Decrease In Action Potential Duration Research Articles

Mechanism of Ventricular Tachycardia Occurring in Chronic Myocardial Infarction Scar.

Cardiac arrest is the leading cause of death in the more economically developed countries. Ventricular tachycardia associated with myocardial infarct is a prominent cause of cardiac arrest. Ventricular arrhythmias occur in 3 phases of infarction: during the ischemic event, during the healing phase, and after the scar matures. Mechanisms of arrhythmias in these phases are distinct. This review focuses on arrhythmia mechanisms for ventricular tachycardia in mature myocardial scar. Available data have shown that postinfarct ventricular tachycardia is a reentrant arrhythmia occurring in circuits found in the surviving myocardial strands that traverse the scar. Electrical conduction follows a zigzag course through that area. Conduction velocity is impaired by decreased gap junction density and impaired myocyte excitability. Enhanced sympathetic tone decreases action potential duration and increases sarcoplasmic reticular calcium leak and triggered activity. These elements of the ventricular tachycardia mechanism are found diffusely throughout scar. A distinct myocyte repolarization pattern is unique to the ventricular tachycardia circuit, setting up conditions for classical reentry. Our understanding of ventricular tachycardia mechanisms continues to evolve as new data become available. The ultimate use of this information would be the development of novel diagnostics and therapeutics to reliably identify at-risk patients and prevent their ventricular arrhythmias.

Pollutants, including Organophosphorus and Organochloride Pesticides, May Increase the Risk of Cardiac Remodeling and Atrial Fibrillation: A Narrative Review.

Atrial fibrillation (AF) is the most common type of cardiac rhythm disorder. Recent clinical and experimental studies reveal that environmental pollutants, including organophosphorus-organochloride pesticides and air pollution, may contribute to the development of cardiac arrhythmias including AF. Here, we discussed the unifying cascade of events that may explain the role of pollutant exposure in the development of AF. Following ingestion and inhalation of pollution-promoting toxic compounds, damage-associated molecular pattern (DAMP) stimuli activate the inflammatory response and oxidative stress that may negatively affect the respiratory, cognitive, digestive, and cardiac systems. Although the detailed mechanisms underlying the association between pollutant exposure and the incidence of AF are not completely elucidated, some clinical reports and fundamental research data support the idea that pollutant poisoning can provoke perturbed ion channel function, myocardial electrical abnormalities, decreased action potential duration, slowed conduction, contractile dysfunction, cardiac fibrosis, and arrhythmias including AF.

Beneficial effects of chronic mexiletine treatment in a human model of SCN5A overlap syndrome.

SCN5A mutations are associated with various cardiac phenotypes, including long QT syndrome type 3 (LQT3), Brugada syndrome (BrS), and cardiac conduction disease (CCD). Certain mutations, such as SCN5A-1795insD, lead to an overlap syndrome, with patients exhibiting both features of BrS/CCD [decreased sodium current (INa)] and LQT3 (increased late INa). The sodium channel blocker mexiletine may acutely decrease LQT3-associated late INa and chronically increase peak INa associated with SCN5A loss-of-function mutations. However, most studies have so far employed heterologous expression systems and high mexiletine concentrations. We here investigated the effects of a therapeutic dose of mexiletine on the mixed phenotype associated with the SCN5A-1795insD mutation in HEK293A cells and human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). To assess only the chronic effects on trafficking, HEK293A cells transfected with wild-type (WT) SCN5A or SCN5A-1795insD were incubated for 48 h with 10 µm mexiletine followed by wash-out, which resulted in an increased peak INa for both SCN5A-WT and SCN5A-1795insD and an increased late INa for SCN5A-1795insD. Acute re-exposure of HEK293A cells to 10 µm mexiletine did not impact on peak INa but significantly decreased SCN5A-1795insD late INa. Chronic incubation of SCN5A-1795insD hiPSC-CMs with mexiletine followed by wash-out increased peak INa, action potential (AP) upstroke velocity, and AP duration. Acute re-exposure did not impact on peak INa or AP upstroke velocity, but significantly decreased AP duration. These findings demonstrate for the first time the therapeutic benefit of mexiletine in a human cardiomyocyte model of SCN5A overlap syndrome.

Sex-specific contribution of small-conductance Ca2+-activated K+ current to atrial electrophysiology and arrhythmogenesis.

