Atrial tachycardia/fibrillation in the connexin 43 G60S mutant (Oculodentodigital dysplasia) mouse

Atrial fibrillation (AF), the most common cardiac arrhythmia seen in general practice, can be promoted by conduction slowing. Cardiac impulse conduction depends on gap junction channels, which are composed of connexins (Cxs). While atrial Cx40 and Cx43 are equally expressed, AF studies have primarily focused on Cx40 reductions. The G60S Cx43 mutant (Cx43(G60S/+)) mouse model of Oculodentodigital dysplasia has a 60% reduction in Cx43 in the atria. Cx43(G60S/+) mice were compared with Cx40-deficient (Cx40(-/-)) mice to determine the role of Cxs in atrial tachycardia/fibrillation (AT/F). Intracardiac electrophysiological studies were done in 6-mo-old male C57BL/6 Cx43(G60S/+) mutant, littermate (Cx43(+/+)), Cx40(-/-), and C57BL/6 wild-type (WT) mice. AT/F induction used an extra stimulus during sinus rhythm, programmed electrical stimulation, or burst pacing (1-ms pulses, 50-Hz, 400-ms train) in the absence and presence of carbachol (CCh). Atrial effective refractory periods did not differ between strains. Cx43(G60S/+) mice were more susceptible to induction of sustained AT/F (duration >2 min, 9 of 12; maximum >35 min) compared with Cx43(+/+) mice (3 of 11; χ(2) = 5.24; P = 0.02). CCh enhanced sustained AT/F susceptibility in WT (from 1 of 12 without, to 7 of 10 with CCh; χ(2) = 8.98; P < 0.01) but not in Cx40(-/-) mice (1 of 13 without vs. 2 of 9 with CCh; χ(2) = 0.95; P = NS). The pattern of epicardial recordings during AT/F in Cx43(G60S/+) mice was left preceding right, with left atrial fractionated activation patterns consistent with clinical observations of AF. In conclusions, while Cx43(G60S/+) mice had severe AT/F, Cx40(-/-) mice were resistant to CCh-induced AT/F.     

Conduction left-to-right and right-to-left across the crista terminalis

A line of block between the vena cava and the crista terminalis (CT) region is important for atrial flutter (AFL), but whether it is fixed or functional is controversial. To test the hypothesis that conduction across the CT normally occurs, but when block occurs in this region it is functional, we analyzed atrial activation during right and left atrial pacing (cycle lengths of 500--130 ms), AFL, and atrial fibrillation in 15 dogs with sterile pericarditis and 7 normal dogs. Electrograms from 396 right, left, and septal atrial sites were simultaneously recorded. Activation across the CT occurred during atrial pacing, AFL, and atrial fibrillation. Activation wave fronts from the right to the left atrium and vice versa traveled over several routes, including Bachmann's bundle and inferior to the inferior vena cava, as well as across the CT. In these models, there is no fixed conduction block across the CT, and when block in the CT region occurs, as during AFL, it is functional.     

Direction-dependent conduction abnormalities in a canine model of atrial fibrillation due to chronic atrial dilatation

Chronic rapid atrial pacing (RAP) leads to changes that perpetuate atrial fibrillation (AF). Chronic atrial dilatation due to mitral regurgitation (MR) also increases AF inducibility, but it is not clear whether the underlying mechanism is similar. Therefore, we have investigated atrial electrophysiology in a canine MR model (mitral valve avulsion, 1 mo) using high-resolution optical mapping and compared it with control dogs and with the canine RAP model (6-8 wk of atrial pacing at 600 beats/min, atrioventricular block, and ventricular pacing at 100 beats/min). At followup, optical action potentials were recorded using a 16 x 16 photodiode array from 2 x 2-cm left atrial (LA) and right atrial (RA) areas in perfused preparations, with pacing electrodes around the field of view to study direction dependency of conduction. Action potential duration at 80% repolarization (APD(80)) was not different between control and MR but was reduced in RAP atria. Conduction velocities during normal pacing were not different between groups. However, the MR LA showed increased conduction heterogeneity during pacing at short cycle lengths and during premature extrastimuli, which frequently caused pronounced regional conduction slowing. Conduction in the MR LA during extrastimulation also displayed a marked dependence on propagation direction. These phenomena were not observed in the MR RA and in control and RAP atria. Thus both models form distinctly different AF substrates; in RAP dogs, the decrease in APD(80) may stabilize reentry. In MR dogs, regional LA conduction slowing and increased directional dependency, allowing unidirectional conduction block and preferential paths of conduction, may account for increased AF inducibility.     

