Late Sodium Current Research Articles

Abstract 4138799: SGK1 inhibition shortens action potential duration in LQT3 mice and LQT2 rabbits via inhibition of late sodium current

Background: Serum and glucocorticoid regulated kinase 1 (SGK1) activation during pathological conditions has been shown to increase late sodium current, prolong action potential duration (APD), and induce ventricular arrhythmias, thus recapitulating a long QT syndrome (LQTS) phenotype. Hence, pharmacological SGK1 inhibition may be a novel therapeutic approach in inherited LQTS. Aim: We aimed to investigate potential effects of SGK1 inhibition in animal models of different LQTS subtypes, and the potential involvement of late I Na inhibition. Methods: Cardiomyocytes (CMs) isolated from wild-type (WT) and LQT3 mice ( Scn5a -1798insD) as well as from WT, LQT1 ( KCNQ1 -Y315S) and LQT2 ( KCNH2 -G628S) rabbits, were incubated for 3-5 hours with SGK1-inhibitor (SGK1-inh, 300 nM-3 μM) or DMSO (vehicle). Patch-clamp experiments were performed to assess AP properties, short-term variability (STV) of APD and late I Na . Result: SGK1-inh incubation significantly shortened the prolonged APD 90 in LQT3 CMs by 23% and in LQT2 CMs by 25% (restoring them towards the WT level), without affecting other AP properties. In contrast, in WT and LQT1 CMs, no effect of SGK1-inh on APD 90 was observed. Furthermore, short-term-variability (STV) of APD, a measure of pro-arrhythmia, was pathologically increased in LQT3 (5.6±0.2 ms vs WT 2.7±0.3 ms, p<0.05) and LQT2 (14.7±2.8 ms vs WT 5.9 ±1.1 ms, p < 0.0001), but not in LQT1. SGK1-Inh markedly reduced STV in LQT3 (to 2.9±0.1 ms) and LQT2 CMs (to 7.2± 1.2 ms), but had no effect in WT and LQT1 CMs. While enhanced late I Na was previously demonstrated in LQT3 CMs, we now also observed increased late I Na in LQT2 CMs as compared to WT (-0.14 ± 0.02 pA/pF vs WT -0.08 ± 0.02 pA/pF, p < 0.005), but not in LQT1 CMs. SGK1-Inh significantly reduced late I Na in LQT3 (by 32%) and LQT2 CMs (by 43%), but not in LQT1 CMs, confirming that SGK1-inh directly targets late I Na . Conclusion: SGK1-inhibition exerts a beneficial APD-shortening and anti-arrhythmic effect in LQT3 and LQT2 by reducing the enhanced late I Na associated with these LQTS genetic subtypes. These findings furthermore underscore the contribution of increased late I Na to the LQT2 phenotype.

Sulcardine, an investigational multichannel blocker for atrial fibrillation with uniquely intense JTp inhibition to enhance safety

Abstract Background Sulcardine is a multi-ion channel blocker for treating atrial fibrillation (AF) with similar inhibitory potency on peak/late sodium, L-type calcium, and rapidly activating delayed-rectifier potassium (IKr) currents. The therapeutic effect of inhibiting IKr includes QT prolongation (accompanied by a proportional J to T peak [JTp] interval prolongation), which, when excessive, may cause malignant arrhythmias. As previously reported, sulcardine strongly mitigates the JTp increase that may contribute to excessive QT prolongation by inhibiting late sodium and L-type calcium currents, an intrinsic mechanism to protect from excessive IKr block. Purpose We explored the JTp mechanism, pharmacokinetics (PK), electrocardiogram (ECG) changes, safety, and drug-drug interaction (DDI) of a single sulcardine (as HBI-3000) intravenous (IV) infusion in healthy subjects. Methods An open-label Phase 1 study evaluated the PK interaction between sulcardine, dosed as a 350 mg 30-minute infusion of the sulfate salt HBI-3000, and a cytochrome P450 CYP2D6 enzyme inhibitor (paroxetine). Triplicate ECGs were extracted from continuous 12-lead Holter ECG recordings at baseline and 10 time points thereafter. Mean baseline-subtracted and heart rate-corrected QTcF (dQTcF), JTpc (dJTpc), and other ECG changes were calculated at each time point. Left ventricular ejection fraction (LVEF) was assessed at baseline, 30-, and 120-minutes post-infusion start by transthoracic echocardiography (TTE). Results 39 subjects received HBI-3000, 33 in the paired PK population. Expected ECG parameter changes proportional to sulcardine plasma concentrations were observed in the HBI-3000-alone portion of the study, including modest QTcF prolongation (Fig 1, blue slope, confidence band) and JTpc inhibition (Fig 1, red). The usual JTpc increase associated with IKr blockers, even those with mitigating channel block (such as the late sodium current), was not only reduced but actually reversed. We calculated the dQTcF change that would have been observed in the absence of JTpc reduction by sulcardine (Fig 1, green). Sulcardine reduced the QTcF increase that might have occurred by about 21 msec at the highest concentrations, and this mitigating effect was observed at all concentrations. No clinically significant safety findings or arrhythmias were observed. The most frequent TEAEs were nausea and headache (n=2 each [5.1%]). No LVEF changes were clinically significant. Sulcardine had minimal metabolic interactions. Conclusion Acute IV HBI-3000 administration resulted in no clinically significant safety findings, LVEF depression, or DDI with CYP2D6. Sulcardine facilitates QT prolongation, which has a therapeutic benefit in terminating AF, with simultaneous uncoupling and reversing of the JTpc interval increase, a potential mechanism for reducing proarrhythmic risk due to excessive QT prolongation.Figure 1.ECG Intervals vs Sulcardine Conc

