Arrhythmogenic Research Articles

Lower sarcoplasmic reticulum Ca2+ threshold for triggering afterdepolarizations in diabetic rat hearts

Type 2 diabetes (T2D) increases arrhythmia risk through incompletely elucidated mechanisms. Ventricular arrhythmias could be initiated by delayed afterdepolarizations (DADs) resulting from elevated spontaneous sarcoplasmic reticulum (SR) Ca2+ release (SR Ca2+ leak). The purpose of this study was to test the role of DADs and SR Ca2+ leak in triggering arrhythmias in T2D hearts. We compared rats with late-onset T2D that display pancreatic and cardiac phenotypes similar to those in humans with T2D (HIP rats) and their nondiabetic littermates (wild type [WT]). HIP rats showed higher propensity for premature ventricular complexes and ventricular tachyarrhythmias, whereas HIP myocytes displayed more frequent DADs and had lower SR Ca2+ content than WT. However, the threshold SR Ca2+ at which depolarizing transient inward currents (Itis) are generated was also significantly decreased in HIP myocytes and was below the actual SR Ca2+ load, which explains the increased DAD incidence despite reduced Ca2+ in SR. In agreement with these findings, Ca2+ spark frequency was augmented in myocytes from HIP vs WT rats, which suggests activation of ryanodine receptors (RyRs) in HIP hearts. Indeed, RyR phosphorylation (by CaMKII and protein kinase A) and oxidation are enhanced in HIP hearts, whereas there is no RyR O-GlcNAcylation in either HIP or control hearts. CaMKII inhibition dissipated the difference in Ca2+ spark frequency between HIP and WT myocytes. The threshold SR Ca2+ for generating depolarizing Itis is lower in T2D because of RyR activation after hyperphosphorylation and oxidation, which favors the occurrence of DADs despite low SR Ca2+ loads.

The Mitochondrial Translocator Protein and the Emerging Link Between Oxidative Stress and Arrhythmias in the Diabetic Heart.

The mitochondrial translocator protein (TSPO) is a key outer mitochondrial membrane protein that regulates the activity of energy-dissipating mitochondrial channels in response to oxidative stress. In this article, we provide an overview of the role of TSPO in the systematic amplification of reactive oxygen species (ROS) through an autocatalytic process known as ROS-induced ROS-release (RIRR). We describe how this TSPO-driven process destabilizes the mitochondrial membrane potential leading to electrical instability at the cellular and whole heart levels. Finally, we provide our perspective on the role of TSPO in the pathophysiology of diabetes, in general and diabetes-related arrhythmias, in particular.

Modulation of Cardiac Ventricular Excitability by GLP-1 (Glucagon-Like Peptide-1).

BackgroundGlucagon-like peptide-1 receptor (GLP-1R) agonists improve cardiovascular outcomes in patients with type 2 diabetes mellitus. However, systemic actions of these agents cause sympathetic activation, which is generally considered to be detrimental in cardiovascular disease. Despite significant research interest in cardiovascular biology of GLP-1, the presence of GLP-1R in ventricular cardiomyocytes remains a controversial issue, and the effects of this peptide on the electrical properties of intact ventricular myocardium are unknown. We sought to determine the effects of GLP-1R agonist exendin-4 (Ex4) on ventricular action potential duration (APD) and susceptibility to ventricular arrhythmia in the rat heart in vivo and ex vivo.MethodsVentricular monophasic action potentials were recorded in anaesthetized (urethane) rats in vivo and isolated perfused rat hearts during sinus rhythm and ventricular pacing.ResultsIn vivo, systemic administration of Ex4 (5 μg/kg intravenously) increased heart rate, and this effect was abolished by β-adrenoceptor blockade. Despite causing sympathetic activation, Ex4 increased APD at 90% repolarization during ventricular pacing by 7% (P=0.044; n=6) and reversed the effect of β-adrenoceptor agonist dobutamine on APD at 90% repolarization. In isolated perfused hearts, Ex4 (3 nmol/L) increased APD at 90% repolarization by 14% (P=0.015; n=6) with no effect on heart rate. Ex4 also reduced ventricular arrhythmia inducibility in conditions of β-adrenoceptor stimulation with isoproterenol. Ex4 effects on APD and ventricular arrhythmia susceptibility were prevented in conditions of muscarinic receptor blockade or inhibition of nitric oxide synthase.ConclusionsThese data demonstrate that GLP-1R activation effectively opposes the effects of β-adrenoceptor stimulation on cardiac ventricular excitability and reduces ventricular arrhythmic potential. The effect of GLP-1R activation on the ventricular myocardium is indirect, mediated by acetylcholine and nitric oxide and, therefore, can be explained by stimulation of cardiac parasympathetic (vagal) neurons.

