Effects of experimental heart failure on atrial cellular and ionic electrophysiology

Atrial fibrillation and heart failure in the elderly

Atrial fibrillation (AF) is a common clinical problem in elderly patients and especially in those with heart failure (HF). It is a major risk factor for serious cardiovascular events, such as stroke, HF and premature death. Both the prevalence and incidence of AF increase with age and its prevalence in the United States are estimated at more than 2.2 million, with nearly 75% of patients aged >65 years. Aging-related atrial remodeling with fibrosis, dilation and mitochondrial DNA mutations predispose elderly patients to AF. Current management options for AF, including rate control and anticoagulation therapy, can be successfully applied to the elderly population. New antiarrhythmic and anticoagulation medications such as dronedarone and dabigatran, respectively, can impact the approach to therapy in the elderly. Non-pharmacological options such as catheter-based ablation have also gained prominence and have been incorporated into the guidelines for management of AF. However, more trials in the elderly and very elderly segments are needed to clarify the safety and long-term efficacy of the new treatment options.

Arrhythmogenic remodelling of activation and repolarization in the failing human heart

Heart failure is a major cause of disability and death worldwide, and approximately half of heart failure-related deaths are sudden and presumably due to ventricular arrhythmias. Patients with heart failure have been shown to be at 6- to 9-fold increased risk of sudden cardiac death compared to the general population. (AHA. Heart Disease and Stroke Statistics-2003 Update. Heart and Stroke Facts. Dallas, TX: American Heart Association; 2002) Thus, electrophysiological remodelling associated with heart failure is a leading cause of disease mortality and has been a major investigational focus examined using many animal models of heart failure. While these studies have provided an important foundation for understanding the arrhythmogenic pathophysiology of heart failure, the need for corroborating studies conducted on human heart tissue has been increasingly recognized. Many human heart studies of conduction and repolarization remodelling have now been published and shed some light on important, potentially arrhythmogenic, changes in human heart failure. These studies are being conducted at multiple experimental scales from isolated cells to whole-tissue preparations and have provided insight into regulatory mechanisms such as decreased protein expression, alternative mRNA splicing of ion channel genes, and defective cellular trafficking. Further investigations of heart failure in the human myocardium will be essential for determining possible therapeutic targets to prevent arrhythmia in heart failure and for facilitating the translation of basic research findings to the clinical realm.

Chronic myocardial infarction promotes atrial action potential alternans, afterdepolarizations, and fibrillation

Aims

Atrial fibrillation (AF) is increased in patients with heart failure resulting from myocardial infarction (MI). We aimed to determine the effects of chronic ventricular MI in rabbits on the susceptibility to AF, and underlying atrial electrophysiological and Ca²⁺-handling mechanisms.

Methods and results

In Langendorff-perfused rabbit hearts, under β-adrenergic stimulation with isoproterenol (ISO; 1 µM), 8 weeks MI decreased AF threshold, indicating increased AF susceptibility. This was associated with increased atrial action potential duration (APD)-alternans at 90% repolarization, by 147%, and no significant change in the mean APD or atrial global conduction velocity (CV; n = 6–13 non-MI hearts, 5–12 MI). In atrial isolated myocytes, also under β-stimulation, L-type Ca²⁺ current (I_CaL) density and intracellular Ca²⁺-transient amplitude were decreased by MI, by 35 and 41%, respectively, and the frequency of spontaneous depolarizations (SDs) was substantially increased. MI increased atrial myocyte size and capacity, and markedly decreased transverse-tubule density. In non-MI hearts perfused with ISO, the I_CaL-blocker nifedipine, at a concentration (0.02 µM) causing an equivalent I_CaL reduction (35%) to that from the MI, did not affect AF susceptibility, and decreased APD.

Conclusion

Chronic MI in rabbits remodels atrial structure, electrophysiology, and intracellular Ca²⁺ handling. Increased susceptibility to AF by MI, under β-adrenergic stimulation, may result from associated production of atrial APD alternans and SDs, since steady-state APD and global CV were unchanged under these conditions, and may be unrelated to the associated reduction in whole-cell I_CaL. Future studies may clarify potential contributions of local conduction changes, and cellular and subcellular mechanisms of alternans, to the increased AF susceptibility.

Antiarrhythmic effects of the highly selective late sodium channel current blocker GS-458967

BACKGROUND

Previous studies have shown that late sodium channel current (INa) blockers such as ranolazine can exert antiarrhythmic effects by suppressing early and delayed afterdepolarization (EAD and DAD)-induced triggered activity.

OBJECTIVE

To evaluate the electrophysiological properties of GS-458967 (GS967), a potent and highly selective late INa blocker, in canine Purkinje fibers (PFs) and pulmonary vein (PV) and superior vena cava (SVC) sleeve preparations.

METHODS

Transmembrane action potentials were recorded from canine PFs and PV and SVC sleeve preparations by using standard microelectrode techniques. The rapidly activating delayed rectifier potassium channel current blocker E-4031 (2.5-5 µM) and the late INa agonist ATX-II (10 nM) were used to induce EADs in PFs. Isoproterenol (1 µM), high calcium ([Ca(2+)]o = 5.4 mM), or their combination was used to induce DADs and triggered activity.

RESULTS

In PFs, GS967 (10-300 nM) caused a significant concentration-dependent reduction in action potential duration without altering the maximum rate of rise of the action potential upstroke, action potential amplitude, or resting membrane potential at any rate studied (basic cycle lengths of 1000, 500, and 300 ms) or concentration evaluated (n = 5; P < .05). GS967 (30-100 nM) abolished EADs and EAD-induced triggered activity elicited in PFs by exposure to E-4031 (n = 4) or ATX-II (n = 4). In addition, GS967 reduced or abolished DADs and suppressed DAD-induced triggered activity elicited in PFs (n = 4) and PV (n = 4) and SVC (n = 3) sleeve preparations by exposure to isoproterenol, high calcium, or their combination.

CONCLUSIONS

Our data suggest that the selective inhibition of late INa with GS967 can exert antiarrhythmic effects by suppressing EAD- and DAD-mediated extrasystolic activity in PFs and PV and SVC sleeve preparations.

Cardiac ion channels and mechanisms for protection against atrial fibrillation

Atrial fibrillation (AF) is recognised as the most common sustained cardiac arrhythmia in clinical practice. Ongoing drug development is aiming at obtaining atrial specific effects in order to prevent pro-arrhythmic, devastating ventricular effects. In principle, this is possible due to a different ion channel composition in the atria and ventricles. The present text will review the aetiology of arrhythmias with focus on AF and include a description of cardiac ion channels. Channels that constitute potentially atria-selective targets will be described in details. Specific focus is addressed to the recent discovery that Ca(2+)-activated small conductance K(+) channels (SK channels) are important for the repolarisation of atrial action potentials. Finally, an overview of current pharmacological treatment of AF is included.

Atrial Fibrillation in Patients with Ischemic and Non-Ischemic Left Ventricular Dysfunction

Atrial fibrillation (AF) and left ventricular dysfunction (LVD) are increasingly common clinical problems, affecting millions of people worldwide. It is well established that the presence of AF portends a poor prognosis in the setting of both ischemic and non-ischemic LVD, and frequently results in worsening clinical status. Many clinical studies and trials have attempted to address treatment options and efficacy; despite this treatment for AF in LVD is still controversial.