Ranolazine therapy is associated with a reduced new diagnosis of diabetes mellitus in chronic coronary syndrome patients: the results of a real-world analysis of an Italian population

Abstract Background Ranolazine (Ran) is an anti-anginal drug acting as a late sodium current inhibitor at a cell membrane level. Some experimental and clinical studies showed an antihyperglycemic effect of the drug, possibly mediated by the increase of insulin secretion, the inhibition of glucagon release, and the protective action on beta-cell function. These effects, additive to the anti-anginal properties of Ran in patients with chronic coronary syndrome (CCS), could modulate and improve glucose metabolic balance. Indeed, data exploring at a population level the association of the drug with diabetes mellitus (DM) incidence are effectively lacking. Aims Aim of this study was to evaluate in CCS patients if the use of Ran, compared with other treatments (No-Ran), was associated with a lower incidence of DM in an Italian real-world clinical setting. Methods Patients included in the database of the National Health System were evaluated (N=6.1 million; about 10% of the Italian population); the recorded information concerned hospitalizations with the related diseases, clinical events, outpatient visits and drug therapy. Patients hospitalized for any cause and discharged with a diagnosis of angina between 2011 and 2020 (ICD-9-CM codes: 413-414) were included in the study after having proved the absence of DM in their medical history. Study population was divided into the Ran (at least one drug prescription) and in the No-Ran cohorts. The mean follow up was 4.3 years for the Ran and 5.0 years for the No-Ran cohorts, respectively. Results The study population included N=171,015 patients (mean age: 72 years, men: 66%). Among them, N=148,808 (No-Ran cohort) were treated with other therapies while N=22,207 (Ran cohort) received Ran. After propensity score matching, Ran (N=6,384) and No-Ran (N=25,536) cohorts did not differ for age, Charlson comorbidity index, use of aspirin and other antithrombotic agents, statins, beta-blockers, Ca-antagonists and nitrates. The incidence of a new diagnosis of DM during the follow-up period was 10.5 and 15.8% in the Ran and in the No-Ran cohorts, respectively (p<0.001). The Cox-regression analysis showed that Ran therapy was associated with a 30% risk reduction to present a new diagnosis of DM (HR=0.70, 95%CI: 0.64-0.76, p<0.001), independently of age, Charlson comorbidity index, heart failure, previous stroke, use of ivabradine, Ca-antagonists and nitrates. Conclusions This real-world study, performed in a whole subset of the Italian population, showed that, in the long-term, the incidence of a new DM diagnosis was lower in Ran users. This association could contribute to explain the benefits of the drug in patients with CCS.

Eleclazine Suppresses Ventricular Fibrillation in Failing Rabbit Hearts with Ischemia- Reperfusion Injury Undergoing Therapeutic Hypothermia.