Abstract 520: Overexpression of a Non-muscle RBFOX2 Splice Isoform Induce Cardiac Arrhythmias in Myotonic Dystrophy

Myotonic dystrophy type 1 (DM1) is a dominantly inherited neuromuscular disease caused by a CTG repeat expansion in 3’-UTR of DMPK gene. DM1 affects multiple tissues, but cardiac dysfunctions are the second leading cause of death which often include conduction defects and arrhythmias. The best characterized pathogenic mechanism of DM1 is toxic gain-of-function of expanded CUG repeat (CUG exp ) RNA that accumulates to form ribonuclear foci causing sequestration of MBNL and overexpression of CELF1 family of splicing factors. However, loss of MBNL or gain of CELF1 activity does not explain the cardiac phenotypes observed in DM1. Here we report that steady-state protein levels of RBFOX2, a critical splicing regulator, are drastically upregulated in DM1 heart tissue. This is accompanied by aberrant skipping of a muscle-specific 43bp exon in RBFOX2 transcript, resulting in selective up-regulation of the non-muscle RBFOX2 splice isoform in adult cardiomyocytes. We demonstrate that expression of CUG exp RNA in DM1 affects RBFOX2 in two distinct ways: reduced expression of a subset of microRNAs causes de-repression of RBFOX2 protein production; and CELF1 overexpression promotes skipping of the muscle-specific exon of RBFOX2 . Remarkably, tet -inducible overexpression of the non-muscle RBFOX2 isoform, or CRISPR/ Cas9 mediated deletion of the muscle-specific RBFOX2 exon in the mouse heart results in prolonged PR and QRS intervals, slower conduction velocity and spontaneous cardiac arrhythmias that mirror human DM1 pathology. Integration of RBFOX2-CLIP with RNA-seq data from cardiomyocytes of mice expressing the non-muscle RBFOX2 isoform identified a core network of mRNA splicing defects in genes encoding sodium, potassium, and calcium ion channels and key excitation-contraction coupling proteins that are similarly misspliced in hearts of DM1 patients. Thus, we have uncovered a central role for RBFOX2 isoform switching in DM1 cardiac pathogenesis and identified the molecular mechanisms that may alter electrophysiological properties and thereby induce cardiac arrhythmias in the DM1 heart.

Electrical stimulation-based renal nerve mapping exacerbates ventricular arrhythmias during acute myocardial ischaemia.

Blood pressure elevation in response to transient renal nerve stimulation (RNS) has been used to determine the ablation target and endpoint of renal denervation. This study aimed to evaluate the safety of transient RNS in canines with normal or ischaemic hearts. In ten normal (Group 1) and six healed myocardial infarction (HMI) (Group 2) canines, a large-tip catheter was inserted into the left or right renal artery to perform transient RNS. The left stellate ganglion neural activity (LSGNA) and ventricular electrophysiological parameters were measured at baseline and during transient RNS. In another 20 acute myocardial infarction (AMI) canines, RNS (Group 3, n = 10) or sham RNS (Group 4, n = 10) was intermittently (1 min ON and 4 min OFF) performed for 1 h following AMI induction. The LSGNA and AMI-induced ventricular arrhythmias were analysed. In normal and HMI canines, although transient RNS significantly increased the LSGNA and facilitated the action potential duration (APD) alternans, it did not induce any ventricular arrhythmias and did not change the ventricular effective refractory period, APD or maximum slope of the APD restitution curve. In AMI canines, transient RNS significantly exacerbated LSG activation and promoted the incidence of ventricular arrhythmias. Transient RNS did not increase the risk of ventricular arrhythmias in normal or HMI hearts, but it significantly promoted the occurrence of ventricular arrhythmias in AMI hearts. Therefore, electrical stimulation-based renal nerve mapping may be unsafe in AMI patients and in patients with a high risk for malignant ventricular arrhythmias.