Epithelial-to-mesenchymal transformation alters electrical conductivity of human epicardial cells

The myocardium of the developing heart tube is covered by epicardium. These epicardial cells undergo a process of epithelial-to-mesenchymal transformation (EMT) and develop into epicardium-derived cells (EPDCs). The ingrowing EPDCs differentiate into several celltypes of which the cardiac fibroblasts form the main group. Disturbance of EMT of the epicardium leads to serious hypoplasia of the myocardium, abnormal coronary artery differentiation and Purkinje fibre paucity. Interestingly, the electrophysiological properties of epicardial cells and whether EMT influences electrical conductivity of epicardial cells is not yet known. We studied the electrophysiological aspects of epicardial cells before and after EMT in a dedicated in vitro model, using micro-electrode arrays to investigate electrical conduction across epicardial cells. Therefore, human adult epicardial cells were placed between two neonatal rat cardiomyocyte populations. Before EMT the epicardial cells have a cobblestone (epithelium-like) phenotype that was confirmed by staining for the cell-adhesion molecule β-catenin. After spontaneous EMT in vitro the EPDCs acquired a spindle-shaped morphology confirmed by vimentin staining. When comparing both types we observed that the electrical conduction is influenced by EMT, resulting in significantly reduced conductivity of spindle-shaped EPDCs, associated with a conduction block. Furthermore, the expression of both gap junction (connexins 40, Cx43 and Cx45) and ion channel proteins (SCN5a, CACNA1C and Kir2.1) was down-regulated after EMT. This study shows for the first time the conduction differences between epicardial cells before and after EMT. These differences may be of relevance for the role of EPDCs in cardiac development, and in EMT-related cardiac dysfunction.     

Comparative effects of single- and linear triple-site rapid bipolar pacing on atrial activation in canine models

Nonuniform conduction may cause block and/or delay, thereby providing a substrate for the onset and maintenance of reentrant atrial arrhythmias. We tested the hypothesis that linear triple-site, bipolar, rapid pacing (LTSBRP) of the right atrium generates more uniform wave-front propagation compared with single-site, bipolar, rapid pacing (SSBRP), thereby reducing and/or eliminating conduction block and delay that is otherwise present. Five dogs with pericarditis and three normal dogs were studied. Three plunge-wire electrode pairs were placed 5-7 mm apart in both perpendicular and parallel configurations at the superior aspect of the crista terminalis and were used to pace at 200- and 300-ms cycle lengths for < or =6 s. During pacing, 380 electrograms were recorded simultaneously from electrode arrays placed epicardially on the atria, which produced activation sequence maps for each pacing episode. Local conduction-velocity vectors were computed for each site during each episode. Histograms of absolute velocity vector angles from the x-axis (of the crista terminalis) were plotted to assess uniformity of wave-front propagation, and the magnitude of each vector was computed to assess the local speed. LTSBRP showed 1) more uniform linear activation wave fronts compared with SSBRP, 2) velocity vectors with a more uniform magnitude and direction compared with SSBRP, 3) a predominant absolute velocity vector angle vs. a scattered angle distribution with SSBRP, and 4) shorter right atrial activation time and faster mean epicardial speed than SSBRP for each pacing cycle length. LTSBRP created a more uniform wave-front propagation with less or no conduction block and/or delay compared with SSBRP.     

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.     