Eleclazine is a highly selective late sodium current inhibitor, possibly effective in reducing ventricular fibrillation (VF) in heart failure (HF) with ischemia-reperfusion (IR) injury. The electrophysiological effects of eleclazine at therapeutic hypothermia (TH) are unknown. We investigated the effects of eleclazine in suppressing VF in failing rabbit hearts with IR injury undergoing TH. HF was induced by right ventricular pacing. An IR model was created using coronary artery ligation for 60 min, followed by reperfusion for 30 min. Hearts were excised and Langendorff-perfused for optical mapping and electrophysiological studies. Electrophysiological studies were repeated after TH (33 oC) for 30 min or eleclazine (1 μM) infusion for 20 min. In failing IR-injured hearts, eleclazine reduced action potential duration (APD) dispersion and accelerated intracellular Ca2+ uptake to suppress arrhythmogenic alternans, but also exacerbated rate-dependent conduction slowing, resulting in neutral effects on VF inducibility at normothermia. TH increased VF severity. Eleclazine after TH ameliorated TH-induced APD dispersion and further depressed conduction to reduce VF inducibility and severity. TH after eleclazine also slowed conduction to a greater extent to reduce VF inducibility and severity by extrastimulus pacing. In control IR-injured hearts, eleclazine increased VF severity by dynamic pacing at normothermia, which was counteracted by TH. Eleclazine does not prevent VF at normothermia, but reduces VF inducibility and severity by extrastimulus pacing at TH in isolated failing hearts with regional IR injury.

Ranolazine New Potential Therapeutic Aspects

Since Ranolazine approval as an antianginal medication by the US Food and Drug Administration in 2006, it has been administered to specific patient populations with stable angina. Originally, it was thought that ranolazine's therapeutic actions were due to its inhibition of the metabolism of fatty acids. However, as research progressed, it became clear that ranolazine's primary advantageous benefits come from its influence on the heart's late sodium current. Numerous experimental and clinical studies have tested ranolazine either directly or indirectly on heart failure since late-sodium currents have been found to be involved in various heart pathologies such as ischemia, arrhythmias, systolic and diastolic dysfunctions, and all these conditions are associated with heart failure. The inhibition of the underlying mechanisms of cardiac remodeling, such as ion disruptions, oxidative stress, inflammation, apoptosis, fibrosis, metabolic dysregulation, and neurohormonal dysfunction, by ranolazine is reviewed after any kind of severe injury, along with open questions. Ranolazine additional metabolic properties contribute to its efficacy in treating conditions other than coronary artery disease. Our publication will focus on potential therapeutic applications of ranolazine, extending beyond cardiology.

Structural basis for the rescue of hyperexcitable cells by the amyotrophic lateral sclerosis drug Riluzole

Neuronal hyperexcitability is a key element of many neurodegenerative disorders including the motor neuron disease Amyotrophic Lateral Sclerosis (ALS), where it occurs associated with elevated late sodium current (INaL). INaL results from incomplete inactivation of voltage-gated sodium channels (VGSCs) after their opening and shapes physiological membrane excitability. However, dysfunctional increases can cause hyperexcitability-associated diseases. Here we reveal the atypical binding mechanism which explains how the neuroprotective ALS-treatment drug riluzole stabilises VGSCs in their inactivated state to cause the suppression of INaL that leads to reversed cellular overexcitability. Riluzole accumulates in the membrane and enters VGSCs through openings to their membrane-accessible fenestrations. Riluzole binds within these fenestrations to stabilise the inactivated channel state, allowing for the selective allosteric inhibition of INaL without the physical block of Na+ conduction associated with traditional channel pore binding VGSC drugs. We further demonstrate that riluzole can reproduce these effects on a disease variant of the non-neuronal VGSC isoform Nav1.4, where pathologically increased INaL is caused directly by mutation. Overall, we identify a model for VGSC inhibition that produces effects consistent with the inhibitory action of riluzole observed in models of ALS. Our findings will aid future drug design and supports research directed towards riluzole repurposing.