Lnc’ing Ca2+, SERCA and cardiac disease

Loss of SERCA function contributes to reduced contractility, Ca2+ overload and arrhythmias in the diseased heart. A long non-coding RNA (ZFAS1) upregulated in cardiac disease is reported to directly inhibit SERCA function. The implications for cardiac disease and the wider roles of SERCA are discussed.

Small-conductance Ca2+-activated K+ channel activation deteriorates hypoxic ventricular arrhythmias via CaMKII in cardiac hypertrophy.

The molecular and electrophysiological mechanisms of acute ischemic ventricular arrhythmias in hypertrophied hearts are not well known. We hypothesized that small-conductance Ca2+-activated K+ (SK) channels are activated during hypoxia via the Ca2+/calmodulin-dependent protein kinase II (CaMKII)-dependent pathway. We used normotensive Wistar-Kyoto (WKY) rats and spontaneous hypertensive rats (SHRs) as a model of cardiac hypertrophy. The inhibitory effects of SK channels and ATP-sensitive K+ channels on electrophysiological changes and genesis of arrhythmias during simulated global hypoxia (GH) were evaluated. Hypoxia-induced abbreviation of action potential duration (APD) occurred earlier in ventricles from SHRs versus. WKY rats. Apamin, a SK channel blocker, prevented this abbreviation in SHRs in both the early and delayed phase of GH, whereas in WKY rats only the delayed phase was prevented. In contrast, SHRs were less sensitive to glibenclamide, a ATP-sensitive K+ channel blocker, which inhibited the APD abbreviation in both phases of GH in WKY rats. SK channel blockers (apamin and UCL-1684) reduced the incidence of hypoxia-induced sustained ventricular arrhythmias in SHRs but not in WKY rats. Among three SK channel isoforms, SK2 channels were directly coimmunoprecipitated with CaMKII phosphorylated at Thr286 (p-CaMKII). We conclude that activation of SK channels leads to the APD abbreviation and sustained ventricular arrhythmias during simulated hypoxia, especially in hypertrophied hearts. This mechanism may result from p-CaMKII-bound SK2 channels and reveal new molecular targets to prevent lethal ventricular arrhythmias during acute hypoxia in cardiac hypertrophy. NEW & NOTEWORTHY We now show a new pathophysiological role of small-conductance Ca2+-activated K+ channels, which shorten the action potential duration and induce ventricular arrhythmias during hypoxia. We also demonstrate that small-conductance Ca2+-activated K+ channels interact with phosphorylated Ca2+/calmodulin-dependent protein kinase II at Thr286 in hypertrophied hearts.

Cardiac electrophysiological adaptations in the equine athlete-Restitution analysis of electrocardiographic features.

Exercising horses uniquely accommodate 7–8-fold increases in heart rate (HR). The present experiments for the first time analysed the related adaptations in action potential (AP) restitution properties recorded by in vivo telemetric electrocardiography from Thoroughbred horses. The horses were subjected to a period of acceleration from walk to canter. The QRS durations, and QT and TQ intervals yielded AP conduction velocities, AP durations (APDs) and diastolic intervals respectively. From these, indices of active, λ = QT/(QRS duration), and resting, λ0 = TQ/(QRS duration), AP wavelengths were calculated. Critical values of QT and TQ intervals, and of λ and λ0 at which plots of these respective pairs of functions showed unity slope, were obtained. These were reduced by 38.9±2.7% and 86.2±1.8%, and 34.1±3.3% and 85.9±1.2%, relative to their resting values respectively. The changes in λ were attributable to falls in QT interval rather than QRS duration. These findings both suggested large differences between the corresponding critical (129.1±10.8 or 117.4±5.6 bpm respectively) and baseline HRs (32.9±2.1 (n = 7) bpm). These restitution analyses thus separately identified concordant parameters whose adaptations ensure the wide range of HRs over which electrophysiological activation takes place in an absence of heart block or arrhythmias in equine hearts. Since the horse is amenable to this in vivo electrophysiological analysis and displays a unique wide range of heart rates, it could be a novel cardiac electrophysiology animal model for the study of sudden cardiac death in human athletes.

