Calcium-activated potassium current: a novel ion channel candidate in atrial fibrillation

Dual effects of the small-conductance Ca2+-activated K+ current on human atrial electrophysiology and Ca2+-driven arrhythmogenesis: an in silico study

By sensing changes in intracellular Ca2+, small-conductance Ca2+-activated K+ (SK) channels dynamically regulate the dynamics of the cardiac action potential (AP) on a beat-to-beat basis. Given their predominance in atria versus ventricles, SK channels are considered a promising atrial-selective pharmacological target against atrial fibrillation (AF), the most common cardiac arrhythmia. However, the precise contribution of SK current (ISK) to atrial arrhythmogenesis is poorly understood, and may potentially involve different mechanisms that depend on species, heart rates, and degree of AF-induced atrial remodeling. Both reduced and enhanced ISK have been linked to AF. Similarly, both SK channel up- and downregulation have been reported in chronic AF (cAF) versus normal sinus rhythm (nSR) patient samples. Here, we use our multiscale modeling framework to obtain mechanistic insights into the contribution of ISK in human atrial cardiomyocyte electrophysiology. We simulate several protocols to quantify how ISK modulation affects the regulation of AP duration (APD), Ca2+ transient, refractoriness, and occurrence of alternans and delayed afterdepolarizations (DADs). Our simulations show that ISK activation shortens the APD and atrial effective refractory period, limits Ca2+ cycling, and slightly increases the propensity for alternans in both nSR and cAF conditions. We also show that increasing ISK counteracts DAD development by enhancing the repolarization force that opposes the Ca2+-dependent depolarization. Taken together, our results suggest that increasing ISK in human atrial cardiomyocytes could promote reentry while protecting against triggered activity. Depending on the leading arrhythmogenic mechanism, ISK inhibition may thus be a beneficial or detrimental anti-AF strategy.NEW & NOTEWORTHY Using our established framework for human atrial myocyte simulations, we investigated the role of the small-conductance Ca2+-activated K+ current (ISK) in the regulation of cell function and the development of Ca2+-driven arrhythmias. We found that ISK inhibition, a promising atrial-selective pharmacological strategy against atrial fibrillation, counteracts the reentry-promoting abbreviation of atrial refractoriness, but renders human atrial myocytes more vulnerable to delayed afterdepolarizations, thus potentially increasing the propensity for ectopic (triggered) activity.

Small-conductance calcium-activated potassium channels in the heart: expression, regulation and pathological implications

Ca2+-activated K+ channels are critical to cellular Ca2+ homeostasis and excitability; they couple intracellular Ca2+ and membrane voltage change. Of these, the small, 4-14 pS, conductance SK channels include three, KCNN1-3 encoded, SK1/KCa2.1, SK2/KCa2.2 and SK3/KCa2.3, channel subtypes with characteristic, EC50 ∼ 10 nM, 40 pM, 1 nM, apamin sensitivities. All SK channels, particularly SK2 channels, are expressed in atrial, ventricular and conducting system cardiomyocytes. Pharmacological and genetic modification results have suggested that SK channel block or knockout prolonged action potential durations (APDs) and effective refractory periods (ERPs) particularly in atrial, but also in ventricular, and sinoatrial, atrioventricular node and Purkinje myocytes, correspondingly affect arrhythmic tendency. Additionally, mitochondrial SK channels may decrease mitochondrial Ca2+ overload and reactive oxygen species generation. SK channels show low voltage but marked Ca2+ dependences (EC50 ∼ 300-500 nM) reflecting their α-subunit calmodulin (CaM) binding domains, through which they may be activated by voltage-gated or ryanodine-receptor Ca2+ channel activity. SK function also depends upon complex trafficking and expression processes and associations with other ion channels or subunits from different SK subtypes. Atrial and ventricular clinical arrhythmogenesis may follow both increased or decreased SK expression through decreased or increased APD correspondingly accelerating and stabilizing re-entrant rotors or increasing incidences of triggered activity. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Steady-state and transient effects of SK channel block and adrenergic stimulation to counteract acetylcholine-induced arrhythmogenic effects in the human atria: A computational study

