The Contribution of Ionic Currents to Rate-Dependent Action Potential Duration and Pattern of Reentry in a Mathematical Model of Human Atrial Fibrillation

Persistent atrial fibrillation (PeAF) in humans is characterized by shortening of action potential duration (APD) and attenuation of APD rate-adaptation. However, the quantitative influences of particular ionic current alterations on rate-dependent APD changes, and effects on patterns of reentry in atrial tissue, have not been systematically investigated. Using mathematical models of human atrial cells and tissue and performing parameter sensitivity analysis, we evaluated the quantitative contributions to action potential (AP) shortening and APD rate-adaptation of ionic current remodeling seen with PeAF. Ionic remodeling in PeAF was simulated by reducing L-type Ca²⁺ channel current (I_CaL), increasing inward rectifier K⁺ current (I_K1) and modulating five other ionic currents. Parameter sensitivity analysis, which quantified how each ionic current influenced APD in control and PeAF conditions, identified interesting results, including a negative effect of Na⁺/Ca²⁺ exchange on APD only in the PeAF condition. At high pacing rate (2 Hz), electrical remodeling in I_K1 alone accounts for the APD reduction of PeAF, but at slow pacing rate (0.5 Hz) both electrical remodeling in I_CaL alone (-70%) and I_K1 alone (+100%) contribute equally to the APD reduction. Furthermore, AP rate-adaptation was affected by I_Kur in control and by I_NaCa in the PeAF condition. In a 2D tissue model, a large reduction (-70%) of I_CaL becomes a dominant factor leading to a stable spiral wave in PeAF. Our study provides a quantitative and unifying understanding of the roles of ionic current remodeling in determining rate-dependent APD changes at the cellular level and spatial reentry patterns in tissue.     

Conditions for Waveblock Due to Anisotropy in a Model of Human Ventricular Tissue

Waveblock formation is the main cause of reentry. We have performed a comprehensive numerical modeling study of block formation due to anisotropy in Ten Tusscher and Panfilov (2006) ionic model for human ventricular tissue. We have examined the border between different areas of myocardial fiber alignment and have shown that blockage can occur for a wave traveling from a transverse fiber area to a longitudinal one. Such blockage occurs for reasonable values of the anisotropy ratio (AR): from 2.4 to 6.2 with respect to propagation velocities. This critical AR decreases by the suppression of I _Na and I _Ca, slightly decreases by the suppression of I _Kr and I _Ks, and substantially increases by the suppression of I _K1. Hyperkalemia affects the block formation in a complex, biphasic way. We provide examples of reentry formation due to the studied effects and have concluded that the suppression of I _K1 should be the most effective way to prevent waveblock at the areas of abrupt change in anisotropy.     

Reflected reentry, delayed conduction, and electrotonic inhibition in segmentally depressed atrial tissues

Reflection is a subclass of reentrant cardiac arrhythmias in which reexcitation of the heart occurs as a result of to and fro electrotonically mediated transmission of impulses across a narrow zone of impaired conductivity. Although relatively well characterized in ventricular tissues, the reflection mechanism has not been studied in atrial tissues. In this study we examine the possibility of reflected reentry in segmentally depressed atrial tissues and evaluate conduction characteristics in these preparations. Narrow strips of atrial pectinate muscle or crista terminalis (canine and calf) were placed in a three-chambered bath and the central segment was superfused with an isotonic sucrose solution or an "ischemic" Tyrode's solution. Proximal to distal conduction across the 1.0- to 1.2-mm wide ischemic gap showed step delays as long as 210 ms. Reflected reentry was readily demonstrable when prominent step delays occurred during anterograde conduction of the impulse across the gap. Progressive acceleration of the stimulation rate resulted in progressively greater impairment of anterograde conduction until complete block occurred. The incidence and patterns of reflected reentry were therefore a sensitive function of the stimulation rate. Other features exhibited by these preparations include a slow recovery of excitability following the action potential, postrepolarization refractoriness, and electrotonic inhibition and summation. Our data suggest that the characteristics of conduction and reflection in segmentally depressed atrial tissues are qualitatively similar to those in ventricular tissues. The presence of electrotonic inhibition in atrial may also help to explain the functionally inexcitable zone seen in the vortex of the leading circle model of atrial flutter.     