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

We hypothesized that partial cellular uncoupling produced by low concentrations of heptanol increases the vulnerability to inducible atrial fibrillation (AF). The epicardial surface of 12 isolated-perfused canine left atria was optically mapped before and after 1-50 microM heptanol infusion. At baseline, no sustained (>30 s) AF could be induced in any of the 12 tissues. However, after 2 microM heptanol infusion, sustained AF was induced in 9 of 12 tissues (P < 0.001). Heptanol >5 microM caused loss of 1:1 capture during rapid pacing, causing no AF to be induced. AF was initiated by conduction block across the fiber leading to reentry, which broke up after one to two rotations into two to four independent wavelets that sustained the AF. Heptanol at 2 microM had no effect on the cellular action potential duration restitution or on the maximal velocity rate over time of the upstroke. The effects of heptanol were reversible. We conclude that partial cellular uncoupling by heptanol without changing atrial active membrane properties promotes wavebreak, reentry, and AF during rapid pacing.     

Loss of atrial contractility is primary cause of atrial dilatation during first days of atrial fibrillation

Atrial fibrillation (AF) induces a progressive dilatation of the atria which in turn might promote the arrhythmia. The mechanism of atrial dilatation during AF is not known. To test the hypothesis that loss of atrial contractile function is a primary cause of atrial dilatation during the first days of AF, eight goats were chronically instrumented with epicardial electrodes, a pressure transducer in the right atrium, and piezoelectric crystals to measure right atrial diameter. AF was induced with the use of repetitive burst pacing. Atrial contractility was assessed during sinus rhythm, atrial pacing (160-, 300-, and 400-ms cycle length), and electrically induced AF. The compliance of the fibrillating right atrium was measured during unloading the atria with diuretics and loading with 1 liter of saline. All measurements were repeated after 6, 12, and 24 h of AF and then once a day during the first 5 days of AF. Recovery of the observed changes after spontaneous cardioversion was also studied. After 5 days of AF, atrial contractility during sinus rhythm or slow atrial pacing was greatly reduced. During rapid pacing (160 ms) or AF, the amplitude of the atrial pressure waves had declined to 20% of control. The compliance of the fibrillating atria increased twofold, whereas the right atrial pressure was unchanged. As a result, the mean right atrial diameter increased by approximately 12%. All changes were reversible within 3 days of sinus rhythm. We conclude that atrial dilatation during the first days of AF is due to an increase in atrial compliance caused by loss of atrial contractility during AF. Atrial compliance and size are restored when atrial contractility recovers after cardioversion of AF.     

Structural atrial remodeling alters the substrate and spatiotemporal organization of atrial fibrillation: a comparison in canine models of structural and electrical atrial remodeling

Several animal models of atrial fibrillation (AF) have been developed that demonstrate either atrial structural remodeling or atrial electrical remodeling, but the characteristics and spatiotemporal organization of the AF between the models have not been compared. Thirty-nine dogs were divided into five groups: rapid atrial pacing (RAP), chronic mitral regurgitation (MR), congestive heart failure (CHF), methylcholine (Meth), and control. Right and left atria (RA and LA, respectively) were simultaneously mapped during episodes of AF in each animal using high-density (240 electrodes) epicardial arrays. Multiple 30-s AF epochs were recorded in each dog. Fast Fourier transform was calculated every 1 s over a sliding 2-s window, and dominant frequency (DF) was determined. Stable, discrete, high-frequency areas were seen in none of the RAP or control dogs, four of nine MR dogs, four of six CHF dogs, and seven of nine Meth dogs in either the RA or LA or both. Average DFs in the Meth model were significantly greater than in all other models in both LA and RA except LA DFs in the RAP model. The RAP model was the only one with a consistent LA-to-RA DF gradient (9.5 +/- 0.2 vs. 8.3 +/- 0.3 Hz, P < 0.00005). The Meth model had a higher spatial and temporal variance of DFs and lower measured organization levels compared with the other AF models, and it was the only model to show a linear relationship between the highest DF and dispersion (R(2) = 0.86). These data indicate that structural remodeling of atria (models known to have predominantly altered conduction) leads to an AF characterized by a stable high-frequency area, whereas electrical remodeling of atria (models known to have predominantly shortened refractoriness without significant conduction abnormalities) leads to an AF characterized by multiple high-frequency areas and multiple wavelets.     

