Characterization of in vivo and in vitro electrophysiological and antiarrhythmic effects of a novel IKACh blocker, NIP-151: a comparison with an IKr-blocker dofetilide

Recent Advances in Antiarrhythmic Drug Therapy

Cardiac arrhythmias remain a common cause of death and disability. Antiarrhythmic drugs (AADs) and antiarrhythmic agents remain a cornerstone of current cardiac arrhythmia management, despite moderate efficacy and the potential for significant adverse proarrhythmic effects. Due to conceptual, regulatory and financial considerations, the number of novel antiarrhythmic targets and agents in the development pipeline has decreased substantially during the last few decades. However, several promising candidates remain and there are exciting developments in repurposing and reformulating already existing drugs for indications related to cardiac arrhythmias. This review discusses the key conceptual considerations for the development of new antiarrhythmic agents, summarizes new compounds and formulations currently in clinical development for rhythm control of atrial fibrillation, and highlights the potential for drug repurposing. Finally, future directions in AAD development are discussed. Together with an ever-increasing understanding of the molecular mechanisms underlying cardiac arrhythmias, these components support a cautiously optimistic outlook towards improved pharmacological treatment opportunities for patients suffering from cardiac arrhythmias.

The Relevance of GIRK Channels in Heart Function

Among the large number of potassium-channel families implicated in the control of neuronal excitability, G-protein-gated inwardly rectifying potassium channels (GIRK/Kir3) have been found to be a main factor in heart control. These channels are activated following the modulation of G-protein-coupled receptors and, although they have been implicated in different neurological diseases in both human and animal studies of the central nervous system, the therapeutic potential of different subtypes of these channel families in cardiac conditions has remained untapped. As they have emerged as a promising potential tool to treat a variety of conditions that disrupt neuronal homeostasis, many studies have started to focus on these channels as mediators of cardiac dynamics, thus leading to research into their implication in cardiovascular conditions. Our aim is to review the latest advances in GIRK modulation in the heart and their role in the cardiovascular system.

Emerging Antiarrhythmic Drugs for Atrial Fibrillation

Atrial fibrillation (AF), the most common cardiac arrhythmia worldwide, is driven by complex mechanisms that differ between subgroups of patients. This complexity is apparent from the different forms in which AF presents itself (post-operative, paroxysmal and persistent), each with heterogeneous patterns and variable progression. Our current understanding of the mechanisms responsible for initiation, maintenance and progression of the different forms of AF has increased significantly in recent years. Nevertheless, antiarrhythmic drugs for the management of AF have not been developed based on the underlying arrhythmia mechanisms and none of the currently used drugs were specifically developed to target AF. With the increased knowledge on the mechanisms underlying different forms of AF, new opportunities for developing more effective and safer AF therapies are emerging. In this review, we provide an overview of potential novel antiarrhythmic approaches based on the underlying mechanisms of AF, focusing both on the development of novel antiarrhythmic agents and on the possibility of repurposing already marketed drugs. In addition, we discuss the opportunity of targeting some of the key players involved in the underlying AF mechanisms, such as ryanodine receptor type-2 (RyR2) channels and atrial-selective K⁺-currents (I_K2P and I_SK) for antiarrhythmic therapy. In addition, we highlight the opportunities for targeting components of inflammatory signaling (e.g., the NLRP3-inflammasome) and upstream mechanisms targeting fibroblast function to prevent structural remodeling and progression of AF. Finally, we critically appraise emerging antiarrhythmic drug principles and future directions for antiarrhythmic drug development, as well as their potential for improving AF management.

New Strategies for the Treatment of Atrial Fibrillation

Atrial fibrillation (AF) is the most prevalent cardiac arrhythmia in the clinical practice. It significantly contributes to the morbidity and mortality of the elderly population. Over the past 25-30 years intense effort in basic research has advanced the understanding of the relationship between the pathophysiology of AF and atrial remodelling. Nowadays it is clear that the various forms of atrial remodelling (electrical, contractile and structural) play crucial role in initiating and maintaining the persistent and permanent types of AF. Unlike in ventricular fibrillation, in AF rapid ectopic firing originating from pulmonary veins and re-entry mechanism may induce and maintain (due to atrial remodelling) this complex cardiac arrhythmia. The present review presents and discusses in detail the latest knowledge on the role of remodelling in AF. Special attention is paid to novel concepts and pharmacological targets presumably relevant to the drug treatment of atrial fibrillation.

Kir Channel Molecular Physiology, Pharmacology, and Therapeutic Implications

For the past two decades several scholarly reviews have appeared on the inwardly rectifying potassium (Kir) channels. We would like to highlight two efforts in particular, which have provided comprehensive reviews of the literature up to 2010 (Hibino et al., Physiol Rev 90(1):291-366, 2010; Stanfield et al., Rev Physiol Biochem Pharmacol 145:47-179, 2002). In the past decade, great insights into the 3-D atomic resolution structures of Kir channels have begun to provide the molecular basis for their functional properties. More recently, computational studies are beginning to close the time domain gap between in silico dynamic and patch-clamp functional studies. The pharmacology of these channels has also been expanding and the dynamic structural studies provide hope that we are heading toward successful structure-based drug design for this family of K⁺ channels. In the present review we focus on placing the physiology and pharmacology of this K⁺ channel family in the context of atomic resolution structures and in providing a glimpse of the promising future of therapeutic opportunities.