An altered expression of genes involved in the regulation of ion channels in atrial myocytes is correlated with the risk of atrial fibrillation in patients with heart failure

The aim of this study was to investigate the correlation between the altered expression of genes involved in the regulation of ion channels in atrial myocytes and the risk of atrial fibrillation (AF) in patients with heart failure (HF). Right atrial appendages were obtained from 18 HF patients and 18 patients with normal cardiac functions who had undergone surgery. The mRNA expression levels of Kv4.3α, KvLQT1, Kv1.5, L-Caα_1c and NCX were measured by reverse transcription-PCR (RT-PCR). Protein expression levels were also detected by western blotting. In comparison with the control group exhibiting normal cardiac functions, the mRNA and protein expression levels of Kv4.3α, KvLQT1 and L-Caα_1c were significantly reduced in HF patients. By contrast, the mRNA and protein expression levels of NCX were significantly increased in HF patients compared with the control group (P<0.01). The mRNA expression levels of Kv1.5 were not evidently altered. We demonstrated that increased levels of Kv4.3α, KvLQT1 and L-Caα_1c and decreased levels of NCX are correlated with the risk of AF in HF patients. Changes in the gene expression of ion channel-related proteins may therefore be used as biological markers of AF occurring in HF patients in future studies.

Role of late sodium channel current block in the management of atrial fibrillation

The anti-arrhythmic efficacy of the late sodium channel current (late I(Na)) inhibition has been convincingly demonstrated in the ventricles, particularly under conditions of prolonged ventricular repolarization. The value of late I(Na) block in the setting of atrial fibrillation (AF) remains poorly investigated. All sodium channel blockers inhibit both peak and late I(Na) and are generally more potent in inhibiting late vs. early I(Na). Selective late I(Na) block does not prolong the effective refractory period (ERP), a feature common to practically all anti-AF agents. Although the late I(Na) blocker ranolazine has been shown to be effective in suppression of AF, it is noteworthy that at concentrations at which it blocks late I(Na) in the ventricles, it also potently blocks peak I(Na) in the atria, thus causing rate-dependent prolongation of ERP due to development of post-repolarization refractoriness. Late I(Na) inhibition in atria is thought to suppress intracellular calcium (Ca(i))-mediated triggered activity, secondary to a reduction in intracellular sodium (Na(i)). However, agents that block late I(Na) (ranolazine, amiodarone, vernakalant, etc) are also potent atrial-selective peak I(Na) blockers, so that the reduction of Na(i) loading in atrial cells by these agents can be in large part due to the block of peak I(Na). The impact of late I(Na) inhibition is reduced by the abbreviation of the action potential that occurs in AF patients secondary to electrical remodeling. It stands to reason that selective late I(Na) block may contribute more to inhibition of Ca(i)-mediated triggered activity responsible for initiation of AF in clinical pathologies associated with a prolonged atrial APD (such as long QT syndrome). Additional studies are clearly needed to test this hypothesis.

Chronic potentiation of cardiac L-type Ca(2+) channels by pirfenidone

AIMS

On the basis of its ability to inhibit fibrosis, pirfenidone has drawn the attention as an intriguing candidate for treating cardiac disease. However, its precise electrophysiological effects have yet to be elucidated. Here, we have investigated its potential to modulate ion channels.

METHODS AND RESULTS

Adult rat cardiac myocytes were investigated using whole-cell patch-clamp, western-blot and qRT-PCR techniques. Pirfenidone increased the density of L-type Ca(2+) current (I(CaL,) 50-100%), without significantly altering Na(+), K(+), or T-type Ca(2+) currents. The effect was dose-dependent, with an EC(50) of 2.8 µM. Its onset was slow, with a lag period larger than 1 h and time to maximum of 24-48 h. Concomitant changes were observed in the voltage-dependent activation of I(CaL) (-5 mV shift in both V(1/2) and k). In contrast, the following properties of I(CaL) remained normal: steady-state inactivation, Ca(V)1.2 levels (mRNA and protein), and intramembrane charge movement. Indeed, the conductance-to-charge ratio, or G(max)/Q(max), was increased by 80%. The effect on I(CaL) was mimicked by an inhibitor of nitric oxide (NO) synthase (NOS), and attenuated by both cyclic adenosine monophosphate (cAMP) and cAMP-dependent protein kinase (PKA) inhibitors. Conversely, cytokines, reactive oxygen species, and Ca(2+) were all ruled out as possible intermediaries. Additional experiments suggest that pirfenidone increases action potential duration by ∼50%.

CONCLUSION

Pirfenidone augments I(CaL), not through higher expression of L-type channels, but through promoting their Ca(2+)-conducting activity. A possible inhibition of NOS expression is likely involved, with subsequent reduced NO production and stimulated cAMP/PKA signalling. These findings may be relevant to the cardioprotective effect of pirfenidone.

Modeling atrial arrhythmias: impact on clinical diagnosis and therapies

Atrial arrhythmias are the most frequent sustained rhythm disorders in humans and often lead to severe complications such as heart failure and stroke. Despite the important insights provided by animal models into the mechanisms of atrial arrhythmias, direct translation of experimental findings to new therapies in patients has not been straightforward. With the advances in computer technology, large-scale electroanatomical computer models of the atria that integrate information from the molecular to organ scale have reached a level of sophistication that they can be used to interpret the outcome of experimental and clinical studies and aid in the rational design of therapies. This paper reviews the state-of-the-art of computer models of the electrical dynamics of the atria and discusses the evolving role of simulation in assisting the clinical diagnosis and treatment of atrial arrhythmias.

Heart failure epicardial fat increases atrial arrhythmogenesis

BACKGROUND

Obesity is an important risk factor for atrial fibrillation (AF) and heart failure (HF). The effects of epicardial fat on atrial electrophysiology were not clear. This study was to evaluate whether HF may modulate the effects of epicardial fat on atrial electrophysiology.

METHODS

Conventional microelectrodes recording was used to record the action potential in left (LA) and right (RA) atria of healthy (control) rabbits before and after application of epicardial fat from control or HF (ventricular pacing of 360-400 bpm for 4 weeks) rabbits. Adipokine profiles were checked in epicardial fat of control and HF rabbits.

RESULTS

The LA 90% of AP duration was prolonged by control epicardial fat (from 77 ± 6 to 87 ± 7 ms, p<0.05, n=7), and by HF epicardial fat (from 78 ± 3 to 98 ± 4 ms, p<0.001, n=9). However, control or HF epicardial fat did not change the AP morphology in RA. HF epicardial fat increased the contractility in LA (61 ± 11 vs. 35 ± 6 mg, p=0.001), but not in RA. Control fat did not change the LA or RA contractility. Moreover, control and HF epicardial fat induced early and delayed afterdepolarizations in LA and RA, but only HF epicardial fat provoked spontaneous activity and burst firing in LA (n=3/9, 33.3% vs. n=0/7, 0%, n=0/9, 0%, p<0.05). Compared to control fat, HF epicardial fat, had lower resistin, C-reactive protein and serum amyloid A, but similar interleukin-6, leptin, monocyte chemotactic protein-1, adiponectin and adipsin.

CONCLUSIONS

HF epicardial fat increases atrial arrhythmogenesis, which may contribute to the higher atrial arrhythmia in obesity.