Hyperactivity of the parasympathetic nervous system has been linked to the development of paroxysmal atrial fibrillation (AF). The parasympathetic neurotransmitter acetylcholine (ACh) causes a reduction in action potential (AP) duration (APD) and an increase in resting membrane potential (RMP), both of which contribute to enhance the risk for reentry. Research suggests that small-conductance calcium activated potassium (SK) channels may be an effective target for treating AF. Therapies targeting the autonomic nervous system, either alone or in combination with other drugs, have been explored and have been shown to decrease the incidence of atrial arrhythmias. This study uses computational modeling and simulation to examine the impact of SK channel block (SKb) and β-adrenergic stimulation through Isoproterenol (Iso) on countering the negative effects of cholinergic activity in human atrial cell and 2D tissue models. The steady-state effects of Iso and/or SKb on AP shape, APD at 90% repolarization (APD₉₀) and RMP were evaluated. The ability to terminate stable rotational activity in cholinergically-stimulated 2D tissue models of AF was also investigated. A range of SKb and Iso application kinetics, which reflect varying drug binding rates, were taken into consideration. The results showed that SKb alone prolonged APD₉₀ and was able to stop sustained rotors in the presence of ACh concentrations up to 0.01 μM. Iso terminated rotors under all tested ACh concentrations, but resulted in highly-variable steady-state outcomes depending on baseline AP morphology. Importantly, the combination of SKb and Iso resulted in greater APD₉₀ prolongation and showed promising anti-arrhythmic potential by stopping stable rotors and preventing re-inducibility.

The Inhibition of the Small-Conductance Ca2+-Activated Potassium Channels Decreases the Sinus Node Pacemaking during Beta-Adrenergic Activation

Sinus pacemaking is based on tight cooperation of intracellular Ca²⁺ handling and surface membrane ion channels. An important player of this synergistic crosstalk could be the small-conductance Ca²⁺-activated K⁺-channel (I_SK) that could contribute to the sinoatrial node (SAN) pacemaking driven by the intracellular Ca²⁺ changes under normal conditions and beta-adrenergic activation, however, the exact role is not fully clarified. SK2 channel expression was verified by immunoblot technique in rabbit SAN cells. Ionic currents and action potentials were measured by patch-clamp technique. The ECG R-R intervals were obtained by Langendorff-perfusion method on a rabbit heart. Apamin, a selective inhibitor of SK channels, was used during the experiments. Patch-clamp experiments revealed an apamin-sensitive current. When 100 nM apamin was applied, we found no change in the action potential nor in the ECG R-R interval. In experiments where isoproterenol was employed, apamin increased the cycle length of the SAN action potentials and enhanced the ECG R-R interval. Apamin did not amplify the cycle length variability or ECG R-R interval variability. Our data indicate that I_SK has no role under normal condition, however, it moderately contributes to the SAN automaticity under beta-adrenergic activation.

HDAC2-dependent remodeling of KCa2.2 (KCNN2) and KCa2.3 (KCNN3) K+ channels in atrial fibrillation with concomitant heart failure

AIMS

Atrial fibrillation (AF) with concomitant heart failure (HF) is associated with prolonged atrial refractoriness. Small-conductance, calcium-activated K⁺ (K_Ca, KCNN) channels promote action potential (AP) repolarization. KCNN2 and KCNN3 variants are associated with AF risk. In addition, histone deacetylase (HDAC)-related epigenetic mechanisms have been implicated in AP regulation. We hypothesized that HDAC2-dependent remodeling of KCNN2 and KCNN3 expression contributes to atrial arrhythmogenesis in AF complicated by HF. The objectives were to assess HDAC2 and KCNN2/3 transcript levels in AF/HF patients and in a pig model, and to investigate cellular epigenetic effects of HDAC2 inactivation on KCNN expression.

MATERIALS AND METHODS

HDAC2 and KCNN2/3 transcript levels were quantified in patients with AF and HF, and in a porcine model of atrial tachypacing-induced AF and reduced left ventricular function. Tachypacing and anti-Hdac2 siRNA treatment were employed in HL-1 atrial myocytes to study effects on KCNN2/3 mRNA and K_Ca protein abundance.