Small-conductance Ca2+-activated K+ (SK) channels are predominantly expressed in the atria, thus offering a unique therapeutic target for atrial fibrillation (AF) independent of ventricular function. Several studies have reported remodeling of SK channels in AF, whereby SK current is increased during AF-induced remodeling in several animal models. Nevertheless, data on SK current (ISK) in atrial myocytes from patients remain scarce, and the AF-associated changes are controversial. Both upregulation and downregulation have been reported in chronic AF vs. normal sinus rhythm, accompanied by increased or decreased action potential duration (APD) lengthening by SK-channel inhibition. A potential confounding factor is the observation that sex-specific differences exist in ISK function in the ventricles, which could also be present in the atria, though this remains unknown.

Acetylcholine Reduces L-Type Calcium Current without Major Changes in Repolarization of Canine and Human Purkinje and Ventricular Tissue.

Vagal nerve stimulation (VNS) holds a strong basis as a potentially effective treatment modality for chronic heart failure, which explains why a multicenter VNS study in heart failure with reduced ejection fraction is ongoing. However, more detailed information is required on the effect of acetylcholine (ACh) on repolarization in Purkinje and ventricular cardiac preparations to identify the advantages, risks, and underlying cellular mechanisms of VNS. Here, we studied the effect of ACh on the action potential (AP) of canine Purkinje fibers (PFs) and several human ventricular preparations. In addition, we characterized the effects of ACh on the L-type Ca2+ current (ICaL) and AP of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) and performed computer simulations to explain the observed effects. Using microelectrode recordings, we found a small but significant AP prolongation in canine PFs. In the human myocardium, ACh slightly prolonged the AP in the midmyocardium but resulted in minor AP shortening in subepicardial tissue. Perforated patch-clamp experiments on hiPSC-CMs demonstrated that 5 µM ACh caused an ≈15% decrease in ICaL density without changes in gating properties. Using dynamic clamp, we found that under blocked K+ currents, 5 µM ACh resulted in an ≈23% decrease in AP duration at 90% of repolarization in hiPSC-CMs. Computer simulations using the O'Hara-Rudy human ventricular cell model revealed that the overall effect of ACh on AP duration is a tight interplay between the ACh-induced reduction in ICaL and ACh-induced changes in K+ currents. In conclusion, ACh results in minor changes in AP repolarization and duration of canine PFs and human ventricular myocardium due to the concomitant inhibition of inward ICaL and outward K+ currents, which limits changes in net repolarizing current and thus prevents major changes in AP repolarization.

Effect of sotagliflozin on ventricular arrhythmias in mice with myocardial infraction

BackgroundVentricular arrhythmias (VAs) after myocardial infarction (MI) are an important cause of death in patients. Inhibition of sodium-glucose ligand transporter type 2 (SGLT-2) has been shown to reduce cardiovascular mortality in myocardial infarction independent of glycemic sham and prevent ventricular arrhythmias, and it is unclear whether these effects can be further enhanced by additional SGLT-1 inhibition. We investigated the effects of chronic treatment with the dual SGLT-1 and 2 inhibitor sotagliflozin (Sto) on VAs and their underlying mechanisms after MI. MethodsMI was induced by ligation of the left anterior coronary artery in 6–8 weeks-old male mice, which were fed Sto at a dose of 30 mg/kg/day for 4 weeks after surgery. The effects were assessed by echocardiography, histopathology, Langendorff Heart Perfusion System, ambulatory electrocardiography, and Western blot. In vitro experiments were performed to assess the effect of Sto on cardiomyocytes subjected to hypoxic. ResultsCompared to the MI group, Sto treatment significantly improved cardiac function and reduced infarct size and fibrosis levels. Sto also decreased action potential duration (APD) and APD alternation thresholds. Ambulatory ECG and BURST stimulation showed that Sto reduced the incidence of VAs. In addition, Sto significantly improved the expression levels of ion channels and CX43 and ameliorated the abnormal expression of Ca2+ handling proteins after MI. Mechanistically, in vivo and in vitro experiments confirmed that TLR4/CaMKII activation was enhanced after MI, while Sto significantly inhibited the activation of the TLR4/CaMKII signaling pathway. ConclusionSto improved LV remodeling and reduced the incidence of VAs after MI, which was regulated by modulating the TLR4/CaMKII signaling pathway.