Rate-dependent effects of vernakalant in the isolated non-remodeled canine left atria are primarily due to block of the sodium channel: comparison with ranolazine and dl-sotalol

BACKGROUND

Several clinical trials have shown that vernakalant is effective in terminating recent onset atrial fibrillation (AF). The electrophysiological actions of vernakalant are not fully understood.

METHODS AND RESULTS

Here we report the results of a blinded study comparing the in vitro canine atrial electrophysiological effects of vernakalant, ranolazine, and dl-sotalol. Action potential durations (APD(50,75,90)), effective refractory period (ERP), post repolarization refractoriness (PRR), maximum rate of rise of the action potential (AP) upstroke (V(max)), diastolic threshold of excitation (DTE), conduction time (CT), and the shortest S(1)-S(1) permitting 1:1 activation (S(1)-S(1)) were measured using standard stimulation and microelectrode recording techniques in isolated normal, non-remodeled canine arterially perfused left atrial preparations. Vernakalant caused variable but slight prolongation of APD(90) (P=not significant), but significant prolongation of APD(50) at 30 μmol/L and rapid rates. In contrast, ranolazine and dl-sotalol produced consistent concentration- and reverse rate-dependent prolongation of APD(90). Vernakalant and ranolazine caused rate-dependent, whereas dl-sotalol caused reverse rate-dependent, prolongation of ERP. Significant rate-dependent PRR developed with vernakalant and ranolazine, but not with dl-sotalol. Other sodium channel-mediated parameters (ie, V(max), CT, DTE, and S(1)-S(1)) also were depressed significantly by vernakalant and ranolazine, but not by dl-sotalol. Only vernakalant elevated AP plateau voltage, consistent with blockade of ultrarapid delayed rectified potassium current and transient outward potassium current.

CONCLUSIONS

In isolated canine left atria, the effects of vernakalant and ranolazine were characterized by use-dependent inhibition of sodium channel-mediated parameters, and those of dl-sotalol by reverse rate-dependent prolongation of APD(90) and ERP. This suggests that during the rapid activation rates of AF, the I(Na) blocking action of the mixed ion channel blocker vernakalant takes prominence. This mechanism may explain vernakalant's anti-AF efficacy.

Heart failure enhances arrhythmogenesis in pulmonary veins

1. Heart failure (HF) predisposes to atrial fibrillation (AF) as a result of substrate remodelling. The present study aimed to investigate the impact of HF on the electrical remodelling of the pulmonary veins (PV) and left atrium (LA). 2. The electrical activity was recorded in LA and PV from control rabbits and rabbits with rapid ventricular pacing-induced HF, using a multi-electrode array system and conventional microelectrodes. 3. Compared with the control-PV (n = 21), the HF-PV (n = 13) had a higher incidence and frequency of rapid pacing-induced spontaneous activity (85 vs 29%, P = 0.005; 3.5 ± 0.2 vs 1.7 ± 0.1 Hz, P < 0.001) and high-frequency irregular electrical activity (92 vs 38%, P = 0.01; 23 ± 1 vs 19 ± 1 Hz, P = 0.003), greater depolarized resting membrane potential (-59 ± 1 vs -70 ± 2 mV, P < 0.001), higher incidence of early afterdepolarizations (EAD; 69 vs 6%, P = 0.001) and delayed afterdepolarizations (DAD; 92 vs 25%, P = 0.001), and slower conduction velocity (38 ± 2 vs 63 ± 2 cm/s, P < 0.05). In comparison to the HF-LA, the HF-PV had a higher incidence of spontaneous activity and high-frequency irregular electrical activity (85 vs 39%, P = 0.04; 92 vs 46%, P = 0.03), and higher incidence of EAD and DAD, and those differences were not found between the control-LA and control-PV. The control-PV with high-frequency irregular electrical activity had a higher incidence of DAD and spontaneous activity as compared with those without it. 4. HF contributed to an increased automaticity, triggered activity and conduction disturbance in the PV. The PV possessed more arrhythmogenic properties, which might play an important role in the genesis of AF in HF.

The role of fibroblasts in complex fractionated electrograms during persistent/permanent atrial fibrillation: implications for electrogram-based catheter ablation

RATIONALE

Electrogram-based catheter ablation, targeting complex fractionated atrial electrograms (CFAEs), is empirically known to be effective in halting persistent/permanent atrial fibrillation (AF). However, the mechanisms underlying CFAEs and electrogram-based ablation remain unclear.

OBJECTIVE

Because atrial fibrosis is associated with persistent/permanent AF, we hypothesized that electrotonic interactions between atrial myocytes and fibroblasts play an important role in CFAE genesis and electrogram-based catheter ablation.

METHODS AND RESULTS

We used a human atrial tissue model in heart failure and simulated propagation and spiral wave reentry with and without regionally proliferated fibroblasts. Coupling of fibroblasts to atrial myocytes resulted in shorter action potential duration, slower conduction velocity, and lower excitability. Consequently, heterogeneous fibroblast proliferation in the myocardial sheet resulted in frequent spiral wave breakups, and the bipolar electrograms recorded at the fibroblast proliferation area exhibited CFAEs. The simulations demonstrated that ablation targeting such fibroblast-derived CFAEs terminated AF, resulting from the ablation site transiently pinning the spiral wave and then pushing it out of the fibroblast proliferation area. CFAEs could not be attributed to collagen accumulation alone.

CONCLUSIONS

Fibroblast proliferation in atria might be responsible for the genesis of CFAEs during persistent/permanent AF. Our findings could contribute to better understanding of the mechanisms underlying CFAE-targeted AF ablation.

Novel pharmacological targets for the rhythm control management of atrial fibrillation

Atrial fibrillation (AF) is a growing clinical problem associated with increased morbidity and mortality. Development of safe and effective pharmacological treatments for AF is one of the greatest unmet medical needs facing our society. In spite of significant progress in non-pharmacological AF treatments (largely due to the use of catheter ablation techniques), anti-arrhythmic agents (AADs) remain first line therapy for rhythm control management of AF for most AF patients. When considering efficacy, safety and tolerability, currently available AADs for rhythm control of AF are less than optimal. Ion channel inhibition remains the principal strategy for termination of AF and prevention of its recurrence. Practical clinical experience indicates that multi-ion channel blockers are generally more optimal for rhythm control of AF compared to ion channel-selective blockers. Recent studies suggest that atrial-selective sodium channel block can lead to safe and effective suppression of AF and that concurrent inhibition of potassium ion channels may potentiate this effect. An important limitation of the ion channel block approach for AF treatment is that non-electrical factors (largely structural remodeling) may importantly determine the generation of AF, so that "upstream therapy", aimed at preventing or reversing structural remodeling, may be required for effective rhythm control management. This review focuses on novel pharmacological targets for the rhythm control management of AF.

Heart failure enhanced pulmonary vein arrhythmogenesis and dysregulated sodium and calcium homeostasis with increased calcium sparks

Late sodium currents and intracellular Ca(2+) (Ca(2+) (i)) dynamics play an important role in arrhythmogenesis of pulmonary vein (PV) and heart failure (HF). It is not clear whether HF enhances PV arrhythmogenesis through modulation of Ca(2+) homeostasis and increased late sodium currents. The aim of this study was to investigate the sodium and calcium homeostasis in PV cardiomyocytes with HF.

