Characterization of the in vivo and in vitro electrophysiological effects of the novel antiarrhythmic agent AZD7009 in atrial and ventricular tissue of the dog

This study evaluated the effects of the novel antiarrhythmic agent AZD7009 on atrial and ventricular repolarization and on the Na+-current system, using Vmax as an index. Anesthetized dogs were infused with AZD7009 or azimilide to produce three pseudo steady-state plasma concentrations in vivo. Microelectrode techniques were used to record action potentials and effective refractory period (ERP) in vitro. Whereas AZD7009 concentration-dependently increased atrial ERP (AERP, by 48 +/- 7 milliseconds maximum, P < 0.001 versus vehicle), the increases in ventricular ERP (VERP, 8 +/- 4 milliseconds) and QT interval (2 +/- 5.5 milliseconds) were small and not concentration-dependent. For azimilide, the AERP increase was less, whereas VERP and QT increases were substantially larger than with AZD7009. In vitro, AZD7009 concentration-dependently reduced Vmax and increased action potential duration (APD). ERP was increased through APD lengthening and post-repolarization refractoriness. The suppression of Vmax, but not APD prolongation, showed frequency-dependence. APD and ERP increases were more pronounced in atrial than ventricular tissue: in atria, 2 microM AZD7009 increased APD90 and ERP from 224 +/- 7 to 318 +/- 7 milliseconds and 241 +/- 7 milliseconds to 378 +/- 17 milliseconds; versus 257 +/- 5 to 283 +/- 7 milliseconds and 253 +/- 12 to 300 +/- 11 milliseconds respectively in ventricles. Thus, AZD7009 potently and predominantly increases atrial refractoriness in the dog, with actions mediated by combined effects on repolarization and the Na+-current system.

Kir3-Based Inward Rectifier Potassium Current

Background— We previously characterized a novel K + current ( I KH ) with properties of constitutively active acetylcholine-related current in dog atrium. I KH is sensitive to tertiapin-Q (IC 50 ≈10 nmol/L), a highly selective Kir3 current blocker. This study assessed the role of I KH in atrial tachycardia (AT)–remodeled canine left atrium (LA) with the use of tertiapin-Q as a probe.

Methods and Results— Dogs were subjected to 7 to 13 days of AT (400 bpm). Coronary-perfused LA preparations were studied intact or subjected to cardiomyocyte isolation. I KH was recorded with patch-clamp methods. AT pacing increased time-dependent hyperpolarization-activated current ( I KH ) at −110 mV from −1.8±0.3 (control) to −3.4±0.5 pA/pF (AT) and the 100-nmol/L tertiapin-sensitive component from −1.5±0.4 (control) to −3.3±0.6 pA/pF (AT). Prolonged atrial tachyarrhythmias could be induced with single extrastimuli in AT-remodeled, but not control, preparations, reflecting the atrial fibrillation–promoting effects of AT remodeling. In AT-remodeled preparations, tachyarrhythmia duration averaged 11.0±5.2 seconds, with a cycle length of 108±6 ms. Tertiapin-Q decreased tachyarrhythmia duration (to 0.6±0.1 second; P <0.001) and increased tachyarrhythmia cycle length (to 175±10 ms; P <0.001). Atrial action potential duration (APD) was increased 65±6% by tertiapin in AT-remodeled hearts versus 19±2% ( P <0.001) in control. In 2 AT-remodeled preparations, tachyarrhythmia lasted uninterrupted for >20 minutes; tertiapin-Q slowed and then terminated arrhythmia in both. Tertiapin had no effect on left ventricular cardiomyocyte currents or APD.

Conclusions— AT remodeling increases I KH , and a highly selective Kir3 current antagonist, tertiapin-Q, increases APD and suppresses atrial tachyarrhythmias in AT-remodeled preparations without affecting ventricular electrophysiology. Constitutive acetylcholine-related K + current contributes to AT-remodeling effects in dogs and is a potentially interesting antiarrhythmic target.

Potassium Channel Subunit Remodeling in Rabbits Exposed to Long-Term Bradycardia or Tachycardia

Background— Sustained heart rate abnormalities produce electrical remodeling and susceptibility to arrhythmia. Uncontrolled tachycardia produces heart failure and ventricular tachyarrhythmia susceptibility, whereas bradycardia promotes spontaneous torsade de pointes (TdP). This study compared arrhythmic phenotypes and molecular electrophysiological remodeling produced by tachycardia versus bradycardia in rabbits.

