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A premature truncation of MYBPHL in humans and a loss of Mybphl in mice is associated with dilated cardiomyopathy, atrial and ventricular arrhythmias, and atrial enlargement. MYBPHL encodes myosin binding protein H-like (MyBP-HL). Prior work in mice indirectly identified Mybphl expression in the atria and in small puncta throughout the ventricle. Because of its genetic association with human and mouse cardiac conduction system disease, we evaluated the anatomical localization of MyBP-HL and the consequences of loss of MyBP-HL on conduction system function. Immunofluorescence microscopy of normal adult mouse ventricles identified MyBP-HL-positive ventricular cardiomyocytes that co-localized with the ventricular conduction system marker contactin-2 near the atrioventricular node and in a subset of Purkinje fibers. Mybphl heterozygous ventricles had a marked reduction of MyBP-HL-positive cells compared to controls. Lightsheet microscopy of normal perinatal day 5 mouse hearts showed enrichment of MyBP-HL-positive cells within and immediately adjacent to the contactin-2-positive ventricular conduction system, but this association was not apparent in Mybphl heterozygous hearts. Surface telemetry of Mybphl-null mice revealed atrioventricular block and atrial bigeminy, while intracardiac pacing revealed a shorter atrial relative refractory period and atrial tachycardia. Calcium transient analysis of isolated Mybphl-null atrial cardiomyocytes demonstrated an increased heterogeneity of calcium release and faster rates of calcium release compared to wild type controls. Super-resolution microscopy of Mybphl heterozygous and homozygous null atrial cardiomyocytes showed ryanodine receptor disorganization compared to wild type controls. Abnormal calcium release, shorter atrial refractory period, and atrial dilation seen in Mybphl null, but not wild type control hearts, agree with the observed atrial arrhythmias, bigeminy, and atrial tachycardia, whereas the proximity of MyBP-HL-positive cells with the ventricular conduction system provides insight into how a predominantly atrial expressed gene contributes to ventricular arrhythmias and ventricular dysfunction.

The cardiac conduction system is a network of heterogeneous cell population that initiates and propagates electric excitations in the myocardium. Purkinje fibers, a network of specialized myocardial cells, comprise the distal end of the conduction system in the ventricles. The developmental origins of Purkinje fibers and their roles during cardiac physiology and arrhythmia have been reported. However, it is not clear if they play a role during ischemic injury and heart regeneration. Here we introduce a novel tamoxifen-inducible Cre allele that specifically labels a broad range of components in the cardiac conduction system while excludes other cardiac cell types and vital organs. Using this new allele, we investigated the cellular and molecular response of Purkinje fibers to myocardial injury. In a neonatal mouse myocardial infarction model, we observed significant increase in Purkinje cell number in regenerating myocardium. RNA-Seq analysis using laser-captured Purkinje fibers showed a unique transcriptomic response to myocardial infarction. Our finds suggest a novel role of cardiac Purkinje fibers in heart injury.

Background Purkinje ectopics (PurkEs) are major triggers of idiopathic ventricular fibrillation (VF). Identifying clinical factors associated with specific PurkE characteristics could yield insights into the mechanisms of Purkinje-mediated arrhythmogenicity. Objective The purpose of this study was to examine the associations of clinical, environmental, and genetic factors with PurkE origin in patients with PurkE-initiated idiopathic VF. Methods Consecutive patients with PurkE-initiated idiopathic VF from 4 arrhythmia referral centers were included. We evaluated demographic characteristics, medical history, clinical circumstances associated with index VF events, and electrophysiological characteristics of PurkEs. An electrophysiology study was performed in most patients to confirm the Purkinje origin. Results Eighty-three patients were included (mean age 38 ± 14 years; 44 [53%] women), of whom 32 had a history of syncope. Forty-four patients had VF at rest. PurkEs originated from the right ventricle (RV) in 41 patients (49%), from the left ventricle (LV) in 36 (44%), and from both ventricles in 6 (7%). Seasonal and circadian distributions of VF episodes were similar according to PurkE origin. The clinical characteristics of patients with RV vs LV PurkE origins were similar, except for sex. RV PurkEs were more frequent in men than in women (76% vs 24%), whereas LV and biventricular PurkEs were more frequent in women (81% vs 19% and 83% vs 17%, respectively) (P Conclusion PurkEs triggering idiopathic VF originate dominantly from the RV in men and from the LV or both ventricles in women, adding to other sex-related arrhythmias such as Brugada syndrome or long QT syndrome. Sex-based factors influencing Purkinje arrhythmogenicity warrant investigation.

Background During His bundle (HB) pacing, the active fixation of HB lead may cause disruption of the enclosed conduction fibers and cause conduction delay. His Purkinje conduction delays have been shown to revert to normal with higher voltage pacing. Objective To determine if conduction delays seen with penetrative HB pacing are voltage dependent and can resolve with higher stimulus voltage. Methods In 17 patients undergoing HBP, the effect of voltage on a composite His Purkinje system-ventricular activation time (hVAT) as well as electrocardiographic (ECG) evidence of conduction delay, was systematically evaluated. Results There was highly significant prolongation of hVAT indicative of conduction delay in 13/17 patients. ECG changes of rightward delay were also seen in 9/13 patients. Both types of delays we manifest at lower voltages and we uniquely resolved by higher stimulation voltage. A pre-excitation like abnormality was also seen in 15/17 patients. This conduction abnormality was manifest at higher voltage without prolongation of hVAT. In 6/15 patients the pre-excitation like abnormality ceased at lower voltage. Conclusion HB pacing is associated with evidence of significant conduction delay noted at low voltage, which resolves with higher stimulation voltage. The more common pre-excitation like abnormality appears to occur at high voltage and, in of itself, does not appear to prolong hVAT.

Modelling 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 modelling. 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.

BACKGROUND: The cardiac sodium channel (SCN5A) mutation, R222Q, neutralizes a positive charge in the domain I voltage sensor. Mutation carriers display very frequent ectopy and dilated cardiomyopathy (DCM). OBJECTIVES: To describe the effect of SCN5A R222Q on murine myocyte and Purkinje fiber electrophysiology, and identify underlying mechanisms. METHODS: We generated mice carrying humanized wild-type (H) and mutant (RQ) SCN5A channels. We characterized whole heart and isolated ventricular and Purkinje myocyte properties. RESULTS: RQ/RQ mice were not viable. I(Na) from RQ/H ventricular myocytes displayed increased “window current” and hyperpolarizing shifts in both inactivation and activation compared to H/H, as previously reported in heterologous expression systems. Surprisingly, action potentials were markedly abbreviated in RQ/H myocytes (APD90: 12.6±1.3 ms versus 29.1±1.0 ms in H/H, p

The Purkinje network is the specialized conduction system in the heart. It ensures the physiological spread of the electrical wave in the ventricles. In this work, in an insulated heart framework, we model the free running Purkinje system, using the monodomain equation. The intra-myocardium part of the Purkinje fiber is coupled to the ventricular tissue using the bidomain equation. The coupling is performed through the extracellular potential. We discretize the problem in time using a semi-implicit scheme. Then, we write a variational formulation of the semi discrete problem in a non standard weighted Sobolev functional spaces. We prove the existence and uniqueness of the solution of the Purkinje/myocardium semi-discretized problem. We discretize in space by the finite element P 1 − L a g r a n g e and conduct some numerical tests showing the anterograde and retrograde propagation of the electrical wave between the tissue and the Purkinje fibers.