Exploring the interplay between extracellular pH and Dronedarone's pharmacological effects on cardiac function

Dronedarone (DRN) is a clinically used drug to mitigate arrhythmias with multichannel block properties, including the sodium channel Nav1.5. Extracellular acidification is known to change the pharmacological properties of several antiarrhythmic drugs. Here, we explore how modification in extracellular pH (pHe) shapes the pharmacological profile of DRN upon Nav1.5 sodium current (INa) and in the ex vivo heart preparation. Embryonic human kidney cells (HEK293T/17) were used to transiently express the human isoform of Nav1.5 α-subunit. Patch-Clamp technique was employed to study INa. Neurotoxin-II (ATX-II) was used to induce the late sodium current (INaLate). Additionally, ex vivo Wistar male rat preparations in the Langendorff system were utilized to study electrocardiogram (ECG) waves. DRN preferentially binds to the closed state inactivation mode of Nav1.5 at pHe 7.0. The recovery from INa inactivation was delayed in the presence of DRN in both pHe 7.0 and 7.4, and the use-dependent properties were distinct at pHe 7.0 and 7.4. However, the potency of DRN upon the peak INa, the voltage dependence for activation, and the steady-state inactivation curves were not altered in both pHe tested. Also, the pHe did not change the ability of DRN to block INaLate. Lastly, DRN in a concentration and pH dependent manner modulated the QRS complex, QT and RR interval in clinically relevant concentration. Thus, the pharmacological properties of DRN upon Nav1.5 and ex vivo heart preparation partially depend on the pHe. The pHe changed the biological effect of DRN in the heart electrical function in relevant clinical concentration.

Ranolazine Unveiled: Rediscovering an Old Solution in a New Light.

Ranolazine is an anti-anginal medication that has demonstrated antiarrhythmic properties by inhibiting both late sodium and potassium currents. Studies have shown promising results for ranolazine in treating both atrial fibrillation and ventricular arrhythmias, particularly when used in combination with other medications. This review explores ranolazine's mechanisms of action and its potential role in cardiac arrhythmias treatment in light of previous clinical studies.

Characterization of a novel SCN5A mutation associated with long QT syndrome and arrhythmogenic right ventricular cardiomyopathy in a family.

Sudden cardiac death represents a significant diagnostic challenge for forensic pathologists, particularly in inherited arrhythmia syndromes or cardiomyopathies resulting from genetic defects. Molecular autopsies can reveal the underlying molecular etiology in such cases. In this study, we investigated a family with a history of sudden cardiac death to elucidate the molecular basis responsible for sudden cardiac death. The proband underwent a comprehensive forensic examination. Family members received thorough clinical evaluations, including electrocardiogram, Holter monitoring, echocardiography, and cardiac magnetic imaging. Whole exome sequencing and genetic analysis were performed on the deceased and her parents. In addition, Western blotting and patch-clamp recordings were employed to evaluate the expression and function of the mutant protein in vitro. Forensic examination diagnosed arrhythmogenic right ventricular cardiomyopathy (ARVC) as the cause of sudden death. Genetic analysis identified a novel missense mutation in SCN5A (p.V1323L), which was assessed as likely pathogenic by the ACMG guideline. Another family member carrying the mutation manifested long QT syndrome and mild cardiac fibrosis. The cellular electrophysiological study demonstrated that the mutation resulted in an enhanced late sodium current, suggesting it was a gain-of-function mutation. This study characterizes a novel SCN5A mutation that putatively causes long QT syndrome and may contribute to the development of ARVC. Our work expands the pathogenic spectrum of SCN5A variants and underscores the importance of molecular autopsy in sudden death cases, especially in those with suspected genetic disorders.

The inotropic and arrhythmogenic effects of acutely increased late INa are associated with elevated ROS but not oxidation of PKARIα.