Methods and Results— We evaluated mRNA and protein expression of subunits underlying rapid ( I Kr ) and slow ( I Ks ) delayed-rectifier and transient-outward K + currents in ventricular tissues from sinus rhythm control rabbits and rabbits with AV block submitted to 3-week ventricular pacing either at 60 to 90 bpm (bradypaced) or at 350 to 370 bpm (tachypaced). QT intervals at matched ventricular pacing rates were longer in bradypaced than tachypaced rabbits (eg, by ≈50% at 60 bpm; P <0.01). KvLQT1 and minK mRNA and protein levels were downregulated in both bradypaced and tachypaced rabbits, whereas ERG was significantly downregulated in bradypaced rabbits only. Kv4.3 and Kv1.4 were downregulated by tachypacing only. Patch-clamp experiments showed that I Ks was reduced in both but I Kr was decreased in bradypaced rabbits only. Continuous monitoring revealed spontaneous TdP in 75% of bradypaced but only isolated ventricular ectopy in tachypaced rabbits. Administration of dofetilide (0.02 mg/kg) to mimic I Kr downregulation produced ultimately lethal TdP in all tachypaced rabbits.

Conclusions— Sustained tachycardia and bradycardia downregulate I Ks subunits, but bradycardia also suppresses ERG/ I Kr , causing prominent repolarization delays and spontaneous TdP. Susceptibility of tachycardia/heart failure rabbits to malignant tachyarrhythmias is induced by exposure to I Kr blockers. These results point to a crucial role for delayed-rectifier subunit remodeling in TdP susceptibility associated with rate-related cardiac remodeling.

Controversies in atrial fibrillation

Atrial fibrillation is the most common sustained cardiac arrhythmia, and contributes greatly to cardiovascular morbidity and mortality. Many aspects of the management of atrial fibrillation remain controversial. We address nine specific controversies in atrial fibrillation management, briefly focusing on the relations between mechanisms and therapy, the roles of rhythm and rate control, the definition of optimum rate control, the need for early cardioversion to prevent remodelling, the comparison of electrical with pharmacological cardioversion, the selection of patients for long-term oral anticoagulation, the roles of novel long-term anticoagulation approaches and ablation therapy, and the potential usefulness of upstream therapy targeting substrate development. The background of every controversy is reviewed and our opinions expressed. Here, we hope to inform physicians about the most important controversies in this specialty and stimulate investigators to address unresolved issues.

Involvement of lipid rafts and caveolae in cardiac ion channel function

A variety of lipid microdomains, including caveolae, have been shown to play an important role in both protein targetting and in controlling protein-protein interactions. There is increasing evidence for significant ion channel localization in lipid rafts. Cardiac channel subunits known to localize in lipid rafts include Kv1.4, Kv1.5, Kv2.1, Kv4, Kir2, Kir3, K(ATP), Nav and Cav subunits. This article reviews what is known about the occurrence and functional significance of cardiac ion channel/lipid raft interactions. Much remains to be learned about this area of potentially enormous importance to cardiac function in health and disease.

Prednisone prevents atrial fibrillation promotion by atrial tachycardia remodeling in dogs

BACKGROUND

There is evidence suggesting involvement of oxidative stress, inflammation, and calcineurin/nuclear factor of activated T cell pathways in atrial fibrillation. This study evaluated the efficacy of anti-inflammatory and calcineurin-inhibitory drugs on promotion of atrial fibrillation by atrial tachycardia-induced remodeling in dogs.