Abstract P1090: Polycystin-1 Regulates Calcium Handling And Contractility In Human IPSC-derived Ventricular Cardiomyocytes

Mutations in polycystin-1 (PC1) cause 85% of autosomal dominant polycystic kidney disease cases. This disease is characterized by progressive renal disease and multiple cardiovascular alterations. Evidence suggests that cardiovascular disease precedes the onset of renal failure. We recently demonstrated that PC1 ablation shortens cardiomyocyte action potential, decreases Ca 2+ transient amplitude, and reduces contractility in adult cardiomyocytes isolated from cardiomyocyte-specific PC1 knock-out mice compared to control mice. Alterations in Ca 2+ cycling in PC1-deficient cardiomyocytes were demonstrated to be due to action potential duration shortening and impaired sarcoendoplasmic reticulum ATPase activity. However, mouse and human cardiomyocytes have considerable differences in their action potential shapes and the ion channels involved. It remains unknown whether PC1 regulates these events in human cardiomyocytes. Here, we sought to investigate whether PC1 regulates excitation-contraction coupling in human ventricular cardiomyocytes. We differentiated commercially available human-induced pluripotent stem cell lines (hiPSC) into ventricular cardiomyocytes (hiPSC-CM) using published protocols (measurements were performed 60-90 days after differentiation). We developed a protocol to simultaneously measure action potentials (FluoVolt), intracellular Ca 2+ (Calbryte-590) and contractility using the IonOptix algorithm to detect changes in contrast as contractility measurements. PC1 ablation using siRNA decreased action potential duration and impaired Ca 2+ transients and contractility in hiPSC-CMs. Because hiPSC-CMs have several limitations due to their immature state, we employed a well-validated method to improve their maturation that utilizes a Matrigel mattress and hormone supplementation (dexamethasone; Dexa / thyroid hormone; T3; PMID: 28974554; developed by Dr. Bjorn Knollmann’s lab). Using the Matrigel mattress/Dexa/T3 technique, we increased hiPSC-CM maturation measured as follows: cardiomyocytes were oriented and contracted in the longitudinal axis, ryanodine receptor type 2 and alpha-actinin localized with a striated pattern, exhibited robust inward K + rectifier currents (I K1 ) and fired action potentials that resemble human adult cardiomyocytes. In this model, PC1 knockdown also decreased action potential duration and impaired Ca 2+ transients and decreased contractility measured as fractional shortening. Altogether, this evidence shows for the first time that PC1 regulates human cardiomyocyte physiology and suggests that cardiac alterations in patients with autosomal dominant polycystic kidney disease may emerge as a result of impaired cardiomyocyte contractile function.

Abstract P1088: Polycystin-1 Protects Against Experimental Atrial Fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia with a 1 in 4 lifetime risk. AF impairs cardiac function and increases the risk of stroke and heart attack. Although this is a common and harmful disease, little is known about the molecular mechanisms driving this disease. We have previously demonstrated that the protein polycystin-1 (PC1) plays a role in atrial cardiomyocyte excitation-contraction coupling in ventricular cardiomyocytes. Interestingly, patients with mutations in PC1 encoding gene have increased rates of atrial arrhythmias, including atrial fibrillation. Here, we sought to study the role of PC1 in atrial cardiomyocyte function and atrial fibrillation inducibility. We performed intracardiac electrophysiology with burst pacing in two mouse models with impaired PC1 function: 1) a mouse model harboring R3277C mutation (RC/RC) known to cause autosomal polycystic kidney disease, and 2) cardiomyocyte-specific PC1 KO (CKO). We observed increased rates of atrial fibrillation inducibility in both RC/RC and PC1 CKO mice compared to their respective control littermates (76.9% in RC/RC and 81.8% in CKO vs 20.0% and 11.1% in their respective WT controls). To elucidate molecular mechanisms driving increased susceptibility to atrial fibrillation, we determined whether PC1 affects excitation-contraction coupling in atrial cardiomyocytes. Human induced pluripotent stem cells-derived atrial cardiomyocytes (hiPSC-aCM) were treated with control or PC1 siRNA. Action potentials and intracellular calcium were measured using Fluovolt and Fluo-4, respectively. PC1 ablation decreased action potential duration and calcium transient peak amplitude elicited by electrical stimulation in hiPSC-CM. Decreased calcium transient peaks were also observed in HL-1 atrial cell line treated with siRNA and adult atrial cardiomyocytes isolated from RC/RC mice compared to their control counterparts. Unexpectedly, RNA-sequencing revealed altered DNA Damage Response gene expression. DNA damage has been implicated in the pathogenesis of atrial fibrillation. Therefore, we studied whether tachypacing elicited DNA damage in hiPSC-aCM. We observed increased levels of H2AX phosphorylation (gamma H2AX) in hiPSC-aCM with reduced PC1 expression. Our data show that PC1 mutations increase atrial fibrillation inducibility in multiple mouse models, likely due to impaired DNA damage response and altered action potential and calcium handling.