Catheter ablation (CA) targeting premature ventricular contraction (PVC) from Purkinje fibers can be an effective therapy for refractory ventricular fibrillation (VF) after myocardial infarction (MI). We experienced two cases in which catheter ablation targeting PVC initiating VF after percutaneous coronary intervention (PCI) in post-MI patients was effective despite transient early recurrences of VF. The first patient (a 68-year-old woman with MI) developed drug-refractory VF 3 days after PCI to the left anterior descending artery (LAD) and left circumflex artery. CA targeting Purkinje potential preceding PVC at the infarcted area eliminated both the PVCs and VF. Three days after the procedure, the VF attacks relapsed by a different type of PVC. However, the VF responded to conventional treatments and disappeared thereafter. In the second patient (an 83-year-old woman with old MI), refractory VF attacks occurred after PCI to the LAD. CA targeting Purkinje potential preceding two distinct types of PVC successfully suppressed the VF. Although the VF relapsed 2 days after CA, it was suppressed by conventional treatment and disappeared the next day.

BACKGROUND The atrioventricular conduction system (AVCS) is a network of specialized cardiomyocytes responsible for coordinated spreading of electrical impulses throughout the ventricles to allow synchronized contractions. Specifically, it is composed of the AV node (AVN), which delays the electrical signal, and the ventricular conduction system (VCS) (i.e. His bundle, bundle branches and Purkinje fibers), which rapidly propagates the signal throughout the ventricles. Different AVCS regions possess different electrophysiological properties, such as conduction velocity. Overlapping and nonoverlapping expression patterns of some key developmental transcription factors (TFs), including Irx3, Tbx3, Etv1 and Nkx2.5 within the AVCS, suggest heterogeneous gene expression profiles. Although these structures of the AVCS were discovered >100 years ago, our understanding about its molecular constituents remains largely limited. METHODS AND RESULTS AVCS cells, which make up ∼0.05% of myocytes (∼500 cells/1 million cardiomyocytes), were isolated from early postnatal hearts (P1 to P4) of fluorescent reporter mice (Cntn2-Cre;Rosa26TdTomato), purified using fluorescence-activated cell sorting, and were subjected to single-cell RNA-sequencing. Unbiased cluster analysis on ∼7000 AVCS cells demonstrated distinct transcriptomic profiles in the AVN, proximal-VCS (i.e. His bundle and bundle branches) and distal-VCS (i.e. Purkinje fibers). Conforming to previous studies, well-known conduction markers, such as Etv1 and Kcne1, were present throughout the entire AVCS, whereas a VCS-specific TF, Irx3, was expressed throughout only the VCS. Additionally, Tbx3, Pcp4, Nppa and Lyz2 were enriched in the proximal, not distal, VCS. Sub-clustering analysis of AVN and proximal-VCS further identified unique expression patterns of molecular markers, such as Shox2 (exclusively in compact AVN) and Rspo3 (likely in lower nodal region connecting AVN and proximal-VCS). Moreover, we uncovered a potential regulatory relationship between Irx3 and Tbx3 in mediating their downstream targets, gap junction Gja5 (Cx40) and Gja1 (Cx43). In Irx3- and Tbx3-positive proximal-VCS, Gja5 and Gja1 expressions were lower compared to Irx3-positive/Tbx3-negative distal-VCS. In only Irx3-negative/Tbx3-positive AVN, both Gja5 and Gja1 expressions were undetected. Importantly, this regulatory relationship was recapitulated in cultured neonatal mouse atrial and ventricular myocytes overexpressing Irx3 and/or Tbx3. We also identified a cluster of cells which co-expressed conductive genes (i.e. Irx3, Kcne1, Gja5, Etv1) as well as G2/M cell cycle genes (i.e. Birc5, Ube2c, Cdk1, Ccnb1/2), suggesting the presence of committed conduction cells undergoing proliferation after birth. CONCLUSION Our study provides a comprehensive view of the cellular and molecular heterogeneity within the postnatal AVCS at a high-resolution and identifies potential novel markers with unique and overlapping transcriptomic patterns.

Hydroxychloroquine (HCQ) is a derivative of the antimalaria drug chloroquine primarily prescribed for autoimmune diseases. Recent attempts to repurpose HCQ in the treatment of corona virus disease 2019 has raised concerns because of its propensity to prolong the QT-segment on the electrocardiogram, an effect associated with increased pro-arrhythmic risk. Since chirality can affect drug pharmacological properties, we have evaluated the functional effects of the R(-) and S(+) enantiomers of HCQ on six ion channels contributing to the cardiac action potential and on electrophysiological parameters of isolated Purkinje fibers. We found that R(-)HCQ and S(+)HCQ block human Kir2.1 and hERG potassium channels in the 1 μM–100 μM range with a 2–4 fold enantiomeric separation. NaV1.5 sodium currents and CaV1.2 calcium currents, as well as KV4.3 and KV7.1 potassium currents remained unaffected at up to 90 μM. In rabbit Purkinje fibers, R(-)HCQ prominently depolarized the membrane resting potential, inducing autogenic activity at 10 μM and 30 μM, while S(+)HCQ primarily increased the action potential duration, inducing occasional early afterdepolarization at these concentrations. These data suggest that both enantiomers of HCQ can alter cardiac tissue electrophysiology at concentrations above their plasmatic levels at therapeutic doses, and that chirality does not substantially influence their arrhythmogenic potential in vitro.

Background Pulmonary vein (PV) myocardium is a known source of atrial fibrillation. A debated question is whether myocardial extensions into caval veins and coronary sinus (CS) have similar properties. No studies have documented specific pacemaker and/or conducting properties of human extracardiac myocardium. Objective The purpose of this study was to characterize the histology and immunohistochemical features of myocardial sleeves in the wall of cardiac veins. Methods Sections of 32 human hearts were investigated. Specimens of PVs, superior caval vein (SVC), CS, sinoatrial and atrioventricular nodes, and left ventricle were stained with Best's Carmine for selective staining of intracellular glycogen. Anti-connexin45 (Cx45)– and Cx43–specific antibodies were used to determine the conduction properties of extracardiac myocardium. Results Myocardial sleeve was found in the wall of PVs of 15 of 16 hearts, 21 of 22 SVCs, and 8 of 8 CSs. Bundles of glycogen-positive cardiomyocytes exhibiting pale cytoplasm and peripheral myofibrils were observed in the venous sleeves. Strong Cx45 and weak Cx43 labeling was detected in the extracardiac myocardium. Similar staining pattern was observed for the pacemaker and conduction system, whereas ventricular myocardium exhibited prominent Cx43 and no Cx45 immunoreactivity. Conclusion Myocardial fibers of PVs, SVC, and CS exhibit morphology similar to that of Purkinje fibers and are enriched in glycogen. We provide data for the first time on prominent positive staining for Cx45 in the extracardiac myocardium, indicating its potential pacemaker and/or conducting nature.

