Human Atrial Myocyte Research Articles

Influence of genetic mutations to atria vulnerability to atrial fibrillation: An in-silico 3D human atria study

Background and ObjectivePersonalized 3D computer models of atria have been extensively implemented in the last yearsas a tool to facilitate the understanding of the mechanisms underlying different forms of arrhythmia, such as atrial fibrillation (AF). Meanwhile, genetic mutations acting on potassium channel dynamics were demonstrated to induce fibrillatory episodes in asymptomatic patients. This research study aims at assessing the effects and the atrial susceptibility to AF of three gain-of-function mutations – namely, KCNH2 T895M, KCNH2 T436M, and KCNE3-V17M – associated with AF outbreaks, using highly detailed 3D atrial models with realistic wall thickness and heterogenous histological properties. MethodsThe 3D atrial model was generated by reconstructing segmented anatomical structures from CT scans of an AF patient. Modified versions of the Courtemanche human atrial myocyte model were used to reproduce the electrophysiological activity of the WT and of the three mutant cells. Ectopic foci (EF) were simulated in sixteen locations across the atrial mesh using an S1-S2 protocol with two S2 basic cycle lengths (BCL) and eleven coupling intervals in order to induce arrhythmias. ResultsThe three genetic mutations at 3D level reduced the APD90. The KCNE3-V17M mutation provoked the highest shortening (55 % in RA and LA with respect to WT), followed by KCNH2 T895M (14 % in RA and 18 % LA with respect to WT)and KCNH2 T436M (7 % in RA and 9 % LA with respect to WT). The KCNE3-V17M mutation led to arrhythmia in 67 % of the cases simulated and in 94 % of ectopic foci considered, at S2 BCL equal to 100 ms. The KCNH2 T436M and KCNH2 T895M mutations increased the vulnerability to AF in a similar way, leading to arrhythmic episodes in 7 % of the simulated conditions, at S2 BCL set to 160 ms. Overall, 60 % of the arrhythmic events generated arise in the left atrium. Spiral waves, multiple rotors and disordered electrical pattern were elicited in the presence of the KCNE3-V17M mutation, exhibiting an instantaneous mean frequency of 7.6 Hz with a mean standard deviation of 1.12 Hz. The scroll waves induced in the presence of the KCNH2 T436M and KCNH2 T895M mutations showed steadiness and regularity with an instantaneous mean frequencies in the range of 4.9 – 5.1 Hz and a mean standard deviation within 0.19 – 0.53 Hz. ConclusionsThe pro-arrhythmogenicity of the KCNE3-V17M, KCNH2 T895M and KCNH2 T436M mutations was studied and proved on personalized 3D cardiac models. The three genetic mutations were demonstrated to increase the predisposition of atrial tissue to the formation of AF-susceptible substrate in different ways based on their effects on electrophysiological properties of the atria.

Modeling sex differences in human atrial myocyte structural and Ca2+ handling properties: Impact on Ca2+-driven arrhythmia.

Substantial sex differences have been reported in the prevalence, pathophysiology, treatment, and prognosis of atrial fibrillation (AF), the most common cardiac arrhythmia. Knowledge of sex differences in the underlying arrhythmogenic substrate may improve therapy. Recent experimental studies revealed differences in cell geometry, transverse-axial tubule system (TATS) density, and expression, function, and phosphorylation status of Ca2+ handling proteins in female vs. male atrial myocytes, and in their remodeling in AF. However, it remains unknown how these sex-dependent differences contribute (individually or collectively) to arrhythmia propensity in females vs. males. Here, we used our new human atrial myocyte model coupling electrophysiology and spatially-detailed Ca2+ cycling governed by the TATS to integrate these sex-specific features and build sex-specific models of both normal sinus rhythm (nSR) and AF conditions. We tested the responses of the models to a pace-pause protocol and found that in nSR the female myocyte model displays 1) higher Ca2+ spark frequency, and 2) greater and more frequent spontaneous Ca2+ release (SCR) compared to male. These sex differences in Ca2+-driven arrhythmias are exacerbated in AF conditions. A systematic parametric study revealed the precise contribution of each sex-dependent change (i.e., TATS density, cell dimension, and Ca2+-handling protein expression or function) to SCR and Ca2+ spark properties. Both sparse TATS and changes in Ca2+-handling proteins (i.e., larger L-type Ca2+ current, reduced calsequestrin expression, higher RyR phosphorylation, and increased sarcoplasmic reticulum Ca2+-ATPase) promote greater Ca2+-driven arrhythmia in female myocytes, which was slightly attenuated by the smaller cell dimension vs male myocytes. Our study demonstrates the interactive contributions of subcellular structural and ionic sex differences to intracellular Ca2+ instability and provides novel model-based mechanistic insight that may guide future therapeutic sex-specific anti-AF strategies.