Acute stimulation of the late sodium current (INaL) as pharmacologically induced by Anemonia toxin II (ATX-II) results in Na+-dependent Ca2+ overload and enhanced formation of reactive oxygen species (ROS). This is accompanied by an acute increase in the amplitude of the systolic Ca2+ transient. Ca2+ transient amplitude is determined by L-type Ca2+-mediated transsarcolemmal Ca2+ influx (ICa) into the cytosol and by systolic Ca2+ release from the sarcoplasmic reticulum (SR). Type-1 protein kinase A (PKARIα) becomes activated upon increased ROS and is capable of stimulating ICa, thereby sustaining the amplitude of the systolic Ca2+ transient upon oxidative stress. We aimed to investigate whether the increase of the systolic Ca2+ transient as acutely induced by INaL (by ATX-II) may involve stimulation of ICa through oxidized PKARIα. We used a transgenic mouse model in which PKARIα was made resistant to oxidative activation by homozygous knock-in replacement of redox-sensitive Cysteine 17 with Serine within the regulatory subunits of PKARIα (KI). ATX-II (at 1 nmol/L) was used to acutely enhance INaL in freshly isolated ventricular myocytes from KI and wild-type (WT) control mice. Epifluorescence and confocal imaging were used to assess intracellular Ca2+ handling and ROS formation. A ruptured-patch whole-cell voltage-clamp was used to measure INaL and ICa. The impact of acutely enhanced INaL on RIα dimer formation and PKA target structures was studied using Western blot analysis. ATX-II increased INaL to a similar extent in KI and WT cells, which was associated with significant cytosolic and mitochondrial ROS formation in both genotypes. Acutely activated Ca2+ handling in terms of increased Ca2+ transient amplitudes and elevated SR Ca2+ load was equally present in KI and WT cells. Likewise, cellular arrhythmias as approximated by non-triggered Ca2+ elevations during Ca2+ transient decay and by diastolic SR Ca2+-spark frequency occurred in a comparable manner in both genotypes. Most importantly and in contrast to our initial hypothesis, ATX-II did not alter the magnitude or inactivation kinetics of ICa in neither WT nor KI cells and did not result in PKARIα dimerization (i.e., oxidation) despite a clear prooxidant intracellular environment. The inotropic and arrhythmogenic effects of acutely increased INaL are associated with elevated ROS, but do not involve oxidation of PKARIα.

Quantitative 3D electron microscopy characterization of mitochondrial structure, mitophagy, and organelle interactions in murine atrial fibrillation

Atrial fibrillation (AF) is the most common clinical arrhythmia, however there is limited understanding of its pathophysiology including the cellular and ultrastructural changes rendered by the irregular rhythm, which limits pharmacological therapy development. Prior work has demonstrated the importance of reactive oxygen species (ROS) and mitochondrial dysfunction in the development of AF. Mitochondrial structure, interactions with other organelles such as sarcoplasmic reticulum (SR) and T-tubules (TT), and degradation of dysfunctional mitochondria via mitophagy are important processes to understand ultrastructural changes due to AF. However, most analysis of mitochondrial structure and interactome in AF has been limited to two-dimensional (2D) modalities such as transmission electron microscopy (EM), which does not fully visualize the morphological evolution of the mitochondria during mitophagy. Herein, we utilize focused ion beam-scanning electron microscopy (FIB-SEM) and perform reconstruction of three-dimensional (3D) EM from murine left atrial samples and measure the interactions of mitochondria with SR and TT. We developed a novel 3D quantitative analysis of FIB-SEM in a murine model of AF to quantify mitophagy stage, mitophagosome size in cardiomyocytes, and mitochondrial structural remodeling when compared with control mice. We show that in our murine model of spontaneous and continuous AF due to persistent late sodium current, left atrial cardiomyocytes have heterogenous mitochondria, with a significant number which are enlarged with increased elongation and structural complexity. Mitophagosomes in AF cardiomyocytes are located at Z-lines where they neighbor large, elongated mitochondria. Mitochondria in AF cardiomyocytes show increased organelle interaction, with 5X greater contact area with SR and are 4X as likely to interact with TT when compared to control. We show that mitophagy in AF cardiomyocytes involves 2.5X larger mitophagosomes that carry increased organelle contents. In conclusion, when oxidative stress overcomes compensatory mechanisms, mitophagy in AF faces a challenge of degrading bulky complex mitochondria, which may result in increased SR and TT contacts, perhaps allowing for mitochondrial Ca2+ maintenance and antioxidant production.

Optimized J to T peak and T peak to T end measurements in nonclinical species administered moxifloxacin and amiodarone