Structural Heterogeneity Modulates Effective Refractory Period: A Mechanism of Focal Arrhythmia Initiation

Reductions in electrotonic loading around regions of structural and electrophysiological heterogeneity may facilitate capture of focal triggered activity, initiating reentrant arrhythmias. How electrotonic loading, refractoriness and capture of focal ectopics depend upon the intricate nature of physiological structural anatomy, as well as pathological tissue remodelling, however, is not well understood. In this study, we performed computational bidomain simulations with anatomically-detailed models representing the rabbit left ventricle. We used these models to quantify the relationship between local structural anatomy and spatial heterogeneity in action potential (AP) characteristics, electrotonic currents and effective refractory periods (ERPs) under pacing and restitution protocols. Regions surrounding vessel cavities, in addition to tissue surfaces, had significantly lower peak downstream electrotonic currents than well coupled myocardium ( vs A/cm²), with faster maximum AP upstroke velocities ( vs mV/ms), although noticeably very similar APDs ( vs ms) and AP restitution properties. Despite similarities in APDs, ERPs in regions of low electrotonic load in the vicinity of surfaces, intramural vessel cavities and endocardial structures were up to ms shorter compared to neighbouring well-coupled tissue, leading to regions of sharp ERP gradients. Consequently, focal extra-stimuli timed within this window of ERP heterogeneity between neighbouring regions readily induced uni-directional block, inducing reentry. Most effective induction sites were within channels of low ERPs between large vessels and epicardium. Significant differences in ERP driven by reductions in electrotonic loading due to fine-scale physiological structural heterogeneity provides an important mechanism of capture of focal activity and reentry induction. Application to pathological ventricles, particularly myocardial infarction, will have important implications in anti-arrhythmia therapy.     

I<Subscript>CaL</Subscript> and I<Subscript>to</Subscript> mediate rate-dependent repolarization in rabbit atrial myocytes

Rate-dependent repolarization (RDR) of action potential (AP) in cardiomyocyte plays a critical role in the genesis of arrhythmias and RDR in atrium has been linked with atrial fibrillation. However, detailed studies focusing on the role of RDR in rabbit atrium are scant. In this study, atrial cells were isolated from rabbit heart and rate-dependent property was explored in single atrial cell to elucidate the underlying mechanism. Our results indicated that rate-dependent prolongation was evident at the action potential duration at 20% (APD20) and 50% (APD50) repolarization but not at 90% repolarization (APD90) under control condition. Using transient outward potassium current (I_to) inhibitor 4-Aminopyridine (4-AP, 2 mM) effectively eliminated the changes in APD20 and APD50, and unmasked the rate-dependent reduction of APD90 which could be diminished by further adding L-type calcium current (I_CaL) inhibitor nifedipine (30 μM). However, using the selective late sodium current (I_NaL) inhibitor GS-458967 (GS967, 1 μM) caused minimal effect on APD90 of atrial cells both in the absence and presence of 4-AP. In consistence with results from APs, I_to and I_CaL displayed significant rate-dependent reduction because of their slow reactivation kinetics. In addition, the magnitude of I_NaL in rabbit atrium was so small that its rate-dependent changes were negligible. In conclusion, our study demonstrated that I_to and I_CaL mediate RDR of AP in rabbit atrium, while minimal effect of I_NaL was seen.     

Mathematical analysis of canine atrial action potentials: rate, regional factors, and electrical remodeling

Dogs have been used extensively to study atrial arrhythmias, but there are no published mathematical models of the canine atrial action potential (AP). To obtain insights into the ionic mechanisms governing canine atrial AP properties, we incorporated formulations of K(+), Na(+), Ca(2+), and Cl(-) currents, based on measurements in canine atrial myocytes, into a mathematical model of the AP. The rate-dependent behavior of model APs corresponded to experimental measurements and pointed to a central role for L-type Ca(2+) current inactivation in rate adaptation. Incorporating previously described regional ionic current variations into the model largely reproduced AP forms characteristic of the corresponding right atrial regions (appendage, pectinate muscle, crista terminalis, and atrioventricular ring). When ionic alterations induced by tachycardia-dependent remodeling were incorporated, the model reproduced qualitatively the AP features constituting the cellular substrate for atrial fibrillation. We conclude that this ionic model of the canine atrial AP agrees well with experimental measurements and gives potential insights into mechanisms underlying functionally important electrophysiological phenomena in canine atrium.     