METHODS AND RESULTS

Dogs were subjected to atrial tachypacing at 400 bpm in the absence and presence of treatment with prednisone (15 or 50 mg/day) or ibuprofen (anti-inflammatory) or cyclosporine-A (calcineurin inhibitor). Serial closed-chest electrophysiological studies were performed in each dog at baseline and 2, 4, and 7 days after tachypacing onset. A final open-chest study was performed on day 8. Serum C-reactive protein was measured by ELISA and nitric oxide synthase by Western blotting. The mean duration of induced atrial fibrillation was markedly increased by tachypacing alone, from 26+/-8 to 962+/-317 s (p<0.01), and the atrial effective refractory period was decreased from 117+/-4 to 73+/-7 ms (p<0.001; cycle-length 300 ms). Tachypacing-induced effective refractory period shortening and atrial fibrillation promotion were unaffected by ibuprofen or cyclosporine-A; however, both doses of prednisone suppressed tachypacing-remodeling effects (atrial fibrillation duration to 96+/-60 s and 28+/-11 s at higher and lower doses, respectively; effective refractory period to 101+/-6 ms for higher-dose and 105+/-3 ms for lower-dose group). In addition, prednisone (but not ibuprofen or cyclosporine) significantly decreased C-reactive protein concentrations and attenuated the increase in endothelial nitric oxide synthase expression caused by atrial tachypacing.

CONCLUSIONS

Prednisone prevents the electrophysiological and atrial fibrillation-promoting effects of atrial tachycardia-remodeling, possibly by an anti-inflammatory action, whereas the less potent anti-inflammatory ibuprofen and the calcineurin inhibitor cyclosporine-A are without effect.

Beta-blockers as antiarrhythmic agents

Drugs that suppress beta-adrenergic signaling by competitively inhibiting agonist binding to beta-adrenergic receptors ("beta-blockers") have important antiarrhythmic properties. They differ from most other antiarrhythmic agents by not directly modifying ion channel function; rather, they prevent the arrhythmia-promoting actions of beta-adrenergic stimulation. beta-Blockers are particularly useful in preventing sudden death due to ventricular tachyarrhythmias associated with acute myocardial ischemia, congenital long QT syndrome, and congestive heart failure. They are also quite valuable in controlling the ventricular rate in patients with atrial fibrillation. This chapter reviews the properties of beta-adrenoceptor signaling, the basic mechanisms of cardiac arrhythmias on which beta-blockers act, the ion channel mediators of beta-adrenergic responses, the evidence for clinical antiarrhythmic indications for beta-blocker therapy and the specific pharmacodynamic and pharmacokinetic properties of beta-blockers that differentiate the various agents of this class.

Role of angiotensin system and effects of its inhibition in atrial fibrillation: clinical and experimental evidence

Atrial fibrillation (AF) is a common arrhythmia that is difficult to treat. Anti-arrhythmic drug therapy, to maintain sinus-rhythm, is limited by inadequate efficacy and potentially serious adverse effects. There is increasing interest in novel therapeutic approaches that target AF-substrate development. Recent trials suggest that angiotensin converting-enzyme (ACE)-inhibitors and angiotensin-receptor blockers (ARBs) may be useful, particularly in patients with left ventricular hypertrophy or failure. The clinical potential and mechanisms of this approach are under active investigation. Angiotensin-II is involved in remodelling and may have direct electrophysiological actions. Experimental studies show protection from atrial structural and possibly electrical remodelling with ACE-inhibitors and ARBs, as well as potential effects on cardiac ion-channels. This article reviews information pertaining to the clinical use and mechanism of action of ACE-inhibitors and ARBs in AF. A lack of prospective randomized double-blind trials data limits their application in AF patients without another indication for their use, but studies under way may alter this in the near future. This exciting field of investigation may lead to significant improvements in therapeutic options for AF patients.

Functional expression of Kir2.x in human aortic endothelial cells: the dominant role of Kir2.2

Inward rectifier K+channels (Kir) are a significant determinant of endothelial cell (EC) membrane potential, which plays an important role in endothelium-dependent vasodilatation. In the present study, several complementary strategies were applied to determine the Kir2 subunit composition of human aortic endothelial cells (HAECs). Expression levels of Kir2.1, Kir2.2, and Kir2.4 mRNA were similar, whereas Kir2.3 mRNA expression was significantly weaker. Western blot analysis showed clear Kir2.1 and Kir2.2 protein expression, but Kir2.3 protein was undetectable. Functional analysis of endothelial inward rectifier K+current ( IK) demonstrated that 1) IKcurrent sensitivity to Ba2+and pH were consistent with currents determined using Kir2.1 and Kir2.2 but not Kir2.3 and Kir2.4, and 2) unitary conductance distributions showed two prominent peaks corresponding to known unitary conductances of Kir2.1 and Kir2.2 channels with a ratio of ∼4:6. When HAECs were transfected with dominant-negative (dn)Kir2.x mutants, endogenous current was reduced ∼50% by dnKir2.1 and ∼85% by dnKir2.2, whereas no significant effect was observed with dnKir2.3 or dnKir2.4. These studies suggest that Kir2.2 and Kir2.1 are primary determinants of endogenous K+conductance in HAECs under resting conditions and that Kir2.2 provides the dominant conductance in these cells.

