INaCa and ICl(Ca) contribute to isoproterenol-induced delayed after depolarizations in midmyocardial cells

Exploring the role of TRPM4 in calcium-dependent triggered activity and cardiac arrhythmias

Cardiac arrhythmias pose a major threat to a patient's health, yet prove to be often difficult to predict, prevent and treat. A key mechanism in the occurrence of arrhythmias is disturbed Ca2+ homeostasis in cardiac muscle cells. As a Ca2+-activated non-selective cation channel, TRPM4 has been linked to Ca2+-induced arrhythmias, potentially contributing to translating an increase in intracellular Ca2+ concentration into membrane depolarisation and an increase in cellular excitability. Indeed, evidence from genetically modified mice, analysis of mutations in human patients and the identification of a TRPM4 blocking compound that can be applied in vivo further underscore this hypothesis. Here, we provide an overview of these data in the context of our current understanding of Ca2+-dependent arrhythmias.

TRPM4 inhibition by meclofenamate suppresses Ca2+-dependent triggered arrhythmias

AIMS

Cardiac arrhythmias are a major factor in the occurrence of morbidity and sudden death in patients with cardiovascular disease. Disturbances of Ca2+ homeostasis in the heart contribute to the initiation and maintenance of cardiac arrhythmias. Extrasystolic increases in intracellular Ca2+ lead to delayed afterdepolarizations and triggered activity, which can result in heart rhythm abnormalities. It is being suggested that the Ca2+-activated nonselective cation channel TRPM4 is involved in the aetiology of triggered activity, but the exact contribution and in vivo significance are still unclear.

METHODS AND RESULTS

In vitro electrophysiological and calcium imaging technique as well as in vivo intracardiac and telemetric electrocardiogram measurements in physiological and pathophysiological conditions were performed. In two distinct Ca2+-dependent proarrhythmic models, freely moving Trpm4-/- mice displayed a reduced burden of cardiac arrhythmias. Looking further into the specific contribution of TRPM4 to the cellular mechanism of arrhythmias, TRPM4 was found to contribute to a long-lasting Ca2+ overload-induced background current, thereby regulating cell excitability in Ca2+ overload conditions. To expand these results, a compound screening revealed meclofenamate as a potent antagonist of TRPM4. In line with the findings from Trpm4-/- mice, 10 µM meclofenamate inhibited the Ca2+ overload-induced background current in ventricular cardiomyocytes and 15 mg/kg meclofenamate suppressed catecholaminergic polymorphic ventricular tachycardia-associated arrhythmias in a TRPM4-dependent manner.

CONCLUSION

The presented data establish that TRPM4 represents a novel target in the prevention and treatment of Ca2+-dependent triggered arrhythmias.

Late Sodium Current of the Heart: Where Do We Stand and Where Are We Going?

Late sodium current has long been linked to dysrhythmia and contractile malfunction in the heart. Despite the increasing body of accumulating information on the subject, our understanding of its role in normal or pathologic states is not complete. Even though the role of late sodium current in shaping action potential under physiologic circumstances is debated, it’s unquestioned role in arrhythmogenesis keeps it in the focus of research. Transgenic mouse models and isoform-specific pharmacological tools have proved useful in understanding the mechanism of late sodium current in health and disease. This review will outline the mechanism and function of cardiac late sodium current with special focus on the recent advances of the area.

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.

Intracellular calcium leak in heart failure and atrial fibrillation: a unifying mechanism and therapeutic target

Ca²⁺ is a fundamental second messenger in all cell types and is required for numerous essential cellular functions, including cardiac and skeletal muscle contraction. The intracellular concentration of free Ca²⁺ ([Ca²⁺]) is regulated primarily by ion channels, pumps (ATPases), exchangers and Ca²⁺-binding proteins. Defective regulation of [Ca²⁺] is found in a diverse spectrum of pathological states that affect all the major organs. In the heart, abnormalities in the regulation of cytosolic and mitochondrial [Ca²⁺] occur in heart failure (HF) and atrial fibrillation (AF), two common forms of heart disease and leading contributors to morbidity and mortality. In this Review, we focus on the mechanisms that regulate ryanodine receptor 2 (RYR2), the major sarcoplasmic reticulum (SR) Ca²⁺-release channel in the heart, how RYR2 becomes dysfunctional in HF and AF, and its potential as a therapeutic target. Inherited RYR2 mutations and/or stress-induced phosphorylation and oxidation of the protein destabilize the closed state of the channel, resulting in a pathological diastolic Ca²⁺ leak from the SR that both triggers arrhythmias and impairs contractility. On the basis of our increased understanding of SR Ca²⁺ leak as a shared Ca²⁺-dependent pathological mechanism in HF and AF, a new class of drugs developed in our laboratory, known as rycals, which stabilize RYR2 channels and prevent Ca²⁺ leak from the SR, are undergoing investigation in clinical trials.