Assessment of cardiac toxicity of manganese chloride for cardiovascular magnetic resonance

MRI is widely used in cardiology to visualize the structure and function of the heart. Currently gadolinium is the contrast agents use to improve sensitivity and specificity of diagnostic images, enabling the assessment of scar and fibrosis. Manganese-based contrast molecules are being developed with the hope to improve the diagnosis of cardiac pathologies. Manganese, a calcium analogue, allows detection of viable cells within altered tissue. However, the cardiac safety of manganese is not fully assessed. To characterize the effect of MnCl2 on cardiac function and electrical parameters. Hemodynamic function was determined ex vivo using the isolated working rat heart preparation. HL-1 cell line was used to investigate cell viability (loading with calcein AM) and calcium cycling (loading with CAL-520 AM). Rat ventricular cardiomyocytes were dissociated by enzymatic digestion. Action potentials and calcium currents were recorded using the patch clamp technique. The effect of ascending concentrations of MnCl2 (1, 10, 25, 50 and 100 μM) were evaluated. MRI experiments (1.5 T and 9.4 T) were performed on formalin-fixed rat hearts, previously perfused with 25 or 100 μM of MnCl2. Left ventricular hemodynamic parameters from isolated working hearts show that heart rate is not modified by μM [MnCl2]. Similar observation was made for contractility index and aortic flow up to 50 μM; 100 μM MnCl2 induced a significant decrease of the two parameters. At the cellular level, μM [MnCl2] did not impact HL-1 cell viability. The amplitude of the calcium transients was significantly reduced only at 100 μM of MnCl2 ( P < 0.05). In isolated ventricular myocytes, action potential duration at 20%, 50% and 90% of repolarization were not modified up to 10 μM of MnCl2. A small but significant decrease of action potential duration was observed at 25, 50 and 100 μM (APD90 decrease by 23 ± 6% vs control). Peak L-type calcium current amplitude was significantly decreased by 50 and 100 μM of MnCl2. MRI on heart perfused with 25 μM of MnCl2 (no impact hemodynamic function) and 100 μM of MnCl2 showed a dose dependent decrease in the T1 relaxation time, a parameter for altered and disease state. Our results show that μM [MnCl2] (up to 25 μM) can be used as a contrast agent in MRI, without effects on cardiac hemodynamic parameters. MRI with manganese should therefore enable visualization of viable and stunned myocardium.

Abstract P498: Polycystin-1 Regulates Excitation-contraction Coupling In Atrial Cardiomyocytes

Patients with Autosomal Dominant Polycystic Kidney disease (ADPKD) have multiple cardiovascular manifestations, including increased susceptibility to arrhythmias. Mutations in polycystin-1 (PC1) encoding gene accounts for 85% cases of ADPKD, whereas mutations in polycystin-2 (PC2) only accounts for 15%. In kidney cells, PC1 interacts with PC2 to form a protein complex at the primary cilia to regulate calcium influx via PC2. However, cardiomyocytes are non-ciliated cells and the role of both PC1 and PC2 in atrial cardiomyocytes remains unknown. We have previously demonstrated that PC1 regulates action potentials and calcium handling to fine-tune ventricular cardiomyocyte contraction. Here, we hypothesize that PC1 regulates action potentials and calcium handling in atrial cardiomyocytes independent of PC2 actions. To test this hypothesis, we differentiated human induced pluripotent stem cells (iPSC) into atrial cardiomyocytes (iPSC-aCM) using previously published protocols. To determine the contribution of PC1/PC2 in atrial excitation-contraction coupling, protein expression was knocked down utilizing specific siRNA constructs, for each protein, or a universal control siRNA transfected using lipofectamine RNAiMAX. We measured action potentials using the potentiometric dye FluoVolt and intracellular calcium with Fura-2 AM or Fluo-4. Changes in fluorescence were monitored using a multiwavelength IonOptix system. iPSC-aCM were paced at 2 Hz to synchronize the beating pattern using field electrical stimulation. Our data shows that PC1 ablation significantly decreased action potential duration at 50% and 80% of repolarization, by 24% and 23%, respectively. Moreover, we observed that PC1 knockdown significantly reduced calcium transient amplitude elicited by field electrical stimulation without changes in calcium transient decay. Interestingly, PC2 knockdown did not modify calcium transients in atrial cardiomyocytes (iPSC-aCM). Our data suggest that PC1 regulates atrial excitation-contraction coupling independent of PC2 actions. This study warrants further investigation into atrial dysfunction in ADPKD patients with PC1 mutations.

Phenanthrene impacts zebrafish cardiomyocyte excitability by inhibiting IKr and shortening action potential duration.