Gene expression profile associated with chronic atrial fibrillation and underlying valvular heart disease in man

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia in humans. The pathophysiology of AF involves electrical, structural and contractile remodeling, which is associated with changes in cardiac gene expression. Previous studies of gene-expression changes in clinical AF have mostly been limited to a small number of candidate genes and have not all been well controlled for underlying heart disease. The present study assessed AF-related gene-expression changes in valve-disease patients with microarrays representing the cardiac transcriptome. Right atrial appendages from 11 patients with chronic AF and underlying valvular heart disease (AF-VHD) and seven patients in sinus rhythm with VHD (SR-VHD) were individually compared to an age-matched sinus-rhythm control group (SR-CTRL, 11 patients) using cardiac-specific microarray analysis. One-class statistical analysis was used to identify genes differentially expressed between SR-VHD and SR-CTRL patients. Two-class statistical analysis was used to identify genes differentially expressed between AF-VHD and SR-VHD patients. Out of 3863 analyzed genes, 832 genes were differentially expressed between SR-VHD and SR-CTRL patients, and 169 genes were differentially expressed between AF-VHD and SR-VHD patients. Striking AF-related changes included altered expression of nine genes pointing towards the development of fibrosis (e.g. upregulation of transforming growth factor beta1), and changes in eight genes potentially related to an increased risk of thromboembolic events (e.g. upregulation of alpha2 macroglobulin). Microarray results were confirmed by quantitative PCR. Our results suggest that AF produces a characteristic profile of gene-expression changes that may be related to the pathophysiology of the arrhythmia.

The pathology of atrial fibrillation in dogs

Atrial fibrillation (AF) occurs in dogs in a number of settings, the most common of which is congestive heart failure. This paper reviews what is known about the pathology of AF in dogs, as well as its clinical relevance. We begin by discussing several experimental AF paradigms in dogs, the associated pathology, and its relevance to AF mechanisms. We then discuss clinical AF in dogs and its relationship to experimental pathology. Finally, we conclude by assessing the potential therapeutic relevance of understanding AF-related pathology in dogs, as well as its potential to have practical applications in the future.

Atrial fibrillation: basic and clinical research at the Montreal Heart Institute

The present article reviews pertinent contributions from the Montreal Heart Institute, Montreal, Quebec, to the understanding of the mechanisms and treatment of atrial fibrillation. The article discusses the usefulness of anticoagulant therapy, antiarrhythmic drug therapy for sinus rhythm maintenance, the electrophysiological basis of atrial fibrillation and the investigation of new energy sources for catheter ablation. Future directions at the Montreal Heart Institute are also briefly addressed.

Wave block formation in homogeneous excitable media following premature excitations: dependence on restitution relations

Spiral wave formation and disorganized activity in excitable media require the existence of broken waves and are related to partial wave block. The determinants of wave block in excitable systems are incompletely understood, especially for cardiac excitable tissue. Previous work in one-dimensional cardiac models has suggested that wave break of a premature excitation (PE) requires critical timing and that the conditions for broken waves are improbable. We analyzed the mechanism of unidirectional wave block that occurs when two consecutive PEs interact with a normal plane wave in a generic one-dimensional spatial excitable medium. A nondimensional coupled-map model built from mesoscopic characteristics of the substrate (the velocity and action potential duration restitution functions) shows that block can occur over a large interval of timing between the two PEs and leads to wave break in two-dimensional media. This mechanism may be an important determinant of spiral wave formation by the response to premature excitations.