Arrhythmogenic Mechanisms in Heart Failure: Linking β-Adrenergic Stimulation, Stretch, and Calcium

Heart failure (HF) is associated with elevated sympathetic tone and mechanical load. Both systems activate signaling transduction pathways that increase cardiac output, but eventually become part of the disease process itself leading to further worsening of cardiac function. These alterations can adversely contribute to electrical instability, at least in part due to the modulation of Ca²⁺ handling at the level of the single cardiac myocyte. The major aim of this review is to provide a definitive overview of the links and cross talk between β-adrenergic stimulation, mechanical load, and arrhythmogenesis in the setting of HF. We will initially review the role of Ca²⁺ in the induction of both early and delayed afterdepolarizations, the role that β-adrenergic stimulation plays in the initiation of these and how the propensity for these may be altered in HF. We will then go onto reviewing the current data with regards to the link between mechanical load and afterdepolarizations, the associated mechano-sensitivity of the ryanodine receptor and other stretch activated channels that may be associated with HF-associated arrhythmias. Furthermore, we will discuss how alterations in local Ca²⁺ microdomains during the remodeling process associated the HF may contribute to the increased disposition for β-adrenergic or stretch induced arrhythmogenic triggers. Finally, the potential mechanisms linking β-adrenergic stimulation and mechanical stretch will be clarified, with the aim of finding common modalities of arrhythmogenesis that could be targeted by novel therapeutic agents in the setting of HF.

Role of abnormal repolarization in the mechanism of cardiac arrhythmia

In cardiac patients, life-threatening tachyarrhythmia is often precipitated by abnormal changes in ventricular repolarization and refractoriness. Repolarization abnormalities typically evolve as a consequence of impaired function of outward K⁺ currents in cardiac myocytes, which may be caused by genetic defects or result from various acquired pathophysiological conditions, including electrical remodelling in cardiac disease, ion channel modulation by clinically used pharmacological agents, and systemic electrolyte disorders seen in heart failure, such as hypokalaemia. Cardiac electrical instability attributed to abnormal repolarization relies on the complex interplay between a provocative arrhythmic trigger and vulnerable arrhythmic substrate, with a central role played by the excessive prolongation of ventricular action potential duration, impaired intracellular Ca²⁺ handling, and slowed impulse conduction. This review outlines the electrical activity of ventricular myocytes in normal conditions and cardiac disease, describes classical electrophysiological mechanisms of cardiac arrhythmia, and provides an update on repolarization-related surrogates currently used to assess arrhythmic propensity, including spatial dispersion of repolarization, activation-repolarization coupling, electrical restitution, TRIaD (triangulation, reverse use dependence, instability, and dispersion), and the electromechanical window. This is followed by a discussion of the mechanisms that account for the dependence of arrhythmic vulnerability on the location of the ventricular pacing site. Finally, the review clarifies the electrophysiological basis for cardiac arrhythmia produced by hypokalaemia, and gives insight into the clinical importance and pathophysiology of drug-induced arrhythmia, with particular focus on class Ia (quinidine, procainamide) and Ic (flecainide) Na⁺ channel blockers, and class III antiarrhythmic agents that block the delayed rectifier K⁺ channel (dofetilide).

Ca2+-activated Cl- current is antiarrhythmic by reducing both spatial and temporal heterogeneity of cardiac repolarization

The role of Ca²⁺-activated Cl^- current (I_Cl(Ca)) in cardiac arrhythmias is still controversial. It can generate delayed afterdepolarizations in Ca²⁺-overloaded cells while in other studies incidence of early afterdepolarization (EAD) was reduced by I_Cl(Ca). Therefore our goal was to examine the role of I_Cl(Ca) in spatial and temporal heterogeneity of cardiac repolarization and EAD formation. Experiments were performed on isolated canine cardiomyocytes originating from various regions of the left ventricle; subepicardial, midmyocardial and subendocardial cells, as well as apical and basal cells of the midmyocardium. I_Cl(Ca) was blocked by 0.5mmol/L 9-anthracene carboxylic acid (9-AC). Action potential (AP) changes were tested with sharp microelectrode recording. Whole-cell 9-AC-sensitive current was measured with either square pulse voltage-clamp or AP voltage-clamp (APVC). Protein expression of TMEM16A and Bestrophin-3, ion channel proteins mediating I_Cl(Ca), was detected by Western blot. 9-AC reduced phase-1 repolarization in every tested cell. 9-AC also increased AP duration in a reverse rate-dependent manner in all cell types except for subepicardial cells. Neither I_Cl(Ca) density recorded with square pulses nor the normalized expressions of TMEM16A and Bestrophin-3 proteins differed significantly among the examined groups of cells. The early outward component of I_Cl(Ca) was significantly larger in subepicardial than in subendocardial cells in APVC setting. Applying a typical subepicardial AP as a command pulse resulted in a significantly larger early outward component in both subepicardial and subendocardial cells, compared to experiments when a typical subendocardial AP was applied. Inhibiting I_Cl(Ca) by 9-AC generated EADs at low stimulation rates and their incidence increased upon beta-adrenergic stimulation. 9-AC increased the short-term variability of repolarization also. We suggest a protective role for I_Cl(Ca) against risk of arrhythmias by reducing spatial and temporal heterogeneity of cardiac repolarization and EAD formation.