Mechanisms of spontaneous Ca2+ release-mediated arrhythmia in a novel 3D human atrial myocyte model: I. Transverse-axial tubule variation.

Intracellular calcium (Ca2+ ) cycling is tightly regulated in the healthy heart ensuring effective contraction. This is achieved by transverse (t)-tubule membrane invaginations that facilitate close coupling of key Ca2+ -handling proteins such as the L-type Ca2+ channel and Na+ -Ca2+ exchanger (NCX) on the cell surface with ryanodine receptors (RyRs) on the intracellular Ca2+ store. Although less abundant and regular than in the ventricle, t-tubules also exist in atrial myocytes as a network of transverse invaginations with axial extensions known as the transverse-axial tubule system (TATS). In heart failure and atrial fibrillation, there is TATS remodelling that is associated with aberrant Ca2+ -handling and Ca2+ -induced arrhythmic activity; however, the mechanism underlying this is not fully understood. To address this, we developed a novel 3D human atrial myocyte model that couples electrophysiology and Ca2+ -handling with variable TATS organization and density. We extensively parameterized and validated our model against experimental data to build a robust tool examining TATS regulation of subcellular Ca2+ release. We found that varying TATS density and thus the localization of key Ca2+ -handling proteins has profound effects on Ca2+ handling. Following TATS loss, there is reduced NCX that results in increased cleft Ca2+ concentration through decreased Ca2+ extrusion. This elevated Ca2+ increases RyR open probability causing spontaneous Ca2+ releases and the promotion of arrhythmogenic waves (especially in the cell interior) leading to voltage instabilities through delayed afterdepolarizations. In summary, the present study demonstrates a mechanistic link between TATS remodelling and Ca2+ -driven proarrhythmic behaviour that probably reflects the arrhythmogenic state observed in disease. KEY POINTS: Transverse-axial tubule systems (TATS) modulate Ca2+ handling and excitation-contraction coupling in atrial myocytes, with TATS remodelling in heart failure and atrial fibrillation being associated with altered Ca2+ cycling and subsequent arrhythmogenesis. To investigate the poorly understood mechanisms linking TATS variation and spontaneous Ca2+ release, we built, parameterized and validated a 3D human atrial myocyte model coupling electrophysiology and spatially-detailed subcellular Ca2+ handling governed by the TATS. Simulated TATS loss causes diastolic Ca2+ and voltage instabilities through reduced Na+ -Ca2+ exchanger-mediated Ca2+ removal, cleft Ca2+ accumulation and increased ryanodine receptor open probability, resulting in spontaneous Ca2+ release and promotion of arrhythmogenic waves and delayed afterdepolarizations. At fast electrical rates typical of atrial tachycardia/fibrillation, spontaneous Ca2+ releases are larger and more frequent in the cell interior than at the periphery. Our work provides mechanistic insight into how atrial TATS remodelling can lead to Ca2+ -driven instabilities that may ultimately contribute to the arrhythmogenic state in disease.

Mechanisms of spontaneous Ca2+ release-mediated arrhythmia in a novel 3D human atrial myocyte model: II. Ca2+ -handling protein variation.