Air pollution is an environmental hazard that is associated with cardiovascular dysfunction. Phenanthrene is a three-ringed polyaromatic hydrocarbon that is a significant component of air pollution and crude oil and has been shown to cause cardiac dysfunction in marine fishes. We investigated the cardiotoxic effects of phenanthrene in zebrafish (Danio rerio), an animal model relevant to human cardiac electrophysiology, using whole-cell patch-clamp of ventricular cardiomyocytes. First, we show that phenanthrene significantly shortened action potential duration without altering resting membrane potential or upstroke velocity (dV/dt). L-type Ca2+ current was significantly decreased by phenanthrene, consistent with the decrease in action potential duration. Phenanthrene blocked the hERG orthologue (zfERG) native current, IKr, and accelerated IKr deactivation kinetics in a dose-dependent manner. Furthermore, we show that phenanthrene significantly inhibits the protective IKr current envelope, elicited by a paired ventricular AP-like command waveform protocol. Phenanthrene had no effect on other IK. These findings demonstrate that exposure to phenanthrene shortens action potential duration, which may reduce refractoriness and increase susceptibility to certain arrhythmia triggers, such as premature ventricular contractions. These data also reveal a previously unrecognized mechanism of polyaromatic hydrocarbon cardiotoxicity on zfERG by accelerating deactivation and decreasing IKr protective current.

Cobalt oxide nanoparticles induce oxidative stress and alter electromechanical function in rat ventricular myocytes

BackgroundNanotoxicology is an increasingly relevant field and sound paradigms on how inhaled nanoparticles (NPs) interact with organs at the cellular level, causing harmful conditions, have yet to be established. This is particularly true in the case of the cardiovascular system, where experimental and clinical evidence shows morphological and functional damage associated with NP exposure. Giving the increasing interest on cobalt oxide (Co3O4) NPs applications in industrial and bio-medical fields, a detailed knowledge of the involved toxicological effects is required, in view of assessing health risk for subjects/workers daily exposed to nanomaterials. Specifically, it is of interest to evaluate whether NPs enter cardiac cells and interact with cell function. We addressed this issue by investigating the effect of acute exposure to Co3O4-NPs on excitation-contraction coupling in freshly isolated rat ventricular myocytes.ResultsPatch clamp analysis showed instability of resting membrane potential, decrease in membrane electrical capacitance, and dose-dependent decrease in action potential duration in cardiomyocytes acutely exposed to Co3O4-NPs. Motion detection and intracellular calcium fluorescence highlighted a parallel impairment of cell contractility in comparison with controls. Specifically, NP-treated cardiomyocytes exhibited a dose-dependent decrease in the fraction of shortening and in the maximal rate of shortening and re-lengthening, as well as a less efficient cytosolic calcium clearing and an increased tendency to develop spontaneous twitches. In addition, treatment with Co3O4-NPs strongly increased ROS accumulation and induced nuclear DNA damage in a dose dependent manner. Finally, transmission electron microscopy analysis demonstrated that acute exposure did lead to cellular internalization of NPs.ConclusionsTaken together, our observations indicate that Co3O4-NPs alter cardiomyocyte electromechanical efficiency and intracellular calcium handling, and induce ROS production resulting in oxidative stress that can be related to DNA damage and adverse effects on cardiomyocyte functionality.

Study of electrophysiological, structural, and mitochondrial metabolism remodelling in a sheep model of atrial fibrillation monitored by telemetry

Atrial Fibrillation (AF) stabilization depends on remodelling processes affecting the electrophysiological, metabolic, and structural properties of the atrial muscle. Develop an AF Sheep model to study the electrophysiogical, structural and bioenergetic remodelling of the cardiac tissue. An algorithm controlled pacemaker stimulating repeatedly the right atria at 10 Hz during 30 sec followed by 5 sec of sinus rhythm (SR) detection was used to induce AF. Its development was monitored by telemetry. Electrophysiological remodelling was studied in vivo by contact mapping and ex vivo by optical mapping. Structural remodelling was studied at the organ level using computed tomography (μCT) and at the mitochondrial level by electronic microscopy. Bioenergetic remodelling was evaluated on isolated mitochondria. AF sheep were kept 75 days before being studied. AF induction rate was 80%. AF developed and stabilized after 25 and 48 days, respectively. Animals remained less than 0.02 ± 0.03% in SR once AF was installed. In vivo contact mapping revealed a decreased voltage in AF atria compared to Sham. The highest AF frequencies were localised in the pulmonary veins. When compared to Sham, AF induced: (1) a 50% decrease of the atrial effective refractory periods; (2) a left atrial decrease in action potential duration; (3) an increase in atrial volumes; (4) Fat infiltration; (5) a decrease in mitochondrial cristae density; (6) a decrease in ATP synthesis rate; (7) an increased oxidation of the mitochondrial NAD(P)H pool; (9) an increased mitochondrial H2O2 emission, independently of the respiratory substrates used to fuel the electron transport chain. The AF Sheep model developed is characterized by occurrence of remodelling processes at different levels. Further studies will establish the time course of development of remodelling processes in order to improve AF therapeutics strategies by targeting atrial remodelling.