METHODS AND RESULTS

Whole-cell patch clamp was used to investigate the action potentials and ionic currents in isolated rabbit single PV cardiomyocytes with and without rapid pacing induced HF. The Ca(2+) (i) dynamics were evaluated through fluorescence and confocal microscopy. As compared to control PV cardiomyocytes (n = 18), HF PV cardiomyocytes (n = 13) had a higher incidence of delayed afterdepolarization (45% vs 13%, P < 0.05) and faster spontaneous activity (3.0 ± 0.2 vs 2.1 ± 0.2 Hz, P < 0.05). HF PV cardiomyocytes had increased late Na(+) currents, Na(+) /Ca(2+) exchanger currents, and transient inward currents, but had decreased Na(+) currents or L-type calcium currents. HF PV cardiomyocytes with pacemaker activity had larger Ca(2+) (i) transients (R410/485, 0.18 ± 0.04 vs 0.11 ± 0.02, P < 0.05), and sarcoplasmic reticulum Ca(2+) stores. Moreover, HF PV cardiomyocytes with pacemaker activity (n = 18) had higher incidence (95% vs 70%, P < 0.05), frequency (7.8 ± 3.1 vs 2.3 ± 1.2 spark/mm/s, P < 0.05), amplitude (F/F(0) , 3.2 ± 0.8 vs 1.9 ± 0.5, P < 0.05), and longer decay time (65 ± 3 vs 48 ± 4 ms, P < 0.05) of Ca(2+) sparks than control PV cardiomyocytes with pacemaker activity (n = 18).

CONCLUSIONS

Dysregulated sodium and calcium homeostasis, and enhanced calcium sparks promote arrhythmogenesis of PV cardiomyocytes in HF, which may play an important role in the development of atrial fibrillation.

20-Hydroxyeicosatetraenoic acid induces apoptosis in neonatal rat cardiomyocytes through mitochondrial-dependent pathways

OBJECTIVE

20-Hydroxyeicosatetraenoic acid (20-HETE), a [omega]-hydroxylation product of arachidonic acid catalyzed by cytochrome P450 4A, may play a role in the cardiovascular system. It is well known that cytochrome P450 [omega]-hydroxylase inhibitors markedly reduced the cardiac ischemia reperfusion injury. However, the direct effect of 20-HETE on cardiomyocytes is still poorly investigated. Here, we studied the effect of 20-HETE on cardiomyocyte apoptosis and the apoptosis-associated signaling pathways.

METHODS AND RESULTS

The cardiomyocyte apoptosis was measured by fluorescein isothiocyanate conjugated annexin V/propidium iodide double staining cytometry, indicating that the percentage of early apoptotic cells increased from 15.6% +/- 2.6% to 25.5% +/- 2.5% in control and 20-HETE-treated cells, respectively. The mitochondrial membrane potential ([DELTA][PSI]m) was measured by detecting the ratio of JC-1 green/red emission intensity. A significant decrease in the ratio was observed after treatment with 20-HETE for 24 hours in comparison with control group, suggesting the disruptive effect of 20-HETE on mitochondrial [DELTA][PSI]m. In addition, 20-HETE stimulated caspase-3 activity and Bax mRNA expression in cardiomyocytes. In contrast, the Bcl-2 mRNA levels were significantly decreased by 20-HETE treatment.

CONCLUSION

These results demonstrate that 20-HETE induces cardiomyocyte apoptosis by activation of several intrinsic apoptotic pathways. The 20-HETE-induced apoptosis could contribute to the cytochrome P450 [omega]-hydroxylase-dependent cardiac injure during cardiac ischemia-reperfusion.

Targeted nonviral gene-based inhibition of Gα(i/o)-mediated vagal signaling in the posterior left atrium decreases vagal-induced atrial fibrillation

BACKGROUND

Pharmacologic and ablative therapies for atrial fibrillation (AF) have suboptimal efficacy. Newer gene-based approaches that target specific mechanisms underlying AF are likely to be more efficacious in treating AF. Parasympathetic signaling appears to be an important contributor to AF substrate.

OBJECTIVE

The purpose of this study was to develop a nonviral gene-based strategy to selectively inhibit vagal signaling in the left atrium and thereby suppress vagal-induced AF.

METHODS

In eight dogs, plasmid DNA vectors (minigenes) expressing Gα(i) C-terminal peptide (Gα(i)ctp) was injected in the posterior left atrium either alone or in combination with minigene expressing Gα(o)ctp, followed by electroporation. In five control dogs, minigene expressing scrambled peptide (Gα(R)ctp) was injected. Vagal- and carbachol-induced left atrial effective refractory periods (ERPs), AF inducibility, and Gα(i/o)ctp expression were assessed 3 days following minigene delivery.

RESULTS

Vagal stimulation- and carbachol-induced effective refractory period shortening and AF inducibility were significantly attenuated in atria receiving a Gα(i2)ctp-expressing minigene and were nearly eliminated in atria receiving both Gα(i2)ctp- and Gα(o1)ctp-expressing minigenes.

CONCLUSION

Inhibition of both G(i) and G(o) proteins is necessary to abrogate vagal-induced AF in the left atrium and can be achieved via constitutive expression of Gα(i/o)ctps expressed by nonviral plasmid vectors delivered to the posterior left atrium.

Recent clinical and experimental advances in atrial fibrillation

Atrial fibrillation (AF) is the most common arrhythmia in clinical settings (Fuster et al., 2001), and it is often associated with congestive heart diseases (Issac et al., 2007). Many studies in both laboratory and clinical settings have sought to analyze the mechanisms of AF, develop treatments based on these mechanisms, and examine atrial remodeling in chronic AF. The aim of this paper is to analyze recent findings regarding the atrial remodeling that occurs in AF. In particular, we will describe the electrical and structural changes that involve atrial myocytes and the extracellular matrix. We will also describe the general classification and basic pathophysiology of AF and its surgical treatments.

Effects of KATP channel openers diazoxide and pinacidil in coronary-perfused atria and ventricles from failing and non-failing human hearts

This study compared the effects of ATP-regulated potassium channel (K(ATP)) openers, diazoxide and pinacidil, on diseased and normal human atria and ventricles. We optically mapped the endocardium of coronary-perfused right (n=11) or left (n=2) posterior atrial-ventricular free wall preparations from human hearts with congestive heart failure (CHF, n=8) and non-failing human hearts without (NF, n=3) or with (INF, n=2) infarction. We also analyzed the mRNA expression of the K(ATP) targets K(ir)6.1, K(ir)6.2, SUR1, and SUR2 in the left atria and ventricles of NF (n=8) and CHF (n=4) hearts. In both CHF and INF hearts, diazoxide significantly decreased action potential durations (APDs) in atria (by -21±3% and -27±13%, p<0.01) and ventricles (by -28±7% and -28±4%, p<0.01). Diazoxide did not change APD (0±5%) in NF atria. Pinacidil significantly decreased APDs in both atria (-46 to -80%, p<0.01) and ventricles (-65 to -93%, p<0.01) in all hearts studied. The effect of pinacidil on APD was significantly higher than that of diazoxide in both atria and ventricles of all groups (p<0.05). During pinacidil perfusion, burst pacing induced flutter/fibrillation in all atrial and ventricular preparations with dominant frequencies of 14.4±6.1 Hz and 17.5±5.1 Hz, respectively. Glibenclamide (10 μM) terminated these arrhythmias and restored APDs to control values. Relative mRNA expression levels of K(ATP) targets were correlated to functional observations. Remodeling in response to CHF and/or previous infarct potentiated diazoxide-induced APD shortening. The activation of atrial and ventricular K(ATP) channels enhances arrhythmogenicity, suggesting that such activation may contribute to reentrant arrhythmias in ischemic hearts.