Anti-arrhythmic investigations in large animal models of atrial fibrillation

Atrial fibrillation (AF) constitutes an increasing health problem in the aging population. Animal models reflecting human phenotypes are needed to understand the mechanisms of AF, as well as to test new pharmacological interventions. In recent years, a number of large animal models, primarily pigs, goats, dog and horses have been used in AF research. These animals can to a certain extent recapitulate the human pathophysiological characteristics and serve as valuable tools in investigating new pharmacological interventions for treating AF. This review focuses on anti-arrhythmic investigations in large animals. Initially, spontaneous AF in small and large mammals is discussed. This is followed by a short presentation of frequently used methods for inducing short- and long-term AF. The major focus of the review is on anti-arrhythmic compounds either frequently used in the human clinic (ranolazine, flecainide, vernakalant and amiodarone) or being promising new AF medicine candidates (I_K,Ach , I_SK,Ca and I_K2P blockers).

A benzopyran with antiarrhythmic activity is an inhibitor of Kir3.1-containing potassium channels

Atrial fibrillation (AF) is the most commonly diagnosed cardiac arrhythmia and is associated with increased morbidity and mortality. Currently approved AF antiarrhythmic drugs have limited efficacy and/or carry the risk of ventricular proarrhythmia. The cardiac acetylcholine activated inwardly rectifying K+ current (IKACh), composed of Kir3.1/Kir3.4 heterotetrameric and Kir3.4 homotetrameric channel subunits, is one of the best validated atrial-specific ion channels. Previous research pointed to a series of benzopyran derivatives with potential for treatment of arrhythmias, but their mechanism of action was not defined. Here, we characterize one of these compounds termed Benzopyran-G1 (BP-G1) and report that it selectively inhibits the Kir3.1 (GIRK1 or G1) subunit of the KACh channel. Homology modeling, molecular docking, and molecular dynamics simulations predicted that BP-G1 inhibits the IKACh channel by blocking the central cavity pore. We identified the unique F137 residue of Kir3.1 as the critical determinant for the IKACh-selective response to BP-G1. The compound interacts with Kir3.1 residues E141 and D173 through hydrogen bonds that proved critical for its inhibitory activity. BP-G1 effectively blocked the IKACh channel response to carbachol in an in vivo rodent model and displayed good selectivity and pharmacokinetic properties. Thus, BP-G1 is a potent and selective small-molecule inhibitor targeting Kir3.1-containing channels and is a useful tool for investigating the role of Kir3.1 heteromeric channels in vivo. The mechanism reported here could provide the molecular basis for future discovery of novel, selective IKACh channel blockers to treat atrial fibrillation with minimal side effects.

Drug Therapy for Vagally-Mediated Atrial Fibrillation and Sympatho-Vagal Balance in the Genesis of Atrial Fibrillation: A Review of the Current Literature

Objective

The presence of both sympathetic activation-mediated triggers and parasympathetic activation-mediated substrates are required to initiate and maintain some forms of atrial fibrillation (AF). AF predominantly precipitated by parasympathetic stimulation is known as vagally-mediated AF (VM-AF). The role of novel drugs and molecular targeted gene therapy that modulate the autonomic nervous system are therapeutic options in this unique population with VM-AF. Here, we review the role of the sympatho-vagal balance in the genesis of AF and consider drug therapy for VM-AF.

Methods

In accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Statement, literature search was conducted using the keywords "vagal", "vagal nerve", "vagus", "vagus nerve", and "atrial fibrillation". Retrieved citations were first screened independently by 2 reviewers for inclusion and exclusion criteria.

Results

A total of 14 studies and 3 practice guidelines from 1986-2017 were included. Only two clinical investigations evaluated the effectiveness of disopyramide and sotalol in human subjects with VM-AF. The potential role of antiarrhythmic drugs has been studied in animal models.

Conclusions

Growing evidence suggests that the autonomic nervous system is integral in the development of VM-AF. Novel medications and genetic targets are undergoing investigation with promising results.

Effect of selective IK,ACh inhibition by XAF-1407 in an equine model of tachypacing-induced persistent atrial fibrillation

BACKGROUND AND PURPOSE

Inhibition of the G-protein gated ACh-activated inward rectifier potassium current, I_K,ACh may be an effective atrial selective treatment strategy for atrial fibrillation (AF). Therefore, the anti-arrhythmic and electrophysiological properties of a novel putatively potent and highly specific I_K,ACh inhibitor, XAF-1407 (3-methyl-1-[5-phenyl-4-[4-(2-pyrrolidin-1-ylethoxymethyl)-1-piperidyl]thieno[2,3-d]pyrimidin-6-yl]azetidin-3-ol), were characterised for the first time in vitro and investigated in horses with persistent AF.

EXPERIMENTAL APPROACH

The pharmacological ion channel profile of XAF-1407 was investigated using cell lines expressing relevant ion channels. In addition, eleven horses were implanted with implantable cardioverter defibrillators enabling atrial tachypacing into self-sustained AF. The electrophysiological effects of XAF-1407 were investigated after serial cardioversions over a period of 1 month. Cardioversion success, drug-induced changes of atrial tissue refractoriness, and ventricular electrophysiology were assessed at baseline (day 0) and days 3, 5, 11, 17, and 29 after AF induction.