Ca(2+)-activated chloride channel activity during Ca(2+) alternans in ventricular myocytes

Cardiac alternans, defined beat-to-beat alternations in contraction, action potential (AP) morphology or cytosolic Ca transient (CaT) amplitude, is a high risk indicator for cardiac arrhythmias. We investigated mechanisms of cardiac alternans in single rabbit ventricular myocytes. CaTs were monitored simultaneously with membrane currents or APs recorded with the patch clamp technique. A strong correlation between beat-to-beat alternations of AP morphology and CaT alternans was observed. During CaT alternans application of voltage clamp protocols in form of pre-recorded APs revealed a prominent Ca(2+)-dependent membrane current consisting of a large outward component coinciding with AP phases 1 and 2, followed by an inward current during AP repolarization. Approximately 85% of the initial outward current was blocked by Cl(-) channel blocker DIDS or lowering external Cl(-) concentration identifying it as a Ca(2+)-activated Cl(-) current (ICaCC). The data suggest that ICaCC plays a critical role in shaping beat-to-beat alternations in AP morphology during alternans.

Effect of α-Allocryptopine on Delayed Afterdepolarizations and Triggered Activities in Mice Cardiomyocytes Treated with Isoproterenol

Objective. To investigate the effect of α-allocryptopine (ALL) on delayed afterdepolarization (DAD) incidence and triggered activity (TA) in mice administered isoproterenol (ISO). Methods. Mouse ventricular myocytes were isolated. And the cellular electrophysiological properties of ventricular myocytes were investigated. Results. We found that the incidences of DADs and TA in mouse myocytes were increased by ISO treatment. In sharp contrast, triggered arrhythmia events were rarely observed in myocytes with 10 μM ALL treatment. Transient inward current (I ti) was reduced significantly with ALL treatment, which contributed to DAD-related triggered arrhythmia. Compared to Iso-treated group, the L-type calcium current (I Ca,L) densities were decreased after exposure to ALL, along with slower activation, quicker inactivation, and longer time constant of recovery from inactivation kinetics. Conclusion. There is less triggered arrhythmia events in ventricular myocytes treated with ALL. This effect may be associated with the inhibition of I ti and I Ca,L.

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.

Explaining calcium-dependent gating of anoctamin-1 chloride channels requires a revised topology

RATIONALE

Ca2+ -activated Cl channels play pivotal roles in the cardiovascular system. They regulate vascular smooth muscle tone and participate in cardiac action potential repolarization in some species. Ca2+ -activated Cl channels were recently discovered to be encoded by members of the anoctamin (Ano, also called Tmem16) superfamily, but the mechanisms of Ano1 gating by Ca2+ remain enigmatic.

OBJECTIVE

The objective was to identify regions of Ano1 involved in channel gating by Ca2+.

METHODS AND RESULTS

The Ca2+ sensitivity of Ano1 was estimated from rates of current activation, and deactivation in excised patches rapidly switched between zero and high Ca2+ on the cytoplasmic side. Mutation of glutamates E702 and E705 dramatically altered Ca2+ sensitivity. E702 and E705 are predicted to be in an extracellular loop, but antigenic epitopes introduced into this loop are not accessible to extracellular antibodies, suggesting this loop is intracellular. Cytoplasmically applied membrane-impermeant sulfhydryl reagents alter the Ca2+ sensitivity of Ano1 E702C and E705C as expected if E702 and E705 are intracellular. Substituted cysteine accessibility mutagenesis of the putative re-entrant loop suggests that E702 and E705 are located adjacent to the Cl conduction pathway.

CONCLUSIONS

We propose an alternative model of Ano1 topology based on mutagenesis, epitope accessibility, and cysteine-scanning accessibility. These data contradict the popular re-entrant loop model by showing that the putative fourth extracellular loop (ECL 4) is intracellular and may contain a Ca2+ binding site. These studies provide new perspectives on regulation of Ano1 by Ca2+.