Direction-dependent conduction abnormalities in a canine model of atrial fibrillation due to chronic atrial dilatation

Chronic rapid atrial pacing (RAP) leads to changes that perpetuate atrial fibrillation (AF). Chronic atrial dilatation due to mitral regurgitation (MR) also increases AF inducibility, but it is not clear whether the underlying mechanism is similar. Therefore, we have investigated atrial electrophysiology in a canine MR model (mitral valve avulsion, 1 mo) using high-resolution optical mapping and compared it with control dogs and with the canine RAP model (6-8 wk of atrial pacing at 600 beats/min, atrioventricular block, and ventricular pacing at 100 beats/min). At followup, optical action potentials were recorded using a 16 x 16 photodiode array from 2 x 2-cm left atrial (LA) and right atrial (RA) areas in perfused preparations, with pacing electrodes around the field of view to study direction dependency of conduction. Action potential duration at 80% repolarization (APD(80)) was not different between control and MR but was reduced in RAP atria. Conduction velocities during normal pacing were not different between groups. However, the MR LA showed increased conduction heterogeneity during pacing at short cycle lengths and during premature extrastimuli, which frequently caused pronounced regional conduction slowing. Conduction in the MR LA during extrastimulation also displayed a marked dependence on propagation direction. These phenomena were not observed in the MR RA and in control and RAP atria. Thus both models form distinctly different AF substrates; in RAP dogs, the decrease in APD(80) may stabilize reentry. In MR dogs, regional LA conduction slowing and increased directional dependency, allowing unidirectional conduction block and preferential paths of conduction, may account for increased AF inducibility.     

Reexcitation mechanisms in epicardial tissue: Role of Ito density heterogeneities and INa inactivation kinetics

Dispersion of action potential repolarization is known to be an important arrhythmogenic factor in cardiopathies such as Brugada syndrome. In this work, we analyze the effect of a variation in sodium current (I_Na) inactivation and a heterogeneous rise of transient outward current (I_to) in the probability of reentry in epicardial tissue. We use the Luo-Rudy model of epicardial ventricular action potential to study wave propagation in a one-dimensional fiber. Spatial dispersion in repolarization is introduced by splitting the fiber into zones with different strength of I_to. We then analyze the pro-arrhythmic effect of a variation in the relaxation time and steady-state of the sodium channel fast inactivating gate h. We quantify the probability of reentry measuring the percentage of reexcitations that occurs in 200 beats. We find that, for high stimulation rates, this percentage is negligible, but increases notably for pacing periods above 700 ms. Surprisingly, with decreasing I_Na inactivation time, the percentage of reexcitations does not grow monotonically, but presents vulnerable windows, separated by values of the I_Na inactivation speed-up where reexcitation does not occur. By increasing the strength of L-type calcium current I_CaL above a certain threshold, reexcitation disappears. Finally, we show the formation of reentry in stimulated two-dimensional epicardial tissue with modified I_Na kinetics and I_to heterogeneity. Thus, we confirm that while I_to dispersion is necessary for phase-2 reentry, altered sodium inactivation kinetics influences the probability of reexcitation in a highly nonlinear fashion.     