KEY RESULTS

XAF-1407 potently and selectively inhibited K_ir 3.1/3.4 and K_ir 3.4/3.4, underlying the I_K,ACh current. XAF-1407 treatment in horses prolonged atrial effective refractory period as well as decreased atrial fibrillatory rate significantly (~20%) and successfully cardioverted AF, although with a decreasing efficacy over time. XAF-1407 shortened atrioventricular-nodal refractoriness, without effect on QRS duration. QTc prolongation (4%) within 15 min of drug infusion was observed, however, without any evidence of ventricular arrhythmia.

CONCLUSION AND IMPLICATIONS

XAF-1407 efficiently cardioverted sustained tachypacing-induced AF of short duration in horses without notable side effects. This supports I_K,ACh inhibition as a potentially safe treatment of paroxysmal AF in horses, suggesting potential clinical value for other species including humans.

Mutant KCNJ3 and KCNJ5 Potassium Channels as Novel Molecular Targets in Bradyarrhythmias and Atrial Fibrillation

BACKGROUND

Bradyarrhythmia is a common clinical manifestation. Although the majority of cases are acquired, genetic analysis of families with bradyarrhythmia has identified a growing number of causative gene mutations. Because the only ultimate treatment for symptomatic bradyarrhythmia has been invasive surgical implantation of a pacemaker, the discovery of novel therapeutic molecular targets is necessary to improve prognosis and quality of life.

METHODS

We investigated a family containing 7 individuals with autosomal dominant bradyarrhythmias of sinus node dysfunction, atrial fibrillation with slow ventricular response, and atrioventricular block. To identify the causative mutation, we conducted the family-based whole exome sequencing and genome-wide linkage analysis. We characterized the mutation-related mechanisms based on the pathophysiology in vitro. After generating a transgenic animal model to confirm the human phenotypes of bradyarrhythmia, we also evaluated the efficacy of a newly identified molecular-targeted compound to upregulate heart rate in bradyarrhythmias by using the animal model.

RESULTS

We identified one heterozygous mutation, KCNJ3 c.247A>C, p.N83H, as a novel cause of hereditary bradyarrhythmias in this family. KCNJ3 encodes the inwardly rectifying potassium channel Kir3.1, which combines with Kir3.4 (encoded by KCNJ5) to form the acetylcholine-activated potassium channel ( I_KACh channel) with specific expression in the atrium. An additional study using a genome cohort of 2185 patients with sporadic atrial fibrillation revealed another 5 rare mutations in KCNJ3 and KCNJ5, suggesting the relevance of both genes to these arrhythmias. Cellular electrophysiological studies revealed that the KCNJ3 p.N83H mutation caused a gain of I_KACh channel function by increasing the basal current, even in the absence of m₂ muscarinic receptor stimulation. We generated transgenic zebrafish expressing mutant human KCNJ3 in the atrium specifically. It is interesting to note that the selective I_KACh channel blocker NIP-151 repressed the increased current and improved bradyarrhythmia phenotypes in the mutant zebrafish.

CONCLUSIONS

The I_KACh channel is associated with the pathophysiology of bradyarrhythmia and atrial fibrillation, and the mutant I_KACh channel ( KCNJ3 p.N83H) can be effectively inhibited by NIP-151, a selective I_KACh channel blocker. Thus, the I_KACh channel might be considered to be a suitable pharmacological target for patients who have bradyarrhythmia with a gain-of-function mutation in the I_KACh channel.

GIRK channel activity of Hungarian mushrooms: From screening to biologically active metabolites

In the current study effects of fungal extracts on the G-protein-activated inwardly rectifying potassium channel (GIRK1/4) were screened using the automated patch-clamp method. 40 organic (n-hexane, chloroform, and 50% methanol) and aqueous extracts were prepared from 10 mushroom species native to Hungary. Among the examined fungal fractions of different polarities some n-hexane and chloroform extracts exerted considerable ion channel activity. One of the most active fungal species, Hypholoma lateritium was selected for further detailed examination to determine the compounds responsible for the observed pharmacological property. Evaluation of the ion channel activity of mushroom metabolites 1-10 revealed that lanosta-7,9(11)-diene-12β,21α-epoxy-2α,3β,24β,25-tetraol (5) demonstrates remarkable blocking activity on GIRK current (IC₅₀ 395.1 ± 31.8 nM). Investigation of the selectivity of the GIRK inhibitory effect proved that lanosta-7,9(11)-diene-12β,21α-epoxy-2α,3β,24β,25-tetraol (5) has only weak inhibitory activity on hERG channel (7.9 ± 2.8% at 100 μM), exerting more than three orders of magnitude lower blocking activity on hERG channel than on GIRK channel.

Cardiac Potassium Channels: Physiological Insights for Targeted Therapy

The development of novel drugs specifically directed at the ion channels underlying particular features of cardiac action potential (AP) initiation, recovery, and refractoriness would contribute to an optimized approach to antiarrhythmic therapy that minimizes potential cardiac and extracardiac toxicity. Of these, K⁺ channels contribute numerous and diverse currents with specific actions on different phases in the time course of AP repolarization. These features and their site-specific distribution make particular K⁺ channel types attractive therapeutic targets for the development of pharmacological agents attempting antiarrhythmic therapy in conditions such as atrial fibrillation. However, progress in the development of such temporally and spatially selective antiarrhythmic drugs against particular ion channels has been relatively limited, particularly in view of our incomplete understanding of the complex physiological roles and interactions of the various ionic currents. This review summarizes the physiological properties of the main cardiac potassium channels and the way in which they modulate cardiac electrical activity and then critiques a number of available potential antiarrhythmic drugs directed at them.