Computational Study to Identify the Effects of the KCNJ2 E299V Mutation in Cardiac Pumping Capacity.

The KCNJ2 gene mutations induce short QT syndrome (SQT3) by directly increasing the IK1 current. There have been many studies on the electrophysiological effects of mutations such as the KCNJ2 D172N that cause the SQT3. However, the KCNJ2 E299V mutation is distinguished from other representative gene mutations that can induce the short QT syndrome (SQT3) in that it increased IK1 current by impairing the inward rectification of K+ channels. The studies of the electromechanical effects on myocardial cells and mechanisms of E299V mutations are limited. Therefore, we investigated the electrophysiological changes and the concomitant mechanical responses according to the expression levels of the KCNJ2 E299V mutation during sinus rhythm and ventricular fibrillation. We performed excitation-contraction coupling simulations using a human ventricular model with both electrophysiological and mechanical properties. In order to observe the electromechanical changes due to the expression of KCNJ2 E299V mutation, the simulations were performed under normal condition (WT), heterogeneous mutation condition (WT/E299V), and pure mutation condition (E299V). First, a single-cell simulation was performed in three types of ventricular cells (endocardial cell, midmyocardial cell, and epicardial cell) to confirm the electrophysiological changes and arrhythmogenesis caused by the KCNJ2 E299V mutation. In three-dimensional sinus rhythm simulations, we compared electrical changes and the corresponding changes in mechanical performance caused by the expression level of E299V mutation. Then, we observed the electromechanical properties of the E299V mutation during ventricular fibrillation using the three-dimensional reentry simulation. The KCNJ2 E299V mutation accelerated the opening of the IK1 channel and increased IK1 current, resulting in a decrease in action potential duration. Accordingly, the QT interval was reduced by 48% and 60% compared to the WT condition, for the WT/E299V and E299V conditions, respectively. During sustained reentry, the wavelength was reduced due to the KCNJ2 E299V mutation. Furthermore, there was almost no ventricular contraction in both WT/E299V and E299V conditions. We concluded that in both sinus rhythm and fibrillation, the KCNJ2 E299V mutation results in very low contractility regardless of the expression level of mutation and increases the risk of cardiac arrest and cardiac death.

P6591Arrhythmogenic effects of non-steroidal anti-inflammatory drugs (NSAID)

Abstract Background Clinical and experimental data suggest potential deleterious effects of non-steroidal anti-inflammatory drugs (NSAID) in cardiac disease. The aim of the present study was to characterize potential direct arrhythmogenic effects of ibuprofen or indometacin on cardiac electrophysiology. Methods and results In 13 isolated rabbit hearts, indometacin was administered in increasing concentrations (10, 30 and 50μM) after obtaining baseline data. In further 13 hearts, ibuprofen was infused (10, 30 and 50μM). Eight endo- and epicardial monophasic action potentials and a simultaneously recorded 12-lead ECG showed a significant decrease of QT-interval in indometacin- (50μM: −60ms, p&lt;0.05) and ibuprofen-treated hearts (50μM: −81ms, p&lt;0.01). This was accompanied by a similar decrease of action potential duration (APD). Effective refractory period (ERP) was also reduced by indometacin (−24ms, p&lt;0.05) and ibuprofen (−22ms, p&lt;0.05) while spatial dispersion of repolarisation remained stable with both drugs. Programmed ventricular stimulation and burst stimulation were employed for provocation of ventricular arrhythmias. In the indomethacin group 7 episodes of ventricular arrhythmias were inducible at baseline. The incidence was increased to 22 (10μM), 37 (30μM) and 51 (50μM) with higher concentrations of indometacin. In the ibuprofen group 15 episodes were inducible at baseline. After infusion of 10μM ibuprofen, also 15 episodes of ventricular arrhythmias occurred while 34 episodes were inducible with 30μM ibuprofen. Conclusion Infusion of the NSAID ibuprofen and indometacin resulted in a significant abbreviation of myocardial repolarization and a shortening of ventricular refractory periods. This resulted in an enhanced inducibility of ventricular arrhythmias and may contribute to the proposed deleterious effects of NSAID in the presence of cardiac disease.

