Key Words: atrial fibrillation Ⅲ heart failure Ⅲ fibrosis Ⅲ gap junction Ⅲ connexin C ongestive heart failure (CHF) predisposes to atrial fibrillation (AF), although the underlying mechanisms remain incompletely understood. 1 Ventricular tachypacing produces a clinically relevant animal model of CHF. 2 The atria of dogs with ventricular tachypacing-induced CHF are characterized by structural remodeling, conduction abnormalities and the ability to sustain AF. 3 Myocardial electric continuity is assured by gap junctions, cell-to-cell connections that maintain low-resistance intercellular coupling via specialized hemichannel subunit proteins called connexins. Connexin (Cx)43 and Cx40 are the principle atrial gap junctional subunits 4 ; abnormalities in their expression and localization are commonly observed in patients and experimental animals with AF. 5 Phosphorylation of Cx43 can regulate channel assembly, 6 degradation, 7 and conductance. 8 In the ventricles, CHF produces hypophosphorylation of Cx43 and redistribution to lateral cell membranes, associated with proarrhythmic conduction slowing. 9,10 Little is known, however, about connexin changes during CHF-related atrial remodeling and their role in AF maintenance. The present study was designed to assess the changes in atrial connexin expression caused by tachypacing-induced CHF in the dog.In initial experiments, we noted significant changes in connexin phosphorylation and sought to understand their role in AF. We previously noted that the cessation of ventricular tachypacing, which is followed by the reversal of CHF, dissociates atrial size and function changes from structural remodeling and AF sustainability. 11,12 Both structural (particularly tissue fibrosis) and connexin remodeling could contribute to CHF-associated AF. We therefore exploited CHF reversal to assess the reversibility of atrial connexin alterations and evaluate their contribution to CHF-related conduction disturbances and AF maintenance. MethodsAn expanded Methods section is available in the Online Data Supplement at http://circres.ahajournals.org. Animal ModelAnimal-handling procedures followed National Institutes of Health guidelines. Animals were prepared and studied as described previously. 11,12 Forty-nine mongrel dogs (18 to 34 kg) were instrumented with a right ventricular tachypacemaker. Dogs were assigned to 3 groups: (1) pacemaker inactive sham controls (CTL group, nϭ19); (2) 2-week ventricular tachypacing at 240 bpm to induce CHF (CHF group, nϭ15); (3) 2-week ventricular tachypacing followed by 4-week recovery (REC group, nϭ16). Atrial effective refractory period (ERP) was measured with 10 basic (S1) stimuli. ERP was measured at multiple basic cycle lengths (BCLs) in the left atrial (LA) appendage of all dogs and at 7 additional sites (BCL, 300 ms) in 5 dogs per group. AF was induced by burst pacing and mean AF duration estimated based on 10 inductions. AF Ն30 minutes was considered sustained and was cardioverted. Five plastic arrays containing a total of 240 bipolar elect...