An atrial-fibrillation-linked connexin40 mutant is retained in the endoplasmic reticulum and impairs the function of atrial gap-junction channels

Connexin40 (Cx40)-containing gap-junction channels are expressed in the atrial myocardium and provide a low-resistance passage for rapid impulse propagation. A germline mutation in the GJA5 gene, which encodes Cx40, resulting in a truncated Cx40 (Q49X) was identified in a large Chinese family with lone (idiopathic) atrial fibrillation (AF). This mutation co-segregated with seven AF probands in an autosomal-dominant way over generations. To test the hypothesis that this Cx40 mutant affects the distribution and function of atrial gap junctions, we studied the Q49X mutant in gap-junction-deficient HeLa and N2A cells. The Q49X mutant, unlike wild-type Cx40, was typically localized in the cytoplasm and failed to form gap-junction plaques at cell-cell interfaces. When the Q49X mutant was co-expressed with Cx40 or Cx43, the mutant substantially reduced the gap-junction plaque formation of Cx40 and Cx43. Electrophysiological studies revealed no electrical coupling of cell pairs expressing the mutant alone and a significant decrease in the coupling conductance when the mutant was co-expressed with Cx40 or Cx43. Further colocalization experiments with the organelle residential proteins indicate that Q49X was retained in the endoplasmic reticulum. These findings provide evidence that the Q49X mutant is capable of impairing gap-junction distribution and function of key atrial connexins, which might play a role in the predisposition to and onset of AF.KEY WORDS: Atrial fibrillation, Genetics, Gap junctions, Connexin40, Germline mutation     

山羊房颤模型中左心房与肺静脉外膜碎裂电位的动态比较

目的: 比较在持续性房颤发生、发展过程中,房颤模型山羊左心房与肺静脉外膜碎裂电位(CFAEs)的变化,以期探讨肺静脉外膜碎裂电位(CFAEs)在持续性房颤中的作用。方法: 选取10只雌性山羊,使用左心房快速刺激,发送输出电压为6 V、周长为20 ms的脉冲1 s,间隔2 s后重复发放,以此方法建立持续性房颤模型(房颤持续＞24 h),平均刺激时间为(9.5±2.3)d。在整个房颤发生、发展过程中,记录并比较刺激前后左心房及肺静脉外膜CFAEs的动态变化。结果: 随着刺激时间延长,心房不应期(AERP)逐渐缩短,房颤持续时间逐渐延长,左心房及肺静脉外膜的CFAEs逐渐增多,肺静脉外膜CFAEs的比例始终多于左心房。当房颤持续24 h,肺静脉外膜CFAEs几乎持续存在(房颤初发与房颤持续24 h相比,2.7%±3.6% vs 92.6%±6.4%,P＜0.05),肺静脉外膜CFAEs比例明显高于左心房(P＜0.05)。结论: 心外膜CFAEs具有位置特异性,与电重构相关联。房颤发生发展中心外膜CFAEs逐渐增多,可能在房颤的维持中起重要作用。     

Termination of atrial flutter by directed transesophageal atrial pacing during transesophageal echocardiography.

INTRODUCTION: The purpose of this study was to evaluate termination of atrial flutter (AFL) by directed rapid transesophageal atrial pacing (TAP) with and without simultaneous transesophageal echocardiography (TEE) performed using a novel TEE tube electrode. MATERIALS AND METHODS, AND RESULTS: A total of 16 AFL patients (age 63+/-12 years; 13 males) with mean AFL cycle length of 224+/-24 ms (n=12) and mean ventricular cycle length of 448+/-47 ms (n=12) were analyzed using either an esophageal TO electrode (n=10) or a novel TEE tube electrode consisting of a tube with four hemispherical electrodes that is pulled over the echo probe (n=6). AFL could be terminated by directed rapid TAP using an esophageal TO electrode, leading to induction of atrial fibrillation (AF) (n=6), induction of AF and spontaneous conversion to sinus rhythm (SR) (n=3), and with conversion to SR (n=1). AFL could also be terminated by directed rapid TAP using the TEE tube electrode, with induction of AF (n=3) or induction of AF and spontaneous conversion to SR (n=3). CONCLUSION: AFL can be terminated by directed rapid TAP with hemispherical electrodes with and without simultaneous TEE. TAP with the directed TEE tube electrode is a safe, simple, and useful method for terminating AFL.     