Substrate size as a determinant of fibrillatory activity maintenance in a mathematical model of canine atrium

Tissue size has been considered an important determinant of atrial fibrillation (AF), but recent work has questioned the critical size hypothesis. Here, we use a previously developed mathematical model of the two-dimensional canine atrium with realistic action potential, ionic, and conduction properties to address substrate size effects on the maintenance of fibrillatory activity. Cholinergic AF was simulated at different acetylcholine (ACh) concentrations ([ACh]) and distributions, with substrate area varied 11.1-fold. Automated phase singularity detection was used to facilitate the analysis of arrhythmic activity. The duration of activity induced by a single extrastimulus increased with increasing substrate dimensions. Two general mechanisms underlying activity were observed and were differentially affected by substrate size. For large mean [ACh], single primary rotors anchored in low-[ACh] zones maintained activity and substrate dimensions were not critical. At lower mean [ACh], extensive spiral wave meander prevented the emergence of single stable rotors. Prolonged activity was favored when substrate size permitted a sufficiently large number of simultaneous longer-lasting rotors that extinction of all was unlikely. Thus either single dominant rotor or multiple reentrant spiral generator mechanisms could maintain fibrillatory activity in this model and were differentially dependent on substrate size. These results speak to recent debates about the role in AF of single driver rotors versus multiple reentrant circuit mechanisms by suggesting that either may maintain fibrillatory atrial activity depending on atrial size and electrophysiological properties.

Human atrial ion channel and transporter subunit gene-expression remodeling associated with valvular heart disease and atrial fibrillation

BACKGROUND

Valvular heart disease (VHD), which often leads to atrial fibrillation (AF), and AF both cause ion-channel remodeling. We evaluated the ion-channel gene expression profile of VHD patients, in permanent AF (AF-VHD) or in sinus rhythm (SR-VHD), in comparison with patients without AF or VHD, respectively.

METHODS AND RESULTS

We used microarrays containing probes for human ion-channel and Ca2+-regulator genes to quantify mRNA expression in atrial tissues from 7 SR-VHD patients and 11 AF-VHD patients relative to 11 control patients in SR without structural heart disease (SR-CAD). From the data set, we selected for detailed analysis 59 transcripts expressed in the human heart. SR-VHD patients differentially expressed 24/59 ion-channel and Ca2+-regulator transcripts. There was significant overlap between VHD groups, with 66% of genes altered in SR-VHD patients being similarly modified in AF-VHD. Statistical differences between the AF- and SR-VHD groups identified the specific molecular portrait of AF, which involved 12 genes that were further confirmed by real-time reverse transcription-polymerase chain reaction. For example, phospholamban, the beta-subunit MinK (KCNE1) and MIRP2 (KCNE3), and the 2-pore potassium channel TWIK-1 were upregulated in AF-VHD compared with SR-VHD, whereas the T-type calcium-channel Cav3.1 and the transient-outward potassium channel Kv4.3 were downregulated. Two-way hierarchical clustering separated SR-VHD from AF-VHD patients. AF-related changes in L-type Ca2+-current and inward-rectifier current were confirmed at protein and functional levels. Finally, for 13 selected genes, SR restoration reversed ion-channel remodeling.

CONCLUSIONS

VHD extensively remodels cardiac ion-channel and transporter expression, and AF alters ion-channel expression in VHD patients.

Mechanisms of Atrial Fibrillation: Lessons From Animal Models

Studies in animal models have provided extremely important insights about atrial fibrillation (AF). The classic mechanisms that still form the framework for our understanding of AF (focal activity, single-circuit or "mother wave" reentry, and multiple circuit reentry) were established based on animal studies almost 100 years ago. The past 10 years have witnessed a tremendous acceleration of animal work in this area, including the development of a range of AF models in clinically relevant pathological substrates (eg, atrial tachycardia remodeling, congestive heart failure, pericarditis, ischemic heart disease, mitral valve disease, volume overload states, respiratory failure) and the establishment of an increasing number of genetically defined transgenic mouse models. This article reviews the contribution of animal models to our knowledge about AF mechanisms and to clinical management, dealing with such issues as the theory of reentry; the specific applications of various animal models and their contribution to our understanding of electrophysiologic, ionic, and molecular mechanisms; the role of the autonomic nervous system and regional factors; and the development of novel therapeutic approaches. The complementary nature of animal research and clinical investigation is emphasized and the clinical relevance of findings in experimental models is highlighted.