Potassium currents in the heart: functional roles in repolarization, arrhythmia and therapeutics

This is the second of the two White Papers from the fourth UC Davis Cardiovascular Symposium Systems Approach to Understanding Cardiac Excitation-Contraction Coupling and Arrhythmias (3-4 March 2016), a biennial event that brings together leading experts in different fields of cardiovascular research. The theme of the 2016 symposium was 'K⁺ channels and regulation', and the objectives of the conference were severalfold: (1) to identify current knowledge gaps; (2) to understand what may go wrong in the diseased heart and why; (3) to identify possible novel therapeutic targets; and (4) to further the development of systems biology approaches to decipher the molecular mechanisms and treatment of cardiac arrhythmias. The sessions of the Symposium focusing on the functional roles of the cardiac K⁺ channel in health and disease, as well as K⁺ channels as therapeutic targets, were contributed by Ye Chen-Izu, Gideon Koren, James Weiss, David Paterson, David Christini, Dobromir Dobrev, Jordi Heijman, Thomas O'Hara, Crystal Ripplinger, Zhilin Qu, Jamie Vandenberg, Colleen Clancy, Isabelle Deschenes, Leighton Izu, Tamas Banyasz, Andras Varro, Heike Wulff, Eleonora Grandi, Michael Sanguinetti, Donald Bers, Jeanne Nerbonne and Nipavan Chiamvimonvat as speakers and panel discussants. This article summarizes state-of-the-art knowledge and controversies on the functional roles of cardiac K⁺ channels in normal and diseased heart. We endeavour to integrate current knowledge at multiple scales, from the single cell to the whole organ levels, and from both experimental and computational studies.

Atrial fibrillation: Therapeutic potential of atrial K+ channel blockers

Despite the epidemiological scale of atrial fibrillation, current treatment strategies are of limited efficacy and safety. Ideally, novel drugs should specifically correct the pathophysiological mechanisms responsible for atrial fibrillation with no other cardiac or extracardiac actions. Atrial-selective drugs are directed toward cellular targets with sufficiently different characteristics in atria and ventricles to modify only atrial function. Several potassium (K⁺) channels with either predominant expression in atria or distinct electrophysiological properties in atria and ventricles can serve as atrial-selective drug targets. These channels include the ultra-rapidly activating, delayed outward-rectifying Kv1.5 channel conducting I_Kur, the acetylcholine-activated inward-rectifying Kir3.1/Kir3.4 channel conducting I_K,ACh, the Ca²⁺-activated K⁺ channels of small conductance (SK) conducting I_SK, and the two pore domain K⁺ (K2P) channels TWIK-1, TASK-1 and TASK-3 that are responsible for voltage-independent background currents I_TWIK-1, I_TASK-1, and I_TASK-3. Here, we briefly review the characteristics of these K⁺ channels and their roles in atrial fibrillation. The antiarrhythmic potential of drugs targeting the described channels is discussed as well as their putative value in treatment of atrial fibrillation.

Atrial-Selective Potassium Channel Blockers

Atrial fibrillation (AF) is associated with increased morbidity and mortality. Atrial-selective potassium (K(+)) channel blockers may represent a novel therapeutic target. The best validated atrial-specific ion currents are the acetylcholine-activated inward-rectifier K(+) current IK,ACh and ultrarapidly activating delayed-rectifier K(+) current IKur. Two-pore domain and small-conductance Ca(2+)-activated K(+) channels and Kv1.1 channels may also contribute to the atrial repolarization. We review the molecular and electrophysiologic characteristics of atrial-selective K(+) channels and their potential pathophysiologic role in AF. We summarize currently available K(+) channel blockers focusing on the most important compounds.

G protein-gated IKACh channels as therapeutic targets for treatment of sick sinus syndrome and heart block

Dysfunction of pacemaker activity in the sinoatrial node (SAN) underlies "sick sinus" syndrome (SSS), a common clinical condition characterized by abnormally low heart rate (bradycardia). If untreated, SSS carries potentially life-threatening symptoms, such as syncope and end-stage organ hypoperfusion. The only currently available therapy for SSS consists of electronic pacemaker implantation. Mice lacking L-type Cav1.3 Ca(2+) channels (Cav1.3(-/-)) recapitulate several symptoms of SSS in humans, including bradycardia and atrioventricular (AV) dysfunction (heart block). Here, we tested whether genetic ablation or pharmacological inhibition of the muscarinic-gated K(+) channel (IKACh) could rescue SSS and heart block in Cav1.3(-/-) mice. We found that genetic inactivation of IKACh abolished SSS symptoms in Cav1.3(-/-) mice without reducing the relative degree of heart rate regulation. Rescuing of SAN and AV dysfunction could be obtained also by pharmacological inhibition of IKACh either in Cav1.3(-/-) mice or following selective inhibition of Cav1.3-mediated L-type Ca(2+) (ICa,L) current in vivo. Ablation of IKACh prevented dysfunction of SAN pacemaker activity by allowing net inward current to flow during the diastolic depolarization phase under cholinergic activation. Our data suggest that patients affected by SSS and heart block may benefit from IKACh suppression achieved by gene therapy or selective pharmacological inhibition.