Electrophysiologic basis for the antiarrhythmic actions of ranolazine

Ranolazine is a Food and Drug Administration-approved antianginal agent. Experimental and clinical studies have shown that ranolazine has antiarrhythmic effects in both ventricles and atria. In the ventricles, ranolazine can suppress arrhythmias associated with acute coronary syndrome, long QT syndrome, heart failure, ischemia, and reperfusion. In atria, ranolazine effectively suppresses atrial tachyarrhythmias and atrial fibrillation (AF). Recent studies have shown that the drug may be effective and safe in suppressing AF when used as a pill-in-the pocket approach, even in patients with structurally compromised hearts, warranting further study. The principal mechanism underlying ranolazine's antiarrhythmic actions is thought to be primarily via inhibition of late I(Na) in the ventricles and via use-dependent inhibition of peak I(Na) and I(Kr) in the atria. Short- and long-term safety of ranolazine has been demonstrated in the clinic, even in patients with structural heart disease. This review summarizes the available data regarding the electrophysiologic actions and antiarrhythmic properties of ranolazine in preclinical and clinical studies.

Autonomic remodeling in the left atrium and pulmonary veins in heart failure: creation of a dynamic substrate for atrial fibrillation

BACKGROUND

Atrial fibrillation (AF) is commonly associated with congestive heart failure (CHF). The autonomic nervous system is involved in the pathogenesis of both AF and CHF. We examined the role of autonomic remodeling in contributing to AF substrate in CHF.

METHODS AND RESULTS

Electrophysiological mapping was performed in the pulmonary veins and left atrium in 38 rapid ventricular-paced dogs (CHF group) and 39 control dogs under the following conditions: vagal stimulation, isoproterenol infusion, β-adrenergic blockade, acetylcholinesterase (AChE) inhibition (physostigmine), parasympathetic blockade, and double autonomic blockade. Explanted atria were examined for nerve density/distribution, muscarinic receptor and β-adrenergic receptor densities, and AChE activity. In CHF dogs, there was an increase in nerve bundle size, parasympathetic fibers/bundle, and density of sympathetic fibrils and cardiac ganglia, all preferentially in the posterior left atrium/pulmonary veins. Sympathetic hyperinnervation was accompanied by increases in β(1)-adrenergic receptor R density and in sympathetic effect on effective refractory periods and activation direction. β-Adrenergic blockade slowed AF dominant frequency. Parasympathetic remodeling was more complex, resulting in increased AChE activity, unchanged muscarinic receptor density, unchanged parasympathetic effect on activation direction and decreased effect of vagal stimulation on effective refractory period (restored by AChE inhibition). Parasympathetic blockade markedly decreased AF duration.

CONCLUSIONS

In this heart failure model, autonomic and electrophysiological remodeling occurs, involving the posterior left atrium and pulmonary veins. Despite synaptic compensation, parasympathetic hyperinnervation contributes significantly to AF maintenance. Parasympathetic and/or sympathetic signaling may be possible therapeutic targets for AF in CHF.

Pathophysiological mechanisms of atrial fibrillation: a translational appraisal

Atrial fibrillation (AF) is an arrhythmia that can occur as the result of numerous different pathophysiological processes in the atria. Some aspects of the morphological and electrophysiological alterations promoting AF have been studied extensively in animal models. Atrial tachycardia or AF itself shortens atrial refractoriness and causes loss of atrial contractility. Aging, neurohumoral activation, and chronic atrial stretch due to structural heart disease activate a variety of signaling pathways leading to histological changes in the atria including myocyte hypertrophy, fibroblast proliferation, and complex alterations of the extracellular matrix including tissue fibrosis. These changes in electrical, contractile, and structural properties of the atria have been called "atrial remodeling." The resulting electrophysiological substrate is characterized by shortening of atrial refractoriness and reentrant wavelength or by local conduction heterogeneities caused by disruption of electrical interconnections between muscle bundles. Under these conditions, ectopic activity originating from the pulmonary veins or other sites is more likely to occur and to trigger longer episodes of AF. Many of these alterations also occur in patients with or at risk for AF, although the direct demonstration of these mechanisms is sometimes challenging. The diversity of etiological factors and electrophysiological mechanisms promoting AF in humans hampers the development of more effective therapy of AF. This review aims to give a translational overview on the biological basis of atrial remodeling and the proarrhythmic mechanisms involved in the fibrillation process. We pay attention to translation of pathophysiological insights gained from in vitro experiments and animal models to patients. Also, suggestions for future research objectives and therapeutical implications are discussed.

Angiotensin II induces complex fractionated electrogram in a cultured atrial myocyte monolayer mediated by calcium and sodium-calcium exchanger

Angiotensin II (AngII) has been implicated in the mechanism of atrial fibrillation (AF). There may be calcium-dependent pro-fibrillatory effect of AngII on atrial myocytes. We used cultured confluent HL-1 atrial myocyte monolayer with spontaneously propagated depolarization to study direct pro-fibrillatory effect of AngII and its molecular mechanism. AngII stimulation induced fibrillatory-like complex electrogram and calcium wave propagation. AngII shortened action potential duration and augmented calcium transient, thus increasing electrochemical gradient of forward-mode sodium-calcium exchanger (NCX) current and induced frequent irregular afterdepolarizations. AngII increased expression of sodium-calcium exchanger (NCX), further increasing calcium-membrane voltage coupling gain. The fibrillatory effect of AngII was attenuated by NCX blocker SEA0400 and NCX siRNA knockdown. AngII increased expression of L-type calcium channel and augmented calcium transient through PKC and CREB. The fibrillatory effect of AngII was also attenuated by PKC inhibitor chelerythrine and dominant negative form of CREB. In conclusions, AngII itself may electrically contribute to the mechanism of AF through increasing NCX expression and augmenting calcium transient, which is PKC and CREB dependent. Specific genetic knockdown of NCX attenuated calcium mediated afterdepolarization and complex electrogram.

Alterations of atrial Ca(2+) handling as cause and consequence of atrial fibrillation

Atrial fibrillation (AF) is the most prevalent sustained arrhythmia. As the most important risk factor for embolic stroke, AF is associated with a high morbidity and mortality. Despite decades of research, successful (pharmacological and interventional) 'ablation' of the arrhythmia remains challenging. AF is characterized by a diverse aetiology, including heart failure, hypertension, and valvular disease. Based on this understanding, new treatment strategies that are specifically tailored to the underlying pathophysiology of a certain 'type' of AF are being developed. One important aspect of AF pathophysiology is altered intracellular Ca(2+) handling. Due to the increase in the atrial activation rate and the subsequent initial [Ca(2+)](i) overload, AF induces 'remodelling' of intracellular Ca(2+) handling. Current research focuses on unravelling the contribution of altered intracellular Ca(2+) handling to different types of AF. More specifically, changes in intracellular Ca(2+) homeostasis preceding the onset of AF, in conditions which predispose to AF (e.g. heart failure), appear to be different from changes in Ca(2+) handling developing after the onset of AF. Here we review and critique altered intracellular Ca(2+) handling and its contribution to three specific aspects of AF pathophysiology, (i) excitation-transcription coupling and Ca(2+)-dependent signalling pathways, (ii) atrial contractile dysfunction, and (iii) arrhythmogenicity.

