Action Potential Dome Research Articles

Significance of J wave with inverted ST-T polarity induced by ill-timed atrial premature beats and its association with Torsade de Pointes in patients with acute acquired long QT syndrome

Abstract Background When ventricular cardiomyocytes become depolarized during their relative refractory period (RRP), the amplitude of the action potential (AP) becomes lower than that of preceding AP, since some sodium channels are in their refractory state. If the membrane voltage does not reach a level high enough to activate voltage-dependent calcium channels, the AP duration becomes markedly shortened due to loss of AP dome. In patients with long QT syndrome (LQTS), intrinsic differences in AP duration and/or AP restitution kinetics exist between ventricular endocardium and epicardium. Thus, we hypothesized that atrial premature beats (APBs) can capture the RRP and amplify a transmural voltage gradient that manifests as a J wave with inverted ST-T polarity (APB-induced J wave). Purpose To investigate the prevalence of APB-induced J wave and examine the contribution to the development of Torsade de Pointes (TdP) in patients with acute acquired LQTS. Methods A total of 34 consecutive patients with acute acquired LQTS (14 men; median age, 69 years; median QTc, 645.5 ms) and documented TdP were enrolled. APBs were observed before the onset of TdP in 18 (52.9%) patients. The 34 patients were divided into 2 groups based on the presence or absence of APB-induced J wave with inverted ST-T polarity: APB-J wave group (n=8) and non-APB-J wave group (n=26). Results APB-J wave group constituted 23.5% (8/34) of the studied patients. There were no significant differences in clinical characteristics (sex, age and QTc) between the 2 groups. Macroscopic T-wave alternans was more frequently observed before TdP onset in APB-J wave group than non-APB-J wave group (75% vs 15.4%, p=0.003). The ill-timed APBs with J wave directly initiated TdP in all patients in APB-J wave group. APB-J wave group experienced TdP even after initiating conventional therapy more frequently than non-APB-J wave group (3.5 vs 1 event, p=0.04). Conclusions APB-induced J wave is observed in approximately one-fourth of acute acquired LQTS patients with TdP and is associated with an increased risk of TdP. Our observations suggest that APBs can amplify the transmural dispersion of repolarization manifested as a J wave.Figure1Figure2

Mechanisms underlying the spontaneous termination of torsades de pointes in an experimental model of long QT syndrome

BackgroundTorsades de pointes (TdP) represent a complex polymorphic ventricular tachycardia. While the triggering mechanisms of early afterdepolarization and increased dispersion of repolarization are well investigated, the sudden self-limiting termination remains poorly understood. ObjectiveThe purpose of this study was to perform analysis of TdP to investigate factors causing spontaneous termination. MethodsWe used a large data set from Langendorff experiments in isolated rabbit hearts in which drug-induced QT prolongation, bradycardia, and hypokalemia provoke TdP. We included 427 episodes with typical TdP characteristics of polymorphic self-terminating beats and twisting QRS complexes occurring in the presence of abnormal QT prolongation due to various different QT-prolonging drugs. The use of 8 monophasic action potential catheters allowed the characterization of action potential duration, configuration, and dispersion of repolarization beyond the capabilities of the surface electrocardiogram. To identify possible mechanisms of arrhythmia termination, the initial, midpoint, and terminal 3 ventricular complexes were analyzed for each episode. ResultsAn abrupt decrease in spatial dispersion over the course of a TdP episode was identified as a precursor for termination of TdP. Within the last 3 beats, a sudden significant decrease in the dispersion of repolarization was observed as a predictor of termination. In parallel, there was a decrease in action potential duration (action potential duration at 90% repolarization) before termination. Also, a change in action potential configuration (action potential duration at 90% repolarization/action potential duration at 50% repolarization ratio) in terms of the loss of action potential dome with a restitution of action potential triangulation was observed. ConclusionIn >400 TdP episodes, homogenization of myocardial repolarization with the recovery of an action potential configuration determines the termination of TdP episodes.