IntroductionCardiovascular safety and the risk of developing the potentially fatal ventricular tachyarrhythmia, Torsades de Pointes (TdP), have long been major concerns of drug development. TdP is associated with a delayed ventricular repolarization represented by QT interval prolongation in the electrocardiogram (ECG), typically due to block of the potassium channel encoded by the human ether-a-go-go related gene (hERG). Importantly however, not all drugs that prolong the QT interval are torsadagenic and not all hERG blockers prolong the QT interval. Recent clinical reports suggest that partitioning the QT interval into early (J to T peak; JTp) and late repolarization (T peak to T end; TpTe) components may be valuable for distinguishing low-risk mixed ion channel blockers (hERG plus calcium and/or late sodium currents) from high-risk pure hERG channel blockers. This strategy, if true for nonclinical animal models, could be used to de-risk QT prolonging compounds earlier in the drug development process. MethodsTo explore this, we investigated JTp and TpTe in ECG data collected from telemetered dogs and/or monkeys administered moxifloxacin or amiodarone at doses targeting relevant clinical exposures. An optimized placement of the Tpeak fiducial mark was utilized, and all intervals were corrected for heart rate (QTc, JTpc, TpTec). ResultsIncreases in QTc and JTpc intervals with administration of the pure hERG blocker moxifloxacin and an initial QTc and JTpc shortening followed by prolongation with the mixed ion channel blocker amiodarone were detected as expected, aligning with clinical data. However, anticipated increases in TpTec by both standard agents were not detected. DiscussionThe inability to detect changes in TpTec reduces the utility of these subintervals for prediction of arrhythmias using continuous single‑lead ECGs collected from freely moving dogs and monkeys.

Ibrutinib, a Bruton's tyrosine kinase inhibitor, regulates ventricular electromechanical activities and enhances arrhythmogenesis

BackgroundIbrutinib, a Bruton's tyrosine kinase inhibitor used in cancer therapy, exerts ventricular proarrhythmic effects; however, the underlying mechanisms remain unclear. Excitation-contraction coupling (E-C) disorders are pivotal for the genesis of ventricular arrhythmias (VAs), which arise mainly from the right ventricular outflow tract (RVOT). In this study, we aimed to comprehensively investigate whether ibrutinib regulates the electromechanical activities of the RVOT, leading to enhanced arrhythmogenesis, and explore the underlying mechanisms. MethodsWe utilized conventional microelectrodes to synchronously record electrical and mechanical responses in rabbit RVOT tissue preparations before and after treatment with ibrutinib (10, 50, and 100 nM) and investigated their electromechanical interactions and arrhythmogenesis during programmed electrical stimulation. The fluorometric ratio technique was used to measure intracellular calcium concentration in isolated RVOT myocytes. ResultsIbrutinib (10–100 nM) shortened the action potential duration. Ibrutinib at 100 nM significantly increased pacing-induced ventricular tachycardia (VT) (from 0% to 62.5%, n = 8, p = 0.025). Comparisons between pacing-induced VT and non-VT episodes demonstrated that VT episodes had a greater increase in contractility than that of non-VT episodes (402.1 ± 41.4% vs. 232.4 ± 29.2%, p = 0.003). The pretreatment of ranolazine (10 μM, a late sodium current blocker) prevented the occurrence of ibrutinib-induced VAs. Ibrutinib (100 nM) increased late sodium current, reduced intracellular calcium transients, and enhanced calcium leakage in RVOT myocytes. ConclusionIbrutinib increased the risk of VAs in the RVOT due to dysregulated electromechanical responses, which can be attenuated by ranolazine or apamin.

Effects of Acute Hypernatremia on the Electrophysiology of Single Human Ventricular Cardiomyocytes: An In Silico Study.

Clinical and experimental data on the cardiac effects of acute hypernatremia are scarce and inconsistent. We aimed to determine and understand the effects of different levels of acute hypernatremia on the human ventricular action potential. We performed computer simulations using two different, very comprehensive models of the electrical activity of a single human ventricular cardiomyocyte, i.e., the Tomek-Rodriguez model following the O'Hara-Rudy dynamic (ORd) model and the Bartolucci-Passini-Severi model as published in 2020 (known as the ToR-ORd and BPS2020 models, respectively). Mild to extreme levels of hypernatremia were introduced into each model based on experimental data on the effects of hypernatremia on cell volume and individual ion currents. In both models, we observed an increase in the intracellular sodium and potassium concentrations, an increase in the peak amplitude of the intracellular calcium concentration, a hyperpolarization of the resting membrane potential, a prolongation of the action potential, an increase in the maximum upstroke velocity, and an increase in the threshold stimulus current at all levels of hypernatremia and all stimulus rates tested. The magnitude of all of these effects was relatively small in the case of mild to severe hypernatremia but substantial in the case of extreme hypernatremia. The effects on the action potential were related to an increase in the sodium-potassium pump current, an increase in the sodium-calcium exchange current, a decrease in the rapid and slow delayed rectifier potassium currents, and an increase in the fast and late sodium currents. The effects of mild to severe hypernatremia on the electrical activity of human ventricular cardiomyocytes are relatively small. In the case of extreme hypernatremia, the effects are more pronounced, especially regarding the increase in threshold stimulus current.