An important ailment that requires treatment by the physician is cardiac failure (“Heart Failure (HF)”), which usually decreases the contractility of the myocardium. The Purkinje Fibre Cells (PFC) augurs to be an important biological network of the human cardiac ventricular conduction system. It assuages to be tertiary pacemaker of the heart which synchronizes the ventricular conduction. This work analyzes the effects of calcium ionic mechanism of propagation using the Zhang model. The strength of contraction of cardiac muscle depends to a great extent on the concentration of calcium ions in the extracellular fluids. The calcium ions also play vital role in excitation, contraction coupling and cardiac muscle relaxation. The model attempts to increase the influx of calcium concentration (0.000156 mM) from the normal Ca 2+ (0.000102 mM) value and also to increase the extracellular calcium concentration ([Ca 2+ ]o) (3 mM). The result show that the internal rise of calcium concentration increased excitability of PFC and maintain plateau potential throughout propagation. Similarly, the high calcium level in extracellular and low calcium level in intracellular decreased excitability of PFC and affect propagation diastole relaxation period. The Calcium channels appear to play a major role in sustaining conduction and propagation when low calcium level in intracellular fluids. The abnormal calcium concentration may affect the PFC excitability and Conduction velocity, which in turn can lead to cardiac ventricular arrhythmias.

BACKGROUND Left ventricular non-compaction cardiomyopathy (LVNC) is a congenital cardiomyopathy, characterized by excessive ventricular trabeculation, with a ‘spongy’ appearance of the myocardium, deep intertrabecular recesses, and a thin compacted myocardial layer. LVNC has recently gained more attention because it has been increasingly recognized as a cause of heart failure, thromboembolism, or ventricular arrhythmia, which can all lead to sudden cardiac death. However, due to its heterogeneous anatomical and clinical features, LVNC is still a poorly understood condition. In addition, LVNC is suggested to be due to a premature arrest in the myocardial compaction process, but its genetic basis and pathologic mechanism remain largely unknown. METHODS AND RESULTS Here we introduce novel genetic factors implicated in ventricular compaction. Specifically, combinatorial loss of two Iroquois homeobox (Irx) transcription factors, Irx3, critical for development and function of the ventricular conduction system (e.g., His-bundle, bundle branches and Purkinje fibers) and Irx4, essential for establishing ventricular identity, recapitulates the clinical hallmarks of LVNC in mice. While mice lacking either Irx3 or Irx4 do not exhibit gross congenital cardiac malformations, mice lacking both Irx3 and Irx4 (Irx3;Irx4DKO) show postnatal lethality with abnormal muscular growths that resemble the polypoid pattern of LVNC, characterized by multiple muscular nodes in the left ventricle. Optical projection tomography in postnatal day 14 (P14) mutant hearts unveiled increased trabeculation, abnormal bridging below the papillary muscles, and disorganized muscle structures (Fig. 1). During embryogenesis, Irx3 and Irx4 exhibit overlapping expression in the developing ventricles. Consistent with the hypothesis that LVNC can be caused by a defect in embryonic heart growth, Irx3;Irx4DKO embryonic hearts showed abnormal trabeculation with thinner ventricular walls as early as E14.5. RNA sequencing on E14.5 Irx3;Irx4DKO ventricles revealed altered ventricular identity with atrial gene activation and enriched Bmp2/Tbx2 pathway that is critical for endocardial cushion development by suppressing cardiomyocyte differentiation. Moreover, co-immunoprecipitation discovered that Irx4 physically interacts with Irx3 and Gata4, and promoter assay further showed that Irx3, Irx4, and Gata4 synergistically suppress Tbx2 promoter activity, suggesting the repressive role of this transcriptional regulatory complex in the developing myocardium. CONCLUSION Our study demonstrates a novel mechanistic interaction of Irx3 and Irx4, in cooperation with Gata4, during the ventricular compaction process, thereby ensuring proper ventricular development. This finding can promote improvement of genetic testing for LVNC and prediction of individual outcomes with LVNC.

Fascicular ventricular arrhythmias represent a spectrum of ventricular tachycardias dependent on the specialized conduction system. Although they are more common in structurally abnormal hearts, there is an increasing body of literature describing their role in normal hearts. In this review, the authors present data from both basic and clinical research that explore the current understanding of idiopathic fascicular ventricular arrhythmias. Evaluation of the cellular electrophysiology of the Purkinje cells shows clear evidence of enhanced automaticity and triggered activity as potential mechanisms of arrhythmias. Perhaps more importantly, heterogeneity in conduction system velocity and refractoriness of the left ventricular conduction system in animal models are in line with clinical descriptions of re-entrant fascicular arrhythmias in humans. Further advances in our understanding of the conduction system will help bridge the current gap between basic science and clinical fascicular arrhythmias.

Drug-induced QT prolongation is a major safety issue in the drug discovery process. This study was conducted to assess the electrophysiological responses of four substances using established preclinical assays usually used in regulatory studies (hERG channel or Purkinje fiber action potential) and a new assay (human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs)-field potential). After acute exposure, moxifloxacin and dofetilide concentration-dependently decreased IKr amplitude (IC50 values: 102 μM and 40 nM, respectively) and lengthened action potential (100 μM moxifloxacin: +23% and 10 nM dofetilide: +18%) and field potential (300 μM moxifloxacin: +76% and 10 nM dofetilide: +38%) durations. Dofetilide starting from 30 nM induced arrhythmia in hiPSC-CMs. Overnight application of pentamidine (10 and 100 μM) and arsenic (1 and 10 μM) decreased IKr, whereas they were devoid of effects after acute application. Long-term pentamidine incubation showed a time- and concentration-dependent effect on field potential duration. In conclusion, our data suggest that hiPSC-CMs represent a fully functional cellular electrophysiology model which may significantly improve the predictive validity of in vitro safety studies. Thereafter, lead candidates may be further investigated in patch-clamp assays for mechanistic studies on individual ionic channels or in a multicellular Purkinje fiber preparation for confirmatory studies on cardiac conduction.