Modifying L-type calcium current kinetics: consequences for cardiac excitation and arrhythmia dynamics

The L-type Ca current (I_Ca,L), essential for normal cardiac function, also regulates dynamic action potential (AP) properties that promote ventricular fibrillation. Blocking I_Ca,L can prevent ventricular fibrillation, but only at levels suppressing contractility. We speculated that, instead of blocking I_Ca,L, modifying its shape by altering kinetic features could produce equivalent anti-fibrillatory effects without depressing contractility. To test this concept experimentally, we overexpressed a mutant Ca-insensitive calmodulin (CaM₁₂₃₄) in rabbit ventricular myocytes to inhibit Ca-dependent I_Ca,L inactivation, combined with the ATP-sensitive K current agonist pinacidil or I_Ca,L blocker verapamil to maintain AP duration (APD) near control levels. Cell shortening was enhanced in pinacidil-treated myocytes, but depressed in verapamil-treated myocytes. Both combinations flattened APD restitution slope and prevented APD alternans, similar to I_Ca,L blockade. To predict the arrhythmogenic consequences, we simulated the cellular effects using a new AP model, which reproduced flattening of APD restitution slope and prevention of APD/Ca_i transient alternans but maintained a normal Ca_i transient. In simulated two-dimensional cardiac tissue, these changes prevented the arrhythmogenic spatially discordant APD/Ca_i transient alternans and spiral wave breakup. These findings provide a proof-of-concept test that I_Ca,L can be targeted to increase dynamic wave stability without depressing contractility, which may have promise as an antifibrillatory strategy.     

In silico investigation of pro-arrhythmic effects of azithromycin on the human ventricle

The macrolide antibiotic azithromycin (AZM) is widely used for respiratory infections and has been suggested to be a possible treatment for the Coronavirus Disease of 2019 (COVID-19). However, AZM-associated QT interval prolongation and arrhythmias have been reported. Integrated mechanistic information on AZM actions on human ventricular excitation and conduction is lacking. Therefore, this study was undertaken to investigate the actions of AZM on ventricular cell and tissue electrical activity. The O'Hara- Virag-Varro-Rudy dynamic (ORd) model of human ventricular cells was modified to incorporate experimental data on the concentration-dependent actions of AZM on multiple ion channels, including I_Na, I_CaL, I_Kr, I_Ks, I_K1 and I_NaL in both acute and chronic exposure conditions. In the single cell model, AZM prolonged the action potential duration (APD) in a concentration-dependent manner, which was predominantly attributable to I_Kr reduction in the acute condition and potentiated I_NaL in the chronic condition. High concentrations of AZM also increased action potential (AP) triangulation (determined as an increased difference between APD₃₀ and APD₉₀) which is a marker of arrhythmia risk. In the chronic condition, the potentiated I_NaL caused a modest intracellular Na ⁺ concentration accumulation at fast pacing rates. At the 1D tissue level, the AZM-prolonged APD at the cellular level was reflected by an increased QT interval in the simulated pseudo-ECG, consistent with clinical observations. Additionally, AZM reduced the conduction velocity (CV) of APs in the acute condition due to a reduced I_Na, and it augmented the transmural APD dispersion of the ventricular tissue, which is also pro-arrhythmic. Such actions were markedly augmented when the effects of chronic exposure of AZM were also considered, or with additional I_Kr block, as may occur with concurrent use of other medications. This study provides insights into the ionic mechanisms by which high concentrations of AZM may modulate ventricular electrophysiology and susceptibility to arrhythmia.     

Conduction and refractory disorders in the diabetic atrium

Diabetes mellitus (DM) is an independent risk of atrial fibrillation. However, its arrhythmogenic substrates remain unclear. This study sought to examine the precise propagation and the spatiotemporal dispersion of the action potential (AP) in the diabetic atrium. DM was induced by streptozotocin (65 mg/kg) in 8-wk-old male Wister rats. Optical mapping and histological analysis were performed in the right atrium (RA) from control (n = 26) and DM (n = 27) rats after 16 wk. Rate-dependent alterations of conduction velocity (CV) and its heterogeneity and the spatial distribution of AP were measured in RA using optical mapping. The duration of atrial tachyarrhythmia (AT) induced by rapid atrial stimulation was longer in DM (2.4 ± 0.6 vs. 0.9 ± 0.3 s, P < 0.05). CV was decreased, and its heterogeneity was greater in DM than control. Average action potential duration of 80% repolarization (APD(80)) at pacing cycle length (PCL) of 200 ms from four areas within the RA was prolonged (53 ± 2 vs. 40 ± 3 ms, P < 0.01), and the coefficient of variation of APD(80) was greater in DM than control (0.20 ± 0.02 vs. 0.15 ± 0.01%, P < 0.05). The ratio of APD(80) at PCL shorter than 200 ms to that at 200 ms was smaller (P < 0.001), and the incidence of APD alternans was higher in DM than control (100 vs. 0%, P < 0.001). Interstitial fibrosis was greater and connexin 40 expression was lower in DM than control. The remodeling of the diabetic atrium was characterized as follows: greater vulnerability to AT, increased conduction slowing and its heterogeneity, the prolongation of APD, the increase in spatial dispersion and frequency-dependent shortening of APD, and increased incidence of APD alternans.     