O-GlcNAcylation of cardiac Nav1.5 contributes to the development of arrhythmias in diabetic hearts

BackgroundCardiovascular complications are major causes of mortality and morbidity in diabetic patients. The mechanisms underlying the progression of diabetic heart (DH) to ventricular arrhythmias are unclear. O-linked GlcNAcylation (O-GlcNAc) is a reversible post-translational modification for the regulation of diverse cellular processes. The purpose of this study was to assess whether the cardiac voltage-gated sodium channel (Nav1.5) is subjected to O-linked GlcNAcylation (O-GlcNAc), which plays an essential role in DH-induced arrhythmias. Methods and resultsIn this study, Sprague-Dawley rats (male, 200-230 g) were treated with a single high-dose of streptozotocin (STZ, 80 mg/kg) to generate a rat model of diabetes. STZ-induced 3-month diabetic rats displayed increased susceptibility to ventricular arrhythmias. The elevated O-GlcNAc modification was correlated with decreases in both total and cytoplasmic Nav1.5 expression in vivo and in vitro. In addition, both co-immunoprecipitation and immunostaining assays demonstrated that hyperglycemia could increase the O-GlcNAc-modified Nav1.5 levels and decrease the interaction between Nav1.5 and Nav1.5-binding proteins Nedd4–2/SAP-97. Furthermore, patch-clamp measurements in HEK-293 T cells showed that Nav1.5 current densities decreased by 30% after high-glucose treatment, and the sodium currents increased via O-GlcNAc inhibition. ConclusionOur data suggested that hyperglycemia increased the O-GlcNAc modification of Nav1.5 expression and decreased the interaction between Nav1.5 and Nedd4–2/SAP-97, which led to the abnormal expression and distribution of Nav1.5, loss of function of the sodium channel, and prolongation of the PR/QT interval. Excessive O-GlcNAc modification of Nav1.5 is a novel signaling event, which may be an underlying contributing factor for the development of the arrhythmogenesis in DH.

Function of L-Type Calcium Channel Microdomain in Human Myocytes from Hearts with Ischemic versus Dilated Cardiomyopathies

Introduction: Recently we have revealed how the imbalance of L-type calcium channels (LTCC) at the cell membrane could lead to arrhythmia in the whole heart, in a rat model of myocardial infarction as well as in a group of patients with dilated cardiomyopathy (DCM). In this study, we conduct a more extensive comparison of myocytes from hearts with ischemic (ICM) versus DCM. Also, the LTCC function is assessed in myocytes from patients with Left Ventricular Assistance Device (LVAD). Methods: Cardiomyocytes were isolated from ventricular tissue of donor normal hearts and hearts of patients with DCM or ICM with and without LVAD. LTCC current was recorded in distinct cellular compartments: T-tubules (TT) and the areas between them or “crests”, using super-resolution scanning patch-clamp technique. Cell size and T-tubules density were measured with confocal microscopy under Di-8-ANEPPS staining. Results: Cardiomyocytes from failing hearts develop a hypertrophic phenotype, more severe in DCM than in ICM patients, which is partially reversed with LVAD. In addition, these cells have less Z-grooves, lower TT density and a significantly increased LTCC density in the crest microdomains. Functionally, LTCCs open probability revealed to be abnormally high in failing patients. These pro-arrhythmic LTCCs, as it has been proposed in relation with the AP prolongation, are, interestingly, in the crests of DCM and in the TT of ICM myocytes. Inhibitor analysis showed that hyperphosphorylation of LTCC occurs via calmodulin-dependent kinase II in DCM and via protein kinase A in ICM. Implantation of LVAD was found to reduce the LTCC activity independently of the disease. Conclusion: These findings suggest that both ICM and DCM myocytes contain pro-arrhythmic hyper-active LTCCs but their increased activity occurs as the result of two different phosphorylation pathways targeting different microdomains.