Background-Adenosine acutely reconnects pulmonary veins (PVs) after radiofrequency application, revealing "dormant conduction" and identifying PVs at risk of reconnection, but the underlying mechanisms are unknown. Methods and Results-Canine PV and left-atrial (LA) action potentials were recorded with standard microelectrodes and ionic currents with whole-cell patch clamp before and after adenosine perfusion. PVs were isolated with radiofrequency current application in coronary-perfused LA-PV preparations. Adenosine abbreviated action potential duration similarly in PV and LA but significantly hyperpolarized resting potential (by 3.9Ϯ0.5%; PϽ0.05) and increased dV/dt max (by 34Ϯ10%) only in PV. Increased dV/dt max was not due to direct effects on I Na , which was reduced similarly by adenosine in LA and PV but correlated with resting-potential hyperpolarization (rϭ0.80). Adenosine induced larger inward rectifier K ϩ current (I KAdo ) in PV (eg, -2.28Ϯ0.04 pA/pF; -100 mV) versus LA (-1.28Ϯ0.16 pA/pF). Radiofrequency ablation isolated PVs by depolarizing resting potential to voltages positive to -60 mV. Adenosine restored conduction in 5 dormant PVs, which had significantly more negative resting potentials (-57Ϯ6 mV) versus nondormant (-46Ϯ5 mV, nϭ6; PϽ0.001) before adenosine. Adenosine hyperpolarized both, but more negative resting-potential values after adenosine in dormant PVs (-66Ϯ6 mV versus -56Ϯ6 mV in nondormant; PϽ0.001) were sufficient to restore excitability. Adenosine effects on resting potential and conduction reversed on washout. Spontaneous recovery of conduction occurring in dormant PVs after 30 to 60 minutes was predicted by the adenosine response. Key Words: arrhythmia ablation Ⅲ adenosine Ⅲ atrium Ⅲ conduction Ⅲ electrophysiology Ⅲ ion channels P ulmonary vein isolation (PVI) is an effective treatment for atrial fibrillation (AF). 1,2 Nevertheless, many patients require repeated ablation procedures because of AF recurrence, which in most cases are associated with reconnection of previously isolated PVs. 3,4 It has recently been noted that intravenous purinergic agonists such as adenosine can transiently restore conduction through a previously isolated PV, a phenomenon called "dormant conduction." [5][6][7][8] The demonstration of dormant conduction has predictive value for eventual reconnection, and additional radiofrequency (RF) applications to veins showing dormant conduction at the time of initial PVI may prevent reconnection and AF recurrence. 6 -8 The mechanisms by which adenosine restores conduction to dormant PVs are unknown. The objectives of this study were to (1) explore the effects of adenosine on ionic currents and action potentials (APs) in canine left-atrial (LA) and PV cardiomyocytes, and (2) relate these effects to changes in conduction between the PV and LA after RF ablation in an in vitro model. Conclusions-Adenosine Clinical Perspective on p 972 Materials and MethodsSee the online-only Data Supplement for the complete Materials and Methods section. The following text summarize...

Key points• Cardiac repolarization, through which heart-cells return to their resting state after having fired, is a delicate process, susceptible to disruption by common drugs and clinical conditions. • Animal models, particularly the dog, are often used to study repolarization properties and responses to drugs, with the assumption that such findings are relevant to humans. However, little is known about the applicability of findings in animals to man.• Here, we studied the contribution of various ion-currents to cardiac repolarization in canine and human ventricle.• Humans showed much greater repolarization-impairing effects of drugs blocking the rapid delayed-rectifier current I Kr than dogs, because of lower repolarization-reserve contributions from two other important repolarizing currents (the inward-rectifier I K1 and slow delayed-rectifier I Ks ).• Our findings clarify differences in cardiac repolarization-processes among species, highlighting the importance of caution when extrapolating results from animal models to man.Abstract The species-specific determinants of repolarization are poorly understood. This study compared the contribution of various currents to cardiac repolarization in canine and human ventricle. Conventional microelectrode, whole-cell patch-clamp, molecular biological and mathematical modelling techniques were used. Selective I Kr block (50-100 nmol l −1 dofetilide) lengthened AP duration at 90% of repolarization (APD 90 ) >3-fold more in human than dog, suggesting smaller repolarization reserve in humans. Selective I K1 block (10 μmol l −1 BaCl 2 ) and I Ks block (1 μmol l −1 HMR-1556) increased APD 90 more in canine than human right ventricular papillary muscle. Ion current measurements in isolated cardiomyocytes showed that I K1 and I Ks densities were 3-and 4.5-fold larger in dogs than humans, respectively. I Kr density and kinetics were similar in human versus dog. I Ca and I to were respectively ∼30% larger and ∼29% smaller in human, and Na + -Ca 2+ exchange current was comparable. Cardiac mRNA levels for the main I K1 ion channel subunit Kir2.1 and the I Ks accessory subunit minK were significantly lower, but mRNA expression of ERG and KvLQT1 (I Kr and I Ks α-subunits) were not significantly different, in human versus dog. Immunostaining suggested lower Kir2.1 and minK, and higher KvLQT1 protein expression in human versus canine cardiomyocytes. I K1 and I Ks inhibition increased the APD-prolonging effect of I Kr block more in dog (by 56% and 49%, respectively) than human (34 and 16%), indicating that both currents contribute to increased repolarization reserve in the dog. A mathematical model incorporating observed human-canine ion current differences confirmed the role of I K1 and I Ks in repolarization reserve differences. Thus, humans show greater repolarization-delaying effects of I Kr block than dogs, because of lower repolarization reserve contributions from I K1 and I Ks , emphasizing species-specific determinants of repolarization and the limitations of animal models fo...