Atrial proarrhythmia due to increased inward rectifier current (IK1) arising from KCNJ2 mutation – A simulation study

Atrial fibrillation (AF) has been linked to increased inward rectifier potassium current, I_K1, either due to AF-induced electrical remodelling, or from functional changes due to the Kir2.1 V93I mutation. The aim of this simulation study was to identify at cell and tissue levels' mechanisms by which increased I_K1 facilitates and perpetuates AF. The Courtemanche et al. human atrial cell action potential (AP) model was modified to incorporate reported changes in I_K1 induced by the Kir2.1 V93I mutation in both heterozygous (Het) and homozygous (Hom) mutant forms. The modified models for wild type (WT), Het and Hom conditions were incorporated into homogeneous 1D, 2D and 3D tissue models. Restitution curves of AP duration (APD), effective refractory period (ERP) and conduction velocity (CV) were computed and both the temporal and the spatial vulnerability of atrial tissue to re-entry were measured. The lifespan and tip meandering pattern of re-entry were also characterised. For comparison, parallel simulations were performed by incorporating into the Courtmanche et al. model a linear increase in maximal I_K1 conductance. It was found that the gain-in-function of V93I ‘mutant’ I_K1 led to abbreviated atrial APs and flattened APD, ERP and CV restitution curves. It also hyperpolarised atrial resting membrane potential and slowed down intra-atrial conduction. V93I ‘mutant’ I_K1 reduced the tissue's temporal vulnerability but increased spatial vulnerability to initiate and sustain re-entry, resulting in an increased overall susceptibility of atrial tissue to arrhythmogenesis. In the 2D model, spiral waves self-terminated for WT (lifespan < 3.3 s) tissue, but persisted in Het and Hom tissues for the whole simulation period (lifespan > 10 s). The tip of the spiral wave meandered more in WT tissue than in Het and Hom tissues. Increased I_K1 due to augmented maximal conductance produced similar results to those of Het and Hom Kir2.1 V93I mutant conditions. In the 3D model the dynamic behaviour of scroll waves was stabilized by increased I_K1. In conclusion, increased I_K1 current, either by the Kir2.1 V93I mutation or by augmented maximal conductance, increases atrial susceptibility to arrhythmia by increasing the lifespan of re-entrant spiral waves and the stability of scroll waves in 3D tissue, thereby facilitating initiation and maintenance of re-entrant circuits.     

Systematic characterization of the ionic basis of rabbit cellular electrophysiology using two ventricular models

Several mathematical models of rabbit ventricular action potential (AP) have been proposed to investigate mechanisms of arrhythmias and excitation-contraction coupling. Our study aims at systematically characterizing how ionic current properties modulate the main cellular biomarkers of arrhythmic risk using two widely-used rabbit ventricular models, and comparing simulation results using the two models with experimental data available for rabbit. A sensitivity analysis of AP properties, Ca²⁺ and Na⁺ dynamics, and their rate dependence to variations (±15% and ±30%) in the main transmembrane current conductances and kinetics was performed using the Shannon et al. (2004) and the [Mahajan et?al., 2008a] and [Mahajan et?al., 2008b] AP rabbit models. The effects of severe transmembrane current blocks (up to 100%) on steady-state AP and calcium transients, and AP duration (APD) restitution curves were also simulated using both models. Our simulations show that, in both virtual rabbit cardiomyocytes, APD is significantly modified by most repolarization currents, AP triangulation is regulated mostly by the inward rectifier K⁺ current (I_K1) whereas APD rate adaptation as well as [Na⁺]_i rate dependence is influenced by the Na⁺/K⁺ pump current (I_NaK). In addition, steady-state [Ca²⁺]_i levels, APD restitution properties and [Ca²⁺]_i rate dependence are strongly dependent on I_NaK, the L-Type Ca²⁺ current (I_CaL) and the Na⁺/Ca²⁺ exchanger current (I_NaCa), although the relative role of these currents is markedly model dependent. Furthermore, our results show that simulations using both models agree with many experimentally-reported electrophysiological characteristics. However, our study shows that the Shannon et al. model mimics rabbit electrophysiology more accurately at normal pacing rates, whereas Mahajan et al. model behaves more appropriately at faster rates. Our results reinforce the usefulness of sensitivity analysis for further understanding of cellular electrophysiology and validation of cardiac AP models.     