Wavelength and Fibrosis Affect Phase Singularity Locations During Atrial Fibrillation.

The mechanisms underlying atrial fibrillation (AF), the most common sustained cardiac rhythm disturbance, remain elusive. Atrial fibrosis plays an important role in the development of AF and rotor dynamics. Both electrical wavelength (WL) and the degree of atrial fibrosis change as AF progresses. However, their combined effect on rotor core location remains unknown. The aim of this study was to analyze the effects of WL change on rotor core location in both fibrotic and non-fibrotic atria. Three patient specific fibrosis distributions (total fibrosis content: 16.6, 22.8, and 19.2%) obtained from clinical imaging data of persistent AF patients were incorporated in a bilayer atrial computational model. Fibrotic effects were modeled as myocyte-fibroblast coupling + conductivity remodeling; structural remodeling; ionic current changes + conductivity remodeling; and combinations of these methods. To change WL, action potential duration (APD) was varied from 120 to 240ms, representing the range of clinically observed AF cycle length, by modifying the inward rectifier potassium current (IK1) conductance between 80 and 140% of the original value. Phase singularities (PSs) were computed to identify rotor core locations. Our results show that IK1 conductance variation resulted in a decrease of APD and WL across the atria. For large WL in the absence of fibrosis, PSs anchored to regions with high APD gradient at the center of the left atrium (LA) anterior wall and near the junctions of the inferior pulmonary veins (PVs) with the LA. Decreasing the WL induced more PSs, whose distribution became less clustered. With fibrosis, PS locations depended on the fibrosis distribution and the fibrosis implementation method. The proportion of PSs in fibrotic areas and along the borders varied with both WL and fibrosis modeling method: for patient one, this was 4.2–14.9% as IK1 varied for the structural remodeling representation, but 12.3–88.4% using the combination of structural remodeling with myocyte-fibroblast coupling. The degree and distribution of fibrosis and the choice of implementation technique had a larger effect on PS locations than the WL variation. Thus, distinguishing the fibrotic mechanisms present in a patient is important for interpreting clinical fibrosis maps to create personalized models.

Experimental assessment of a myocyte-based multiscale model of cardiac contractile dysfunction

Cardiac contractile dysfunction (CD) is a multifactorial syndrome caused by different acute or progressive diseases which hamper assessing the role of the underlying mechanisms characterizing a defined pathological condition. Mathematical modeling can help to understand the processes involved in CD and analyze their relative impact in the overall response. The aim of this study was thus to use a myocyte-based multiscale model of the circulatory system to simulate the effects of halothane, a volatile anesthetic which at high doses elicits significant acute CD both in isolated myocytes and intact animals. Ventricular chambers built using a human myocyte model were incorporated into a whole circulatory system represented by resistances and capacitances. Halothane-induced decreased sarco(endo)plasmic reticulum Ca2+ (SERCA2a) reuptake pump, transient outward K+ (Ito), Na+-Ca2+ exchanger (INCX) and L-type Ca2+ channel (ICaL) currents, together with ryanodine receptor (RyR2) increased open probability (Po) and reduced myofilament Ca2+ sensitivity, reproduced equivalent decreased action potential duration at 90% repolarization and intracellular Ca2+ concentration at the myocyte level reported in the literature. In the whole circulatory system, model reduction in mean arterial pressure, cardiac output and regional wall thickening fraction was similar to experimental results in open-chest sheep subjected to acute halothane overdose. Effective model performance indicates that the model structure could be used to study other changes in myocyte targets eliciting CD.

The effect of myocardial action potential duration on cardiac pumping efficacy: a computational study