Disruption of the transverse-axial tubule system (TATS) in diseases such as heart failure and atrial fibrillation occurs in combination with changes in the expression and distribution of key Ca2+ -handling proteins. Together this ultrastructural and ionic remodelling is associated with aberrant Ca2+ cycling and electrophysiological instabilities that underlie arrhythmic activity. However, due to the concurrent changes in TATs and Ca2+ -handling protein expression and localization that occur in disease it is difficult to distinguish their individual contributions to the arrhythmogenic state. To investigate this, we applied our novel 3D human atrial myocyte model with spatially detailed Ca2+ diffusion and TATS to investigate the isolated and interactive effects of changes in expression and localization of key Ca2+ -handling proteins and variable TATS density on Ca2+ -handling abnormality driven membrane instabilities. We show that modulating the expression and distribution of the sodium-calcium exchanger, ryanodine receptors and the sarcoplasmic reticulum (SR) Ca2+ buffer calsequestrin have varying pro- and anti-arrhythmic effects depending on the balance of opposing influences on SR Ca2+ leak-load and Ca2+ -voltage relationships. Interestingly, the impact of protein remodelling on Ca2+ -driven proarrhythmic behaviour varied dramatically depending on TATS density, with intermediately tubulated cells being more severely affected compared to detubulated and densely tubulated myocytes. This work provides novel mechanistic insight into the distinct and interactive consequences of TATS and Ca2+ -handling protein remodelling that underlies dysfunctional Ca2+ cycling and electrophysiological instability in disease. KEY POINTS: In our companion paper we developed a 3D human atrial myocyte model, coupling electrophysiology and Ca2+ handling with subcellular spatial details governed by the transverse-axial tubule system (TATS). Here we utilize this model to mechanistically examine the impact of TATS loss and changes in the expression and distribution of key Ca2+ -handling proteins known to be remodelled in disease on Ca2+ homeostasis and electrophysiological stability. We demonstrate that varying the expression and localization of these proteins has variable pro- and anti-arrhythmic effects with outcomes displaying dependence on TATS density. Whereas detubulated myocytes typically appear unaffected and densely tubulated cells seem protected, the arrhythmogenic effects of Ca2+ handling protein remodelling are profound in intermediately tubulated cells. Our work shows the interaction between TATS and Ca2+ -handling protein remodelling that underlies the Ca2+ -driven proarrhythmic behaviour observed in atrial fibrillation and may help to predict the effects of antiarrhythmic strategies at varying stages of ultrastructural remodelling.

Numerical methods for the detection of phase defect structures in excitable media.

Electrical waves that rotate in the heart organize dangerous cardiac arrhythmias. Finding the region around which such rotation occurs is one of the most important practical questions for arrhythmia management. For many years, the main method for finding such regions was so-called phase mapping, in which a continuous phase was assigned to points in the heart based on their excitation status and defining the rotation region as a point of phase singularity. Recent analysis, however, showed that in many rotation regimes there exist phase discontinuities and the region of rotation must be defined not as a point of phase singularity, but as a phase defect line. In this paper, we use this novel methodology and perform a comparative study of three different phase definitions applied to in silico data and to experimental data obtained from optical voltage mapping experiments on monolayers of human atrial myocytes. We introduce new phase defect detection algorithms and compare them with those that appeared in literature already. We find that the phase definition is more important than the algorithm to identify sudden spatial phase variations. Sharp phase defect lines can be obtained from a phase derived from local activation times observed during one cycle of arrhythmia. Alternatively, similar quality can be obtained from a reparameterization of the classical phase obtained from observation of a single timeframe of transmembrane potential. We found that the phase defect line length was (35.9 ± 6.2)mm in the Fenton-Karma model and (4.01 ± 0.55)mm in cardiac human atrial myocyte monolayers. As local activation times are obtained during standard clinical cardiac mapping, the methods are also suitable to be applied to clinical datasets. All studied methods are publicly available and can be downloaded from an institutional web-server.

Quantifying the interactive arrhythmogenic effects of cell ultrastructural remodeling and calcium-handling protein redistribution in a three-dimensional model of the human atrial myocyte

It is well accepted that calcium handling abnormalities can precipitate atrial fibrillation (AF). Both ionic and ultrastructural remodeling have been associated with deranged calcium signaling and AF through altered channel and transporter expression and function, reduced density and regularity of the transverse-axial tubule system (TATS), and subcellular re-distribution of calcium-handling proteins. However, it remains unclear whether ionic and ultrastructural remodeling interact to affect arrhythmogenic propensity.