Proarrhythmia in a non-failing murine model of cardiac-specific Na+/Ca 2+ exchanger overexpression: whole heart and cellular mechanisms

The cardiac Na(+)/Ca(2+) exchanger (NCX) generates an inward electrical current during SR-Ca(2+) release, thus possibly promoting afterdepolarizations of the action potential (AP). We used transgenic mice 12.5 weeks or younger with cardiomyocyte-directed overexpression of NCX (NCX-Tg) to study the proarrhythmic potential and mechanisms of enhanced NCX activity. NCX-Tg exhibited normal echocardiographic left ventricular function and heart/body weight ratio, while the QT interval was prolonged in surface ECG recordings. Langendorff-perfused NCX-Tg, but not wild-type (WT) hearts, developed ventricular tachycardia. APs and ionic currents were measured in isolated cardiomyocytes. Cell capacitance was unaltered between groups. APs were prolonged in NCX-Tg versus WT myocytes along with voltage-activated K(+) currents (K(v)) not being reduced but even increased in amplitude. During abrupt changes in pacing cycle length, early afterdepolarizations (EADs) were frequently recorded in NCX-Tg but not in WT myocytes. Next to EADs, delayed afterdepolarizations (DAD) triggering spontaneous APs (sAPs) occurred in NCX-Tg but not in WT myocytes. To test whether sAPs were associated with spontaneous Ca(2+) release (sCR), Ca(2+) transients were recorded. Despite the absence of sAPs in WT, sCR was observed in myocytes of both genotypes suggesting a facilitated translation of sCR into DADs in NCX-Tg. Moreover, sCR was more frequent in NCX-Tg as compared to WT. Myocardial protein levels of Ca(2+)-handling proteins were not different between groups except the ryanodine receptor (RyR), which was increased in NCX-Tg versus WT. We conclude that NCX overexpression is proarrhythmic in a non-failing environment even in the absence of reduced K(V). The underlying mechanisms are: (1) occurrence of EADs due to delayed repolarization; (2) facilitated translation from sCR into DADs; (3) proneness to sCR possibly caused by altered Ca(2+) handling and/or increased RyR expression.

Beta-adrenoceptor stimulation exacerbates detrimental effects of ischemia and reperfusion in isolated guinea pig ventricular myocytes

We investigated whether beta-adrenoceptor stimulation exacerbates detrimental effects of ischemia and reperfusion on electrical and contractile function and on intracellular Ca(2+) homeostasis in isolated guinea pig ventricular myocytes. Myocytes were exposed to 20 min of simulated ischemia (37 degrees C) in the absence or presence of isoproterenol (10 nM, applied prior to and during ischemia) and reperfused with Tyrode's solution for 30 min. Unloaded cell shortening, Ca(2+) transients (fura-2), and cell viability were recorded at 5 min intervals in field-stimulated cells (2 Hz). In experiments using microelectrodes, membrane potentials, contractions, and transmembrane currents also were recorded at 5 min intervals. In the absence of ischemia, 10 nM isoproterenol had little effect on either contractile function or Ca(2+) homeostasis. In contrast, when cells were exposed to ischemia, isoproterenol increased the size of contractions and Ca(2+) transients and augmented the increase in diastolic Ca(2+) concentration during ischemia in field-stimulated myocytes. Exposure to isoproterenol also promoted contractile depression in reperfusion. In voltage clamp experiments, isoproterenol abolished the decrease in the magnitude of L-type Ca(2+) current caused by ischemia. Isoproterenol also increased the incidence of abnormal contractile activity and induced delayed afterdepolarizations and the arrhythmogenic transient inward current in ischemia. Additionally, the decline in cell viability in ischemia and reperfusion was exacerbated by isoproterenol. This study shows that beta-adrenoceptor stimulation strongly potentiates adverse effects of ischemia and reperfusion on electrical and contractile function. These adverse effects of isoproterenol are likely caused by an increase in intracellular Ca(2+) accumulation during ischemia.

Mechanisms and potential therapeutic targets for ventricular arrhythmias associated with impaired cardiac calcium cycling

The close relationship between life-threatening ventricular arrhythmias and contractile dysfunction in the heart implicates intracellular calcium cycling as an important underlying mechanism of arrhythmogenesis. Despite this close association, however, the mechanisms of arrhythmogenesis attributable to impaired calcium cycling are not fully appreciated or understood. In this report we review some of the current thinking regarding arrhythmia mechanisms associated with either abnormal impulse initiation (i.e. arrhythmia triggers) or impulse propagation (i.e. arrhythmia substrates). In all cases, the mechanisms are primarily related to dysfunction of calcium regulatory proteins associated with the sarcomere. These findings highlight the broad scope of arrhythmias associated with abnormal calcium cycling, and provide a basis for a causal relationship between cardiac electrical instability and contractile dysfunction. Moreover, calcium cycling proteins may provide much needed targets for novel antiarrhythmic therapies.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Cellular basis for the electrocardiographic and arrhythmic manifestations of Timothy syndrome: effects of ranolazine

BACKGROUND

Timothy syndrome is a multisystem disorder associated with QT interval prolongation and ventricular cardiac arrhythmias. The syndrome has been linked to mutations in Ca(V)1.2 resulting in gain of function of the L-type calcium current (I(Ca,L)). Ranolazine is an antianginal agent shown to exert an antiarrhythmic effect in experimental models of long QT syndrome.