Transcriptional regulation of stress kinase JNK2 in pro-arrhythmic CaMKIIδ expression in the aged atrium.

c-jun N-terminal kinase (JNK) is a critical stress response kinase that activates in a wide range of physiological and pathological cellular processes. We recently discovered a pivotal role of JNK in the development of atrial arrhythmias in the aged heart, while cardiac CaMKIIδ, another pro-arrhythmic molecule, was also known to enhance atrial arrhythmogenicity. Here, we aimed to reveal a regulatory role of the stress kinase JNK2 isoform on CaMKIIδ expression. Activated JNK2 leads to increased CaMKIIδ protein expression in aged human and mouse atria, evidenced from the reversal of CaMKIIδ up-regulation in JNK2 inhibitor treated wild-type aged mice. This JNK2 action in CaMKIIδ expression was further confirmed in HL-1 myocytes co-infected with AdMKK7D-JNK2, but not when co-infected with AdMKK7D-JNK1. JNK2-specific inhibition (either by a JNK2 inhibitor or overexpression of inactivated dominant-negative JNK2 (JNK2dn) completely attenuated JNK activator anisomycin-induced CaMKIIδ up-regulation in HL-1 myocytes, whereas overexpression of JNK1dn did not. Moreover, up-regulated CaMKIIδ mRNA along with substantially increased phosphorylation of JNK downstream transcription factor c-jun [but not activating transcription factor2 (ATF2)] were exhibited in both aged atria (humans and mice) and transiently JNK activated HL-1 myocytes. Cross-linked chromatin-immunoprecipitation assays (XChIP) revealed that both c-jun and ATF2 were bound to the CaMKIIδ promoter, but significantly increased binding of c-jun only occurred in the presence of anisomycin and JNK inhibition alleviated this anisomycin-elevated c-jun binding. Mutated CaMKII consensus c-jun binding sites impaired its promoter activity. Enhanced transcriptional activity of CaMKIIδ by anisomycin was also completely reversed to the baseline by either JNK2 siRNA or c-jun siRNA knockdown. JNK2 activation up-regulates CaMKIIδ expression in the aged atrium. This JNK2 regulation in CaMKIIδ expression occurs at the transcription level through the JNK downstream transcription factor c-jun. The discovery of this novel molecular mechanism of JNK2-regulated CaMKII expression sheds new light on possible anti-arrhythmia drug development.

Management of Complex Ventricular Arrhythmias in Heterotopic Heart Transplant Patients

A 55-year-old woman was admitted with palpitations on a background of heterotopic heart transplantation 13 years earlier. Prior to transplant, a dual-chamber permanent pacemaker was implanted in her native heart for a severe ischaemic cardiomyopathy with a left ventricular ejection fraction of 5%. Post-transplant, our patient's donor heart had been affected by both allograft vasculopathy requiring percutaneous coronary intervention and one episode of cellular rejection. Her current donor left ventricular ejection fraction was 40%. Our patient's palpitations were identified as recurrent episodes of native heart ventricular tachycardia lasting up to 21 hours at rates of 180 beats per minute. Direct cardioversion to the native heart was performed twice, unsuccessfully. Amiodarone infusion also failed to control the burden of ventricular tachycardia. Lignocaine was then added, but at this point the patient developed bradycardia in the donor heart. In the setting of competing native tachycardia and donor bradycardia, our patient developed cardiogenic shock due to her doubly low-output state. Further cardioversion to the native heart was considered too high risk given the new donor bradycardia. A dual-chamber permanent pacemaker was implanted into the donor heart. Anti-tachycardic therapy was optimised to bisoprolol, oral amiodarone, and mexiletine. With aggressive anti–arrhythmic therapy, ventricular arrhythmias in the native heart ceased while the donor heart was supported with pacing. This is the first report of double dual-chamber pacemakers being implanted in a heterotopic heart transplant recipient to treat complex competing arrhythmias. This case emphasises the difficult balance of controlling two connected cardiomyopathic hearts developing conflicting arrhythmias.