KEY FINDINGS

Atrial KCNN2 and KCNN3 expression was reduced in AF/HF patients and in a corresponding pig model. HDAC2 displayed significant downregulation in humans and a tendency towards reduced expression in right atrial tissue of pigs. Tachypacing recapitulated downregulation of Kcnn2/K_Ca2.2, Kcnn3/K_Ca2.3 and Hdac2/HDAC2, indicating that high atrial rates trigger epigenetic remodeling mechanisms. Finally, knock-down of Hdac2 in vitro reduced Kcnn3/K_Ca2.3 expression.

SIGNIFICANCE

KCNN2/3 and HDAC2 expression is suppressed in AF complicated by HF. Hdac2 directly regulates Kcnn3 mRNA levels in atrial cells. The mechanistic and therapeutic significance of epigenetic electrophysiological effects in AF requires further validation.

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

SK Channel Block and Adrenergic Stimulation Counteract Acetylcholine-Induced Arrhythmogenic Effects in Human Atria

There is increasing evidence on the role of the autonomic nervous system in the pathogenesis of atrial fibrillation. Interventions targeting autonomic modulation of atrial electrical activity have been shown to reduce the incidence of atrial arrhythmias. Additionally, recent investigations have proved that pharmacological therapies inhibiting small-conductance calcium-activated potassium (SK) channels are able to lessen cholinergic effects in the atria.In this study we use computational modeling and simulation to test individual and combined effects of SK channel block and adrenergic stimulation in counteracting detrimental effects induced by the parasympathetic neurotransmitter acetylcholine (ACh) on human atrial electrophysiology. Cell and tissue models are built that incorporate descriptions of SK channels as well as of isoproterenol (Iso)- and ACh-mediated regulation of the atrial action potential (AP). Three different cellular AP models, representing a range of physiological AP shapes, are considered and both homogeneous and heterogeneous ACh distributions in atrial tissue are simulated.At the cellular level, SK channel block is demonstrated to partially revert shortening of AP duration (APD) mediated by ACh at various doses, whereas 1 µM Iso has a variable response depending on the AP shape. The combination of SK block and Iso is in all cases able to take APD back to baseline levels, recovering between 82% and 120% of the APD shortening induced by 0.1 µM ACh. At the tissue level, SK block and Iso alone or in combination do not exert remarkable effects on conduction velocity, but the combination of the two is able to notably prolong the ACh-mediated APD shortening, thus increasing the wavelength for reentry.In conclusion, the results from this study support the combination of SK channel block and adrenergic stimulation as a potential option to counteract parasympathetically-mediated proarrhythmic effects in the human atria.

Towards chamber specific heart-on-a-chip for drug testing applications

Modeling of human organs has long been a task for scientists in order to lower the costs of therapeutic development and understand the pathological onset of human disease. For decades, despite marked differences in genetics and etiology, animal models remained the norm for drug discovery and disease modeling. Innovative biofabrication techniques have facilitated the development of organ-on-a-chip technology that has great potential to complement conventional animal models. However, human organ as a whole, more specifically the human heart, is difficult to regenerate in vitro, in terms of its chamber specific orientation and its electrical functional complexity. Recent progress with the development of induced pluripotent stem cell differentiation protocols, made recapitulating the complexity of the human heart possible through the generation of cells representative of atrial & ventricular tissue, the sinoatrial node, atrioventricular node and Purkinje fibers. Current heart-on-a-chip approaches incorporate biological, electrical, mechanical, and topographical cues to facilitate tissue maturation, therefore improving the predictive power for the chamber-specific therapeutic effects targeting adult human. In this review, we will give a summary of current advances in heart-on-a-chip technology and provide a comprehensive outlook on the challenges involved in the development of human physiologically relevant heart-on-a-chip.