Background— Atrial tissue fibrosis is often an important component of the atrial fibrillation (AF) substrate. Small noncoding microRNAs are important mediators in many cardiac remodeling paradigms. MicroRNA-21 (miR-21) has been suggested to be important in ventricular fibrotic remodeling by downregulating Sprouty-1, a protein that suppresses fibroblast proliferation. The present study examined the potential role of miR-21 in the atrial AF substrate resulting from experimental heart failure after myocardial infarction (MI). Methods and Results— Large MIs (based on echocardiographic left ventricular wall motion score index) were created by left anterior descending coronary artery ligation in rats. Changes induced by MI versus sham controls were first characterized with echocardiography, histology, biochemistry, and in vivo electrophysiology. Additional MI rats were then randomized to receive anti–miR-21 (KD21) or scrambled control sequence (Scr21) injections into the left atrial myocardium. Progressive left ventricular enlargement, hypocontractility, left atrial dilation, fibrosis, refractoriness prolongation, and AF promotion occurred in MI rats versus sham controls. Atrial tissues of MI rats showed upregulation of miR-21, along with dysregulation of the target genes Sprouty-1, collagen-1, and collagen-3. KD21 treatment reduced atrial miR-21 expression levels in MI rats to values in sham rats, decreased AF duration from 417 (69–1595; median [Q1–Q3]) seconds to 3 (2–16) seconds (8 weeks after MI; P <0.05), and reduced atrial fibrous tissue content from 14.4±1.8% (mean±SEM) to 4.9±1.2% (8 weeks after MI; P <0.05) versus Scr21 controls. Conclusions— MI-induced heart failure leads to AF-promoting atrial remodeling in rats. Atrial miR-21 knockdown suppresses atrial fibrosis and AF promotion, implicating miR-21 as an important signaling molecule for the AF substrate and pointing to miR-21 as a potential target for molecular interventions designed to prevent AF.

Background-Pulmonary vein (PV) -encircling radiofrequency ablation frequently is effective in vagal atrial fibrillation (AF), and there is evidence that PVs may be particularly prone to cholinergically induced arrhythmia mechanisms. However, PV ablation procedures also can affect intracardiac autonomic ganglia. The present study examined the relative role of PVs versus peri-PV autonomic ganglia in an experimental vagal AF model. Methods and Results-Cholinergic AF was studied under carbachol infusion in coronary perfused canine left atrial PV preparations in vitro and with cervical vagal stimulation in vivo. Carbachol caused dose-dependent AF promotion in vitro, which was not affected by excision of all PVs. Sustained AF could be induced easily in all dogs during vagal nerve stimulation in vivo both before and after isolation of all PVs with encircling lesions created by a bipolar radiofrequency ablation clamp device. PV elimination had no effect on atrial effective refractory period or its responses to cholinergic stimulation. Autonomic ganglia were identified by bradycardic and/or tachycardic responses to high-frequency subthreshold local stimulation. Ablation of the autonomic ganglia overlying all PV ostia suppressed the effective refractory period-abbreviating and AF-promoting effects of cervical vagal stimulation, whereas ablation of only left-or right-sided PV ostial ganglia failed to suppress AF. Dominant-frequency analysis suggested that the success of ablation in suppressing vagal AF depended on the elimination of high-frequency driver regions. Conclusions-Intact