Background Three distinct Ca 2 + release channels were identified in dog P-cells: the ryanodine receptor subtype 2 ( RyR2 ) was detected throughout the cell, while the ryanodine receptor subtype 3 ( RyR3 ) and inositol phosphate sensitive Ca 2 + release channel ( InsP3R ) were found in the cell periphery. How each of these channels contributes to the Ca 2 + cycling of P-cells is unclear. Recent modeling of Ca 2 + mobilization in P-cells suggested that Ca 2 + sensitivity of Ca 2 + induced Ca 2 + release ( CICR ) was larger at the P-cell periphery. Our study examined whether this numerically predicted region of Ca 2 + release exists in live P-cells. We compared the regional Ca 2 + dynamics with the arrangement of intracellular Ca 2 + release ( CR ) channels. Methods Gene expression of CR channels was measured by qPCR in Purkinje fibers and myocardium of adult Yucatan pig hearts. We characterized the CR channels protein expression in isolated P-cells by immuno-fluorescence, laser scanning confocal microscopy, and 3D reconstruction. The spontaneous Ca 2 + activity and electrically-evoked Ca 2 + mobilization were imaged by 2D spinning disk confocal microscopy. Functional regions of P-cell were differentiated by the characteristics of local Ca 2 + events. We used the Ca 2 + propagation velocities as indicators of channel Ca 2 + sensitivity. Results RyR2 gene expression was identical in Purkinje fibers and myocardium (6 hearts) while RyR3 and InsP 3 R gene expressions were, respectively, 100 and 16 times larger in the Purkinje fibers. Specific fluorescent immuno-staining of Ca 2 + release channels revealed an intermediate layer of RyR3 expression between a near-membrane InsP3R -region and a central RyR2 -region. We found that cell periphery produced two distinct forms of spontaneous Ca 2 + -transients: (1) large asymmetrical Ca 2 + sparks under the membrane, and (2) typical Ca 2 + -wavelets propagating exclusively around the core of the cell. Larger cell-wide Ca 2 + waves ( CWW s) appeared occasionally traveling in the longitudinal direction through the core of Pcells. Large sparks arose in a micrometric space overlapping the InsP3R expression. The InsP3R antagonists 2-aminoethoxydiphenyl borate (2-APB; 3 μM) and xestospongin C (XeC; 50 μM) dramatically reduced their frequency. The Ca 2 + wavelets propagated in a 5–10 μm thick layered space which matched the intermediate zone of RyR3 expression. The wavelet incidence was unchanged by 2-APB or XeC, but was reduced by 60% in presence of the RyR3 antagonist dantrolene (10 μM). The velocity of wavelets was two times larger (86 ± 16 μm/s; n = 14) compared to CWWs' (46 ± 10 μm/s; n = 11; P 2 + concentration under the membrane which preceded the propagation of Ca 2 + into the interior of the cell. Elevated Ca i propagated at 150 μm/s (147 ± 34 μm/s; n = 5) through the region equivalent to the zone of RyR3 expression. This velocity dropped by 50% (75 ± 24 μm/s; n = 5) in the central region wherein predominant RyR2 expression was detected. Conclusion We identified two layers of distinct Ca 2 + release channels in the periphery of Pcell: an outer layer of InsP 3 Rs under the membrane and an inner layer of RyR3s . The propagation of Ca 2 + events in these layers revealed that Ca 2 + sensitivity of Ca 2 + release was larger in the RyR3 layer compared to that of other sub-cellular regions. We propose that RyR3 expression in P-cells plays a role in the stability of electric function of Purkinje fibers.

A single beat arising as extra systole within the His-Purkinje system or from ventricle or even atrium based on conduction timing can invoke delayed conduction or block within intra-Hisian or infra-Hisian sites. This may be either manifested in the form of premature atrial or ventricular complexes or concealed as with His extra systoles. It appears commonly there is disease within the His-Purkinje system.

The His-Purkinje system (HPS) plays a significant role in human pathophysiology, but knowledge is scattered. This article highlights some of the relevant concepts, phenomena, and mechanisms; clarifies, expands, confirms, or modifies commonly encountered clinical events; and adds new information, which is often available but obscure. Also included are the essentials of HPS anatomy and physiology. It is important to abandon inaccurate concepts that are still taught and occasionally appear in text books.

We describe a case of a 70 year-old woman with effort dyspnea and alternating trifascicular conduction pattern, worsened with exercise to atrioventricular block (AV Block). The alternating conduction at rest resulted in marked T wave changes. We discuss the mechanism of T wave memory changes and the management of exercise-induced AV Block.

Cannabidiol (CBD) is a non-psychoactive component of Cannabis which has recently received regulatory consideration for the treatment of intractable forms of epilepsy such as the Dravet and the Lennox-Gastaut syndromes. The mechanisms of the antiepileptic effects of CBD are unclear, but several pre-clinical studies suggest the involvement of ion channels. Therefore, we have evaluated the effects of CBD on seven major cardiac currents shaping the human ventricular action potential and on Purkinje fibers isolated from rabbit hearts to assess the in vitro cardiac safety profile of CBD. We found that CBD inhibits with comparable micromolar potencies the peak and late components of the NaV1.5 sodium current, the CaV1.2 mediated L-type calcium current, as well as all the repolarizing potassium currents examined except Kir2.1. The most sensitive channels were KV7.1 and the least sensitive were KV11.1 (hERG), which underly the slow (IKs) and rapid (IKr) components, respectively, of the cardiac delayed-rectifier current. In the Purkinje fibers, CBD decreased the action potential (AP) duration more potently at half-maximal than at near complete repolarization, and slightly decreased the AP amplitude and its maximal upstroke velocity. CBD had no significant effects on the membrane resting potential except at the highest concentration tested under fast pacing rate. These data show that CBD impacts cardiac electrophysiology and suggest that caution should be exercised when prescribing CBD to carriers of cardiac channelopathies or in conjunction with other drugs known to affect heart rhythm or contractility.

Introduction Ventricular Fibrillation (VF) is a major cause of sudden cardiac death. Purkinje fibers (PF) are known to play an important role in triggering and maintaining VF. Previous studies have highlighted the role of TRPM4 channels in repolarization properties of PFs, but their potential impact on arrhythmogenesis and VF dynamics remains unknown. Objective To investigate the role of TRPM4 channels in Purkinje-mediated trigger and substrate mechanisms. Method Endocardial optical mapping experiments were performed in sheep perfused left ventricular wedge preparations using a high-resolution CMOS camera. Purkinje-Muscle Junctions (PMJ) were localized through PF stimulation and identification of the breakthrough sites. Arrhythmias were induced either by isoprenaline perfusion (ISO; 1 μM; n = 7) or by endocardial burst pacing (10 Hz; n = 6) and their spatiotemporal dynamics quantified with or without 9-Phenanthrol (9PHE; 45 μM), a known TRPM4 inhibitor. Results ISO perfusion provoked focal tachy-arrhythmias originating from the PFs confirmed by the co-localization of the exit sites with previously identified PMJs. 9PHE reduced the rate of spontaneous focal activity and activation patterns were no longer correlated to the PMJs. However, following burst pacing, inhibition of TRPM4 led to a higher propensity of VF than ventricular tachycardia (VT). For the induced VT episodes, we observed a significant increase of DF (3.7 ± 0.7 Hz vs. 4.8 ± 0.8 Hz) and a significant decrease of regularity index (0.94 ± 0.05 vs. 0.84 ± 0.07) at the PMJs after 9PHE perfusion, while a significant increase in long lasting rotors was observed (P = 0.044; for rotor number and P = 0.005; for rotor duration). A significant increase in focal activity (15 ± 2 μpixel/ms vs. 30 ± 9 μpixel/ms) was found during VF episodes. Conclusion These results suggest that TRPM4 inhibition has anti-arrhythmic effects by preventing VF triggers from PFs, while at the same time increasing the complexity of induced ventricular arrhythmias.