Cardiac-restricted angiotensin-converting enzyme overexpression causes conduction defects and connexin dysregulation

Renin-angiotensin (RAS) system activation is associated with an increased risk of sudden death. Previously, we used cardiac-restricted angiotensin-converting enzyme (ACE) overexpression to construct a mouse model of RAS activation. These ACE 8/8 mice die prematurely and abruptly. Here, we have investigated cardiac electrophysiological abnormalities that may contribute to early mortality in this model. In ACE 8/8 mice, surface ECG voltages are reduced. Intracardiac electrograms showed atrial and ventricular potential amplitudes of 11% and 24% compared with matched wild-type (WT) controls. The atrioventricular (AV), atrio-Hisian (AH), and Hisian-ventricular (HV) intervals were prolonged 2.8-, 2.6-, and 3.9-fold, respectively, in ACE 8/8 vs. WT mice. Various degrees of AV nodal block were present only in ACE 8/8 mice. Intracardiac electrophysiology studies demonstrated that WT and heterozygote (HZ) mice were noninducible, whereas 83% of ACE 8/8 mice demonstrated ventricular tachycardia with burst pacing. Atrial connexin 40 (Cx40) and connexin 43 (Cx43) protein levels, ventricular Cx43 protein level, atrial and ventricular Cx40 mRNA abundances, ventricular Cx43 mRNA abundance, and atrial and ventricular cardiac Na(+) channel (Scn5a) mRNA abundances were reduced in ACE 8/8 compared with WT mice. ACE 8/8 mice demonstrated ventricular Cx43 dephosphorylation. Atrial and ventricular L-type Ca(2+) channel, Kv4.2 K(+) channel alpha-subunit, and Cx45 mRNA abundances and the peak ventricular Na(+) current did not differ between the groups. In isolated heart preparations, a connexin blocker, 1-heptanol (0.5 mM), produced an electrophysiological phenotype similar to that seen in ACE 8/8 mice. Therefore, cardiac-specific ACE overexpression resulted in changes in connexins consistent with the phenotype of low-voltage electrical activity, conduction defects, and induced ventricular arrhythmia. These results may help explain the increased risk of arrhythmia in states of RAS activation such as heart failure.     

Pharmacological modulation and differential regulation of the cardiac gap junction proteins connexin 43 and connexin 40

Gap junction channels provide the basis for the electrical syncytial properties of the heart as a communicating electrical network. Cardiac gap junction channels are predominantly composed of connexin 40 or connexin 43. The conductance of these channels (g(j)) can be regulated pharmacologically: substances which activate protein kinase C, protein kinase A or protein kinase G may alter Cx43 gap junction conductance. However, for PKC, this seems to be subtype specific. Thus, antiarrhythmic peptides can enhance g(j) via activation of PKCepsilon, while FGF-2 reduces g(j) via PKCepsilon. Lipophilic drugs can uncouple the channels. Besides an acute regulation of g(j), the expression of the cardiac connexins can also be regulated. A decrease in Cx43 with a concomitant increase in Cx40 has been found in end-stage failing hearts, while in renovascular hypertension, an increase in Cx43 has been described. Mediators like endothelin-1, angiotensin-II, TGF-beta, VEGF, and cAMP have been shown to increase Cx43. Interestingly, endothelin-1 and angiotensin-II increased Cx43 but did not affect Cx40 expression. In contrast, in humans suffering from atrial fibrillation (AF), the content in Cx40 can be enhanced while Cx43 was unaltered, although in several other studies, other changes of the cardiac connexins were found, which might be related to the type of AF. Regarding the role of calcium, the content in both Cx40 and Cx43 was decreased in cultured neonatal rat cardiomyocytes after 24 h administration of 100 nM verapamil. Thus, gap junctional channels can be affected pharmacologically either acutely by modulating gap junction conductance or chronically by altering gap junction protein expression. Interestingly, it appears that the expression of Cx43 and Cx40 can be differentially regulated.     