The "leading circle model" was the first detailed attempt at understanding the mechanisms of functional reentry, and remains a widely-used notion in cardiac electrophysiology. The "spiral wave" concept was developed more recently as a result of modern theoretical analysis and is the basis for consideration of reentry mechanisms in present biophysical theory. The goal of this paper is to present these models in a way that is comprehensible to both the biophysical and electrophysiology communities, with the idea of helping clinical and experimental electrophysiologists to understand better the spiral wave concept and of helping biophysicists to understand why the leading circle concept is so attractive and widely used by electrophysiologists. To this end, the main properties of the leading circle and spiral wave models of reentry are presented. Their basic assumptions and determinants are discussed and the predictions of the two concepts with respect to pharmacological responses of arrhythmias are reviewed. A major difference between them lies in the predicted responses to Na(+)-channel blockade, for which the spiral wave paradigm appears more closely to correspond to the results of clinical and experimental observations. The basis of this difference is explored in the context of the fundamental properties of the models.

Chronically repeated OSA episodes cause AF-promoting cardiac remodeling, with conduction abnormalities related to connexin dysregulation and fibrosis playing a prominent role. This novel animal model provides mechanistic insights into an important clinical problem and may be useful for further exploration of underlying mechanisms and therapeutic approaches.

Background-Coronary artery disease predisposes to atrial fibrillation (AF), but the effects of chronic atrial ischemia/ infarction on AF-related substrates are unknown. Methods and Results-Regional right atrial myocardial infarction (MI) was created in 40 dogs by ligating an artery that supplies the right atrial free wall and not the ventricles; 35 sham dogs with the same artery isolated but not ligated were controls. Dogs were observed 8 days after MI and subjected to open-chest study, in vitro optical mapping, and/or cell isolation for patch-clamp and Ca 2ϩ imaging on day 8. Holter ECGs showed more spontaneous atrial ectopy in MI dogs (eg, 662Ϯ281 on day 7 versus 34Ϯ25 ectopic complexes per day at baseline; 52Ϯ21 versus 1Ϯ1 atrial tachycardia episodes per day). Triggered activity was increased in MI border zone cells, which had faster decay of caffeine-evoked Ca 2ϩ transients and enhanced (by Ϸ73%) Na ϩ -Ca 2ϩ exchange current. Spontaneous Ca 2ϩ sparks (confocal microscopy) occurred under ␤-adrenergic stimulation in more MI dog cells (66Ϯ9%) than in control cells (29Ϯ4%; PϽ0.01). Burst pacing induced long-lasting AF in MI dogs (1146Ϯ259 versus 30Ϯ14 seconds in shams). Increased border zone conduction heterogeneity was confirmed by both bipolar electrode mapping in vivo and optical mapping. Optical mapping demonstrated stable border zone reentry in all 9 MI preparations but in none of 6 shams. Border zone tissue showed increased fibrous tissue content. Conclusions-Chronic atrial ischemia/infarction creates substrates for both spontaneous ectopy (Ca 2ϩ -release events, increased Na ϩ -Ca 2ϩ exchange current) and sustained reentry (conduction abnormalities that anchor reentry). Thus, chronic atrial infarction in dogs promotes both AF triggers and the substrate for AF maintenance. These results provide novel insights into potential AF mechanisms in patients with coronary artery disease. (Circulation. 2011;123:137-146.) Key Words: atrial fibrillation Ⅲ calcium Ⅲ electrophysiology Ⅲ ischemic heart disease Ⅲ myocardial infarction A trial fibrillation (AF) is an extremely common cardiac arrhythmia associated with increased cardiovascular morbidity and mortality. 1,2 However, our understanding of AF pathophysiology remains incomplete. An improved comprehension of mechanisms underlying AF may lead to the development of novel therapeutic options. 3 Coronary artery disease is a significant risk factor for AF. 4,5 Shortterm (several hours) acute atrial ischemia creates a substrate for AF maintenance. 6,7 However, no data are available on the atrial electrophysiological and arrhythmic changes caused by longer-term atrial ischemia/infarction, as might occur in patients with chronic coronary artery disease. Clinical Perspective on p 146Atrial myocardial infarction (MI) is considered unusual because it is rarely diagnosed; however, it is often undetected. The incidence of atrial MI in autopsy series varies from 0.7% to 42%, depending largely on whether or not the atria were specifically examined. 8 The largest series of aut...