Abstract 13052: Antiarrhythmic Effect of Artemisinin in Canine Experimental Model of Brugada Syndrome

Introduction: Brugada syndrome is an inherited arrhythmia syndrome which presents sudden cardiac death (SCD) without structural heart diseases. One of the mechanisms of SCD has been suggested that uneven dispersion of Ito channel between epicardium and endocardium induces ventricular arrhythmia. Hypothesis: Artemisinin is used as antimalarial drug widely. Its antiarrhythmic effect which included suppression of Ito channel was reported previously. We investigated the effect of Artemisinin for suppression of Brugada syndrome in canine experimental model Methods: Transmural pseudo-ECG and epicardial/endocardial action potentials were recorded from coronary-perfused canine right ventricular wedge preparation (N=7). To mimic the Brugada syndrome model, acetylcholine (3 μM), calcium channel blocker verapamil (1 μM), Ito agonist NS5806 (7-10 μM) were used and waited until ventricular arrhythmia induction. Then, Artemisinin 100-150 μM was perfused to ameliorate the ventricular arrhythmia. Action potential duration, J wave area, notch index and T wave dispersion were measured. Results: In 5 sample models, ventricular arrhythmia was induced. The Artemisinin suppressed ventricular arrhythmia (n=5) and recovering the action potential dome of RV epicardium after provocation of ventricular arrhythmia in all preparations (n=7). J wave area and epicardial notch index were also decreased after Artemisinin perfusion. Action potential duration prolonged as the ventricular arrhythmia provocation and recovered after Artemisinin perfusion. Conclusions: Our findings suggest that Artemisinin alleviate the Brugada syndrome in the wedge preparation model by inhibition of Ito channel and it suppressed the development of phase 2 reentry and ventricular tachycardia/ventricular fibrillation.

Simulating Notch-Dome Morphology of Action Potential of Ventricular Cell: How the Speeds of Positive and Negative Feedbacks on Transmembrane Voltage Can Influence the Health of a Cell?

Ventricular action potential is well-known because of its plateau phase with a spike-notch-dome morphology. As such, the morphology of action potential is necessary for ensuring a correct heart functioning. Any distraction from normal notch-dome morphology may trigger a circus movement reentry in the form of lethal ventricular fibrillation. When the epicardial action potential dome propagates from a site where it is maintained to regions where it has been lost, it gives rise to the proposed mechanism for the Brugada syndrome. Despite the impact of notch-dome dynamics on the heart function, no independent and explicit research has been performed on the simulation of notch-dome dynamics and morphology. In this paper, using a novel mathematical approach, a three-state variable model is proposed; we show that our proposed model not only can simulate morphology of action potential of ventricular cells but also can propose a biological reasonable tool for controlling of the morphology of action potential spike-notch-dome. We show that the processes of activation and inactivation of ionic gating variables (as positive or negative feedbacks on the voltage of cell membrane) and the ratio of their speeds (time constants) can be treated as a reasonable biological tool for simulating ventricular cell notch-dome. This finding may led to a new insight to the quantification of the health of a ventricular cell and may also propose a new drug therapy strategy for cardiac diseases.

Epicardial Substrate as a Target for Radiofrequency Ablation in an Experimental Model of Early Repolarization Syndrome.

Early repolarization syndrome (ERS) is an inherited cardiac arrhythmia syndrome associated with sudden cardiac death. Approaches to therapy are currently very limited. This study probes the mechanisms underlying the electrocardiographic and arrhythmic manifestation of experimental models of ERS and of the ameliorative effect of radiofrequency ablation. Action potentials, bipolar electrograms, and transmural pseudo-ECGs were simultaneously recorded from coronary-perfused canine left ventricular wedge preparations (n=11). The Ito agonist NS5806 (7-10 μmol/L), calcium channel blocker verapamil (3 μmol/L), and acetylcholine (1-3 μmol/L) were used to pharmacologically mimic the effects of genetic defects associated with ERS. The provocative agents induced prominent J waves in the ECG secondary to accentuation of the action potential notch in epicardium but not endocardium. Bipolar recordings displayed low-voltage fractionated potentials in epicardium because of temporal and spatial variability in appearance of the action potential dome. Concealed phase 2 reentry developed when action potential dome was lost at some epicardial sites but not others, appearing in the bipolar electrogram as discrete high-frequency spikes. Successful propagation of the phase 2 reentrant beat precipitated ventricular tachycardia/ventricular fibrillation. Radiofrequency ablation of the epicardium destroyed the cells displaying abnormal repolarization and thus suppressed the J waves and the development of ventricular tachycardia/ventricular fibrillation in 6/6 preparations. Our findings suggest that low-voltage fractionated electrical activity and high-frequency late potentials recorded from the epicardial surface of the left ventricle can identify regions of abnormal repolarization responsible for ventricular tachycardia/ventricular fibrillation in ERS and that radiofrequency ablation of these regions in left ventricular epicardium can suppress ventricular tachycardia/ventricular fibrillation by destroying regions of ER.