Downregulation of connexin40 and increased prevalence of atrial arrhythmias in transgenic mice with cardiac-restricted overexpression of tumor necrosis factor

Atrial arrhythmias, primarily atrial fibrillation, have been independently associated with structural remodeling and with inflammation. We hypothesized that sustained inflammatory signaling by tumor necrosis factor (TNF) would lead to alterations both in underlying atrial myocardial structure and in atrial electrical conduction. We performed ECG recording, intracardiac electrophysiology studies, epicardial mapping, and connexin immunohistochemical analyses on transgenic mice with targeted overexpression of TNF in the cardiac compartment (MHCsTNF) and on wild-type (WT) control mice (age 8-16 wk). Atrial and ventricular conduction abnormalities were always evident on ECG in MHCsTNF mice, including a shortened atrioventricular interval with a wide QRS duration secondary to junctional rhythm. Supraventricular arrhythmias were observed in five of eight MHCsTNF mice, whereas none of the mice demonstrated ventricular arrhythmias. No arrhythmias were observed in WT mice. Left ventricular conduction velocity during apical pacing was similar between the two mouse groups. Connexin40 was significantly downregulated in MHCsTNF mice. In contrast, connexin43 density was not significantly altered in MHCsTNF mice, but rather dispersed away from the intercalated disks. In conclusion, sustained inflammatory signaling contributed to atrial structural remodeling and downregulation of connexin40 that was associated with an increased prevalence of atrial arrhythmias.     

星状神经节电刺激建立交感神经介导的急性房颤犬模型

目的建立交感神经张力异常介导的急性房颤动物模型的方法学。方法将16只随意来源犬分为三组：对照组（n=4）,右侧星状神经节（aSG）组（n=6）和左侧星状神经节（LSG）组（n=6）,测定心房和肺静脉不同部位的房颤诱发率、房颤持续时间。结果RSG刺激显著增加右心房（RA）的房颤诱发率和持续时间（P〈0．05）,LSG刺激显著增加左心房（LA）、左上肺静脉（LSPV）、左下肺静脉（LIPV）的房颤诱发率和持续时间（P〈0．05）;与刺激时相比,RSG切除显著降低RA的房颤诱发率和持续时间（P〈0．05）;LSG切除显著降低LA、LSPV、LIPV的房颤诱发率和持续时间（P〈0．05）。结论星状神经节电刺激同时快速心房起搏6h可成功建立交感神经介导的急性房颤犬模型,星状神经节电刺激使心房和肺静脉部位的房颤诱发率显著升高,房颤持续时间显著延长,去星状神经节支配可减少房颤的发生和维持。     

Atrial fibrillation-linked GJA5/connexin40 mutants impaired gap junctions via different mechanisms

The gap junctions (GJs) formed by Cx40 and Cx43 provide a low resistance passage allowing for rapid propagation of action potentials. Sporadic somatic mutations in GJA5 (encoding Cx40) have been identified in lone atrial fibrillation (AF) patients. More recently germline autosomal dominantly inherited mutations in GJA5 have been found in early onset lone AF patients in several families over generations. Characterizations of these AF-linked Cx40 mutants in model cells and in patient tissues revealed that some of the mutants reduced GJ channel function due to an impaired trafficking or channel formation. While others showed a gain-of-function in hemichannels. These functional alterations in GJs or hemichannel may play an important role in the pathogenesis of AF in the mutant carriers.     

Intermittent hypoxia causes NOX2-dependent remodeling of atrial connexins

Background

Obstructive sleep apnea has been linked to the development of heart disease and arrhythmias, including atrial fibrillation. Since altered conduction through gap junction channels can contribute to the pathogenesis of such arrhythmias, we examined the abundance and distributions of the major cardiac gap junction proteins, connexin40 (Cx40) and connexin43 (Cx43) in mice treated with sleep fragmentation or intermittent hypoxia (IH) as animal models of the components of obstructive sleep apnea.

Results

Wild type C57BL/6 mice or mice lacking NADPH 2 (NOX2) oxidase activity (gp91phox(?/Y)) were exposed to room air or to SF or IH for 6 weeks. Then, the mice were sacrificed, and atria and ventricles were immediately dissected. The abundances of Cx40 or Cx43 in atria and ventricles were unaffected by SF. In contrast, immunoblots showed that the abundance of atrial Cx40 and Cx43 and ventricular Cx43 were reduced in mice exposed to IH. qRT-PCR demonstrated significant reductions of atrial Cx40 and Cx43 mRNAs. Immunofluorescence microscopy revealed that the abundance and size of gap junctions containing Cx40 or Cx43 were reduced in atria by IH treatment of mice. However, no changes of connexin abundance or gap junction size/abundance were observed in IH-treated NOX2-null mice.