Differential Expression of hERG1 Channel Isoforms Reproduces Properties of Native I Kr and Modulates Cardiac Action Potential Characteristics

Background

The repolarizing cardiac rapid delayed rectifier current, I _Kr, is composed of ERG1 channels. It has been suggested that two isoforms of the ERG1 protein, ERG1a and ERG1b, both contribute to I _Kr. Marked heterogeneity in the kinetic properties of native I _Kr has been described. We hypothesized that the heterogeneity of native I _Kr can be reproduced by differential expression of ERG1a and ERG1b isoforms. Furthermore, the functional consequences of differential expression of ERG1 isoforms were explored as a potential mechanism underlying native heterogeneity of action potential duration (APD) and restitution.

Methodology/Principal Findings

The results show that the heterogeneity of native I _Kr can be reproduced in heterologous expression systems by differential expression of ERG1a and ERG1b isoforms. Characterization of the macroscopic kinetics of ERG1 currents demonstrated that these were dependent on the relative abundance of ERG1a and ERG1b. Furthermore, we used a computational model of the ventricular cardiomyocyte to show that both APD and the slope of the restitution curve may be modulated by varying the relative abundance of ERG1a and ERG1b. As the relative abundance of ERG1b was increased, APD was gradually shortened and the slope of the restitution curve was decreased.

Conclusions/Significance

Our results show that differential expression of ERG1 isoforms may explain regional heterogeneity of I _Kr kinetics. The data demonstrate that subunit dependent changes in channel kinetics are important for the functional properties of ERG1 currents and hence I _Kr. Importantly, our results suggest that regional differences in the relative abundance of ERG1 isoforms may represent a potential mechanism underlying the heterogeneity of both APD and APD restitution observed in mammalian hearts.     

Attraction of Rotors to the Pulmonary Veins in Paroxysmal Atrial Fibrillation: A Modeling Study

Maintenance of paroxysmal atrial fibrillation (AF) by fast rotors in the left atrium (LA) or at the pulmonary veins (PVs) is not fully understood. To gain insight into this dynamic and complex process, we studied the role of the heterogeneous distribution of transmembrane currents in the PVs and LA junction (PV-LAJ) in the localization of rotors in the PVs. We also investigated whether simple pacing protocols could be used to predict rotor drift in the PV-LAJ. Experimentally observed heterogeneities in I_K1, I_Ks, I_Kr, I_to, and I_CaL in the PV-LAJ were incorporated into two- and pseudo three-dimensional models of Courtemanche-Ramirez-Nattel-Kneller human atrial kinetics to simulate various conditions and investigate rotor drifting mechanisms. Spatial gradients in the currents resulted in shorter action potential duration, minimum diastolic potential that was less negative, and slower upstroke and conduction velocity for rotors in the PV region than in the LA. Rotors under such conditions drifted toward the PV and stabilized at the shortest action potential duration and less-excitable region, consistent with drift direction under intercellular coupling heterogeneities and regardless of the geometrical constraint in the PVs. Simulations with various I_K1 gradient conditions and current-voltage relationships substantiated its major role in the rotor drift. In our 1:1 pacing protocol, we found that among various action potential properties, only the minimum diastolic potential gradient was a rate-independent predictor of rotor drift direction. Consistent with experimental and clinical AF studies, simulations in an electrophysiologically heterogeneous model of the PV-LAJ showed rotor attraction toward the PV. Our simulations suggest that I_K1 heterogeneity is dominant compared to other currents in determining the drift direction through its impact on the excitability gradient. These results provide a believed novel framework for understanding the complex dynamics of rotors in AF.     