OBJECTIVE

The purpose of this study was to develop and characterize an experimental model of Timothy syndrome by using BayK8644 to mimic the gain of function of I(Ca,L) and to examine the effects of ranolazine.

METHODS

Action potentials from epicardial and M regions and a pseudo-electrocardiogram (ECG) were simultaneously recorded from coronary-perfused left ventricular wedge preparations, before and after addition of BayK8644 (1 microM).

RESULTS

BayK8644 preferentially prolonged action potential duration of the M cell, leading to prolongation of the QT interval and an increase in transmural dispersion of repolarization (from 44.3 +/- 7 ms to 86.5 +/- 25 ms). Stimulation at cycle lengths of 250-500 ms led to ST-T wave alternans due to alternation of the plateau voltage of the M cell action potential as well as development of delayed afterdepolarizations in epicardial and M cell action potentials. Ventricular extrasystoles and tachycardia (monomorphic, bidirectional, or torsades de pointes) developed spontaneously or after rapid pacing. Peak and late I(Na) were unaffected by BayK8644. Clinically relevant concentrations of ranolazine (10 microM) suppressed all actions of BayK8644.

CONCLUSION

A left ventricular wedge model of long QT syndrome created by augmentation of I(Ca,L) recapitulates the ECG and arrhythmic manifestations of Timothy syndrome, which can be suppressed by ranolazine.

Late-phase 3 EAD. A unique mechanism contributing to initiation of atrial fibrillation

Early (EAD) and delayed (DAD) afterdepolarizations-induced triggered activity is capable of initiating and maintaining cardiac arrhythmias. EAD-induced triggered responses are traditionally thought to be involved in the generation of ventricular arrhythmias under long QT conditions and are precipitated by bradycardia or long pauses. In contrast, DAD-induced triggered activity commonly underlies arrhythmias precipitated by tachycardia. Spontaneous release of calcium from the sarcoplasmic reticulum (SR) secondary to cellular calcium overload induces DADs and some forms of EADs. Recent studies from our laboratory have uncovered a novel mechanism giving rise to triggered activity, termed "late-phase 3 EAD," which combines properties of both EAD and DAD, but has its own unique character. Late-phase 3 EAD-induced triggered extrasystoles represent a new concept of arrhythmogenesis in which abbreviated repolarization permits "normal SR calcium release" to induce an EAD-mediated closely coupled triggered response, particularly under conditions permitting intracellular calcium loading. This review briefly describes the mechanisms and properties of late-phase 3 EADs, how they differ from conventional EADs and DADs, as well as their role in the initiation of cardiac arrhythmias, such as atrial fibrillation.

Severe arrhythmia disorder caused by cardiac L-type calcium channel mutations

Timothy syndrome (TS) is a multisystem disorder that causes syncope and sudden death from cardiac arrhythmias. Prominent features include congenital heart disease, immune deficiency, intermittent hypoglycemia, cognitive abnormalities, and autism. All TS individuals have syndactyly (webbing of fingers and toes). We discovered that TS resulted from a recurrent, de novo cardiac L-type calcium channel (CaV1.2) mutation, G406R. G406 is located in alternatively spliced exon 8A, encoding transmembrane segment S6 of domain I. Here, we describe two individuals with a severe variant of TS (TS2). Neither child had syndactyly. Both individuals had extreme prolongation of the QT interval on electrocardiogram, with a QT interval corrected for heart rate ranging from 620 to 730 ms, causing multiple arrhythmias and sudden death. One individual had severe mental retardation and nemaline rod skeletal myopathy. We identified de novo missense mutations in exon 8 of CaV1.2 in both individuals. One was an analogous mutation to that found in exon 8A in classic TS, G406R. The other mutation was G402S. Exon 8 encodes the same region as exon 8A, and the two are mutually exclusive. The spliced form of CaV1.2 containing exon 8 is highly expressed in heart and brain, accounting for approximately 80% of CaV1.2 mRNAs. G406R and G402S cause reduced channel inactivation, resulting in maintained depolarizing L-type calcium currents. Computer modeling showed prolongation of cardiomyocyte action potentials and delayed afterdepolarizations, factors that increase risk of arrhythmia. These data indicate that gain-of-function mutations of CaV1.2 exons 8 and 8A cause distinct forms of TS.