First clinical trial of specific IKACh blocker shows no reduction in atrial fibrillation burden in patients with paroxysmal atrial fibrillation: pacemaker assessment of BMS 914392 in patients with paroxysmal atrial fibrillation

AIMS

To assess the efficacy of BMS 914392 on atrial fibrillation (AF) burden reduction in 20 patients with pacemakers and paroxysmal atrial fibrillation (PAF). BMS 914392 is a potent, selective, oral inhibitor of the IKACh current and has been shown to suppress AF, whilst having no effect on the ventricular refractory period. This is the first efficacy study of BMS 914392 in patients with PAF.

METHODS AND RESULTS

The study was a four-way, crossover, double-blind design. A total of 20 patients with PAF and dual-chamber pacemakers were recruited. The pacemakers allowed beat-to-beat monitoring. Anti-arrhythmic drugs were withdrawn. Patients received low-dose (10 mg OD), medium-dose (10 mg TDS), and high-dose (20 mg TDS) BMS 914392 or placebo for 3 weeks before being crossed to the next phase. Patients underwent a washout period, four treatment phases and a final washout phase. Atrial fibrillation burden was downloaded from their pacemakers at the end of each study phase. BMS 914392 did not reduce AF burden when compared with placebo (10 mg OD P = 0.56, 10 mg TDS P = 0.22, 20 mg TDS P = 0.23). Heart rate and corrected QT (QTc) were not affected by BMS 914392. Adverse event (AE) rates did not differ from placebo in any of the treatment groups, with no serious AEs recorded.

CONCLUSION

BMS 914932 has not been shown to reduce AF burden in patients with PAF and pacemakers using beat-to-beat pacemaker monitoring throughout the study. BMS 914392 was well tolerated and did not affect QTc or reduce heart rate.

TRIAL REGISTRATION

Clinicaltrials.gov: NCT01356914.

Neuronal and Cardiovascular Potassium Channels as Therapeutic Drug Targets: Promise and Pitfalls

Potassium (K(+)) channels, with their diversity, often tissue-defined distribution, and critical role in controlling cellular excitability, have long held promise of being important drug targets for the treatment of dysrhythmias in the heart and abnormal neuronal activity within the brain. With the exception of drugs that target one particular class, ATP-sensitive K(+) (KATP) channels, very few selective K(+) channel activators or inhibitors are currently licensed for clinical use in cardiovascular and neurological disease. Here we review what a range of human genetic disorders have told us about the role of specific K(+) channel subunits, explore the potential of activators and inhibitors of specific channel populations as a therapeutic strategy, and discuss possible reasons for the difficulty in designing clinically relevant K(+) channel modulators.

[New developments in the antiarrhythmic therapy of atrial fibrillation]

Atrial fibrillation often affects elderly people with cardiovascular disease and takes a progressive course with increasing resistance to treatment. For the latter, electrical and structural changes (remodelling) seem to be responsible that are directly related to the high excitatory rate in the atria. Therapeutic strategies for atrial fibrillation consist of (i) treating the underlying cardiovascular disease, (ii) re-establishing sinus rhythm and (iii) reducing ventricular rate. Rapid pharmacological or electrical cardioversion is expected to prevent remodelling. Classical antiarrhythmic drugs are notoriously ineffective and burdened with serious cardiac and extracardiac side effects so that there is an urgent need for effective and safe novel compounds. In this review the three recently introduced drugs dronedarone, vernakalant and ranolazine are discussed with respect to the use in atrial fibrillation. Other new antiarrhythmic agents are still in the developmental phase and aim at atria-selective mechanisms thereby excluding ventricular proarrhythmic effects. The mechanisms of action will be discussed in the context of the present understanding of the pathophysiology of onset and maintenance of atrial fibrillation.

Regional ion channel gene expression heterogeneity and ventricular fibrillation dynamics in human hearts

RATIONALE

Structural differences between ventricular regions may not be the sole determinant of local ventricular fibrillation (VF) dynamics and molecular remodeling may play a role.

OBJECTIVES

To define regional ion channel expression in myopathic hearts compared to normal hearts, and correlate expression to regional VF dynamics.

METHODS AND RESULTS

High throughput real-time RT-PCR was used to quantify the expression patterns of 84 ion-channel, calcium cycling, connexin and related gene transcripts from sites in the LV, septum, and RV in 8 patients undergoing transplantation. An additional eight non-diseased donor human hearts served as controls. To relate local ion channel expression change to VF dynamics localized VF mapping was performed on the explanted myopathic hearts right adjacent to sampled regions. Compared to non-diseased ventricles, significant differences (p<0.05) were identified in the expression of 23 genes in the myopathic LV and 32 genes in the myopathic RV. Within the myopathic hearts significant regional (LV vs septum vs RV) expression differences were observed for 13 subunits: Nav1.1, Cx43, Ca3.1, Cavα2δ2, Cavβ2, HCN2, Na/K ATPase-1, CASQ1, CASQ2, RYR2, Kir2.3, Kir3.4, SUR2 (p<0.05). In a subset of genes we demonstrated differences in protein expression between control and myopathic hearts, which were concordant with the mRNA expression profiles for these genes. Variability in the expression of Cx43, hERG, Na(+)/K(+) ATPase ß1 and Kir2.1 correlated to variability in local VF dynamics (p<0.001). To better understand the contribution of multiple ion channel changes on VF frequency, simulations of a human myocyte model were conducted. These simulations demonstrated the complex nature by which VF dynamics are regulated when multi-channel changes are occurring simultaneously, compared to known linear relationships.