Synthesis of the Ca2+-mobilizing messengers NAADP and cADPR by intracellular CD38 enzyme in the mouse heart: Role in β-adrenoceptor signaling

Nicotinic acid adenine dinucleotide phosphate (NAADP) and cyclic ADP-ribose (cADPR) are Ca2+-mobilizing messengers important for modulating cardiac excitation–contraction coupling and pathophysiology. CD38, which belongs to the ADP-ribosyl cyclase family, catalyzes synthesis of both NAADP and cADPR in vitro. However, it remains unclear whether this is the main enzyme for their production under physiological conditions. Here we show that membrane fractions from WT but not CD38−/− mouse hearts supported NAADP and cADPR synthesis. Membrane permeabilization of cardiac myocytes with saponin and/or Triton X-100 increased NAADP synthesis, indicating that intracellular CD38 contributes to NAADP production. The permeabilization also permitted immunostaining of CD38, with a striated pattern in WT myocytes, whereas CD38−/− myocytes and nonpermeabilized WT myocytes showed little or no staining, without striation. A component of β-adrenoreceptor signaling in the heart involves NAADP and lysosomes. Accordingly, in the presence of isoproterenol, Ca2+ transients and contraction amplitudes were smaller in CD38−/− myocytes than in the WT. In addition, suppressing lysosomal function with bafilomycin A1 reduced the isoproterenol-induced increase in Ca2+ transients in cardiac myocytes from WT but not CD38−/− mice. Whole hearts isolated from CD38−/− mice and exposed to isoproterenol showed reduced arrhythmias. SAN4825, an ADP-ribosyl cyclase inhibitor that reduces cADPR and NAADP synthesis in mouse membrane fractions, was shown to bind to CD38 in docking simulations and reduced the isoproterenol-induced arrhythmias in WT hearts. These observations support generation of NAADP and cADPR by intracellular CD38, which contributes to effects of β-adrenoreceptor stimulation to increase both Ca2+ transients and the tendency to disturb heart rhythm.

Desensitization of the human 5-HT4 receptor in isolated atria of transgenic mice.

In the human cardiovascular system, serotonin (5-HT) exerts positive inotropic and chronotropic effects mediated by 5-HT4 receptors. Moreover, 5-HT4 receptor stimulation can cause arrhythmias in the human heart. Response to 5-HT can fade due to desensitization of the receptor system and/or activation of phosphodiesterases. In this study, we investigated a potential desensitization of the human 5-HT4(a) receptor expressed in the mouse heart. Therefore, we have used atrial preparations of transgenic (TG) mice with cardiac myocyte-specific overexpression of the human 5-HT4(a) receptor and their non-transgenic littermates (WT). Homologous (by 5-HT) and potentially heterologous (by isoprenaline) desensitization of the 5-HT4 receptor was investigated in atria of TG mice. 5-HT increased force of contraction in isolated electrically paced left atria and beating rate in spontaneously beating right atria only in preparations from TG but not from WT. Pre-treatment of isolated atria with high concentrations (10-600μM) of 5-HT for 60min attenuated the positive inotropic effects and the positive chronotropic effects of 5-HT in TG atria. Several inhibitors of desensitization including Zn2+, sucrose, and paroxetine were tested. Whereas sucrose was without any effect and Zn2+ only was partially effective, paroxetine was able to inhibit desensitization favoring at least in part a G-protein receptor-coupled kinase-mediated mechanism of 5-HT4 receptor desensitization in the TG mouse heart. In addition, desensitization of ventricular 5-HT4 receptors was noted in isolated perfused hearts (Langendorff preparations) from TG mice. In summary, we show homologous desensitization of the 5-HT4 receptor in the heart of a transgenic mouse model possibly dependent on active G-protein receptor-coupled kinase. The exact mechanism and a potentially heterologous desensitization have to be elucidated by further investigations.

116LED-based termination of ventricular arrhythmias in the adult rat heart upon optogenetic engineering: Towards biological shockless defibrillation

116LED-based termination of ventricular arrhythmias in the adult rat heart upon optogenetic engineering: Towards biological shockless defibrillation

Catheter ablation for ventricular tachycardia in patients with arrhythmogenic right ventricular dysplasia/cardiomyopathy: a systematic review and meta-analysis.