Background and aimsAlthough studies on the relation between arrhythmias and the action potential duration (APD) have been carried out, most of them are based only on electrophysiological factors of the heart and lack experiments that consider cardiac mechanical and electromechanical characteristics. Therefore, we conducted this study to clarify the relevance of the shortening of APD of a cell in relation to the mechanical contraction activity of the heart and the associated risk of arrhythmia.MethodsThe human ventricular model used in this study has two dynamic characteristics: electrophysiological conduction and mechanical contraction. The model simulating electrophysiological characteristics was consisted of lumped parameter circuit that can mimic the phenomenon of ion exchange through the cell membrane of myocyte and consisted of 214,319 tetrahedral finite elements. In contrast, the model simulating mechanical contraction characteristics was constructed to mimic cardiac contraction by means of the crossbridge of a myofilament and consisted of 14,720 hermite-based finite elements to represent a natural 3D curve of the cardiac surface. First, we performed a single cell simulation and the electrophysiological simulation according to the change of the APD by changing the electrical conductivity of the IKs channel. Thus, we confirmed the correlation between APD and intracellular Ca2+ concentration. Then, we compared mechanical response through mechanical simulation using Ca2+ data from electrical simulation.ResultsThe APD and the sum of the intracellular Ca2+ concentrations showed a positive correlation. The shortened APD reduced the conduction wavelength of ventricular cells by shortening the plateau and early repolarization in myocardial cells. The decrease in APD reduced ventricular pumping efficiency by more than 60% as compared with the normal group (normal conditions). This change is caused by the decline of ventricular output owing to reduced ATP consumption during the crossbridge of myofilaments and decreased tension.ConclusionThe shortening of APD owing to increased electrical conductivity of a protein channel on myocardial cells likely decreases the wavelength and the pumping efficiency of the ventricles. Additionally, it may increase tissue sensitivity to ventricular fibrillation, including reentry, and cause symptoms such as dyspnea and dizziness.

Mitochondrial Ca2+ Influx Contributes to Arrhythmic Risk in Nonischemic Cardiomyopathy.

BackgroundHeart failure (HF) is associated with increased arrhythmia risk and triggered activity. Abnormal Ca2+ handling is thought to underlie triggered activity, and mitochondria participate in Ca2+ homeostasis.Methods and ResultsA model of nonischemic HF was induced in C57BL/6 mice by hypertension. Computer simulations were performed using a mouse ventricular myocyte model of HF. Isoproterenol‐induced premature ventricular contractions and ventricular fibrillation were more prevalent in nonischemic HF mice than sham controls. Isolated myopathic myocytes showed decreased cytoplasmic Ca2+ transients, increased mitochondrial Ca2+ transients, and increased action potential duration at 90% repolarization. The alteration of action potential duration at 90% repolarization was consistent with in vivo corrected QT prolongation and could be explained by augmented L‐type Ca2+ currents, increased Na+‐Ca2+ exchange currents, and decreased total K+ currents. Of myopathic ventricular myocytes, 66% showed early afterdepolarizations (EADs) compared with 17% of sham myocytes (P<0.05). Intracellular application of 1 μmol/L Ru360, a mitochondrial Ca2+ uniporter–specific antagonist, could reduce mitochondrial Ca2+ transients, decrease action potential duration at 90% repolarization, and ameliorate EADs. Furthermore, genetic knockdown of mitochondrial Ca2+ uniporters inhibited mitochondrial Ca2+ uptake, reduced Na+‐Ca2+ exchange currents, decreased action potential duration at 90% repolarization, suppressed EADs, and reduced ventricular fibrillation in nonischemic HF mice. Computer simulations showed that EADs promoted by HF remodeling could be abolished by blocking either the mitochondrial Ca2+ uniporter or the L‐type Ca2+ current, consistent with the experimental observations.ConclusionsMitochondrial Ca2+ handling plays an important role in EADs seen with nonischemic cardiomyopathy and may represent a therapeutic target to reduce arrhythmic risk in this condition.

Leptin injection into the left stellate ganglion augments ischemia-related ventricular arrhythmias via sympathetic nerve activation

Leptin is a peptide hormone produced by adipose tissue whose basic function is regulating energy balance and sympathetic outflow. Previous studies have shown that increased nerve activity of the left stellate ganglion (LSG) promotes ventricular arrhythmia (VA). The purpose of this study was to investigate whether leptin could facilitate VA through activation of the LSG. Sixteen pentobarbital-anesthetized dogs were divided into a control group (saline; n = 8) and a leptin group (leptin; n= 8). Microinjections of either 0.1 mL saline or leptin (18 μg) were injected into the LSG. Action potential duration (APD) of the myocyte and the function and neural activity of the LSG were measured at different time points. VA induced by occlusion of the left anterior descending branch was continuously measured for 1 hour. At the end of the experiment, the LSG tissues were collected for molecular detections. Compared with the control group, leptin microinjection resulted in (1) significant enhancement in the incidence of VA; (2) significant decrease in APD and increase in APD dispersion; and (3) significant increase in the function and neural activity of the LSG. Mechanistically, the leptin receptor was found in the LSG, and its signaling was significantly activated in the leptin-injected group. Additionally, leptin microinjection markedly increased the expression of proinflammatory cytokines. LSG activation induced by leptin microinjection promotes ischemia-induced VAs. Activated leptin receptor signaling and up-regulation of proinflammatory cytokines in the LSG may be responsible for these effects.