The Purkinje network is an integral part of the excitation system in the human heart. Yet, to date, there is no in vivo imaging technique to accurately reconstruct its geometry and structure. Computational modeling of the Purkinje network is increasingly recognized as an alternative strategy to visualize, simulate, and understand the role of the Purkinje system. However, most computational models either have to be generated manually, or fail to smoothly cover the irregular surfaces inside the left and right ventricles. Here we present a new algorithm to reliably create robust Purkinje networks within the human heart. We made the source code of this algorithm freely available online. Using Monte Carlo simulations, we demonstrate that the fractal tree algorithm with our new projection method generates denser and more compact Purkinje networks than previous approaches on irregular surfaces. Under similar conditions, our algorithm generates a network with 1219 ± 61 branches, three times more than a conventional algorithm with 419 ± 107 branches. With a coverage of 11 ± 3 mm, the surface density of our new Purkije network is twice as dense as the conventional network with 22 ± 7 mm. To demonstrate the importance of a dense Purkinje network in cardiac electrophysiology, we simulated three cases of excitation: with our new Purkinje network, with left-sided Purkinje network, and without Purkinje network. Simulations with our new Purkinje network predicted more realistic activation sequences and activation times than simulations without. Six-lead electrocardiograms of the three case studies agreed with the clinical electrocardiograms under physiological conditions, under pathological conditions of right bundle branch block, and under pathological conditions of trifascicular block. Taken together, our results underpin the importance of the Purkinje network in realistic human heart simulations. Human heart modeling has the potential to support the design of personalized strategies for single- or bi-ventricular pacing, radiofrequency ablation, and cardiac defibrillation with the common goal to restore a normal heart rhythm.

Purkinje fibers/cells continue to be a focus of arrhythmologists. Here we review several new ideas that have emerged in the literature and fold them into important new points. These points include the following: some proteins in Purkinje cells are specific to Purkinjes; pacemaker function in Purkinje may be similar to that of the sinus node cell; sink-source concerns about tracts/sheets of Purkinje fibers; role of Ito in arrhythmias; and genetic lesions in Purkinjes and their high impact on cardiac rhythm. Although new ideas about the remodeled Purkinje cell are not the focus of this review, one can easily imagine how Purkinjes and their function may be altered in diseased hearts.

The electrical activation of the heart is a complex physiological process that is essential for the understanding of several cardiac dysfunctions, such as ventricular tachycardia (VT). Nowadays, patient-specific activation times on ventricular chambers can be estimated from electro-anatomical maps, providing crucial information to clinicians for guiding cardiac radio-frequency ablation treatment. However, some relevant electrical pathways such as those of the Purkinje system are very difficult to interpret from these maps due to sparsity of data and the limited spatial resolution of the system. We present here a novel method to estimate these fast electrical pathways from the local activations maps (LATs) obtained from electro-anatomical maps. The location of Purkinje-myocardial junctions (PMJs) is estimated considering them as critical points of a distance map defined by the activation maps, and then minimal cost geodesic paths are computed on the ventricular surface between the detected junctions. Experiments to validate the proposed method have been carried out in simplified and realistic simulated data, showing good performance on recovering the main characteristics of simulated Purkinje networks (e.g. PMJs). A feasibility study with real cases of fascicular VT was also performed, showing promising results.

Optogenetics provides an alternative to electrical stimulation to manipulate membrane voltage, and trigger or modify action potentials (APs) in excitable cells. We compare biophysically and energetically the cellular responses to direct electrical current injection versus optical stimulation mediated by genetically expressed light-sensitive ion channels, e.g., Channelrhodopsin-2 (ChR2). Using a computational model of ChR2(H134R mutant), we show that both stimulation modalities produce similar-in-morphology APs in human cardiomyocytes, and that electrical and optical excitability vary with cell type in a similar fashion. However, whereas the strength-duration curves for electrical excitation in ventricular and atrial cardiomyocytes closely follow the theoretical exponential relationship for an equivalent RC circuit, the respective optical strength-duration curves significantly deviate, exhibiting higher nonlinearity. We trace the origin of this deviation to the waveform of the excitatory current—a nonrectangular self-terminating inward current produced in optical stimulation due to ChR2 kinetics and voltage-dependent rectification. Using a unifying charge measure to compare energy needed for electrical and optical stimulation, we reveal that direct electrical current injection (rectangular pulse) is more efficient at short pulses, whereas voltage-mediated negative feedback leads to self-termination of ChR2 current and renders optical stimulation more efficient for long low-intensity pulses. This applies to cardiomyocytes but not to neuronal cells (with much shorter APs). Furthermore, we demonstrate the cell-specific use of ChR2 current as a unique modulator of intrinsic activity, allowing for optical control of AP duration in atrial and, to a lesser degree, in ventricular myocytes. For self-oscillatory cells, such as Purkinje, constant light at extremely low irradiance can be used for fine control of oscillatory frequency, whereas constant electrical stimulation is not feasible due to electrochemical limitations. Our analysis offers insights for designing future new energy-efficient stimulation strategies in heart or brain.

A female 4-month-old Holstein-Friesian calf was presented in heart failure. Microscopical examination of samples of the cardiac wall taken at necropsy examination revealed numerous aggregates of Purkinje fibres, particularly in the perivascular areas. Some Purkinje fibres were stained strongly with phosphotungstic acid haematoxylin and immunohistochemically were shown to express alpha smooth muscle actin, indicating an embryonic-like Purkinje fibre phenotype. A diagnosis of congenital multifocal increase of Purkinje fibres was made. The histological features of this case resemble multifocal cardiac Purkinje cell tumour of the heart in man.

The synchronization of heartbeats is triggered by electric impulses delivered by specialized cardiomyocytes of the conduction system. In the ventricles, the His-Purkinje system mediates rapid propagation of electrical activity such that contractions propagate from the apex to the base of the heart to increase the efficiency of blood expulsion to the great arteries. Human heterozygous mutations of NKX2-5 are associated with a large spectrum of congenital heart diseases and atrioventricular conduction block. In agreement with these observations, mouse genetics has revealed the complexity of the role of Nkx2-5 during cardiac development. Germline deletion of Nkx2-5 results in cardiac lethality at early stages, while Nkx2-5± embryos and mice appear normal and reproduce the phenotype observed in patients, including atrial septal defects and conduction disturbances. These mice display ventricular conduction defects that can be correlated with hypoplastic development of the His-Purkinje system. To further decipher the role of this transcription factor in the conduction system, we conditionally inactivated Nkx2-5 in the His-Purkinje system using a Cx40-CreERT2 mouse line. Postnatal deletion of Nkx2-5 in the His-Purkinje system induced a progressive loss of expression of conduction markers and hypoplasia of the Purkinje fiber network. These mice develop conduction defects with enlarged QRS intervals and left bundle branch block. Long-term follow-up of cardiac function recorded by MRI and echocardiography shows a progressive reduction of ejection fraction correlated with strain defects. These results highlight the importance of Nkx2-5-dependent maintenance of the His-Purkinje system in preserving cardiac function.