Functional interaction of Junctophilin 2 with small- conductance Ca2+ -activated potassium channel subtype 2(SK2) in mouse cardiac myocytes

Aim

Junctophilins (JPs), a protein family of the junctional membrane complex, maintain the close conjunction between cell surface and intracellular membranes in striate muscle cells mediating the crosstalk between extracellular Ca²⁺ entry and intracellular Ca²⁺ release. The small‐conductance Ca²⁺‐activated K⁺ channels are activated by the intracellular calcium and play an essential role in the cardiac action potential profile. Molecular mechanisms of regulation of the SK channels are still uncertain. Here, we sought to determine whether there is a functional interaction of junctophilin type 2 (JP2) with the SK channels and whether JP2 gene silencing might modulate the function of SK channels in cardiac myocytes.

Methods

Association of JP2 with SK2 channel in mouse heart tissue as well as HEK293 cells was studied using in vivo and in vitro approaches. siRNA knockdown of JP2 gene was assessed by real‐time PCR. The expression of proteins was analysed by Western blotting. Ca²⁺‐activated K⁺ current (I_K,Ca) in infected adult mouse cardiac myocytes was recorded using whole‐cell voltage‐clamp technique. The intracellular Ca²⁺ transient was measured using an IonOptix photometry system.

Results

We showed for the first time that JP2 associates with the SK2 channel in native cardiac tissue. JP2, via the membrane occupation and recognition nexus (MORN motifs) in its N‐terminus, directly interacted with SK2 channels. A colocalization of the SK2 channel with its interaction protein of JP2 was found in the cardiac myocytes. Moreover, we demonstrated that JP2 is necessary for the proper cell surface expression of the SK2 channel in HEK293. Functional experiments indicated that knockdown of JP2 caused a significant decrease in the density of I_K,Ca and reduced the amplitude of the Ca²⁺ transient in infected cardiomyocytes.

Conclusion

The present data provide evidence that the functional interaction between JP2 and SK2 channels is present in the native mouse heart tissue. Junctophilin 2, as junctional membrane complex (JMC) protein, is an important regulator of the cardiac SK channels.

Antiarrhythmic effect of the Ca2+-activated K+ (SK) channel inhibitor ICA combined with either amiodarone or dofetilide in an isolated heart model of atrial fibrillation

Dose is an important parameter in terms of both efficacy and adverse effects in pharmacological treatment of atrial fibrillation (AF). Both of the class III antiarrhythmics dofetilide and amiodarone have documented anti-AF effects. While dofetilide has dose-related ventricular side effects, amiodarone primarily has adverse non-cardiac effects. Pharmacological inhibition of small conductance Ca²⁺-activated K⁺ (SK) channels has recently been reported to be antiarrhythmic in a number of animal AF models. In a Langendorff model of acutely induced AF on guinea pig hearts, it was investigated whether a combination of the SK channel blocker N-(pyridin-2-yl)-4-(pyridin-2-yl)thiazol-2-amine (ICA) together with either dofetilide or amiodarone provided a synergistic effect. The duration of AF was reduced with otherwise subefficacious concentrations of either dofetilide or amiodarone when combined with ICA, also at a subefficacious concentration. At a concentration level effective as monotherapy, dofetilide produced a marked increase in the QT interval. This QT prolonging effect was absent when combined with ICA at non-efficacious monotherapy concentrations. The results thereby reveal that combination of subefficacious concentrations of an SK channel blocker and either dofetilide or amiodarone can maintain anti-AF properties, while the risk of ventricular arrhythmias is reduced.

Role of Calcium-activated Potassium Channels in Atrial Fibrillation Pathophysiology and Therapy

Small-conductance Ca²⁺-activated potassium (SK) channels are relative newcomers within the field of cardiac electrophysiology. In recent years, an increased focus has been given to these channels because they might constitute a relatively atrial-selective target. This review will give a general introduction to SK channels followed by their proposed function in the heart under normal and pathophysiological conditions. It is revealed how antiarrhythmic effects can be obtained by SK channel inhibition in a number of species in situations of atrial fibrillation. On the contrary, the beneficial effects of SK channel inhibition in situations of heart failure are questionable and still needs investigation. The understanding of cardiac SK channels is rapidly increasing these years, and it is hoped that this will clarify whether SK channel inhibition has potential as a new anti–atrial fibrillation principle.