Inducibility of atrial fibrillation depends not on inflammation but on atrial structural remodeling in rat experimental autoimmune myocarditis

INTRODUCTION

There is increasing evidence to support a link between inflammation and atrial fibrillation (AF). However, the role of inflammation on new-onset AF is still to be elucidated.

METHODS

Rats underwent induction of experimental autoimmune myocarditis (EAM). Atrial structural change was evaluated by echocardiography and histological analysis. Electrophysiological data and the in vivo atrial response to burst atrial pacing were evaluated in the acute (2 weeks after EAM induction) and chronic phases (8 weeks after induction). In addition, atrial pacing after 2, 4, and 6 h after lipopolysaccharide (LPS) infusion, when the expression of gap junctions was modified, were challenged with young healthy rats.

RESULTS

AF was induced in 11 of 15 chronic phase EAM rats but not in either acute phase EAM rats or LPS infusion rats (P<.01). Echocardiography showed dilatation of both atrium and ventricle and a decrease in the ejection fraction in the chronic phase. Histology revealed severe inflammatory lesions only in the acute phase. Interstitial atrial fibrosis as well as the area of atrial myocyte increased in the chronic phase but not in the acute phase.

CONCLUSIONS

AF could be induced in the chronic phase of myocarditis rats, but not in the acute phase of myocarditis rats or in rats with LPS infusion. Acute inflammation per se did not increase the occurrence of AF induction. Atrial structural remodeling caused by inflammation and hemodynamic effects is necessary to induce AF.

Extraction of cardiac rhythm devices: indications, techniques and outcomes for the removal of pacemaker and defibrillator leads

Cardiac rhythm management devices (pacemakers) are being increasingly implanted worldwide not only for symptomatic bradycardia, but also for the management of arrhythmia and heart failure. Their use in more elderly patients with significant comorbidities is rising steeply and consequently long-term complications are increasingly arising. Such an increase in device therapy is being paralleled by an increase in the requirement for system extraction. Safe lead extraction is central to the management of much of the complications related to pacemakers. The most common indication for lead extraction is system infection Adhesions in chronically implanted leads can become major obstacles to safe lead extraction and life-threatening bleeding and cardiac perforations may occur. Over the last 20 years, specific tools and techniques for transvenous lead extraction have been developed to assist in freeing the lead body from the adhesions. This article provides a comprehensive review of the indications, tools, techniques and outcomes for transvenous lead extraction. The success rate largely depends on the time from implant. Up to 12 months from implant, it is rare that traction alone will not suffice. For longer lead implant duration, no single technique is sufficient to address all extractions, but laser provides the best chance of extracting the entire lead. Operator experience is vital in determining success as familiarity of a wide array of techniques will increase the likelihood of uncomplicated extraction. Long implantation time, lack of operator experience, ICD lead type and female gender are risk factors for life-threatening complications. Lead extraction should therefore, ideally be performed in high volume centres with experienced staff and on-site support from a cardiothoracic surgical team able to deal with bleeding complications from cardiovascular perforation.

Mathematical models of canine right and left atria cardiomyocytes

The aim of this study is to build two mathematical models of canine ionic currents specific to right atria and left atria. The canine left atria mathematical model was firstly modified from the Ramirez-Nattel-Courtemanche (RNC) model using the recently available experimental data of ionic currents and was further developed based on our own experimental data. A model of right atria was then built by considering the differences between right atria and left atria. The two developed models well reproduced the experimental data on action potential morphology, the rate dependence, and action potential duration restitution. They are useful for investigating the mechanisms underlying the heterogeneity of canine regional action potentials and would help the simulation of whole heart excitation propagation and cardiac arrhythmia in the near future.

Atrial fibrillation in congestive heart failure

Atrial fibrillation and congestive heart failure are morbid conditions that have common risk factors and frequently coexist. Each condition predisposes to the other, and the concomitant presence of the two identifies individuals at increased risk for mortality. Recent data have emerged that help elucidate the complex genetic and nongenetic pathophysiological mechanisms that contribute to the development of atrial fibrillation in individuals with congestive heart failure. Clinical trial results offer insights into the noninvasive prevention and management of these conditions, although newer technologies, such as catheter ablation for atrial fibrillation, have yet to be studied extensively in patients with congestive heart failure.

Cardiac adrenergic control and atrial fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia, and it causes substantial mortality. The autonomic nervous system, and particularly the adrenergic/cholinergic balance, has a profound influence on the occurrence of AF. Adrenergic stimulation from catecholamines can cause AF in patients. In human atrium, catecholamines can affect each of the electrophysiological mechanisms of AF initiation and/or maintenance. Catecholamines may produce membrane potential oscillations characteristic of afterdepolarisations, by increasing Ca(2+) current, [Ca(2+)](i) and consequent Na(+)-Ca(2+) exchange, and may also enhance automaticity. Catecholamines might affect reentry, by altering excitability or conduction, rather than action potential terminal repolarisation or refractory period. However, which arrhythmia mechanisms predominate is unclear, and likely depends on cardiac pathology and adrenergic tone. Heart failure (HF), a major cause of AF, causes adrenergic activation and adaptational changes, remodelling, of atrial electrophysiology, Ca(2+) homeostasis, and adrenergic responses. Chronic AF also remodels these, but differently to HF. Myocardial infarction and AF cause neural remodelling that also may promote AF. beta-Adrenoceptor antagonists (beta-blockers) are used in the treatment of AF, mainly to control the ventricular rate, by slowing atrioventricular conduction. beta-Blockers also reduce the incidence of AF, particularly in HF or after cardiac surgery, when adrenergic tone is high. Furthermore, the chronic treatment of patients with beta-blockers remodels the atria, with a potentially antiarrhythmic increase in the refractory period. Therefore, the suppression of AF by beta-blocker treatment may involve an attenuation of arrhythmic activity that is caused by increased [Ca(2+)](i), coupled with effects of adaptation to the treatment. An improved understanding of the involvement of the adrenergic system and its control in basic mechanisms of AF under differing cardiac pathologies might lead to better treatments.

Atrial fibrillation in heart failure: a comprehensive review

Chronic heart failure and atrial fibrillation are 2 major disorders that are closely linked. Their coexistence is associated with adverse prognosis. Both share several common predisposing conditions, but their interaction involves complex ultrastructural, electrophysiologic, and neurohormonal processes that go beyond mere sharing of mutual risk factors. Rate control approach remains the standard therapy for atrial fibrillation in heart failure because current strategies at rhythm control have so far failed to positively impact mortality and morbidity. This is largely because of the shortcomings of current pharmacologic anti-arrhythmic agents. Surgical and catheter-based therapies are promising, but long-term data are lacking. The role of non-anti-arrhythmic therapeutic agents also is being explored. Further progress toward improved understanding the complex relationship between atrial fibrillation and heart failure should improve management strategies.