A Methodology to Improve Human Ventricular Models for the Investigation of Cardiac Arrhythmias

Introduction: A number of human action potential (AP) models can be found in the literature for the study of ventricular arrhythmias. One of the latest proposed models is the Carro-Rodriguez-Laguna-Pueyo (CRLP). Although this model reproduces many human ventricular physiological features, it lacks the characteristic AP peak and dome. In this work we propose a modification of the L-type Calcium current (ICaL) to improve its AP shape and to best fit the model to experimental results.Methods: Both the time constants and the steady-state value of the ICaL inactivation gates were modified, with validation performed at two different levels: 1)The results for the new ICaL formulation were compared against three different experimental datasets using in silico simulations of the experimental protocols. 2)A set of arrhythmic risk markers were calculated using the modified AP model, including: systolic and diastolic intracellular calcium concentration ([Ca2+]i), AP duration, AP triangulation at different pacing frequencies, and APD adaptation to abrupt cycle length changes.Results: The mean square error of steady-state ICaL inactivation was 25% lower than in the original CRLP model and satisfactorily reproduced the available experimental data. The modified time constants of the ICaL inactivation gates led to physiological AP peak and dome shapes. All the computed arrhythmic risk markers remained in the physiological range or were very similar to the ones reported for the original CRLP model.Conclusions: The proposed modification in the CRLP model improves the behavior of the ICaL current and the AP shape, providing a model suitable for the investigation of ventricular arrhythmias. The strategy proposed in this work can be extended to other human ventricular cell models available in the literature.

Management of Brugada Syndrome 2016: Should All High Risk Patients Receive an ICD? All High-Risk Patients Should Receive an Implantable Cardiac Defibrillator.

Management of Brugada Syndrome 2016: Should All High Risk Patients Receive an ICD? All High-Risk Patients Should Receive an Implantable Cardiac Defibrillator.

Two novel Brugada syndrome-associated mutations increase KV4.3 membrane expression and function.

The human cardiac fast transient outward K+ channel is composed of the KV4.3 α subunit encoded by KCND3 and the K+ channel-interacting protein 2 (KChIP2) β subunit, and determines the early repolarization of the action potential (AP). Two human mutations (G600R and L450F) in KV4.3 are associated with Brugada syndrome and they increase the KV4.3/KChIP2-encoded fast transient outward K+ current (Ito,f) and cause the stable loss of the AP dome. However, the detailed mechanisms underlying the gain of Ito,f function by these two mutations are largely unknown. The experiments in the present study were undertaken to investigate the effect of these mutations and the underlying mechanism. Whole cell patch-clamp recording was performed in HEK-293 cells expressing KV4.3-wild-type (WT) and KV4.3 mutants with KChIP2. The two individual mutant-encoded currents were significantly increased but the kinetics of the channels affected by the two mutations were different. The two mutations slowed KV4.3/KChIP2-encoded channel inactivation; they did not increase the recovery from the KV4.3/KChIP2-encoded channel inactivation. Western blotting showed that total KV4.3 protein was significantly augmented in HEK-293 cells expressing the two individual mutants with KChIP2. Furthermore, immunofluorescence confocal microscopy demonstrated that the KV4.3 channel protein was expressed more in the cell membrane compared to the cytoplasm in cells that expressed individual mutants with KChIP2. Also, KChIP2 increased the amount of channel protein in the cell membrane of KV4.3 mutants significantly more than KV4.3-WT. Reverse transcription-polymerase chain reaction showed that KV4.3 mRNA was not significantly changed by individual mutations in the presence of KChIP2. Taken together, the present study revealed that the mutations cause a gain-of-function of KV4.3/KChIP2-encoded channels by increasing membrane protein expression and slowing channel inactivation.

Management of ventricular arrhythmias in suspected channelopathies.