Mechanism of Proarrhythmic Effects of Potassium Channel Blockers

Any disturbance of electrical impulse formation in the heart and of impulse conduction or action potential (AP) repolarization can lead to rhythm disorders. Potassium (K(+)) channels play a prominent role in the AP repolarization process. In this review we describe the causes and mechanisms of proarrhythmic effects that arise as a response to blockers of cardiac K(+) channels. The largest and chemically most diverse groups of compound targets are Kv11.1 (hERG) and Kv7.1 (KvLQT1) channels. Finally, the proarrhythmic propensity of atrial-selective K(+) blockers inhibiting Kv1.5, Kir3.1/3.4, SK, and K2P channels is discussed.

Identification and Localization of Gold Nanoparticles in Potassium Ion Pores: Implications for Kir Blockade

INTRODUCTION

In our previous study, we found that negatively charged gold nanoparticles with spermidine have the potential of blocking inwardly rectifying potassium channels (Kir), both at the cellular and the tissue level.

METHODS

For the purpose of the present study, we purified the cytoplasmic domain of the Kir 3.1 receptor from Escherichia coli. Using single particles with surface coating by transmission electron microscopy, we identified the gold nanoparticles at the cytoplasmic side of the human Kir channel.

RESULTS

Energy-dispersive X-ray spectroscopy showed the presence of the gold deposits in the cytoplasmic domain of the Kir receptor.

CONCLUSION

In conclusion, we could identify undecagold in the ion pore of the Kir3.1 channel in order to clarify its direct blocking effect in the Kir ion pore by undecagold.

The electrophysiological development of cardiomyocytes

The generation of human cardiomyocytes (CMs) from human pluripotent stem cells (hPSCs) has become an important resource for modeling human cardiac disease and for drug screening, and also holds significant potential for cardiac regeneration. Many challenges remain to be overcome however, before innovation in this field can translate into a change in the morbidity and mortality associated with heart disease. Of particular importance for the future application of this technology is an improved understanding of the electrophysiologic characteristics of CMs, so that better protocols can be developed and optimized for generating hPSC-CMs. Many different cell culture protocols are currently utilized to generate CMs from hPSCs and all appear to yield relatively “developmentally” immature CMs with highly heterogeneous electrical properties. These hPSC-CMs are characterized by spontaneous beating at highly variable rates with a broad range of depolarization-repolarization patterns, suggestive of mixed populations containing atrial, ventricular and nodal cells. Many recent studies have attempted to introduce approaches to promote maturation and to create cells with specific functional properties. In this review, we summarize the studies in which the electrical properties of CMs derived from stem cells have been examined. In order to place this information in a useful context, we also review the electrical properties of CMs as they transition from the developing embryo to the adult human heart. The signal pathways involved in the regulation of ion channel expression during development are also briefly considered.

Apamin-Sensitive K+ Current Upregulation in Volume-Overload Heart Failure is Associated with the Decreased Interaction of CK2 with SK2

Recent studies have shown that the sensitivity of apamin-sensitive K(+) current (I KAS, mediated by apamin-sensitive small conductance calcium-activated potassium channels subunits) to intracellular Ca(2+) is increased in heart failure (HF), leading to I KAS upregulation, action potential duration shortening, early after depolarization, and recurrent spontaneous ventricular fibrillation. We hypothesized that casein kinase 2 (CK2) interacted with small conductance calcium-activated potassium channels (SK) is decreased in HF, and protein phosphatase 2A (PP2A) is increased on the opposite, upregulating the sensitivity of I KAS to intracellular Ca(2+) in HF. Rat model of volume-overload HF was established by an abdominal arteriovenous fistula procedure. The expression of SK channels, PP2A and CK2 was detected by Western blot analysis. Interaction and colocalization of CK2 with SK channel were detected by co-immunoprecipitation analysis and double immunofluorescence staining. In HF rat left ventricle, SK3 was increased by 100 % (P < 0.05), and SK2 was not significantly changed. PP2A protein was increased by 94.7 % in HF rats (P < 0.05), whereas the level of CK2 was almost unchanged. We found that CK2 colocalized with SK2 and SK3 in rat left ventricle. With anti-CK2α antibody, SK2 and SK3 were immunoprecipitated, the level of precipitated SK2 decreased by half, whereas precipitated SK3 was almost unchanged. In conclusion, the increased expression of total PP2A and decreased interaction of CK2 with SK2 may underlie enhanced sensitivity of I KAS to intracellular Ca(2+) in volume-overload HF rat.