Sex-dependent differences in Ca2+-related arrhythmia revealed by human atrial myocyte models

Sex is an established independent risk factor for atrial fibrillation (AF), the most common cardiac arrhythmia. Substantial sex differences have been reported in prevalence, clini-cal representation, pathophysiology, therapy, and prognosis of AF; but the associated mechanisms and causes remain largely unknown. It is well established that AF is associated with remodeling of the Ca2+ handling system. A recent study in human atria revealed significant sex-differences in AF-related perturbations of Ca2+ handling, which could have important implications for understanding sex-specific AF pathophysiology and treatment.

Arrhythmogenic Effects of Genetic Mutations Affecting Potassium Channels in Human Atrial Fibrillation: A Simulation Study.

Genetic mutations in genes encoding for potassium channel protein structures have been recently associated with episodes of atrial fibrillation in asymptomatic patients. The aim of this study is to investigate the potential arrhythmogenicity of three gain-of-function mutations related to atrial fibrillation—namely, KCNH2 T895M, KCNH2 T436M, and KCNE3-V17M—using modeling and simulation of the electrophysiological activity of the heart. A genetic algorithm was used to tune the parameters’ value of the original ionic currents to reproduce the alterations experimentally observed caused by the mutations. The effects on action potentials, ionic currents, and restitution properties were analyzed using versions of the Courtemanche human atrial myocyte model in different tissues: pulmonary vein, right, and left atrium. Atrial susceptibility of the tissues to spiral wave generation was also investigated studying the temporal vulnerability. The presence of the three mutations resulted in an overall more arrhythmogenic substrate. Higher current density, action potential duration shortening, and flattening of the restitution curves were the major effects of the three mutations at the single-cell level. The genetic mutations at the tissue level induced a higher temporal vulnerability to the rotor’s initiation and progression, by sustaining spiral waves that perpetuate until the end of the simulation. The mutation with the highest pro-arrhythmic effects, exhibiting the widest sustained VW and the smallest meandering rotor’s tip areas, was KCNE3-V17M. Moreover, the increased susceptibility to arrhythmias and rotor’s stability was tissue-dependent. Pulmonary vein tissues were more prone to rotor’s initiation, while in left atrium tissues rotors were more easily sustained. Re-entries were also progressively more stable in pulmonary vein tissue, followed by the left atrium, and finally the right atrium. The presence of the genetic mutations increased the susceptibility to arrhythmias by promoting the rotor’s initiation and maintenance. The study provides useful insights into the mechanisms underlying fibrillatory events caused by KCNH2 T895M, KCNH2 T436M, and KCNE3-V17M and might aid the planning of patient-specific targeted therapies.

Quantifying the Arrhythmogenic Effects of Subcellular Structural Variations in a Three-Dimensional Model of the Human Atrial Myocyte

Calcium handling abnormalities can precipitate atrial fibrillation (AF). In recent years, both ionic remodeling (i.e., changes in expression and function of channels and transporters) and ultrastructural remodeling (i.e., decreased density and regularity of the transverse-axial tubule system, TATS, and disrupted channel and transporter distribution) have been associated with deranged calcium signaling and excitation-contraction coupling. Here, we used a three-dimensional computational model of the human atrial myocyte, in which electrophysiology and subcellular calcium signaling are coupled with a spatially-detailed description of TATS structure, to predict the effect of variation in cell structure on atrial myocyte calcium signaling and electrophysiology.Our simulations predict that atrial myocytes with sparse and irregular tubules exhibit greater vulnerability to arrhythmia (e.g., shorter latency and higher amplitude of delayed afterdepolarizations, DADs, and lower pacing threshold for alternans).