Objective Catheter ablation is commonly performed for ventricular tachycardia (VT) in patients with arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C). The success rate of VT ablati...

Inhibition of late sodium current suppresses calcium-related ventricular arrhythmias by reducing the phosphorylation of CaMK-II and sodium channel expressions

Cardiac arrhythmias associated with intracellular calcium inhomeostasis are refractory to antiarrhythmic therapy. We hypothesized that late sodium current (INa) contributed to the calcium-related arrhythmias. Monophasic action potential duration at 90% completion of repolarization (MAPD90) was significantly increased and ventricular arrhythmias were observed in hearts with increased intracellular calcium concentration ([Ca2+]i) by using Bay K 8644, and the increase became greater in hearts treated with a combination of ATX-II and Bay K 8644 compared to Bay K 8644 alone. The prolongations caused by Bay K 8644 and frequent episodes of ventricular tachycardias, both in absence and presence of ATX-II, were significantly attenuated or abolished by late INa inhibitors TTX and eleclazine. In rabbit ventricular myocytes, Bay K 8644 increased ICaL density, calcium transient and myocyte contraction. TTX and eleclazine decreased the amplitude of late INa, the reverse use dependence of MAPD90 at slower heart rate, and attenuated the increase of intracellular calcium transient and myocyte contraction. TTX diminished the phosphorylation of CaMKII-δ and Nav 1.5 in hearts treated with Bay K 8644 and ATX-II. In conclusion, late INa contributes to ventricular arrhythmias and its inhibition is plausible to treat arrhythmias in hearts with increased [Ca2+]i

Mitochondrial Calcium Flux Contributes to Arrhythmia in Mouse Heart during Acute Myocardial Infarction

Introduction: Acute myocardial infarction (AMI) is associated with lethal ventricular arrhythmias. One important mechanism for these arrhythmias is delayed afterdepolarizations resulting from Ca2+ overload in border zone. Mitochondria participated in Ca2+ homeostasis, and the mitochondrial Ca2+ uniporter (MCU) mediates mitochondrial Ca2+ entry. We tested whether mitochondrial Ca2+ flux played a role in ventricular arrhythmias during AMI. Methods: Hearts and ventricular cells were isolated from CD1 wild type (Wt) and MCU knock down (MCU+/-) mice. Whole cell current-clamp was employed to record action potentials (APs). Cytoplasmic Ca2+ and mitochondrial Ca2+ were sampled by loading Indo-1 AM and Rhod-2 AM (recorded synchronously with APs) respectively. Electric pacing was used to induce arrhythmia. MCU mRNA and protein expression were evaluated by Western blotting & Real-Time PCR. Optical mapping was used to detect arrhythmias in the intact heart. Results: In MCU+/− mice, the mRNA (from 1.00 ± 0.02 to 0.42 ± 0.03 ) and protein expression (from 0.73 ± 0.07 to 0.33 ± 0.04) of MCU was substantially reduced, with peak mitochondrial Ca2+ transients (F/Fo) prominently decreased from 0.21 ± 0.02 to 0.14 ± 0.03. Infarct size was similar in the two models: Wt (48% ± 4%) and MCU+/- (44% ± 2%) mice. Two of eight of Wt mice died suddenly within 1 h after AMI, but none of MCU+/- mice died suddenly in this period. Compare to the Wt mice, pacing induced significantly less ventricular arrhythmias in MCU+/- mice during the first stage of coronary artery ligation as VT/VF duration decreased from 0.38 ± 0.09 s in Wt mice to 0.12 ± 0.05 s in MCU+/- mice. Consistent with a role of mitochondria in ischemic ventricular rhythms, inhibition of mitochondrial NCX by CGP-37157 (i.p. 0.4mg/100g) in both Wt and MCU+/− mice could abolish ventricular arrhythmias evoked by electric pacing during AMI. Furthermore, VT/VF was detected only in Wt mice at infarction border zone by optical mapping experiment. Finally, after AMI, action potential durations were longer in Wt mouse hearts/cardiomyocytes than those in MCU+/- mouse hearts/cardiomyocytes. Conclusion: Our study suggest a role for mitochondrial Ca2+ handling in arrhythmic risk during AMI. Reducing mitochondrial Ca2+ cycling improved arrhythmic risk.