Although structural heart disease remains the predominant substrate for ventricular arrhythmia, channelopathies including long QT syndrome (LQTS), short QT syndrome (SQTS), Brugada syndrome (BrS), catecholaminergic polymorphic ventricular tachycardia (CPVT), and early repolarization syndrome (ERS) are less common but important contributing entities. These etiologies require specific therapies potentially contrary to empirical management of arrhythmias associated with structural heart disease. Conventional therapy including antiarrhythmic drug therapy may not only fail to resolve unstable arrhythmias but worsen them. Additionally, channelopathy patients with implantable cardioverter defibrillators (ICD) and arrhythmic storms represent a major challenge, and the acute care team needs to be cognizant of unique circumstances that require specific acute therapies beyond empirical advanced life support algorithm recommendations.1 Successful and considered acute management of ventricular arrhythmias is contingent on a number of variables, including knowledge of the cardiac substrate or potential substrate; form, mechanism, and precipitants of ventricular arrhythmias; and acute effect of potential therapies. In the longer term, an understanding of the natural history of the channelopathy along with the efficacy of long-term therapy will lead to superior outcomes. This review will present the risk of ventricular arrhythmias associated with these uncommon entities, the evolving understanding of the mechanism of arrhythmia, and the mechanistic basis of therapies along with a clinical approach to summarize the evidence pertaining to acute and long-term management. Patients with a prolonged QT interval are at risk of sudden cardiac death (SCD) due to Torsade de Pointes (TdP; Figure 1). Most patients with congenital LQTS are asymptomatic and diagnosed incidentally on electrocardiogram screening or following family screening. However, syncope, aborted SCD, or SCD may be the first presentation. Most arrhythmic events in congenital LQT1 occur during physical or emotional stress, at rest or in association with sudden auditory stimulation in LQT2, and during sleep or rest in LQT3 …

Brugada syndrome: A major cause of sudden cardiac death

In 1992 a new syndrome was described consisting of syncopal episodes or sudden death in patients with a structurally normal heart and an electrocardiogram characteristic of right bundle branch block with ST segment elevation in leads V1 to V3. Brugada syndrome is an autosomal dominant disorder. It has been shown to be associated with mutations in the gene (SCN5A) that encodes for the sodiumion channel in cardiac myocyte. Over 160 mutations of gene SCN5A have been identified. The incidence of the disease is difficult to estimate, but it causes sudden deaths of 5 per 10,000 inhabitants per year and involved much more frequently in people of Asian ancestry. Diagnosis can be easily made by means of genetic analysis and ECG. Recent data suggest that loss of the action potential dome in the right ventricular epicardium underlies ST segment elevation seen in this syndrome. Right ventricular epicardium is preferentially affected because of the predominance of transient outward current in this tissue. Antiarrhythmic drugs like amiodarone and beta-blockers do not prevent death in symptomatic or asymptomatic individuals. Though Implantation of an automatic cardioverter–defibrillator is the only recently proven effective therapy; Quinidine has been found to decrease Ventricular fibrillation and could prove to be a secured option of implantable cardioverter–defibrillator. However, researcher set focus on gene therapy that may offer an enduring cure in future years. The purpose of this brief review is to record the past highlights that have brought us to our present understanding of Brugada syndrome.

Evaluation of genes encoding for the transient outward current (Ito) identifies the KCND2 gene as a cause of J-wave syndrome associated with sudden cardiac death.

J-wave ECG patterns are associated with an increased risk of sudden arrhythmic death, and experimental evidence supports a transient outward current (I(to))-mediated mechanism of J-wave formation. This study aimed to determine the frequency of genetic mutations in genes encoding the I(to) in patients with J waves on ECG. Comprehensive mutational analysis was performed on I(to)-encoding KCNA4, KCND2, and KCND3 genes, as well as the previously described J-wave-associated KCNJ8 gene, in 51 unrelated patients with ECG evidence defining a J-wave syndrome. Only patients with a resuscitated cardiac arrest or type 1 Brugada ECG pattern were included for analysis. A rare genetic mutation of the KCND2 gene, p.D612N, was identified in a single patient. Co-expression of mutant and wild-type KCND2 with KChIP2 in HEK293 cells demonstrated a gain-of-function phenotype, including an increase in peak I(to) density of 48% (P<0.05) in the heterozygous state. Using computer modeling, this increase in Ito resulted in loss of the epicardial action potential dome, predicting an increased ventricular transmural Ito gradient. The previously described KCNJ8-S422L mutation was not identified in this cohort of patients with ECG evidence of J-wave syndrome. These findings are the first to implicate the KCND2 gene as a novel cause of J-wave syndrome associated with sudden cardiac arrest. However, genetic defects in I(to)-encoding genes seem to be an uncommon cause of sudden cardiac arrest in patients with apparent J-wave syndromes.