Effects of Ultrastructural Remodeling on Calcium Signaling and Electrophysiology in a Three-Dimensional Model of the Human Atrial Myocyte

Effects of Ultrastructural Remodeling on Calcium Signaling and Electrophysiology in a Three-Dimensional Model of the Human Atrial Myocyte

Stretch-activated current in human atrial myocytes and Na+ current and mechano-gated channels\u2019 current in myofibroblasts alter myocyte mechanical behavior: a computational study

BackgroundThe activation of stretch-activated channels (SACs) in cardiac myocytes, which changes the phases of action potential repolarization, is proven to be highly efficient for the conversion of atrial fibrillation. The expression of Na+ current in myofibroblasts (Mfbs) regenerates myocytes’ action potentials, suggesting that Mfbs play an active role in triggering cardiac rhythm disturbances. Moreover, the excitation of mechano-gated channels (MGCs) in Mfbs depolarizes their membrane potential and contributes to the increased risk of post-infarct arrhythmia. Although these electrophysiological mechanisms have been largely known, the roles of these currents in cardiac mechanics are still debated. In this study, we aimed to investigate the mechanical influence of these currents via mathematical modeling. A novel mathematical model was developed by integrating models of human atrial myocyte (including the stretch-activated current, Ca2+–force relation, and mechanical behavior of a single segment) and Mfb (including our formulation of Na+ current and mechano-gated channels’ current). The effects of the changes in basic cycle length, number of coupled Mfbs and intercellular coupling conductance on myocyte mechanical properties were compared.ResultsOur results indicated that these three currents significantly regulated myocyte mechanical parameters. In isosarcometric contraction, these currents increased segment force by 13.8–36.6% and dropped element length by 12.1–31.5%. In isotonic contraction, there are 2.7–5.9% growth and 0.9–24% reduction. Effects of these currents on the extremum of myocyte mechanical parameters become more significant with the increase of basic cycle length, number of coupled Mfbs and intercellular coupling conductance.ConclusionsThe results demonstrated that stretch-activated current in myocytes and Na+ current and mechano-gated channels’ current in Mfbs significantly influenced myocyte mechanical behavior and should be considered in future cardiac mechanical mathematical modeling.

Effects of Varying Transverse and Axial Tubules in a Three-Dimensional Model of Calcium Signaling in the Human Atrial Myocyte

T-tubules are invaginations of the sarcolemma that play a key role in excitation-contraction coupling in ventricular myocytes. Atrial myocytes are generally thought to possess sparse irregular transverse-tubular (TT) components, as opposed to the highly dense and regular ventricular TT system. Axial tubules (ATs) with extensive junctions to the sarcoplasmic reticulum that include ryanodine receptor (RyR) clusters, characterized by rapid activation of Ca2+-induced Ca2+ release, have also been identified in atrial myocytes. AT and TT remodeling and changes in distribution, composition, and phosphorylation status of Ca2+ release units are thought to underlie Ca2+ abnormalities in atrial fibrillation (AF), the most common cardiac arrhythmia. Here, we performed a computational analysis to investigate how changes in the tubular network affect human atrial electrophysiology. We modified our well-established three-dimensional model of Ca2+ signaling in the rabbit ventricular myocyte to develop an analogous model in the human atrium. We also coupled the Ca2+ signaling model to our well-established model of membrane electrophysiology in the human atrial myocyte. We systematically varied TT and AT density and RyR distribution and assessed the effect on Ca2+ spark and wave properties. When TT density is low, as shown in isolated atrial myocytes, the model recapitulates the typical U-shaped Ca2+ wave seen experimentally in transverse confocal line-scan images. Introduction of ATs resulted in W-shaped Ca2+ waves, reflecting more synchronous Ca2+ release. The frequency and amplitude of Ca2+ release events were affected by both density of TT and AT, as well as RyR distribution. Our newly developed three-dimensional model of the human atrial myocyte will be employed to investigate whether and how AF-induced cellular electrical and structural remodeling effects collectively contribute to the membrane potential and Ca2+ abnormalities seen in AF.

Transient outward K+ current can strongly modulate action potential duration and initiate alternans in the human atrium.