During cardiac development, transient invaginations of the myocardium, termed trabeculae, appear at the inner surface of the ventricles, which disappeared by compaction to form a functional ventricular wall. Trabeculae also contain progenitor cells of the ventricular conduction system (VCS), a complex network of Purkinje fibers driving the rapid propagation of electrical activity in the ventricles. Defects in ventricular compaction and conduction have been observed in patients and mutant mice carrying mutations in NKX2-5, encoding a key transcriptional regulator of heart development. In order to analyze the link between trabecular fate and VCS differentiation, we carried out genetic tracing and prospective clonal analyses of trabeculae using Cx40-CreERT2 mice and Rosa26-Confetti mice. Our results show that the peripheral VCS segregates progressively during embryonic development. Cells exclusively fated to give rise to the VCS are present in the trabecular compartment as early as E9.5. Specification of new conductive myocytes within the trabecular compartment during subsequent development contributes to the formation of a complex Purkinje fiber network at birth. We performed the same lineage tracing experiments in Nkx2-5 heterozygous mice with severe hypoplasia of the Purkinje fiber network at adult stages. The number of Purkinje fibers originating from early Nkx2-5 hapolinsufficient progenitors is identical to control. In contrast, at later stages of development, we observed an increase in the number of trabecular cells that do not give rise to Purkinje fibers. This suggests that Nkx2-5 controls the progressive recruitment of trabecular cells into the VCS as well as the later maintenance of the conductive phenotype. Overall our study highlights the early segregation of the VCS lineage at the onset of trabeculation and the later role of Nkx2-5 in formation of the Purkinje fiber network.

Background Idiopathic left ventricular tachycardia (ILVT) is a common form of ventricular tachycardia (VT) in structurally normal heart. Different methods have been proposed for radiofrequency ablation (RFA) of ILVT that have good short-term results but recurrence is higher. Termination of tachycardia during RFA and/or noninduciblity has been the procedural end point. Objective To describe electrophysiological markers that add to long-term freedom from recurrences. Methods Fifteen patients with ILVT underwent RFA guided by 3-dimensional electroanatomical mapping. After creating a 3-dimensional geometry of the left ventricle (LV), the conduction system of the LV was mapped by tracing from His recording from the left ventricular outflow tract and distally till the fascicles and perifascicular myocardium. VT was induced by using programmed electrical stimulation. Ablation was done targeting the distal posterior fascicle and extended linearly to the surrounding myocardium till conduction block was achieved between the fascicle-Purkinje network and the left ventricular myocardium. Results All patients (13 men; mean age 32±9 years) had inducible VTs. The mean tachycardia cycle length was 320±28 ms. Radiofrequency energy was given to the distal posterior fascicle and the myocardium, with an aim to achieve a myocardial-fascicular conduction block in addition to the termination of VT and noninducibility. Ablation was successful in all. No recurrence of tachycardia was seen in any patient on follow-up (20.8±8.5 months). Conclusions Distal posterior fascicle and Purkinje-myocardial junction is an effective target site for ILVT ablation. The demonstration of myocardial to fascicle conduction block serves as an important electrophysiological marker of successful ablation and improved long-term success.

Background and aims Origin of idiopathic ventricular fibrillation (IVF) remains unclear. The incorporation of genetic knowledge could improve the diagnosis and could create a better classification of this sub-group of patients. To this end, we collected DNA from 19 patients with VF of Purkinje origin associated with short-coupled premature ventricular beats or torsade de pointes. We aimed to determine the frequency of rare variants in 47 genes implied in ventricular arrhythmias and 20 additional genes highly expressed in Purkinje fibers. Methods Exome sequencing was performed using Illumina technology. We selected rare non-synonymous variants with reads superior to 10, a minor allele frequency inferior to 0.1% and predicted to alter the protein sequence. Potential pathogenicity was assessed by Polyphen-2, Mutation Taster and CADD (Combined Annotation Dependent Depletion) and variants considered as benign for 2 of 3 tests were discarded. Results We identified 10 rare (MAF Conclusions By analyzing 67 candidate genes we identified 19 variants in 13 different genes. This suggests than the idiopathic ventricular fibrillation associated with short-coupled torsades de pointes is an oligogenic disease. We didn’t find any variants for 6 patients. We identified a high proportion of these variants affecting a calcium channel subunit, which could be meaningful since our patients are sensitive to verapamil, a calcium inhibitor. Functional studies should confirm their involvement in ventricular fibrillation with short-coupled torsade de pointes. The author hereby declares no conflict of interest

Left ventricular non-compaction (LVNC), characterized by prominent trabeculations and deep trabecular recesses in the ventricles, is the most common cardiomyopathy with a spectrum ranging from extreme normal variants to a pathological phenotype. In all forms, heart failure and sudden cardiac death represent the most severe complications related with noncompaction and arrhythmias. The presence of ventricular trabeculae is a normal component of ventricular myocardium during embryonic development. Trabeculae are also progenitors of the ventricular conduction system (VCS), which is composed of a complex network of Purkinje fibers controlling the rapid propagation of electrical activity in the ventricles. The transcription factor Nkx2.5 is a key regulator of cardiac development. Conduction disturbances and ventricular non-compaction are traits that have been observed in patients and mutant mice carrying NKX2-5 mutations. However, the relationship between Purkinje fiber differentiation and trabecular compaction is unknown, despite recent evidence that abnormal conduction system morphology underlies conduction defects and arrhythmias. In order to dissect the role of this gene in the apparition of the pathological outcomes of LVNC, we specifically inactivate Nkx2-5 in a time and tissue specific manner. Nkx2-5 floxed mice were crossed with Cx40-creERT2 mice and tamoxifen injections, to induce gene deletion, were performed at different time of trabecular development to detect the impact of this gene in ventricular trabeculation, compaction and in the VCS. Morphological and functional cardiac phenotypes in these different mutant mice were analysed by oupling non-invasive imaging techniques and immunofluorescent studies. Preliminary results showed that the degree of non-compaction and the VCS morphology differ according to the timing of Nkx2-5 inactivation, suggesting multiple roles for this transcription factor during ventricular trabeculae development. The author hereby declares no conflict of interest

Introduction The aim of the present study was to evaluate direct/acute effects of arsenic trioxide on action potentials (APs) in isolated cardiac tissues, and to investigate if the choice of species and tissue and the duration of the perfusion play a role in arsenic-induced acute/direct prolongation of AP/QT. Methods and results Direct electrophysiological effects of arsenic trioxide were measured in cardiac tissues isolated from four different species using micro-electrode recording. Arsenic (after 30 to 95 min perfusion at 10 μM) significantly prolonged APD 90 , increased triangulation of the AP and elicited early afterdepolarizations (EADs) only in isolated guinea-pig and dog Purkinje fibers but not in rabbit and porcine (minipig) Purkinje fibers. Arsenic induced a prolongation of the APD 90 and increases in triangulation and the occurrence of EADs was not observed in papillary muscles of guinea-pigs and rabbits. Arsenic at 4 increasing concentrations from 0.1 μM to 10 μM at the standard perfusion-time of 15 min per concentration, and after a continuous 90-min perfusion at 1 μM and 1 Hz did not induce these direct effects on APD 90 , triangulation and EADs in isolated guinea-pig Purkinje fibers, but it at 1 µM elicited EADs in 2 out of 7 preparations after 90 min at 0.2 Hz. Discussion The present study demonstrates that the choice of species and cardiac tissue as well as perfusion-time play important roles in arsenic-induced direct/acute effects on APD 90 and induction of EADs in vitro .