Cellular Mechanism Underlying Hypothermia-Induced Ventricular Tachycardia/Ventricular Fibrillation in the Setting of Early Repolarization and the Protective Effect of Quinidine, Cilostazol, and Milrinone

Hypothermia has been reported to induce ventricular tachycardia and fibrillation (VT/VF) in patients with early repolarization (ER) pattern. This study examines the cellular mechanisms underlying VT/VF associated with hypothermia in an experimental model of ER syndrome and examines the effectiveness of quinidine, cilostazol, and milrinone to prevent hypothermia-induced arrhythmias. Transmembrane action potentials were simultaneously recorded from 2 epicardial and 1 endocardial site of coronary-perfused canine left ventricular wedge preparations, together with a pseudo-ECG. A combination of NS5806 (3-10 μmol/L) and verapamil (1 μmol/L) was used to pharmacologically model the genetic mutations responsible for ER syndrome. Acetylcholine (3 μmol/L) was used to simulate increased parasympathetic tone, which is known to promote ER. In controls, lowering the temperature of the coronary perfusate to induce mild hypothermia (32°C-34°C) resulted in increased J-wave area on the ECG and accentuated epicardial action potential notch but no arrhythmic activity. In the setting of ER, hypothermia caused further accentuation of the epicardial action potential notch, leading to loss of the action potential dome at some sites but not others, thus creating the substrate for development of phase 2 reentry and VT/VF. Addition of the transient outward current antagonist quinidine (5 μmol/L) or the phosphodiesterase III inhibitors cilostazol (10 μmol/L) or milrinone (5 μmol/L) diminished the ER manifestations and prevented the hypothermia-induced phase 2 reentry and VT/VF. Hypothermia leads to VT/VF in the setting of ER by exaggerating repolarization abnormalities, leading to development of phase 2 reentry. Quinidine, cilostazol, and milrinone suppress the hypothermia-induced VT/VF by reversing the repolarization abnormalities.

Idiopathic Ventricular Fibrillation, Early Repolarization and Other J Wave-Related Ventricular Fibrillation Syndromes

Current clinical and experimental data demonstrate that the electrocardiographic J wave plays a critical role in the pathogenesis of ventricular fibrillation (VF) in patients with Brugada syndrome (BS) and early repolarization (ER) syndrome (ERS). This has generated renewed interest in the presence of J waves and ERS in the general population, yet the identification of high-risk ECG markers and the risk stratification of subjects with ERS remain to be established. More recently, this concept has been expanded to VF mechanisms in patients with structural heart diseases. Some of the fatal arrhythmias in the setting of acute myocardial ischemia or infarction may share a similar, J wave-related electrophysiologic process. In canine arterially perfused wedge preparations, the occurrence of J wave-related arrhythmias is mediated by phase 2 reentry. The stability of the action potential (AP) dome in the ventricular epicardium is dependent on the prominence of the AP phase 1 notch. The ability to maintain the AP dome depends on a delicate balance between inward and outward ionic currents during depolarization and the early phase of repolarization. Outward shifts of the balance and inability to maintain the AP dome result in marked dispersion of repolarization and vulnerability to VF. This review describes the electrocardiographic and clinical features of the J waves in idiopathic VF and other structural heart diseases.