Pharmacologic inhibition of small-conductance calcium-activated potassium (SK) channels by NS8593 reveals atrial antiarrhythmic potential in horses

BACKGROUND

Small-conductance calcium-activated potassium (SK) channels have been found to play an important role in atrial repolarization and atrial fibrillation (AF).

OBJECTIVE

The purpose of this study was to investigate the existence and functional role of SK channels in the equine heart.

METHODS

Cardiac biopsies were analyzed to investigate the expression level of the most prominent cardiac ion channels, with special focus on SK channels, in the equine heart. Subcellular distribution of SK isoform 2 (SK2) was assessed by immunohistochemistry and confocal microscopy. The electrophysiologic and anti-AF effects of the relative selective SK channel inhibitor NS8593 (5 mg/kg IV) were evaluated in anesthetized horses, focusing on the potential of NS8593 to terminate acute pacing-induced AF, drug-induced changes in atrial effective refractory period, AF duration and vulnerability, and ventricular depolarization and repolarization times.

RESULTS

Analysis revealed equivalent mRNA transcript levels of the 3 SK channel isoforms in atria compared to ventricles. Immunohistochemistry and confocal microscopy displayed a widespread distribution of SK2 in both atrial and ventricular cardiomyocytes. NS8593 terminated all induced AF episodes (duration ≥15 minutes), caused pronounced prolongation of atrial effective refractory period, and reduced AF duration and vulnerability. QRS duration and QTc interval were not affected by treatment.

CONCLUSION

SK channels are widely distributed in atrial and ventricular cardiomyocytes and contribute to atrial repolarization. Inhibition by NS8593 terminates pacing-induced AF of short duration and decreases AF duration and vulnerability without affecting ventricular conduction and repolarization. Thus, inhibition by NS8593 demonstrates clear atrial antiarrhythmic properties in healthy horses.

Calcium-activated potassium current modulates ventricular repolarization in chronic heart failure

The role of I_KCa in cardiac repolarization remains controversial and varies across species. The relevance of the current as a therapeutic target is therefore undefined. We examined the cellular electrophysiologic effects of I_KCa blockade in controls, chronic heart failure (HF) and HF with sustained atrial fibrillation. We used perforated patch action potential recordings to maintain intrinsic calcium cycling. The I_KCa blocker (apamin 100 nM) was used to examine the role of the current in atrial and ventricular myocytes. A canine tachypacing induced model of HF (1 and 4 months, n = 5 per group) was used, and compared to a group of 4 month HF with 6 weeks of superimposed atrial fibrillation (n = 7). A group of age-matched canine controls were used (n = 8). Human atrial and ventricular myocytes were isolated from explanted end-stage failing hearts which were obtained from transplant recipients, and studied in parallel. Atrial myocyte action potentials were unchanged by I_KCa blockade in all of the groups studied. I_KCa blockade did not affect ventricular myocyte repolarization in controls. HF caused prolongation of ventricular myocyte action potential repolarization. I_KCa blockade caused further prolongation of ventricular repolarization in HF and also caused repolarization instability and early afterdepolarizations. SK2 and SK3 expression in the atria and SK3 in the ventricle were increased in canine heart failure. We conclude that during HF, I_KCa blockade in ventricular myocytes results in cellular arrhythmias. Furthermore, our data suggest an important role for I_KCa in the maintenance of ventricular repolarization stability during chronic heart failure. Our findings suggest that novel antiarrhythmic therapies should have safety and efficacy evaluated in both atria and ventricles.