CONCLUSIONS

Ion channel expression profile in myopathic human hearts is significantly altered compared to normal hearts. Multi-channel ion changes influence VF dynamic in a complex manner not predicted by known single channel linear relationships.

New insights into the therapeutic potential of Girk channels

G protein-dependent signaling pathways control the activity of excitable cells of the nervous system and heart, and are the targets of neurotransmitters, clinically relevant drugs, and drugs of abuse. G protein-gated inwardly rectifying potassium (K(+)) (Girk/Kir3) channels are a key effector in inhibitory signaling pathways. Girk-dependent signaling contributes to nociception and analgesia, reward-related behavior, mood, cognition, and heart-rate regulation, and has been linked to epilepsy, Down syndrome, addiction, and arrhythmias. We discuss recent advances in our understanding of Girk channel structure, organization in signaling complexes, and plasticity, as well as progress on the development of subunit-selective Girk modulators. These findings offer new hope for the selective manipulation of Girk channels to treat a variety of debilitating afflictions.

Essential role of the m2R-RGS6-IKACh pathway in controlling intrinsic heart rate variability

Normal heart function requires generation of a regular rhythm by sinoatrial pacemaker cells and the alteration of this spontaneous heart rate by the autonomic input to match physiological demand. However, the molecular mechanisms that ensure consistent periodicity of cardiac contractions and fine tuning of this process by autonomic system are not completely understood.

Here we examined the contribution of the m₂R-I_KACh intracellular signaling pathway, which mediates the negative chronotropic effect of parasympathetic stimulation, to the regulation of the cardiac pacemaking rhythm. Using isolated heart preparations and single-cell recordings we show that the m₂R-I_KACh signaling pathway controls the excitability and firing pattern of the sinoatrial cardiomyocytes and determines variability of cardiac rhythm in a manner independent from the autonomic input. Ablation of the major regulator of this pathway, Rgs6, in mice results in irregular cardiac rhythmicity and increases susceptibility to atrial fibrillation. We further identify several human subjects with variants in the RGS6 gene and show that the loss of function in RGS6 correlates with increased heart rate variability. These findings identify the essential role of the m₂R-I_KACh signaling pathway in the regulation of cardiac sinus rhythm and implicate RGS6 in arrhythmia pathogenesis.

Atrial selectivity of antiarrhythmic drugs

New antiarrhythmic drugs for treatment of atrial fibrillation should ideally be atrial selective in order to avoid pro-arrhythmic effects in the ventricles. Currently recognized atrial selective targets include atrial Nav1.5 channels, Kv1.5 channels and constitutively active Kir3.1/3.4 channels, each of which confers atrial selectivity by different mechanisms. Na(+) channel blockers with potential- and frequency-dependent action preferentially suppress atrial fibrillation because of the high excitation rate and less negative atrial resting potential, which promote drug binding in atria. Kv1.5 channels are truly atrial selective because they do not conduct repolarizing current IKur in ventricles. Constitutively active IK,ACh is predominantly observed in remodelled atria from patients in permanent atrial fibrillation (AF). A lot of effort has been invested to detect compounds which will selectively block Kir3.1/Kir3.4 in their remodelled constitutively active form. Novel drugs which have been and are being developed aim at atrial-selective targets. Vernakalant and ranolazine which mainly block atrial Na(+) channels are clinically effective. Newly designed selective IKur blockers and IK,ACh blockers are effective in animal models; however, clinical benefit in converting AF into sinus rhythm (SR) or reducing AF burden remains to be demonstrated. In conclusion, atrial-selective antiarrhythmic agents have a lot of potential, but a long way to go.

Tissue-specific effects of acetylcholine in the canine heart

Acetylcholine (ACh) release from the vagus nerve slows heart rate and atrioventricular conduction. ACh stimulates a variety of receptors and channels, including an inward rectifying current [ACh-dependent K⁺ current (IK,ACh)]. The effect of ACh in the ventricle is still debated. We compared the effect of ACh on action potentials in canine atria, Purkinje, and ventricular tissue as well as on ionic currents in isolated cells. Action potentials were recorded from ventricular slices, Purkinje fibers, and arterially perfused atrial preparations. Whole cell currents were recorded under voltage-clamp conditions, and unloaded cell shortening was determined on isolated cells. The effect of ACh (1-10 μM) as well as ACh plus tertiapin, an IK,ACh-specific toxin, was tested. In atrial tissue, ACh hyperpolarized the membrane potential and shortened the action potential duration (APD). In Purkinje and ventricular tissues, no significant effect of ACh was observed. Addition of ACh to atrial cells activated a large inward rectifying current (from -3.5 ± 0.7 to -23.7 ± 4.7 pA/pF) that was abolished by tertiapin. This current was not observed in other cell types. A small inhibition of Ca²⁺ current (ICa) was observed in the atria, endocardium, and epicardium after ACh. ICa inhibition increased at faster pacing rates. At a basic cycle length of 400 ms, ACh (1 μM) reduced ICa to 68% of control. In conclusion, IK,ACh is highly expressed in atria and is negligible/absent in Purkinje, endocardial, and epicardial cells. In all cardiac tissues, ACh caused rate-dependent inhibition of ICa.