Transient outward current (Ito) gain-of-function mutations in the KCND3-encoded Kv4.3 potassium channel and Brugada syndrome

Brugada syndrome (BrS) is a sudden death-predisposing genetic condition characterized electrocardiographically by ST segment elevation in the leads V(1)-V(3). Given the prominent role of the transient outward current (I(to)) in BrS pathogenesis, we hypothesized that rare gain-of-function mutations in KCND3 may serve as a pathogenic substrate for BrS. Comprehensive mutational analysis of KCND3-encoded Kv4.3 (I(to)) was conducted using polymerase chain reaction, denaturing high performance liquid chromatography, and direct sequencing of DNA derived from 86 unrelated BrS1-8 genotype-negative BrS patients. DNA from 780 healthy individuals was examined to assess allelic frequency for nonsynonymous variants. Putative BrS-associated Kv4.3 mutations were engineered and coexpressed with wild-type KChIP2 in HEK293 cells. Wild-type and mutant I(to) ion currents were recorded using whole-cell patch clamp. Two BrS1-8 genotype-negative cases possessed novel Kv4.3 missense mutations. Both Kv4.3-L450F and Kv4.3-G600R were absent in 1,560 reference alleles and involved residues highly conserved across species. Both Kv4.3-L450F and Kv4.3-G600R demonstrated a gain-of-function phenotype, increasing peak I(to) current density by 146.2% (n = 15, P <.05) and 50.4% (n = 15, P <.05), respectively. Simulations using a Luo-Rudy II action potential (AP) model demonstrated the stable loss of the AP dome as a result of the increased I(to) maximal conductance associated with the heterozygous expression of either L450F or G600R. These findings provide the first molecular and functional evidence implicating novel KCND3 gain-of-function mutations in the pathogenesis and phenotypic expression of BrS, with the potential for a lethal arrhythmia being precipitated by a genetically enhanced I(to) current gradient within the right ventricle where KCND3 expression is the highest.

Late potential at the high ventricular septal level in a patient with Brugada: possible mechanisms and clinical implications

The Brugada syndrome (BS) accounts for approximately 20% of cases of sudden cardiac death in patients with structurally normal hearts. The electrophysiologic basis for ST-segment elevation in the precordial electrocardiogram (ECG) leads that characterize the Brugada phenotype and its strong linkage to ventricular tachycardia (VT)/ventricular fibrillation is still a subject of controversy. Electrocardiographic manifestations of the syndrome have been attributed to one of two basic mechanisms: (1) conduction delay in the right ventricular (RV) epicardial-free wall in the region of the outflow tract or (2) premature repolarization of the RV epicardial action potential secondary to loss of the action potential dome. Signal-averaged ECG recordings have demonstrated late potentials that extend beyond the QRS complex in patients with the BS, especially in the anterior wall of the RV outflow tract. The basis for these epicardial late potentials remains a subject of interest among basic and clinical electrophysiologists. Endocardial late potentials in BS are even less well understood. We present a case of a patient with Brugada syndrome with a distinct endocardial late potential in the high ventricular septum coinciding with the ST-segment elevation. We discuss the possible mechanisms for this intracardiac finding and its clinical significance. We also review the effect of isoproterenol infusion on both the late potential and the surface ECG.

How the knowledge of genetic “makeup” and cellular data can affect the analysis of repolarization in surface electrocardiogram

This review article sought to describe patterns of repolarization on the surface electrocardiogram in inherited cardiac arrhythmias and to discuss how the knowledge of genetic makeup and cellular data can affect the analysis based on the data derived from the experimental studies using arterially perfused canine ventricular wedge preparations. Molecular genetic studies have established a link between a number of inherited cardiac arrhythmia syndromes and mutations in genes encoding cardiac ion channels or membrane components during the past 2 decades. Twelve forms of congenital long QT syndrome have been so far identified, and genotype-phenotype correlations have been investigated especially in the 3 major genotypes—LQT1, LQT2, and LQT3. Abnormal T waves are reported in the LQT1, LQT2, and LQT3, and the differences in the time course of repolarization of the epicardial, midmyocardial, and endocardial cells give rise to voltage gradients responsible for the manifestation of phenotypic appearance of abnormal T waves. Brugada syndrome is characterized by ST-segment elevation in leads V1 to V3 and an episode of ventricular fibrillation, in which 7 genotypes have been reported. An intrinsically prominent transient outward current ( I to)-mediated action potential notch and a subsequent loss of action potential dome in the epicardium, but not in the endocardium of the right ventricular outflow tract, give rise to a transmural voltage gradient, resulting in ST-segment elevation, and a subsequent phase 2 reentry-induced ventricular fibrillation. In conclusion, transmural electrical heterogeneity of repolarization across the ventricular wall profoundly affects the phenotypic manifestation of repolarization patterns on the surface electrocardiogram in inherited cardiac arrhythmias.