Progesterone Protects Against Bisphenol A-Induced Arrhythmias in Female Rat Cardiac Myocytes via Rapid Signaling.

Bisphenol A (BPA) is an estrogenic endocrine-disrupting chemical (EDC) that has a range of potential adverse health effects. Previously we showed that acute exposure to BPA promoted arrhythmias in female rat hearts through estrogen receptor rapid signaling. Progesterone (P4) and estrogen have antagonistic or complementary actions in a number of tissues and systems. In the current study, we examined the influence and possible protective effect of P4 on the rapid cardiac actions of BPA in female rat cardiac myocytes. Preincubation with physiological concentration (1 nM) of P4 abolished BPA-induced triggered activities in female cardiac myocytes. Further, P4 abrogated BPA-induced alterations in Ca2+ handling, including elevated sarcoplasmic reticulum Ca2+ leak and Ca2+ load. Key to the inhibitory effect of P4 is its blockade of BPA-induced increase in the phosphorylation of phospholamban. At myocyte and protein levels, these inhibitory actions of P4 were blocked by pretreatment with the nuclear P4 receptor (nPR) antagonist RU486. Analysis using membrane-impermeable bovine serum albumin-conjugated P4 suggested that the actions of P4 were mediated by membrane-initiated signaling. Inhibitory G (Gi) protein and phophoinositide-3 kinase (PI3K), but not tyrosine protein kinase activation, were involved in the observed effects of P4. In conclusion, P4 exerts an acute protective effect against BPA-induced arrhythmogenesis in female cardiac myocytes through nPR and the Gi/PI3K signaling pathway. Our findings highlight the importance of considering the impact of EDCs in the context of native hormonals and may provide potential therapeutic strategies for protection against the cardiac toxicities associated with BPA exposure.

SK channel enhancers attenuate Ca2+-dependent arrhythmia in hypertrophic hearts by regulating mito-ROS-dependent oxidation and activity of RyR.

Plasmamembrane small conductance Ca2+-activated K+ (SK) channels were implicated in ventricular arrhythmias in infarcted and failing hearts. Recently, SK channels were detected in the inner mitochondria membrane (IMM) (mSK), and their activation protected from acute ischaemia-reperfusion injury by reducing intracellular levels of reactive oxygen species (ROS). We hypothesized that mSK play an important role in regulating mitochondrial function in chronic cardiac diseases. We investigated the role of mSK channels in Ca2+-dependent ventricular arrhythmia using rat model of cardiac hypertrophy induced by banding of the ascending aorta thoracic aortic banding (TAB). Dual Ca2+ and membrane potential optical mapping of whole hearts derived from TAB rats revealed that membrane-permeable SK enhancer NS309 (2 μM) improved aberrant Ca2+ homeostasis and abolished VT/VF induced by β-adrenergic stimulation. Using whole cell patch-clamp and confocal Ca2+ imaging of cardiomyocytes derived from TAB hearts (TCMs) we found that membrane-permeable SK enhancers NS309 and CyPPA (10 μM) attenuated frequency of spontaneous Ca2+ waves and delayed afterdepolarizations. Furthermore, mSK inhibition enhanced (UCL-1684, 1 μM); while activation reduced mitochondrial ROS production in TCMs measured with MitoSOX. Protein oxidation assays demonstrated that increased oxidation of ryanodine receptors (RyRs) in TCMs was reversed by SK enhancers. Experiments in permeabilized TCMs showed that SK enhancers restored SR Ca2+ content, suggestive of substantial improvement in RyR function. These data suggest that enhancement of mSK channels in hypertrophic rat hearts protects from Ca2+-dependent arrhythmia and suggest that the protection is mediated via decreased mitochondrial ROS and subsequent decreased oxidation of reactive cysteines in RyR, which ultimately leads to stabilization of RyR-mediated Ca2+ release.