Advanced glycation end products induce senescence of atrial myocytes and increase susceptibility of atrial fibrillation in diabetic mice

Diabetes mellitus (DM) is a common chronic metabolic disease caused by significant accumulation of advanced glycation end products (AGEs). Atrial fibrillation (AF) is a common cardiovascular complication of DM. Here, we aim to clarify the role and mechanism of atrial myocyte senescence in the susceptibility of AF in diabetes. Rapid transesophageal atrial pacing was used to monitor the susceptibility of mice to AF. Whole‐cell patch‐clamp was employed to record the action potential (AP) and ion channels in single HL‐1 cell and mouse atrial myocytes. More importantly, anti‐RAGE antibody and RAGE‐siRNA AAV9 were used to investigate the relationship among diabetes, aging, and AF. The results showed that elevated levels of p16 and retinoblastoma (Rb) protein in the atrium were associated with increased susceptibility to AF in diabetic mice. Mechanistically, AGEs increased p16/Rb protein expression and the number of SA‐β‐gal‐positive cells, prolonged the action potential duration (APD), reduced protein levels of Cav1.2, Kv1.5, and current density of I _Ca,L , I _Kur in HL‐1 cells. Anti‐RAGE antibody or RAGE‐siRNA AAV9 reversed these effects in vitro and in vivo, respectively. Furthermore, downregulating p16 or Rb by siRNA prevented AGEs‐mediated reduction of Cav1.2 and Kv1.5 proteins expression. In conclusion, AGEs accelerated atrial electrical remodeling and cellular senescence, contributing to increased AF susceptibility by activating the p16/Rb pathway. Inhibition of RAGE or the p16/Rb pathway may be a potential therapeutic target for AF in diabetes.     

Inter-Subject Variability in Human Atrial Action Potential in Sinus Rhythm versus Chronic Atrial Fibrillation

Aims

Human atrial electrophysiology exhibits high inter-subject variability in both sinus rhythm (SR) and chronic atrial fibrillation (cAF) patients. Variability is however rarely investigated in experimental and theoretical electrophysiological studies, thus hampering the understanding of its underlying causes but also its implications in explaining differences in the response to disease and treatment. In our study, we aim at investigating the ability of populations of human atrial cell models to capture the inter-subject variability in action potential (AP) recorded in 363 patients both under SR and cAF conditions.

Methods and Results

Human AP recordings in atrial trabeculae (n = 469) from SR and cAF patients were used to calibrate populations of computational SR and cAF atrial AP models. Three populations of over 2000 sampled models were generated, based on three different human atrial AP models. Experimental calibration selected populations of AP models yielding AP with morphology and duration in range with experimental recordings. Populations using the three original models can mimic variability in experimental AP in both SR and cAF, with median conductance values in SR for most ionic currents deviating less than 30% from their original peak values. All cAF populations show similar variations in G_K1, G_Kur and G_to, consistent with AF-related remodeling as reported in experiments. In all SR and cAF model populations, inter-subject variability in I_K1 and I_NaK underlies variability in APD₉₀, variability in I_Kur, I_CaL and I_NaK modulates variability in APD₅₀ and combined variability in I_to and I_Kur determines variability in APD₂₀. The large variability in human atrial AP triangulation is mostly determined by I_K1 and either I_NaK or I_NaCa depending on the model.

Conclusion

Experimentally-calibrated human atrial AP models populations mimic AP variability in SR and cAF patient recordings, and identify potential ionic determinants of inter-subject variability in human atrial AP duration and morphology in SR versus cAF.     