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.

Effects of the natural flavone trimethylapigenin on cardiac potassium currents

The natural flavones and polymethylflavone have been reported to have cardiovascular protective effects. In the present study, we determined whether quecertin, apigenin and their methylated compounds (3,7,3',4'-tetramethylquecertin, 3,5,7,3',4'-pentamethylquecertin, 7,4'-dimethylapigenin, and 5,7,4'-trimethylapigenin) would block the atrial specific potassium channel hKv1.5 using a whole-cell patch voltage-clamp technique. We found that only trimethylapigenin showed a strong inhibitory effect on hKv1.5 channel current. This compound suppressed hKv1.5 current in HEK 293 cell line (IC₅₀=6.4 μM), and the ultra-rapid delayed rectify K⁺ current I(Kur) in human atrial myocytes (IC₅₀=8.0 μM) by binding to the open channels and showed a use- and frequency-dependent manner. In addition, trimethylapigenin decreased transient outward potassium current (I(to)) in human atrial myocytes, inhibited acetylcholine-activated K⁺ current (IC₅₀=6.8μM) in rat atrial myocytes. Interestingly, trimethylapigenin had a weak inhibition of hERG channel current. Our results indicate that trimethyapigenin significantly inhibits the atrial potassium currents hKv1.5/I(Kur) and I(KACh), which suggests that trimethylapigenin may be a potential candidate for anti-atrial fibrillation.

New pharmacological approaches to atrial fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia facing physicians, afflicting 13% of men and 11% of women over 85 years of age. Epidemiological studies estimate that there are ≥ 11 million AF sufferers in the seven major economies and that its prevalence will increase two- to threefold over the next 50 years. Current strategies for treating AF involve either sinus rhythm (SR) maintenance or heart rate control, combined with anticoagulation therapy. Although SR control is the preferred and most effective treatment of AF, none of the SR control drugs currently available are able to maintain rhythm without significant side effects. In this article we discuss some of the recent advancements in developing new antiarrhythmic drugs for AF.

Targeting GIRK Channels for the Development of New Therapeutic Agents

G protein-coupled inward rectifier K(+) (GIRK) channels represent novel targets for the development of new therapeutic agents. GIRK channels are activated by a large number of G protein-coupled receptors (GPCRs) and regulate the electrical activity of neurons, cardiac myocytes, and β-pancreatic cells. Abnormalities in GIRK channel function have been implicated in the patho-physiology of neuropathic pain, drug addiction, cardiac arrhythmias, and other disorders. However, the pharmacology of these channels remains largely unexplored. In this paper we describe the development of a screening assay for identifying new modulators of neuronal and cardiac GIRK channels. Pituitary (AtT20) and cardiac (HL-1) cell lines expressing GIRK channels were cultured in 96-well plates, loaded with oxonol membrane potential-sensitive dyes and measured using a fluorescent imaging plate reader. Activation of the endogenous GPCRs in the cells caused a rapid, time-dependent decrease in the fluorescent signal; indicative of K(+) efflux through the GIRK channels (GPCR stimulation versus control, Z'-factor = 0.5-0.7). As expected this signal was inhibited by addition of Ba(2+) and the GIRK channel toxin tertiapin-Q. To test the utility of the assay for screening GIRK channel blockers, cells were incubated for 5 min with a compound library of Na(+) and K(+) channel modulators. Ion transporter inhibitors such as 5-(N,N-hexamethylene)-amiloride and SCH-28080 were identified as blockers of the GIRK channel at sub-micromolar concentrations. Thus, the screening assay will be useful for expanding the limited pharmacology of the GIRK channel and in developing new agents for the treatment of GIRK channelopathies.

Small-molecule modulators of inward rectifier K+ channels: recent advances and future possibilities

Inward rectifier potassium (Kir) channels have been postulated as therapeutic targets for several common disorders including hypertension, cardiac arrhythmias and pain. With few exceptions, however, the small-molecule pharmacology of this family is limited to nonselective cardiovascular and neurologic drugs with off-target activity toward inward rectifiers. Consequently, the actual therapeutic potential and 'drugability' of most Kir channels has not yet been determined experimentally. The purpose of this review is to provide a comprehensive summary of publicly disclosed Kir channel small-molecule modulators and highlight recent targeted drug-discovery efforts toward Kir1.1 and Kir2.1. The review concludes with a brief speculation on how the field of Kir channel pharmacology will develop over the coming years and a discussion of the increasingly important role academic laboratories will play in this progress.