Dual Variation in SCN5A and CACNB2b Underlies the Development of Cardiac Conduction Disease without Brugada Syndrome

Inherited loss of function mutations in SCN5A have been linked to overlapping syndromes including cardiac conduction disease and Brugada syndrome (BrS). The mechanisms responsible for the development of one without the other are poorly understood. Direct sequencing was performed in a family with cardiac conduction disease. Wild-type (WT) and mutant channels were expressed in TSA201 cells for electrophysiological study. Green fluorescent protein (GFP)-fused WT or mutant genes were used to assess channel trafficking. A novel SCN5A mutation, P1008S, was identified in all family members displaying first-degree atrioventricular block, but not in unaffected family members nor in 430 reference alleles. Peak P1008S current was 11.77% of WT (P < 0.001). Confocal microscopy showed that WT channels tagged with GFP were localized on the cell surface, whereas GFP-tagged P1008S channels remained trapped in intracellular organelles. Trafficking could be rescued by incubation at room temperature, but not by incubation with mexiletine (300 muM) at 37 degrees C. We also identified a novel polymorphism (D601E) in CACNB2b that slowed inactivation of L-type calcium current (I(Ca,L)), significantly increased total charge. Using the Luo-Rudy action potential (AP) model, we show that the reduction in sodium current (I(Na)) can cause loss of the right ventricular epicardial AP dome in the absence but not in the presence of the slowed inactivation of I(Ca,L). Slowed conduction was present in both cases. Our results suggest genetic variations leading to a loss-of-function in I(Na) coupled with a gain of function in I(Ca,L) may underlie the development of cardiac conduction disease without BrS.

Is there a significant transmural gradient in repolarization time in the intact heart?

Is there a significant transmural gradient in repolarization time in the intact heart?

NS5806 Activates the Transient Outward Potassium Current in the Canine Ventricle and Provides a New Model of the Brugada Syndrome

Background: The Brugada syndrome (BrS) is characterized by elevated ST segments in the right precordial leads, ventricular tachycardia and sudden death. The syndrome has been linked to loss-of-function of sodium and calcium channels, however the transient outward potassium current (Ito) is thought to be central in the pathogenesis of BrS. We assessed the effects of Ito augmentation in a mammalian model using a novel activator of Ito, NS5806. Methods and Results: Voltage-clamp experiments were performed on midmyocardial cells isolated from the canine left ventricular wall. At 40mV NS5806 (10 μM) increased peak Ito by 79±4 % and the time-course of inactivation was slowed (from Tau=12.6±3.2 ms to 20.3±2.9 ms). We next assessed the effect of increased Ito in the development of BrS phenotype using canine ventricular wedge preparations. NS5806 increased the epicardial action potential (AP) phase 1 magnitude, whereas the APs of endocardial cells were largely unaffected. The accentuated epicardial notch was associated with an accentuated J-wave on the ECG. Loss of the epicardial AP dome at some sites but not others, led to development of phase-2-reentry and polymorphic ventricular tachycardia. NS5806 was able to induce the BrS phenotype in wedges from both right and left ventricles of the canine heart; at 15 μM NS5806 BrS developed in 4/6 right ventricular preparations compared to 2/10 left ventricular preparations. Conclusion: The Ito agonist NS5806 recapitulates the electrographic and arrhythmic manifestation of BrS, providing evidence in support of its pivotal role in the genesis of the disease. Our findings suggest that a genetic defect leading to a prominent gain of function of Ito could explain variants of BrS in which ST segment elevation are evident in both right and left ECG leads.

Role of the R1135H KCNH2 mutation in Brugada syndrome

Recently, a novel mutation, R1135H, in KCNH2, the gene encoding the α-subunit of the rapid delayed rectifier K+ channel (IKr), has been identified in a patient with short QT interval as well as Brugada-type ECG. Voltage clamp experiments revealed larger tail currents and slowed deactivation of mutant IKr channels. We have carried out computer simulations to test the effects of the R1135H mutation on the action potential of human left and right ventricular myocytes. Our results demonstrate that this ‘gain-of-function’ mutation in KCNH2 shortens the action potential and enhances the susceptibility of right ventricular myocytes to all-or-none repolarization (epicardial loss of the action potential dome). Thus, the mutation induced changes in IKr cannot only explain the short QT interval but also the Brugada-type ECG of the patient.