Behavior of Ca(2+) waves in multicellular preparations from guinea pig ventricle

Ca(+) waves have been implicated in Ca(2+) overload-induced cardiac arrhythmias. To deepen understanding of the behavior of Ca(2+) waves in a multicellular system, consecutive two-dimensional Ca(2+) images were obtained with a confocal microscope from surface cells of guinea pig ventricular papillary muscles loaded with fluo 3 or rhod 2. In intact muscles, no Ca(2+) waves were detected under the resting condition, whereas they were frequently observed during the rest immediately after high-frequency stimulations where cytoplasmic Ca(2+) concentration and Ca(2+) stored in the sarcoplasmic reticulum (SR) were gradually decreasing. The intervals of Ca(2+) waves increased as they occurred later, their amplitudes and velocities remaining unchanged. A SERCA inhibitor reversibly prolonged the wave intervals. In Na(+)-free/Ca(2+)-free medium where neither Ca(2+) influx nor Na(+)/Ca(2+) exchange took place, recurrent Ca(2+) waves emerged at constant intervals in each cell. These results are consistent with the conclusion that the loading level of the SR is critical for induction of Ca(2+) waves. Each cell independently exhibited its own regular rhythm of Ca(2+) wave with a distinct interval. These waves propagated in either direction along the longitudinal axis within a muscle cell, but seldom beyond the cell boundary. In contrast, in partially damaged muscles that showed spontaneous Ca(2+) waves at rest in normal Krebs solution, their propagation often was unidirectional, decreasing in frequency. In these cases, however, Ca(2+) waves rarely moved beyond the cellular boundary. The gradient of the cytoplasmic Ca(2+) concentration was suggested to be the cause of the one-way propagation.

Ca2+-activated Cl- current reduces transmural electrical heterogeneity within the rabbit left ventricle

OBJECTIVE

Various cationic membrane channels contribute to the heterogeneity of action potential configuration between the transmural layers of the left ventricle. The role of anionic membrane channels is less intensively studied. We investigated the role of the Ca2+-activated Cl- current, ICl(Ca), in transmural electrical heterogeneity.

METHODS AND RESULTS

We determined the density of ICl(Ca) and its physiological role in subepicardial and subendocardial ventricular myocytes of rabbit using the patch-clamp technique. ICl(Ca) was measured as the 4,4'diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) sensitive current. The current-voltage relationships and the densities of ICl(Ca) were similar in subepicardial and subendocardial myocytes. However, the functional role of ICl(Ca) exhibited striking differences. In subendocardial myocytes, blockade of ICl(Ca) by DIDS increased action potential duration (APD) significantly at all measured stimulus frequencies (3.33-0.2 Hz). In subepicardial myocytes, ICl(Ca) blockade increased APD only at 3.33 Hz, but not at the lower stimulus frequencies. At 1 Hz, ICl(Ca) blockade in subepicardial myocytes only caused an APD increase when the transient outward K+ current, Ito1, was blocked.

CONCLUSIONS

The densities and gating properties of ICl(Ca) are similar in subepicardial and subendocardial myocytes. ICl(Ca) contributes to APD shortening in subendocardial, but not in subepicardial myocytes except at 3.33 Hz. These differences in functional expression of ICl(Ca) reduce the electrical heterogeneity in rabbit left ventricle.

Induced automaticity in isolated rat atrial cells by incorporation of a stretch-activated conductance

Stretch of the atrium and sympathetic activity have been implicated as substrates for atrial fibrillation. We investigate how a model of stretch in combination with sympathetic stimulation can induce automaticity in atrial cells. We adapted our coupling clamp circuit so that a model ionic current that represents stretch-activated channels (SACs) was injected into an isolated rat atrial cell in real time. This current was calculated as ISAC= GSAC (Vm-ESAC), where GSAC and ESAC are the conductance and reversal potential of SACs and Vm is the cell's membrane potential. Repetitive automaticity was induced by a sufficiently large GSAC and this critical value of GSAC was decreased by exposure to isoproterenol. The critical value of GSAC decreased from 0.63+/-0.05 nS (mean+/-SE) in control to 0.40+/-0.07 nS in isoproterenol (P<0.05). Additionally, after exposure to isoproterenol, automaticity continued after GSAC was no longer applied and was accompanied by delayed after-depolarizations. In three cells, repetitive automaticity could not be induced at any value of GSAC. Exposure to 10 nM isoproterenol converted these cells to cells with repetitive automaticity in response to GSAC. We conclude that automaticity can be induced in isolated rat atrial cells by application of a model of SACs. Exposure to isoproterenol enhances this effect.

Presence of a calcium-activated chloride current in mouse ventricular myocytes

The properties of several components of outward K(+) currents, including the pharmacological and kinetics profiles as well as the respective molecular correlates, have been identified in mouse cardiac myocytes. Surprisingly little is known with regard to the Ca(2+)-activated ionic currents. We studied the Ca(2+)-activated transient outward currents in mouse ventricular myocytes. We have identified a 4-aminopyridine (4-AP)- and tetraethyl ammonium-resistant transient outward current that is Ca(2+) dependent. The current is carried by Cl(-) and is critically dependent on Ca(2+) influx via voltage-gated Ca(2+) channels and the sarcoplasmic reticulum Ca(2+) store. The current can be blocked by the anion transport blockers niflumic acid and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. Single channel recordings reveal small conductance channels (approximately 1 pS in 140 mM Cl(-)) that can be blocked by anion transport blockers. Ensemble-averaged current faithfully mirrors the transient kinetics observed at the whole level. Niflumic acid (in the presence of 4-AP) leads to prolongation of the early repolarization. Thus this current may contribute to early repolarization of action potentials in mouse ventricular myocytes.