OBJECTIVES: The aim of this study was to describe a new familial cardiac phenotype and to elucidate the electrophysiological mechanism responsible for the disease. BACKGROUND: Mutations in several genes encoding ion channels, especially SCN5A, have emerged as the basis for a variety of inherited cardiac arrhythmias. METHODS: Three unrelated families comprising 21 individuals affected by multifocal ectopic Purkinje-related premature contractions (MEPPC) characterized by narrow junctional and rare sinus beats competing with numerous premature ventricular contractions with right and/or left bundle branch block patterns were identified. RESULTS: Dilated cardiomyopathy was identified in 6 patients, atrial arrhythmias were detected in 9 patients, and sudden death was reported in 5 individuals. Invasive electrophysiological studies demonstrated that premature ventricular complexes originated from the Purkinje tissue. Hydroquinidine treatment dramatically decreased the number of premature ventricular complexes. It normalized the contractile function in 2 patients. All the affected subjects carried the c.665G>A transition in the SCN5A gene. Patch-clamp studies of resulting p.Arg222Gln (R222Q) Nav1.5 revealed a net gain of function of the sodium channel, leading, in silico, to incomplete repolarization in Purkinje cells responsible for premature ventricular action potentials. In vitro and in silico studies recapitulated the normalization of the ventricular action potentials in the presence of quinidine. CONCLUSIONS: A new SCN5A-related cardiac syndrome, MEPPC, was identified. The SCN5A mutation leads to a gain of function of the sodium channel responsible for hyperexcitability of the fascicular-Purkinje system. The MEPPC syndrome is responsive to hydroquinidine.

International audience; The eight pre- or/and post-synaptic metabotropic glutamatergic receptors (mGluRs) modulate rapid excitatory transmission sustained by ionotropic receptors. They are classified in three families according to their percentage of sequence identity and their pharmacological properties. mGluR4 belongs to group III and is mainly localized presynaptically. Activation of group III mGluRs leads to depression of excitatory transmission, a process that is exclusively provided by mGluR4 at parallel fiber-Purkinje cell synapse in rodent cerebellum. This function relies at least partly on an inhibition of presynaptic calcium influx, which controls glutamate release. To improve the understanding of molecular mechanisms of the mGluR4 depressant effect, we decided to identify the proteins interacting with this receptor. Immunoprecipitations using anti-mGluR4 antibodies were performed with cerebellar extracts. 183 putative partners that co-immunoprecipitated with anti-mGluR4 antibodies were identified and classified according to their cellular functions. It appears that native mGluR4 interacts with several exocytosis proteins such as Munc18-1, synapsins, and syntaxin. In addition, native mGluR4 was retained on a Sepharose column covalently grafted with recombinant Munc18-1, and immunohistochemistry experiments showed that Munc18-1 and mGluR4 colocalized at plasma membrane in HEK293 cells, observations in favor of an interaction between the two proteins. Finally, affinity chromatography experiments using peptides corresponding to the cytoplasmic domains of mGluR4 confirmed the interaction observed between mGluR4 and a selection of exocytosis proteins, including Munc18-1. These results could give indications to explain how mGluR4 can modulate glutamate release at parallel fiber-Purkinje cell synapses in the cerebellum in addition to the inhibition of presynaptic calcium influx.

Aims In a previous study we found that A-935142 enhanced hERG current in a concentration-dependent manner by facilitating activation, reducing inactivation, and slowing deactivation (Su et al., 2009). A-935142 also shortened action potential duration (APD 90 ) in canine Purkinje fibers and guinea pig atrial tissue. This study focused on the combined effects of the prototypical hERG enhancer, A-935142 and two hERG current blockers (sotalol and terfenadine). Main methods The whole-cell voltage clamp method with HEK 293 cells heterologously expressing the hERG channel (Kv 11.1) was used. Key findings A-935142 did not compete with 3 H-dofetilide binding, suggesting that A-935142 does not overlap the binding site of typical hERG blockers. In whole-cell voltage clamp studies we found: 1) 60 μM A-935142 enhanced hERG current in the presence of 150 μM sotalol (57.5 ± 5.8%) to a similar extent as seen with A-935142 alone (55.6 ± 5.1%); 2) 150 μM sotalol blocked hERG current in the presence of 60 μM A-935142 (43.5 ± 1.5%) to a similar extent as that seen with sotalol alone (42.0 ± 3.2%) and 3) during co-application, hERG current enhancement was followed by current blockade. Similar results were obtained with 60 nM terfenadine combined with A-935142. Significance These results suggest that the hERG enhancer, A-935142 does not compete with these two known hERG blockers at their binding site within the hERG channel. This selective hERG current enhancement may be useful as a treatment for inherited or acquired LQTS (Casis et al., 2006).

Objectives The purpose of this study was to assess the prevalence of the re-entry circuit within the interventricular septum in post-infarction patients referred for ventricular tachycardia (VT) ablation. Background Post-infarction ventricular tachycardia can involve the endocardial myocardium, the intramural myocardium, the epicardium, or the His Purkinje system. Methods Among 74 consecutive patients with recurrent post-infarction VT, 33 patients (45%) were identified in whom the critical part of the VT involved the interventricular septum. A total of 206 VTs were induced in these 33 patients. In 46 of the 206 VTs, a critical component was identified in the interventricular septum. The critical isthmus of the re-entry circuit was identified by entrainment mapping, activation mapping, or pace-mapping. Results In 32 of 46 VTs (70%), the critical component of the re-entry circuit was confined to the endocardium. In 9 of 46 VTs (20%), the critical component involved the Purkinje system, and in 5 of 46 VTs (11%), an intramural area was critical. Entrainment and/or pace-mapping helped to identify critical areas of endocardial VTs as well as VTs involving the Purkinje fibers, but neither of these mapping techniques localized intramural VTs. Electrocardiographic characteristics were specific for each of the septal locations. All VTs mapped to the interventricular septum were acutely successfully ablated. VTs recurred in 9 of 33 patients with septal VTs during a mean follow-up period of 40 ± 20 months. Conclusions Post-infarction VT involving the interventricular septum can involve the endocardial muscle, Purkinje fibers, or intramural muscle fibers. Electrocardiographic characteristics differ depending on the type of tissue involved.