Efforts to identify the mechanisms for the initiation and maintenance of human atrial fibrillation (AF) often focus on changes in specific elements of the atrial "substrate," i.e., its electrophysiological properties and/or structural components. We used experimentally validated mathematical models of the human atrial myocyte action potential (AP), both at baseline in sinus rhythm (SR) and in the setting of chronic AF, to identify significant contributions of the Ca2+-independent transient outward K+ current ( Ito) to electrophysiological instability and arrhythmia initiation. First, we explored whether changes in the recovery or restitution of the AP duration (APD) and/or its dynamic stability (alternans) can be modulated by Ito. Recent reports have identified disease-dependent spatial differences in expression levels of the specific K+ channel α-subunits that underlie Ito in the left atrium. Therefore, we studied the functional consequences of this by deletion of 50% of native Ito (Kv4.3) and its replacement with Kv1.4. Interestingly, significant changes in the short-term stability of the human atrial AP waveform were revealed. Specifically, this K+ channel isoform switch produced discontinuities in the initial slope of the APD restitution curve and appearance of APD alternans. This pattern of in silico results resembles some of the changes observed in high-resolution clinical electrophysiological recordings. Important insights into mechanisms for these changes emerged from known biophysical properties (reactivation kinetics) of Kv1.4 versus those of Kv4.3. These results suggest new approaches for pharmacological management of AF, based on molecular properties of specific K+ isoforms and their changed expression during progressive disease. NEW & NOTEWORTHY Clinical studies identify oscillations (alternans) in action potential (AP) duration as a predictor for atrial fibrillation (AF). The abbreviated AP in AF also involves changes in K+ currents and early repolarization of the AP. Our simulations illustrate how substitution of Kv1.4 for the native current, Kv4.3, alters the AP waveform and enhances alternans. Knowledge of this "isoform switch" and related dynamics in the AF substrate may guide new approaches for detection and management of AF.

Simulation of Atrial Fibrosis Using Coupled Myocyte-Fibroblast Cellular and Human Atrial Models.

Atrial fibrosis is characterized by expansion of extracellular matrix and increase in the number of fibroblasts which has been associated with the development and maintenance of atrial arrhythmias. However, the mechanisms how the fibrosis contributes to atrial arrhythmia remain incompletely understood. In this study, we used a proposed fibroblast model coupled with the human atrial myocyte to investigate the effects of fibrosis on atrial excitability and repolarization at both cellular and macroscopic levels. The 12-lead electrocardiogram (ECG) was also simulated to explore the index of clinical diagnosis for fibrosis. The simulation results showed that the fibrosis can modify action potential morphology of human atrial myocyte, slow down wave propagation, and have rate adaptation, thus causing the atrial electrical heterogeneity. The fibrosis alone was sufficient to cause arrhythmia, induce reentry wave, and result in low amplitude and wide P waves at normal heart rate and significant prolonged and inverse P waves at high heart rate. All these symptoms aggravated when the level of fibrosis increased. Our simulations demonstrated that fibrosis is the substrate of atrial arrhythmia and thereby may be a potential target in the treatment of atrial arrhythmias.

18-02: Fibrosis explains increased epicardial breakthrough in AF

Introduction: Recent work has shown that the transition from persistent to permanent AF in goats coincides with an increase in fibrosis in the outer millimeter of the atrial wall. Macroscopically this leads to reduced electrical conductivity orthogonal to the dominant fiber orientation. A causal relation has not been established yet. Our purpose was to test if subepicardial fibrosis can explain the increased incidence of epicardial breakthroughs, which is also observed in permanent AF. Methods: We constructed a detailed geometry of the human atria including all major bundle structures and two layers of fiber orientation. Computer simulations were run with a mesh of 0.2 mm resolution and an ionic current formulation specific for the human atrial myocyte. Subepicardial fibrosis was modeled by assigning zero transverse conductivity to a random selection of model elements. Simulations were performed with 0, 50, and 70% affected elements. Fibrillation was induced with incremental rapid pacing. Simulations were repeated with 10 different pacing locations, for a total of 30 simulations of 5 seconds each. We tested in the first place whether AF was induced by this protocol. In those simulations where fibrillation occurred, we computed the number of waves, number of phase singularities, percentage dyssynchrony, and number of breakthroughs in the 3 seconds after pacing. Conclusion: Subepicardial fibrosis can account for an increased susceptibility for AF as well as for the remarkable increase in epicardial breakthrough observed in experimental studies.

MMP9 Rs3918242 Polymorphism Affects Tachycardia-Induced MMP9 Expression in Cultured Atrial-Derived Myocytes but Is Not a Risk Factor for Atrial Fibrillation among the Taiwanese.