Mechanisms of termination and prevention of atrial fibrillation by drug therapy

Atrial fibrillation (AF) is a disorder of the rhythm of electrical activation of the cardiac atria. It is the most common cardiac arrhythmia, has multiple aetiologies, and increases the risk of death from stroke. Pharmacological therapy is the mainstay of treatment for AF, but currently available anti-arrhythmic drugs have limited efficacy and safety. An improved understanding of how anti-arrhythmic drugs affect the electrophysiological mechanisms of AF initiation and maintenance, in the setting of the different cardiac diseases that predispose to AF, is therefore required. A variety of animal models of AF has been developed, to represent and control the pathophysiological causes and risk factors of AF, and to permit the measurement of detailed and invasive parameters relating to the associated electrophysiological mechanisms of AF. The purpose of this review is to examine, consolidate and compare available relevant data on in-vivo electrophysiological mechanisms of AF suppression by currently approved and investigational anti-arrhythmic drugs in such models. These include the Vaughan Williams class I-IV drugs, namely Na(+) channel blockers, β-adrenoceptor antagonists, action potential prolonging drugs, and Ca(2+) channel blockers; the "upstream therapies", e.g., angiotensin converting enzyme inhibitors, statins and fish oils; and a variety of investigational drugs such as "atrial-selective" multiple ion channel blockers, gap junction-enhancers, and intracellular Ca(2+)-handling modulators. It is hoped that this will help to clarify the main electrophysiological mechanisms of action of different and related drug types in different disease settings, and the likely clinical significance and potential future exploitation of such mechanisms.

Atrial fibrillation-the final frontier

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia and represents a significant health care issue. The diagnosis and management of AF uses a significant proportion of the health care budget and is responsible for substantial morbidity and mortality. Restoration and maintenance of sinus rhythm is still an important treatment option for symptomatic AF. Anti-arrhythmic drugs (AADs) have had inconsistent results for the prevention of recurrent AF and have been hampered by significant adverse effects. Catheter ablation has rapidly evolved and is fast becoming an alternative for AF prevention. Although multiple treatment options exist, no single modality is effective for all patients. This review outlines best current practice for AF prevention and future perspectives, focusing on new and promising developments in antiarrhythmic drug therapy, strategies for ablation therapy, and forms of hybrid therapy that may offer improved outcomes in selected patients.

A Real-Time Screening Assay for GIRK1/4 Channel Blockers

The cardiac acetylcholine-activated K+ channel (IK,Ach) represents a novel target for drug therapy in the treatment of atrial fibrillation (AF). This channel is a member of the G-protein-coupled inward rectifier K+ (GIRK) channel superfamily and is composed of the GIRK1/4 (Kir3.1 and Kir3.4) subunits. The goal of this study was to develop a cell-based screening assay for identifying new blockers of the GIRK1/4 channel. The mouse atrial HL-1 cell line, expressing the GIRK1/4 channel, was plated in 96-well plate format, loaded with the fluorescent membrane potential-sensitive dye bis-(1,3-dibutylbarbituric acid) trimethine oxonol (DiBAC4(3)) and measured using a fluorescent imaging plate reader (FLIPR). Application of the muscarinic agonist carbachol to the cells caused a rapid, time-dependent decrease in the fluorescent signal, indicative of K+ efflux through the GIRK1/4 channel (carbachol vs. control solution, Z′ factor = 0.5-0.6). The GIRK1/4 channel fluorescent signal was blocked by BaCl2 and enhanced by increasing the driving force for K+ across the cell membrane. To test the utility of the assay for screening GIRK1/4 channel blockers, cells were treated with a small compound library of Na+ and K+ channel modulators. Analogues of amiloride and propafenone were identified as channel blockers at concentrations less than 1 µM. Thus, the GIRK1/4 channel assay may be used in the development of new and selective agents for treating AF.

Cellular modelling: experiments and simulation to develop a physiological model of G-protein control of muscarinic K+ channels in mammalian atrial cells

The first model of G-protein-K(ACh) channel interaction was developed 14 years ago and then expanded to include both the receptor-G-protein cycle and G-protein-K(ACh) channel interaction. The G-protein-K(ACh) channel interaction used the Monod-Wyman-Changeux allosteric model with the idea that one K(ACh) channel is composed of four subunits, each of which binds one active G-protein subunit (G(betagamma)). The receptor-G-protein cycle used a previous model to account for the steady-state relationship between K(ACh) current and intracellular guanosine-5-triphosphate at various extracellular concentrations of acetylcholine (ACh). However, simulations of the activation and deactivation of K(ACh) current upon ACh application or removal were much slower than experimental results. This clearly indicates some essential elements were absent from the model. We recently found that regulators of G-protein signalling are involved in the control of K(ACh) channel activity. They are responsible for the voltage-dependent relaxation behaviour of K(ACh) channels. Based on this finding, we have improved the receptor-G-protein cycle model to reproduce the relaxation behaviour. With this modification, the activation and deactivation of K(ACh) current in the model are much faster and now fall within physiological ranges.

Voltage-clamp-based methods for the detection of constitutively active acetylcholine-gated I(K,ACh) channels in the diseased heart

Vagal nerve stimulation can promote atrial fibrillation (AF) that requires activation of the acetylcholine (ACh)-gated potassium current I(K,ACh). In chronic AF (cAF), I(K,ACh) shows strong activity despite the absence of ACh or analogous pharmacological stimulation. This receptor-independent, constitutive I(K,ACh) activity is suggested to represent an atrial-selective anti-AF therapeutic target, but the underlying molecular mechanisms are unknown. This chapter provides an overview of the voltage-clamp techniques that can be used to study constitutive I(K,ACh) activity in atrial myocytes and summarizes briefly the current knowledge about the potential underlying mechanism(s) of constitutive I(K,ACh) activity in diseased heart.

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.

Information learned from animal models of atrial fibrillation

Animal models of atrial fibrillation have taught us about mechanisms of this common disease. A variety of animal models exist, including models of lone atrial fibrillation and models of atrial fibrillation in the setting of heart failure, aging, or pericardial inflammation. This article reviews these various models.