Conclusions

These results demonstrate that intermittent hypoxia (but not sleep fragmentation) causes reductions and remodeling of atrial Cx40 and Cx43. These alterations may contribute to the substrate for atrial fibrillation that develops in response to obstructive sleep apnea. Moreover, these connexin changes are likely generated in response to reactive oxygen species generated by NOX2.

     

Transitions among atrial fibrillation, atrial flutter, and sinus rhythm during procainamide infusion and vagal stimulation in dogs with sterile pericarditis

The mechanism of atrial flutter and fibrillation induced by rapid pacing in 22 dogs with 3-day-old sterile pericarditis was investigated by computerized epicardial mapping of atrial activation before and after administration of agents known to modify atrial electrophysiologic properties: procainamide, isoproterenol, and electrical stimulation of the vagosympathetic trunks. Before the administration of any of these agents, a total of 30 episodes of sustained atrial flutter (greater than 1 min duration, monomorphic; regular cycle length, 127 +/- 12 ms, mean +/- SD) was induced in 15 out of 22 dogs and 9 episodes of unstable atrial flutter (duration, less than 1 min; cycle length, 129 +/- 34 ms; monomorphic, alternating with fibrillation) were induced in the remaining 7 preparations. In the latter, administration of procainamide transformed unstable atrial flutter and atrial fibrillation to sustained atrial flutter (cycle length, 142 +/- 33 ms; n = 9 episodes). During control atrial flutter, atrial maps displayed circus movement of excitation in the right atrial free wall with faster conduction parallel to the orientation of intra-atrial myocardial bundles. Vagal stimulation changed atrial flutter to atrial fibrillation in 32 of 73 trials; this was associated with acceleration of conduction in the lower right atrium, leading to fragmentation of the major wave front. Isoproterenol produced a 6-25% increase of the atrial rate in 6 out of 14 trials of atrial flutter and induced atrial fibrillation in 4. After procainamide, the reentrant pathway was lengthened and conduction was slowed further in the right atrium. Maps obtained during unstable atrial flutter showed incomplete circuits involving the right atrium. Following procainamide infusion, the area of functional dissociation or block was enlarged and a stable circus movement pattern, which was similar to the pattern seen in control atrial flutter, was established in the right atrium. We conclude that (1) the transitions among atrial fibrillation, atrial flutter, and sinus rhythm occur between different functional states of the same circus movement substratum primarily located in the lower right atrial free wall, and (2) the anisotropic conduction properties of the right atrium may contribute to these reentrant arrhythmias and may be potentiated by acute pericarditis.     

Left atrial dilatation resulting from chronic mitral regurgitation decreases spatiotemporal organization of atrial fibrillation in left atrium

Atrial conduction properties have been shown to differ among animal atrial fibrillation (AF) models of rapid atrial pacing (RAP), chronic mitral regurgitation (MR), and control. We hypothesized that these conduction differences would continue with the onset of AF, which would affect AF spatiotemporal organization, resulting in model-specific characteristics of AF. With frequency domain analysis of electrograms acquired from high-density optical mapping, AF from the right (RA) and left (LA) atrium in animals with RAP and MR were compared with control animals. At follow-up, the hearts were excised and perfused, and optical action potentials were recorded from a 2 x 2-cm area each of the RA and LA free wall with a 16 x 16 photodiode array. AF was induced with extra stimuli, several 2.4-s AF episodes were recorded in each dog, and a fast Fourier transform was calculated. The dominant frequency (DF) was determined, and the organization (organization index, OI) was calculated as the ratio of the area under the dominant peak and its harmonics to the total area of the spectrum. All possible pairs of electrograms for each episode were cross-correlated. LA AF in the chronic MR model showed an increase in the highest DF, the number of DF domains, and in frequency gradient compared with AF in control or RAP models. In addition, there was a decrease in OI and in the correlation coefficients in the LA of the MR model. These results suggest that the AF substrate in the MR model may be different from that of control or RAP models.