Ca(2+)-activated Cl(-) current in rabbit sinoatrial node cells

The Ca(2+)-activated Cl(-) current (I(Cl(Ca))) has been identified in atrial, Purkinje and ventricular cells, where it plays a substantial role in phase-1 repolarization and delayed after-depolarizations. In sinoatrial (SA) node cells, however, the presence and functional role of I(Cl(Ca)) is unknown. In the present study we address this issue using perforated patch-clamp methodology and computer simulations. Single SA node cells were enzymatically isolated from rabbit hearts. I(Cl(Ca)) was measured, using the perforated patch-clamp technique, as the current sensitive to the anion blocker 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS). Voltage clamp experiments demonstrate the presence of I(Cl(Ca)) in one third of the spontaneously active SA node cells. The current was transient outward with a bell-shaped current-voltage relationship. Adrenoceptor stimulation with 1 microM noradrenaline doubled the I(Cl(Ca)) density. Action potential clamp measurements demonstrate that I(Cl(Ca)) is activate late during the action potential upstroke. Current clamp experiments show, both in the absence and presence of 1 microM noradrenaline, that blockade of I(Cl(Ca)) increases the action potential overshoot and duration, measured at 20 % repolarization. However, intrinsic interbeat interval, upstroke velocity, diastolic depolarization rate and the action potential duration measured at 50 and 90 % repolarization were not affected. Our experimental data are supported by computer simulations, which additionally demonstrate that I(Cl(Ca)) has a limited role in pacemaker synchronization or action potential conduction. In conclusion, I(Cl(Ca)) is present in one third of SA node cells and is activated during the pacemaker cycle. However, I(Cl(Ca)) does not modulate intrinsic interbeat interval, pacemaker synchronization or action potential conduction.

I(NaCa) contributes to electrical heterogeneity within the canine ventricle

This study examines the amplitude of sodium-calcium exchange current (I(NaCa)) in epicardial, midmyocardial, and endocardial canine ventricular myocytes. Whole cell currents were recorded at 37( degrees )C using standard or perforated-patch voltage-clamp techniques in the absence of potassium, calcium-activated chloride, and sodium-pump currents. I(NaCa) was triggered by release of calcium from the sarcoplasmic reticulum or by rapid removal of external sodium. I(NaCa) was large in midmyocardial myocytes and significantly smaller in endocardial myocytes, regardless of the method used to activate I(NaCa). I(NaCa) at -80 mV was -0.316 +/- 0. 013, -0.293 +/- 0.016, and -0.210 +/- 0.007 pC/pF, respectively, in midmyocardial, epicardial, and endocardial myocytes when activated by the calcium transient. When triggered by sodium removal, peak I(NaCa) was 0.74 +/- 0.04, 0.57 +/- 0.04, and 0.50 +/- 0.03 pA/pF, respectively, in midmyocardial, epicardial, and endocardial myocytes. Epicardial I(NaCa) was smaller than midmyocardial I(NaCa) when activated by removal of external sodium but was comparable to epicardial and midmyocardial I(NaCa) when activated by the normal calcium transient, implying possible transmural differences in excitation-contraction coupling. Our results suggest that I(NaCa) differences contribute to transmural electrical heterogeneity under normal and pathological states. A large midmyocardial I(NaCa) may contribute to the prolonged action potential of these cells as well as to the development of triggered activity under calcium-loading conditions.

Intracellular cyclic AMP inhibits native and recombinant volume-regulated chloride channels from mammalian heart

1. ClC-3 encodes a volume-regulated Cl- channel (ICl,vol) in heart. We studied the regulation of native and recombinant cardiac ICl,vol by intracellular cyclic AMP (cAMPi). 2. Symmetrical high Cl- concentrations were used to effectively separate outwardly rectifying ICl,vol from other non-rectifying Cl- currents, such as the cystic fibrosis transmembrane conductance regulator (CFTR) and Ca2+-activated Cl- currents (ICl,CFTR and ICl,Ca, respectively), which are concomitantly expressed in cardiac myocytes. 3. 8-Bromo-cyclic AMP (8-Br-cAMP) significantly inhibited ICl,vol in most guinea-pig atrial myocytes. In approximately 30 % of the atrial myocytes examined, 8-Br-cAMP increased macroscopic Cl- currents. However, the 8-Br-cAMP-stimulated difference currents exhibited a linear current-voltage (I-V ) relation, consistent with activation of ICl,CFTR, not ICl,vol. 4. In canine atrial myocytes, isoprenaline (1 microM) consistently reduced ICl,vol in Ca2+-free hypotonic bath solutions with strong intracellular Ca2+ (Ca2+i) buffering. In Ca2+-containing hypotonic bath solutions with weak Ca2+i buffering, however, isoprenaline increased net macroscopic Cl- currents. Isoprenaline-stimulated difference currents were not outwardly rectifying, consistent with activation of ICl,Ca, not ICl, vol. 5. In NIH/3T3 cells transfected with gpClC-3 (the gene encoding ICl,vol), 8-Br-cAMP consistently inhibited ICl,ClC-3. These effects were prevented by a protein kinase A (PKA) inhibitor, KT5720, or by mutation of a single consensus protein kinase C (PKC) phosphorylation site (S51A) on the N-terminus of ClC-3, which also mediates PKC inhibition of ICl,ClC-3. 6. We conclude that cAMPi causes inhibition of ICl,vol in mammalian heart due to cross-phosphorylation of the same PKC consensus site on ClC-3 by PKA. Our results suggest that contamination of macroscopic ICl,vol by ICl,CFTR and/or ICl,Ca may account for some of the inconsistent and controversial effects of cAMPi on ICl,vol previously reported in native cardiac myocytes.