Gold nanoparticle-spermidine complex blocks the inward rectifier potassium channel

A previous study showed that negatively charged gold nanoparticles block ion pores by binding to the sulfur group of the cysteine loop of the ion channel when small molecules like amine lead the nanoparticles inside the ion pore. Cells were voltage clamped at -100 mV. Subsequently a bath application of 30 μM Ach produced a current followed by the extracellular application of 100 mM spermidine and 50 nM of nanoparticle complex. Peak amplitude was then recorded. The addition of Ach (30 uM) reversed the effect, and we recorded inhibition of the peak amplitude. We also recorded electrocardiogram (EKG) and the atria effective refractory period (AERP) after treatment with the complex in the atrium of a rabbit heart in a Langendorff apparatus. Upon external application of the complex, the Ach-activated current was blocked by 48.8% ± 3.1% with 82.7% ± 3.1% reversal. In recording the EKG and the AERP after the addition of the complex including 30 mM spermidine with 50 nM nanoparticles, the complete resolution of atrial fibrillation at 50 s and the elongation of AERP from 46 to 52 was observed, which unveils a new class 3 anti arrythmic agent using gold nanoparticles with spermidine. Negatively charged gold nanoparticles (0.8 nm) block ion pores after penetrating the cell membrane with spermidine, thus entering the cells with a polyamine transporter and acting at the intracellular face of the channel via binding to the sulfur group of the human inward rectifying potassium channel- I(KAch).

Cardiac potassium channel subtypes: new roles in repolarization and arrhythmia

About 10 distinct potassium channels in the heart are involved in shaping the action potential. Some of the K+ channels are primarily responsible for early repolarization, whereas others drive late repolarization and still others are open throughout the cardiac cycle. Three main K+ channels drive the late repolarization of the ventricle with some redundancy, and in atria this repolarization reserve is supplemented by the fairly atrial-specific KV1.5, Kir3, KCa, and K2P channels. The role of the latter two subtypes in atria is currently being clarified, and several findings indicate that they could constitute targets for new pharmacological treatment of atrial fibrillation. The interplay between the different K+ channel subtypes in both atria and ventricle is dynamic, and a significant up- and downregulation occurs in disease states such as atrial fibrillation or heart failure. The underlying posttranscriptional and posttranslational remodeling of the individual K+ channels changes their activity and significance relative to each other, and they must be viewed together to understand their role in keeping a stable heart rhythm, also under menacing conditions like attacks of reentry arrhythmia.

Small-conductance calcium-activated potassium (SK) channels contribute to action potential repolarization in human atria

AIMS

Small-conductance calcium-activated potassium (SK) channels are expressed in the heart of various species, including humans. The aim of the present study was to address whether SK channels play a functional role in human atria.

METHODS AND RESULTS

Quantitative real-time PCR analyses showed higher transcript levels of SK2 and SK3 than that of the SK1 subtype in human atrial tissue. SK2 and SK3 were reduced in chronic atrial fibrillation (AF) compared with sinus rhythm (SR) patients. Immunohistochemistry using confocal microscopy revealed widespread expression of SK2 in atrial myocytes. Two SK channel inhibitors (NS8593 and ICAGEN) were tested in heterologous expression systems revealing ICAGEN as being highly selective for SK channels, while NS8593 showed less selectivity for these channels. In isolated atrial myocytes from SR patients, both inhibitors decreased inwardly rectifying K(+) currents by ∼15% and prolonged action potential duration (APD), but no effect was observed in myocytes from AF patients. In trabeculae muscle strips from right atrial appendages of SR patients, both compounds increased APD and effective refractory period, and depolarized the resting membrane potential, while only NS8593 induced these effects in tissue from AF patients. SK channel inhibition did not alter any electrophysiological parameter in human interventricular septum tissue.

CONCLUSIONS

SK channels are present in human atria where they participate in repolarization. SK2 and SK3 were down-regulated and had reduced functional importance in chronic AF. As SK current was not found to contribute substantially to the ventricular AP, pharmacological inhibition of SK channels may be a putative atrial-selective target for future antiarrhythmic drug therapy.

