly is becoming a therapy of choice for a select group of symptomatic, drug-refractory patients without structural heart disease, and it is being offered to a rapidly widening patient pool. This article reviews the procedural aspects and clinical evidence supporting this wider use of AF ablation. In addition, new techniques and technologies for AF ablation and new avenues of research in this area are explored.

Fiber orientation and cell-cell coupling influence ventricular fibrillation dynamics

Cell Coupling Influences VF Dynamics.

INTRODUCTION

The structure of ventricular fibrillation (VF) is influenced by regional differences in action potential durations and perhaps restitution kinetics and fiber anisotropy. The spatial organization of VF was investigated by measuring the cross-correlation (CC) and mutual information (MI) of membrane potential (Vm) oscillations recorded from multiple sites.

METHODS AND RESULTS

Rabbit hearts (n = 6) were retrogradely perfused and stained with di-4-ANEPPS, and VF was elicited by burst pacing. Vm oscillations were recorded optically from multiple locations on the epicardium using a 16 x 16 photodiode array or a 72 x 78 CCD camera. The spatial organization of VF was investigated by calculating the maximum CC (CCmax) and MI (MImax) that can be obtained between any two sites. CCmax and MImax were extended to all pixels and served as indices of the similarities between Vm transients at a reference pixel and all other pixels on the map. We found that maps of CCmax and MImax did not contain discrete regions with high CC or MI. However, CCmax and MImax decreased monotonically with increasing distance between any arbitrarily chosen reference pixel and all other pixels. In VF, maps of CCmax and MImax revealed elliptical gradients of CC and MI that were closely aligned with fiber orientation, with major axis at 127 degrees +/- 8 degrees on the left ventricles.

CONCLUSION

CC and MI analysis in fibrillation provides new evidence that anisotropy of fiber orientation and cell-cell coupling have a direct influence on VF dynamics.