Matrix metalloproteinase (MMP) plays an important role in the pathogenesis of atrial fibrillation (AF). The MMP9 promoter has a functional polymorphism rs3918242 that can regulate the level of gene transcription. This study recruited 200 AF patients and 240 controls. The MMP9 rs3918242 was examined by polymerase chain reactions. HL-1 atrial myocytes were cultured and electrically stimulated. Right atrial appendages were obtained from six patients with AF and three controls with sinus rhythm undergoing open heart surgery. The MMP9 expression and activity were determined using immunohistochemical analysis and gelatin zymography, respectively. Rapid pacing induces MMP9 secretion from HL-1 myocytes in a time- and dose-dependent manner. The responsiveness of MMP9 transcriptional activity to tachypacing was significantly enhanced by rs3918242. The expression of MMP9 was increased in fibrillating atrial tissue than in sinus rhythm. However, the distribution of rs3918242 genotypes and allele frequencies did not significantly differ between the control and AF groups. HL-1 myocyte may secrete MMP9 in response to rapid pacing, and the secretion could be modulated by rs3918242. Although the MMP9 expression of human atrial myocyte is associated with AF, our study did not support the association of susceptibility to AF among Taiwanese subjects with the MMP9 rs3918242 polymorphism.

0113 : Epicardial progenitors are source of adipocyte in human atria

and aims The accumulation of the adipose tissue (AT) in the sup- and subepicardium of the atrial myocardium is associated with a high risk of atrial fibrillation. Here we addressed the question of the cellular origin of atrial AT. Human right atrial specimen obtained during cardiac surgery were used for histological, biochemical studies and to harvest epicardial progenitors. Cells were characterized using flow cytometry, proteomic and genic expression assays and maintained in culture conditions, in some experiments they were co cultured with human atrial myocytes using a Transwell system. In atrial section, epicardial progenitors expressing the pre-adipocyte factor 1 (Pref-1) were detected in the epicardial and sub-epicardial layer which contained a number of progenitor cells referred as epicardic progenitorderived cells (EPDCs). Some EPDCs expressed Pref-1 suggesting that they could engaged in the adipocyte pathways. This was tested in vitro using EPDCs harvested from human right atrial samples (n=20). From 1 st to 5-6 cells passages, EPDCs underwent an epithelial-to-mesenchymal transition (EMT), acquired mesenchymal phenotypes and could differentiate into osteocyte or chondrocyte. When cultured using an adipogenic medium, around 40±6% of EPDCs cells showed lipid droplet colored with oil red and expressed mature adipocyte perilipin marker. Culture medium enriched with the secretome of human atria too induced the differentiation of EPDCs in adipocytes (n=6). A shift from pre- to mature adipocytes was indicated by the detection of Pref-1 at 7 but not 30 days of culture, it was the opposite for perilipin. Human atrial myocyte could be the source of pro-adipogenic factor. In fact, after 7 days in coculture, EPDCs are differentiated into adipocytes (n=6). Human EPDCs from atria can differentiate into adipocyte; factors secreted by atrial myocytes regulate this differentiation process. The author hereby declares no conflict of interest

Effects of Na+ Current and Mechanogated Channels in Myofibroblasts on Myocyte Excitability and Repolarization.

Fibrotic remodeling, characterized by fibroblast phenotype switching, is often associated with atrial fibrillation and heart failure. This study aimed to investigate the effects on electrotonic myofibroblast-myocyte (Mfb-M) coupling on cardiac myocytes excitability and repolarization of the voltage-gated sodium channels (VGSCs) and single mechanogated channels (MGCs) in human atrial Mfbs. Mathematical modeling was developed from a combination of (1) models of the human atrial myocyte (including the stretch activated ion channel current, I SAC) and Mfb and (2) our formulation of currents through VGSCs (I Na_Mfb) and MGCs (I MGC_Mfb) based upon experimental findings. The effects of changes in the intercellular coupling conductance, the number of coupled Mfbs, and the basic cycle length on the myocyte action potential were simulated. The results demonstrated that the integration of I SAC, I Na_Mfb, and I MGC_Mfb reduced the amplitude of the myocyte membrane potential (V max) and the action potential duration (APD), increased the depolarization of the resting myocyte membrane potential (V rest), and made it easy to trigger spontaneous excitement in myocytes. For Mfbs, significant electrotonic depolarizations were exhibited with the addition of I Na_Mfb and I MGC_Mfb. Our results indicated that I SAC, I Na_Mfb, and I MGC_Mfb significantly influenced myocytes and Mfbs properties and should be considered in future cardiac pathological mathematical modeling.