The characteristics of action potential and nonselective cation current of cardiomyocytes in rabbit superior vena cava

As a special focus in initiating and maintaining atrial fibrillation (AF), cardiomyocytes in superior vena cava (SVC) have distinctive electrophysiological characters. In this study, we found that comparing with the right atrial (RA) cardiomyoctyes, the SVC cardiomyoctyes had longer APD₉₀ at the different basic cycle lengths; the conduction block could be observed on both RA and SVC cardiomyoctyes. A few of SVC cardiomyoctyes showed slow response action potentials with automatic activity and some others showed early afterdepolarization (EAD) spontaneously. Further more, we found that there are nonselective cation current (I _Ns) in both SVC and RA cardiomyocytes. The peak density of I _Ns in SVC cardiomyocytes was smaller than that in RA cardiomyocytes. Removal of extracellular divalent cation and glucose could increase I _Ns in SVC cardiomyocytes. The agonist or the antagonist of I _Ns may increase or decrease APD. To sum up, some SVC cardiomyocytes possess the ability of spontaneous activity; the difference of transmembrane action potentials between SVC and RA cardiomyocytes is partly because of the different density of I _Ns between them; the agonist or the antagonist of I _Ns can increase or decrease APD leading to the enhancement or reduction of EAD genesis in SVC cardiomyocytes. I _Ns in rabbit myocytes is fairly similar to TRPC3 current in electrophysiological property, which might play an important role in the mechanisms of AF.     

Protective effect of piperine on electrophysiology abnormalities of left atrial myocytes induced by hydrogen peroxide in rabbits

Aims

Piperine had protective effects on oxidative stress damage of ventricular myocytes by hydrogen peroxide (H₂O₂). In this study we aimed to explore the protective effect of piperine on abnormalities of the cardiac action potential (AP) and several ion currents induced by hydrogen peroxide (H₂O₂) in single rabbit left atrial myocyte.

Main methods

Conventional microelectrodes were used to record action potential duration (APD), resting membrane potential (RMP) and some ion currents (I_Ca,L,I_to,I_K1 and I_kur,ect.), before and after H₂O₂ administration with or without piperine.

Key findings

The piperine (7 μmol/L) had no significant effect on APD, I_Ca,L,I_to,I_K1 and I_kur and their channel dynamics. In the presence of 50 μmol/L H₂O₂, APD₅₀ and APD₉₀ shortened (P < 0.01), amplitude of RMP decreased (P < 0.05), the peak of I_Ca,L reduced significantly (P < 0.05). Piperine (7 μmol/L) significantly alleviated the inhibiting effect of H₂O₂ on APD and I_Ca,L (P < 0.01) and protected the changes of I_Ca,L dynamics induced by H₂O₂. The peak current of I_to was reduced significantly (P < 0.05); Piperine (7 μmol/L) significantly alleviated the inhibiting effect of H₂O₂ on I_to (P < 0.01). In addition, piperine protected the changes of I_to dynamics induced by H₂O_2. The peak current of I_K1 and I_KUr was significantly reduced (P < 0.05); Piperine (7 μmol/L) alleviated the inhibiting effect of H₂O₂ on I_K1 and I_KUr significantly (P < 0.01). In addition, piperine protected the changes of I_KUr dynamics induced by H₂O₂.

Significance

These results suggest that piperine effectively protects atrial myocytes from oxidative stress injury in atrial electrophysiology.     

Electrical refractory period restitution and spiral wave reentry in simulated cardiac tissue

Theoretical and experimental studies have shown that restitution of the cardiac action potential (AP) duration (APD) plays a major role in predisposing ventricular tachycardia to degenerate to ventricular fibrillation, whereas its role in atrial fibrillation is unclear. We used the Courtemanche human atrial cell model and the Luo-Rudy guinea pig ventricular model to compare the roles of electrical restitution in destabilizing spiral wave reentry in simulated two-dimensional homogeneous atrial and ventricular tissue. Because atrial AP morphology is complex, we also validated the usefulness of effective refractory period (ERP) restitution. ERP restitution correlated best with APD restitution at transmembrane potentials greater than or equal to -62 mV, and its steepness was a reliable predictor of spiral wave phenotype (stable, meandering, hypermeandering, and breakup) in both atrial and ventricular tissue. Spiral breakup or single hypermeandering spirals occurred when the slope of ERP restitution exceeded 1 at short diastolic intervals. Thus ERP restitution, which is easier to measure clinically than APD restitution, is a reliable determinant of spiral wave stability in simulated atrial and ventricular tissue.