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.

Ability to induce atrial fibrillation in the peri-operative period is associated with phosphorylation-dependent inhibition of TWIK protein-related acid-sensitive potassium channel 1 (TASK-1)

Peri-operative atrial fibrillation (peri-op AF) is a common complication following thoracic surgery. This arrhythmia is thought to be triggered by an inflammatory response and can be reproduced in various animal models. Previous work has shown that the lipid inflammatory mediator, platelet-activating factor (PAF), synthesized by activated neutrophils, can induce atrial and ventricular arrhythmias as well as repolarization abnormalities in isolated ventricular myocytes. We have previously shown that carbamylated PAF-induced repolarization abnormalities result from the protein kinase C (PKC) ε-dependent phosphorylation of the two-pore domain potassium channel TASK-1. We now demonstrate that canine peri-op AF is associated with the phosphorylation-dependent loss of TASK-1 current. Further studies identified threonine 383 in the C terminus of human and canine TASK-1 as the phosphorylation site required for PAF-dependent inhibition of the channel. Using a novel phosphorylation site-specific antibody targeting the phosphorylated channel, we have determined that peri-op AF is associated with the loss of TASK-1 current and increased phosphorylation of TASK-1 at this site.

The multidimensional role of calcium in atrial fibrillation pathophysiology: mechanistic insights and therapeutic opportunities

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia, and its prevalence is increasing with the ageing of the population. Presently available treatment options are far from optimal and new insights into underlying mechanisms are needed to improve therapy. A variety of recent lines of research are converging to reveal important and relatively underappreciated multidimensional roles of cellular Ca(2+) content, distribution, and handling in AF pathophysiology. The objective of the present paper is to review the participation of changes in cell Ca(2+) and related processes in the mechanisms that lead to AF initiation and maintenance, and to consider the relevance of new knowledge in this area to therapeutic innovation. We first review the involvement of Ca(2+)-related functions in the principal arrhythmia mechanisms underlying AF: focal ectopic activity due to afterdepolarizations and re-entrant mechanisms. The detailed molecular pathophysiology of focal ectopic and re-entrant activity is then discussed in relationship to the participation of cell Ca(2+) changes and related Ca(2+)-handling and Ca(2+)-sensitive signalling systems. We then go on to consider the participation of Ca(2+)-related functions in electrical and structural remodelling processes leading to the AF substrate. Finally, we consider the implications for development of new arrhythmia management approaches and future research and development.

Decreased expression of small-conductance Ca2+-activated K+ channels SK1 and SK2 in human chronic atrial fibrillation

AIMS

Small-conductance Ca2+-activated K+ (SK) channels are recognized as new ion channel candidates in atrial fibrillation (AF), with pivotal implications as novel drug targets due to their atrial-selective distribution in humans. The purpose of this study was to investigate whether SK channels and the Ca2+-activated K+ current (IK,Ca) are involved in electrical remodeling of human chronic AF (cAF) and whether they display the differential distribution between the right (RA) and left atria (LA).

MAIN METHODS

The right (RAA) and left atrial appendage (LAA) myocytes were obtained from 29 sinus rhythm (SR) and 22 cAF patients. The IK,Ca and action potential (AP) were recorded using the patch-clamp technique. Three SK channel subtypes (SK1-3) expressions were assayed by western blot and real-time quantitative PCR analysis.

KEY FINDINGS

The IK,Ca was decreased and its role in AP repolarization was attenuated in cAF, concomitant with a significant decrease in protein and mRNA levels of SK1 and SK2. In either SR or cAF, there was no difference in the IK,Ca density and protein and mRNA expression levels of SK1-3 between RAA and LAA myocytes.

SIGNIFICANCE

Our results demonstrated that SK1 and SK2 are involved in electrical remodeling of cAF. SK1-3 and IK,Ca do not display the inter-atrial differential distribution in SR or cAF. These findings provide a new insight into mechanisms of electrical remodeling of human cAF.