Atrial-selective targeting of arrhythmogenic phase-3 early afterdepolarizations in human myocytes

BackgroundWe have previously shown that non-equilibrium Na+ current (INa) reactivation drives isoproterenol-induced phase-3 early afterdepolarizations (EADs) in mouse ventricular myocytes. In these cells, EAD initiation occurs secondary to potentiated sarcoplasmic reticulum Ca2+ release and enhanced Na+/Ca2+ exchange (NCX). This can be abolished by tetrodotoxin-blockade of INa, but not ranolazine, which selectively inhibits ventricular late INa. AimSince repolarization of human atrial myocytes is similar to mouse ventricular myocytes in that it is relatively rapid and potently modulated by Ca2+, we investigated whether similar mechanisms can evoke EADs in human atrium. Indeed, phase-3 EADs have been shown to re-initiate atrial fibrillation (AF) during autonomic stimulation, which is a well-recognized initiator of AF. MethodsWe integrated a Markov model of INa gating in our human atrial myocyte model. To simulate experimental results, we rapidly paced this cell model at 10Hz in the presence of 0.1μM acetylcholine and 1μM isoproterenol, and assessed EAD occurrence upon return to sinus rhythm (1Hz). ResultsCellular Ca2+ loading during fast pacing results in a transient period of hypercontractility after return to sinus rhythm. Here, fast repolarization and enhanced NCX facilitate INa reactivation via the canonical gating mode (i.e., not late INa burst mode), which drives EAD initiation. Simulating ranolazine administration reduces atrial peak INa and leads to faster repolarization, during which INa fails to reactivate and EADs are prevented. ConclusionsNon-equilibrium INa reactivation can critically contribute to arrhythmias, specifically in human atrial myocytes. Ranolazine might be beneficial in this context by blocking peak (not late) atrial INa.

Ranolazine Prevents Phase-3 Early Afterdepolarizations in Human Atrial Myocytes by Inhibiting Na Current Non-Equilibrium Reactivation

Background: We have previously shown that non-equilibrium reactivation of the Na current (INa) drives Isoproterenol-induced phase-3 early afterdepolarizations (EADs) in failing mouse ventricular myocytes. EAD initiation is secondary to potentiated sarcoplasmic reticulum Ca release and enhanced Na/Ca exchange (NCX), and can be abolished by the INa blocker tetrodotoxin, but not by the selective (in ventricles) late INa blocker ranolazine.Aim. Since repolarization in human atrial myocytes is relatively rapid and potently modulated by Ca (as in mouse ventricle), we investigate whether the same EAD mechanism may occur in human atria. Indeed, phase-3 EADs have been suggested to underlie re-initiation of atrial fibrillation (AF) after termination upon autonomic stimuli - well recognized AF triggers.Methods: We integrated a Markov model of INa (also describing the channel interaction with ranolazine, similar to work by Clancy) in our human atrial myocyte model. To recapitulate experimental results, we simulated rapid cell pacing (10 Hz) in the presence of Acetylcholine (0.1 μM) and Isoproterenol (1 μM), and assessed EAD occurrence upon return to sinus rhythm (1 Hz).Results: Cellular Ca overload during fast pacing results in a transient period of hyper-contractility after return to sinus rhythm. Here, fast repolarization and enhanced NCX facilitate INa reactivation via the canonical gating mode (i.e., not late INa burst mode), which drives EAD initiation. Notably, in action potential clamp experiments, an EAD waveform elicits a lidocaine-sensitive inward current during the EAD upstroke in pig atrial cells. Simulating ranolazine administration reduces peak INa and leads to a faster repolarization, during which INa fails to reactivate.Conclusions: Our simulations suggest that non-equilibrium INa reactivation critically contributes to arrhythmias in human atrial myocytes. Ranolazine might be beneficial in this context by blocking peak (not late) atrial INa.