Ca2+-activated non-selective cation current in rabbit ventricular myocytes

Oscillatory currents (OCs) were studied in isolated rabbit ventricular myocytes with whole cell mode voltage clamp using Na+-free intracellular and extracellular solutions under conditions where K+ currents were anticipated to be eliminated or minimized. All OCs were dependent on release of Ca2+ from the sarcoplasmic reticulum (SR) because they were associated with intracellular Ca2+ ([Ca2+]i) transients, and were suppressed by high concentrations of BAPTA (20 mmol l-1) or pretreatment with the SR antagonist agents ryanodine (10 micromol l-1) or thapsigargin (1 micromol l-1). The reversal potential (Vrev) for OCs shifted with changes in the calculated Vrev for Cl- (ECl) but was between ECl and the calculated Vrev for elemental monovalent cations (ECat), indicating that more than one Ca2+-activated current contributed to OCs. Addition of the Ca2+-activated Cl- current (ICl(Ca)) antagonist, niflumic acid, shifted the OC Vrev to ECat, suggesting that ICl(Ca) and a Ca2+-activated non-selective cation current (ICAN) contributed to the observed OCs. A reduced niflumic acid-insensitive Ca2+-activated OC persisted following marked symmetrical reduction of Cl- in the intracellular and extracellular solutions. Subsequent removal of all extracellular monovalent cations, by N-methyl-D-glucamine (NMDG) substitution, eliminated OCs and the inward holding current suggesting that ICAN and ICl(Ca) accounted for all or most of the Ca2+-activated OC in the absence of Na+. The OC Vrev was equal to ECl in the absence of monovalent elemental cations. Under these conditions niflumic acid eliminated all OCs. Macroscopic OC is partially due to ICAN in rabbit ventricular myocytes.

Bimodal control of a Ca(2+)-activated Cl(-) channel by different Ca(2+) signals

Ca(2+)-activated Cl(-) channels play important roles in a variety of physiological processes, including epithelial secretion, maintenance of smooth muscle tone, and repolarization of the cardiac action potential. It remains unclear, however, exactly how these channels are controlled by Ca(2+) and voltage. Excised inside-out patches containing many Ca(2+)-activated Cl(-) channels from Xenopus oocytes were used to study channel regulation. The currents were mediated by a single type of Cl(-) channel that exhibited an anionic selectivity of I(-) > Br(-) > Cl(-) (3.6:1.9:1.0), irrespective of the direction of the current flow or [Ca(2+)]. However, depending on the amplitude of the Ca(2+) signal, this channel exhibited qualitatively different behaviors. At [Ca(2+)] < 1 microM, the currents activated slowly upon depolarization and deactivated upon hyperpolarization and the steady state current-voltage relationship was strongly outwardly rectifying. At higher [Ca(2+)], the currents did not rectify and were time independent. This difference in behavior at different [Ca(2+)] was explained by an apparent voltage-dependent Ca(2+) sensitivity of the channel. At +120 mV, the EC(50) for channel activation by Ca(2+) was approximately fourfold less than at -120 mV (0.9 vs. 4 microM). Thus, at [Ca(2+)] < 1 microM, inward current was smaller than outward current and the currents were time dependent as a consequence of voltage-dependent changes in Ca(2+) binding. The voltage-dependent Ca(2+) sensitivity was explained by a kinetic gating scheme in which channel activation was Ca(2+) dependent and channel closing was voltage sensitive. This scheme was supported by the observation that deactivation time constants of currents produced by rapid Ca(2+) concentration jumps were voltage sensitive, but that the activation time constants were Ca(2+) sensitive. The deactivation time constants increased linearly with the log of membrane potential. The qualitatively different behaviors of this channel in response to different Ca(2+) concentrations adds a new dimension to Ca(2+) signaling: the same channel can mediate either excitatory or inhibitory responses, depending on the amplitude of the cellular Ca(2+) signal.