Increased late sodium current in left atrial myocytes of rabbits with left ventricular hypertrophy: its role in the genesis of atrial arrhythmias

Left ventricular hypertrophy (LVH) is frequently associated with clinical atrial arrhythmias, but little is known about how it causes those arrhythmias. Our previous studies have shown that LVH increases the late sodium current (I(Na-L)) that plays an important role in the genesis of ventricular arrhythmias. We hypothesize that LVH may also induce an upregulation of the I(Na-L) in atrial myocytes, leading to atrial electrical abnormalities. The renovascular hypertension model was used to induce LVH in rabbits. Action potential and membrane current recordings were performed in single myocytes. At a pacing cycle length of 2,000 ms, spontaneous phase-2 early afterdepolarizations (EADs) could be recorded from the left atrial myocytes in 10 of 12 LVH rabbits, whereas no EADs could be elicited in right atrial myocytes of LVH rabbits or atrial myocytes from any of the 12 control rabbits. Spontaneous automaticity (SA) from left atrial myocytes was observed in 9 out of 12 LVH rabbits, but none in right atrial myocytes of LVH rabbits or control rabbits, at a pacing rate of 8,000 ms. The left atrial myocytes of LVH rabbits had a significantly higher density of the I(Na-L) compared with those of control rabbits (0.90 +/- 0.12 in LVH vs. 0.50 +/- 0.08 pA/pF in control, n = 8, P < 0.01). Tetrodotoxin, an I(Na-L) blocker, abolished all atrial EADs and SA at 10 microM. Our results demonstrate that LVH induction results in a significant increase of I(Na-L) in the left atrial myocytes that may render these cells susceptible to the genesis of EADs and SA. The I(Na-L) may serve as a potentially useful ionic target for antiarrhythmic drugs for the treatment of atrial arrhythmias in the setting of LVH.

Atrial fibrillation in congestive heart failure

Atrial fibrillation (AF) and heart failure (HF) are common and interrelated conditions, each promoting the other, and both associated with increased mortality. HF leads to structural and electrical atrial remodeling, thus creating the basis for the development and perpetuation of AF; and AF may lead to hemodynamic deterioration and the development of tachycardia-mediated cardiomyopathy. Stroke prevention by antithrombotic therapy is crucial in patients with AF and HF. Of the 2 principal therapeutic strategies to treat AF, rate control and rhythm control, neither has been shown to be superior to the other in terms of survival, despite better survival in patients with sinus rhythm compared with those in AF. Antiarrhythmic drug toxicity and poor efficacy are concerns. Catheter ablation of AF can establish sinus rhythm without the risks of antiarrhythmic drug therapy, but has important procedural risks, and data from randomized trials showing a survival benefit of this treatment strategy are still lacking. In intractable cases, ablation of the atrioventricular junction and placement of a permanent pacemaker is a treatment alternative; and biventricular pacing may prevent or reduce the negative consequences of chronic right ventricular pacing.

Electrophysiological remodeling in heart failure

Heart failure affects nearly 6 million Americans, with a half-million new cases emerging each year. Whereas up to 50% of heart failure patients die of arrhythmia, the diverse mechanisms underlying heart failure-associated arrhythmia are poorly understood. As a consequence, effectiveness of antiarrhythmic pharmacotherapy remains elusive. Here, we review recent advances in our understanding of heart failure-associated molecular events impacting the electrical function of the myocardium. We approach this from an anatomical standpoint, summarizing recent insights gleaned from pre-clinical models and discussing their relevance to human heart failure.

Animal models for atrial fibrillation: clinical insights and scientific opportunities

Atrial fibrillation (AF) is the most common arrhythmia in clinical practice. A variety of animal models have been used to study the pathophysiology of AF, including molecular basis, ion-current determinants, anatomical features, and macroscopic mechanisms. In addition, animal models play a key role in the development of new therapeutic approaches, whether drug-based, molecular therapeutics, or device-related. This article discusses the various types of animal models that have been used for AF research, reviews the principle mechanisms governing atrial arrhythmias in each model, and provides some guidelines for model selection for various purposes.

Atrial Ca2+ signaling in atrial fibrillation as an antiarrhythmic drug target

Atrial fibrillation (AF) is the most frequent arrhythmia and is associated with increased morbidity and mortality. Current drugs for AF treatment have moderate efficacy and increase the risk of life-threatening antiarrhythmias, making novel drug development crucial. Newer antiarrhythmic drugs like dronedarone and possibly vernakalant are efficient and may have less proarrhythmic potential. Emerging evidence suggests that abnormal intracellular Ca(2+) signaling is the key contributor to focal firing, substrate evolution, and atrial remodeling during AF. Accordingly, identification of the underlying atrial Ca(2+)-handling abnormalities is expected to discover novel mechanistically based therapeutic targets. This article reviews the molecular mechanisms of altered Ca(2+) signaling in AF and discusses the potential value of novel approaches targeting atrial Ca(2+)-handling abnormalities.

Enalapril, irbesartan, and angiotensin-(1-7) prevent atrial tachycardia-induced ionic remodeling

BACKGROUND

Atrial fibrillation (AF) is associated with activation of the renin-angiotensin system (RAS) in the atria. Angiotensin-(1-7) [Ang-(1-7)] is a biologically active component of the RAS, it not only counterbalances the actions of angiotensin II (Ang II) but also is a potential inhibitor of angiotensin-converting enzyme (ACE). The purpose of this study was to investigate the effects of the ACE inhibitor enalapril, the angiotensin-receptor blocker (ARB) irbesartan, and Ang-(1-7) on the chronic atrial ionic remodeling.

METHODS

Thirty dogs were assigned to sham, paced, paced + enalapril, paced + irbesartan or paced + Ang-(1-7) group, 6 dogs in each group. Rapid atrial pacing at 500 beats per minute was maintained for 14 days, but dogs in sham group were instrumented without pacing. During the pacing, enalapril (2 mg · Kg(-1) · d(-1)) and irbesartan (60 mg · Kg(-1) · d(-1)) were given orally and Ang-(1-7) (6 μg · Kg(-1) · h(-1)) was given intravenously. Whole-cell patch-clamp technique was used to record atrial ionic currents and action potential duration (APD). And RT-PCR was applied to assess atrial mRNA expression of I(TO) Kv4.3 and I(CaL)α1C subunits.

RESULTS

Compared with sham, rapid pacing shortened APD90 (P < 0.05) of atrial myocytes, and decreased APD90 rate adaptation (P<0.05). APD90 changes were prevented by irbesartan and Ang-(1-7), but not enalapril. In atria from paced group, the densities and gene expression of I(TO) and I(CaL) were reduced (P < 0.01 vs. sham). Enalapril increased the density and gene expression of I(TO) compared with sham (P < 0.01), Ang-(1-7) prevented the decrease of I(TO) and I(CaL) (P < 0.05 vs. control) and Kv4.3 mRNA expression (P < 0.01 vs. control). Irbesartan had no effect on I(TO) and I(CaL) densities or mRNA expression.

CONCLUSIONS

These results suggest that enalapril, irbesartan, and Ang-(1-7) have differing influences on atrial tachycardia-induced atrial ionic remodeling.