1,4-Disubstituted Piperazin-2-Ones as Selective Late Sodium Current Inhibitors with QT Interval Shortening Properties in Isolated Rabbit Hearts.

Late sodium current (INa) inhibitors are a new subclass of antiarrhythmic agents. To overcome the drawbacks, e.g., low efficacy and inhibition effect on K+ current, of the FDA-approved late INa inhibitor ranolazine, chain amide 6a-6q, 1,4-disubstituted piperazin-2-ones 7a-7s, and their derivatives 8a-8n were successively designed, synthesized, and evaluated in vitro on the NaV1.5-transfected HEK293T cells by the whole-cell patch clamp recording assay at the concentration of 40 μM. Among the new skeleton compounds, 7d showed the highest efficacy (IC50 = 2.7 μM) and good selectivity (peak/late ratio >30 folds), as well as excellent pharmacokinetics properties in mice (T1/2 of 3.5 h, F = 90%, 3 mg/kg, po). It exhibited low hERG inhibition and was able to reverse the ATX-II-induced augmentation of late INa phenotype of LQT3 model in isolated rabbit hearts. These results suggest the application potentials of 7d in the treatments of arrhythmias related to the enhancement of late INa.

REAL WORLD USAGE OF RANOLAZINE FOR ANTIARRHYTHMIC PURPOSES: SINGLE CENTER EXPERIENCE

Abstract Introduction Ranolazine is a late sodium current blocking drug developed as an antianginal agent, with good preliminary efficacy data even for antiarrhythmic purposes (both at atrial and at ventricular level). Objectives To evaluate patients’ clinical characteristics and antiarrhythmic efficacy of ranolazine in subjects with a history of atrial and/or ventricular arrhythmias, isolated or repetitive. Patients and methods we included patients who received a first prescription of ranolazine for antiarrhythmic purposes in our center from 1/2022 to 11/2023 and evaluated the survival free from major arrhythmic relapses/ablation. Results 93 patients were enrolled (66±13 years, 15% women): 59% with arterial hypertension, 26% diabetics, 46% with post–ischemic dilated heart disease, 32% with primary cardiomyopathy (including 11% with hypertrophic cardiomyopathy, 10% with LMNA, 14% with TTN), 46% with implantable cardioverter defibrillator, 9% with pacemaker. The mean LV ejection fraction was 42 ± 15%, the mean serum creatinine 1 ± 0.3 mg/dL. Half (50%) had a history of atrial arrhythmias in the 6 months before starting the drug, mainly paroxysmal atrial fibrillation (AF), 67% of ventricular arrhythmias (VAs), mainly isolated and repetitive premature ventricular beats (PVB, 47% of the total), but also sustained or treated VAs (n=18, 19%). Overall, 18% of patients had already undergone AF ablation, 13% ventricular tachycardia ablation, 3% cardiac sympathetic denervation. Finally, 95% of patients were on beta–blocker therapy and 28% were on amiodarone. Ranolazine was prescribed at an average initial dose of 535± 150 mg. The majority (54%) of patients (n=50) had a follow–up longer than 3 months (median 9 months, IQR 6–14); in 6% the drug was stopped due to intolerance and the average dose at the last FU was 641 ± 150 mg. Twenty–eight percent of patients with paroxysmal/persistent atrial arrhythmias had relapses, requiring ablation in 6%. Among patients with major VAs, 57% suffered recurrences, but only 19% required ablation (invasive in 2 cases, non–invasive in 1 case). Finally, among the patients with PVBs, none suffered major VAs or required PVB/VA ablation. Conclusions This is the largest available case–series reporting about the real–world usage of ranolazine for antiarrhythmic purposes. Our single center experience supports ranolazine’s good safety and tolerability and shows encouraging efficacy results in terms of clinical stabilization of complex patients.

Extracellular acidification reveals the antiarrhythmic properties of amiodarone related to late sodium current-induced atrial arrhythmia.