The aim of this study was to investigate the cellular electrophysiological effects of ranolazine on action potential characteristics. The experiments were carried out in dog and human cardiac preparations using the conventional microelectrode technique. In dog Purkinje fibres ranolazine produced a concentration- and frequency-dependent depression of the maximum rate of depolarization (V(max)) while action potential duration (APD) was shortened. In dog and human right ventricular papillary muscle ranolazine exerted no significant effect on APD, while it produced, like mexiletine, use-dependent depression of V(max) with relatively fast onset and offset kinetics. In dog midmyocardial preparations the drug did not exert statistically significant effect on repolarization at 10 μM, although a tendency toward prolongation was observed at 20 μM. A moderate lengthening of APD(90) by ranolazine was noticed in canine atrial preparations obtained from dogs in sinus rhythm and in tachypacing induced remodelled preparations. Use-dependent depression of V(max) was more pronounced in atria from dogs in sinus rhythm than those in remodelled atria or in the ventricle. These findings indicate that ranolazine, in addition to its known late sodium current blocking effect, also depresses peak I(Na) with class I/B antiarrhythmic characteristics. Although peak I(Na) inhibition by ranolazine is stronger in the atria, it is also substantial (at fast stimulation frequencies) in ventricular preparations. Ranolazine also decreased the dispersion of ventricular repolarization (the difference in APD(90) values between Purkinje fibres and papillary muscles), which can contribute to the antiarrhythmic property of the drug.

Activating autoantibodies to β-adrenergic receptors (AAβ1/2AR) and M2 muscarinic receptors (AAM2R) have been reported in several cardiac diseases and may have pathophysiologic relevance. However, the interactions and relative effects of AAβ1AR, AAβ2AR and AAM2R on contractile function have not been characterized.The inotropic effects of IgG from 18 selected patients with cardiomyopathy and/or atrial tachyarrhythmias positive by ELISA for antibodies to β1/2AR were studied using an isolated canine Purkinje fiber contractility assay. M2R-blockade was tested using atropine while selective β1AR and β2AR blockade used CGP-20712A and ICI-118551 respectively.Fifteen of the 18 anti-β1/2AR ELISA-positive samples demonstrated evidence for negative inotropic muscarinic effects which were blocked using atropine. Atropine failed to uncover a positive inotropic response in 2 of the 18 IgG samples (false positive ELISA for AAβAR). In the remaining 16 AAβAR true-positive subjects, the β1AR-induced increase in contractility (concurrent M2/β2 blockade) was augmented to 140.5±12.2% of baseline compared to 127.4±7.2% of baseline with M2 blockade (atropine) only (p0.001, n=16). The β2AR-induced increase in contractility (concurrent M2/β1 blockade) was only 114.5±4.3% of baseline (p0.001, n=16). Combined M2 and β1/β2 blockade eliminated any increase in contractility.The inherently positive inotropic effect of AAβ1AR was negatively modulated by AAM2R and AAβ2AR. These opposing effects of receptor-activating autoantibodies may alter cardiac performance and influence clinical outcome depending on their receptor type and relative contractile activity.

Purkinje fibers play an essential role in transmitting electrical impulses through the heart, but they may also serve as triggers for arrhythmias linked to defective intracellular calcium (Ca(2+)) regulation. Although prior studies have extensively characterized spontaneous Ca(2+) release in nondriven Purkinje cells, little attention has been paid to rate-dependent changes in Ca(2+) transients. Therefore we explored the behaviors of Ca(2+) transients at pacing rates ranging from 0.125 to 3 Hz in single canine Purkinje cells loaded with fluo3 and imaged with a confocal microscope. The experiments uncovered the following novel aspects of Ca(2+) regulation in Purkinje cells: 1) the cells exhibit a negative Ca(2+)-frequency relationship (at 2.5 Hz, Ca(2+) transient amplitude was 66 ± 6% smaller than that at 0.125 Hz); 2) sarcoplasmic reticulum (SR) Ca(2+) release occurs as a propagating wave at very low rates but is localized near the cell membrane at higher rates; 3) SR Ca(2+) load declines modestly (10 ± 5%) with an increase in pacing rate from 0.125 Hz to 2.5 Hz; 4) Ca(2+) transients show considerable beat-to-beat variability, with greater variability occurring at higher pacing rates. Analysis of beat-to-beat variability suggests that it can be accounted for by stochastic triggering of local Ca(2+) release events. Consistent with this hypothesis, an increase in triggering probability caused a decrease in the relative variability. These results offer new insight into how Ca(2+) release is normally regulated in Purkinje cells and provide clues regarding how disruptions in this regulation may lead to deleterious consequences such as arrhythmias.

The author discusses Wenckebach phenomenon (WP) in the His-Purkinje system (HPS). Subtle PR changes may be interpreted as no change, often seen in the HPS-WP. Changes in the QRS axis help to localize the site of delay and block along fascicles of the left bundle branch. Marked PR changes may occur during HPS-WP. All the QRS complexes that are conducted depict right bundle branch and left anterior superior fascicular block, so the visible changes in the HV interval suggest WP mostly occurring in the left posterior inferior fascicle.

High-frequency signals (e.g., Purkinje potentials) are of interest in arrhythmia syndromes such as fascicular tachycardia and ventricular fibrillation. However, restricted dynamic range and sampling rate in modern electrophysiology recording systems make it challenging to identify high-frequency

A 55-year old woman with recurrent syncope and palpitation experienced polymorphic ventricular tachycardia (VT) and more than three monomorphic VTs with a right bundle branch block configuration, either inferior-, middle-, and superior-axis. During the pleomorphic VT, the diastolic potential (dp) was recorded at the antero-lateral left ventricle. Changes in the QRS morphology were associated with the time between dp and onset of QRS complex (dp-V interval), and prolongation of dp-V interval terminated the VT. In addition, the delayed potentials were seen during sinus rhythm around this area. Delivery of radiofrequency current targeting the delayed potentials abolished all the VTs. Different exits from relatively large area of slow conduction in the left anterior fascicle might have produced the pleomorphic VTs.

The intrinsic heterogeneity of electrical action potential (AP) properties between Purkinje fibers (PFs) and the ventricular wall, as well as within the wall, plays an important role in ensuring successful excitation of the ventricles. It can also be proarrhythmic due to nonuniform repolarization across the Purkinje-ventricular junction. However, the ionic mechanisms that underlie the marked AP differences between PFs and ventricular cells are not fully characterized. We studied such mechanisms by developing a new family of biophysically detailed AP models for rabbit PF cells and three transmural ventricular cell types. The models were based on and validated against experimental data recorded from rabbit at ionic channel, single cell, and tissue levels. They were then used to determine the functional roles of each individual ionic channel current in modulating the AP heterogeneity at the rabbit Purkinje-ventricular junction, and to identify specific currents responsible for the differential response of PFs and ventricular cells to pharmacological interventions.