Cardiac strong inward rectifier potassium channels

Cardiac I(K1) and I(KACh) are the major potassium currents displaying classical strong inward rectification, a unique property that is critical for their roles in cardiac excitability. In the last 15 years, research on I(K1) and I(KACh) has been propelled by the cloning of the underlying inwardly rectifying potassium (Kir) channels, the discovery of the molecular mechanism of strong rectification and the linking of a number of disorders of cardiac excitability to defects in genes encoding Kir channels. Disease-causing mutations in Kir genes have been shown experimentally to affect one or more of the following channel properties: structure, assembly, trafficking, and regulation, with the ultimate effect of a gain- or a loss-of-function of the channel. It is now established that I(K1) and I(KACh) channels are heterotetramers of Kir2 and Kir3 subunits, respectively. Each homomeric Kir channel has distinct biophysical and regulatory properties, and individual Kir subunits often display different patterns of regional, cellular, and membrane distribution. These differences are thought to underlie important variations in the physiological properties of I(K1) and I(KACh). It has become increasingly clear that the contribution of I(K1) and I(KACh) channels to cardiac electrical activity goes beyond their long recognized role in the stabilization of resting membrane potential and shaping the late phase of action potential repolarization in individual myocytes but extends to being critical elements determining the overall electrical stability of the heart.

Novel approaches for pharmacological management of atrial fibrillation

In the light of the progressively increasing prevalence of atrial fibrillation (AF), medical awareness of the need to develop improved therapeutic approaches for the arrhythmia has also risen over the last decade. AF reduces quality of life and is associated with increased morbidity and mortality. Despite several setbacks as a result of negative results from rhythm control trials, the potential advantages of sinus-rhythm (SR) maintenance have motivated continued efforts to design novel pharmacological options aiming to terminate AF and prevent its recurrence, with a hope that optimized medical therapy will improve outcomes in AF patients. Pathophysiologically, AF is associated with electrical and structural changes in the atria, which increase the propensity to arrhythmia perpetuation but may eventually allow for new modalities for therapeutic intervention. Antiarrhythmic drug therapy has traditionally targeted ionic currents that modulate excitability and/or repolarization of cardiac myocytes. Despite efficacious suppression of ventricular and supraventricular arrhythmias, traditional antiarrhythmic drugs present problematic risks of pro-arrhythmia, potentially leading to excess mortality in the case of Na+-channel blockers or IKr (IKr=the rapid component of the delayed rectifier potassium current) blockers. New anti-AF agents in development do not fit well into the classical Singh and Vaughan-Williams formulation, and are broadly divided into 'atrial-selective compounds' and 'multiple-channel blockers'. The prototypic multiple-channel blocker amiodarone is the most efficient presently available compound for SR maintenance, but the drug has extra-cardiac adverse effects and complex pharmacokinetics that limit widespread application. The other available drugs are not nearly as efficient for SR maintenance and have a greater risk of proarrhythmia than amiodarone. Two new antiarrhythmic drugs are on the cusp of introduction into clinical practice. Vernakalant affects several atrially expressed ion channels and has rapid unbinding Na+-channel blocking action along with promising efficacy for AF conversion to SR. Dronedarone is an amiodarone derivative with an electrophysiological profile similar to its predecessor but lacking most amiodarone-associated adverse effects. Furthermore, dronedarone has shown benefits for important clinical endpoints, including cardiovascular mortality in specific AF populations, the first AF-suppressing drug to do so in prospective randomized clinical trials. Agents that modulate non-ionic current targets (termed 'upstream' therapies) may help to modify the substrate for AF maintenance. Among these, drugs such as angiotensin II type 1 (AT1) receptor antagonists, immunosuppressive agents or HMG-CoA reductase inhibitors (statins) deserve mention. Finally, drugs that block atrial-selective ion-channel targets such as the ultra-rapid delayed rectifier current (IKur) and the acetylcholine-regulated K+-current (IKACh) are presently in development. The introduction of novel antiarrhythmic agents for the management of AF may eventually improve patient outcomes. The potential value of a variety of other novel therapeutic options is currently under active investigation.

Chronic heart failure and the substrate for atrial fibrillation

AIMS

We sought to define the underlying mechanisms for atrial fibrillation (AF) during chronic heart failure (HF).

METHODS AND RESULTS

Preliminary studies showed that 4 months of HF resulted in irreversible systolic dysfunction (n = 9) and a substrate for sustained inducible AF (>3 months, n = 3). We used a chronic (4-month) canine model of tachypacing-induced HF (n = 10) to assess atrial electrophysiological remodelling, relative to controls (n = 5). Left ventricular fractional shortening was reduced from 37.2 +/- 0.83 to 13.44 +/- 2.63% (P < 0.05). Left atrial (LA) contractility (fractional area change) was reduced from 34.9 +/- 7.9 to 27.9 +/- 4.23% (P < 0.05). Action potential durations (APDs) at 50 and 90% repolarization were shortened by approximately 60 and 40%, respectively, during HF (P < 0.05). HF-induced atrial remodelling included increased fibrosis, increased I(to), and decreased I(K1), I(Kur), and I(Ks) (P < 0.05). HF induced increases in LA Kv channel interacting protein 2 (P < 0.05), no change in Kv4.3, Kv1.5, or Kir2.3, and reduced Kir2.1 (P < 0.05). When I(Ca-L) was elicited by action potential (AP) clamp, HF APs reduced the integral of I(Ca) in control myocytes, with a larger reduction in HF myocytes (P < 0.05). I(CaL) measured with standard voltage clamp was unchanged by HF. Incubation of myocytes with N-acetylcysteine (a glutathione precursor) attenuated HF-induced electrophysiological alterations. LA angiotensin-1 receptor expression was increased in HF.

CONCLUSION

Chronic HF causes alterations in ion channel expression and ion currents, resulting in attenuation of the APD and atrial contractility and a substrate for persistent AF.

Atrial Remodeling And Atrial Fibrillation: Mechanistic Interactions And Clinical Implications

Atrial fibrillation (AF) is the most common arrhythmia in clinical practice. The prevalence of AF increases dramatically with age and is seen in as high as 9% of individuals by the age of 80 years. In high-risk patients, the thromboembolic stroke risk can be as high as 9% per year and is associated with a 2-fold increase in mortality. Although the pathophysiological mechanism underlying the genesis of AF has been the focus of many studies, it remains only partially understood. Conventional theories focused on the presence of multiple re-entrant circuits originating in the atria that are asynchronous and conducted at various velocities through tissues with various refractory periods. Recently, rapidly firing atrial activity in the muscular sleeves at the pulmonary veins ostia or inside the pulmonary veins have been described as potential mechanism,. AF results from a complex interaction between various initiating triggers and development of abnormal atrial tissue substrate. The development of AF leads to structural and electrical changes in the atria, a process known as remodeling. To have effective surgical or catheter ablation of AF good understanding of the possible mechanism(s) is crucial.Once initiated, AF alters atrial electrical and structural properties that promote its maintenance and recurrence. The role of atrial remodeling (AR) in the development and maintenance of AF has been the subject of many animal and human studies over the past 10-15 years. This review will discuss the mechanisms of AR, the structural, electrophysiologic, and neurohormonal changes associated with AR and it is role in initiating and maintaining AF. We will also discuss briefly the role of inflammation in AR and AF initiation and maintenance, as well as, the possible therapeutic interventions to prevent AR, and hence AF, based on the current understanding of the interaction between AF and AR.