Amiodarone (AMIO) is an antiarrhythmic drug with the pKa in the physiological range. Here, we explored how mild extracellular pH (pHe) changes shape the interaction of AMIO with atrial tissue and impact its pharmacological properties in the classical model of sea anemone sodium channel neurotoxin type 2 (ATX) induced late sodium current (INa-Late) and arrhythmias. Isolated atrial cardiomyocytes from male Wistar rats and human embryonic kidney cells expressing SCN5A Na+ channels were used for patch-clamp experiments. Isolated right atria (RA) and left atria (LA) tissue were used for bath organ experiments. A more acidophilic pHe caused negative inotropic effects on isolated RA and LA atrial tissue, without modification of the pharmacological properties of AMIO. A pHe of 7.0 changed the sodium current (INa) related components of the action potential (AP), which was enhanced in the presence of AMIO. ATXinduced arrhythmias in isolated RA and LA. Also, ATX prolonged the AP duration and enhanced repolarization dispersion in isolated cardiomyocytes in both pHe 7.4 and pHe 7.0. Pre-incubation of the isolated RA and LA and isolated atrial cardiomyocytes with AMIO prevented arrhythmias induced by ATX only at a pHe of 7.0. Moreover, AMIO was able to block INa-Late induced by ATX only at a pHe of 7.0. The pharmacological properties of AMIO concerning healthy rat atrial tissue are not dependent on pHe. However, the prevention of arrhythmias induced by INa-Late is pHe-dependent. The development of drugs analogous to AMIO with charge stabilization may help to create more effective drugs to treat arrhythmias related to the INa-Late.

Cardiac-Specific Deletion of Scn8a Mitigates Dravet Syndrome-Associated Sudden Death in Adults

BackgroundSudden unexpected death in epilepsy (SUDEP) is a fatal complication experienced by otherwise healthy epilepsy patients. Dravet syndrome (DS) is an inherited epileptic disorder resulting from loss of function of the voltage-gated sodium channel, NaV 1.1, and is associated with particularly high SUDEP risk. Evidence is mounting that NaVs abundant in the brain also occur in the heart, suggesting that the very molecular mechanisms underlying epilepsy could also precipitate cardiac arrhythmias and sudden death. Despite marked reduction of NaV 1.1 functional expression in DS, pathogenic late sodium current (INa,L) is paradoxically increased in DS hearts. However, the mechanisms by which DS directly impacts the heart to promote sudden death remain unclear. ObjectivesIn this study, the authors sought to provide evidence implicating remodeling of Na+ - and Ca2+ -handling machinery, including NaV 1.6 and Na+/Ca2+exchanger (NCX) within transverse (T)-tubules in DS-associated arrhythmias. MethodsThe authors undertook scanning ion conductance microscopy (SICM)-guided patch clamp, super-resolution microscopy, confocal Ca2+ imaging, and in vivo electrocardiography studies in Scn1a haploinsufficient murine model of DS. ResultsDS promotes INa,L in T-tubular nanodomains, but not in other subcellular regions. Consistent with increased NaV activity in these regions, super-resolution microscopy revealed increased NaV 1.6 density near Ca2+release channels, the ryanodine receptors (RyR2) and NCX in DS relative to WT hearts. The resulting INa,L in these regions promoted aberrant Ca2+ release, leading to ventricular arrhythmias in vivo. Cardiac-specific deletion of NaV 1.6 protects adult DS mice from increased T-tubular late NaV activity and the resulting arrhythmias, as well as sudden death. ConclusionsThese data demonstrate that NaV 1.6 undergoes remodeling within T-tubules of adult DS hearts serving as a substrate for Ca2+ -mediated cardiac arrhythmias and may be a druggable target for the prevention of SUDEP in adult DS subjects.

Identification of a novel Scn3b mutation in a Chinese Brugada syndrome pedigree: implications for Nav1.5 electrophysiological properties and intracellular distribution of Nav1.5 and Navβ3.

The Scn3b gene encodes for Navβ3, a pivotal regulatory subunit of the fast sodium channel in cardiomyocytes. However, its mutation status in the Chinese population suffering from Brugada Syndrome (BrS) has not been characterized, and the contributory pathophysiological mechanisms to disease pathology remain undefined. A Scn3b (c.260C>T, p.P87l) mutation was identified in a patient with BrS of Chinese descent. Functional analyses demonstrated that sodium channel activation for the wild type, mutant samples, and co-expression of both commenced at -55 mv and peaked at -25 mv. The mutant group exhibited a notable reduction, approximately 60%, in peak sodium channel activation current (INa) at -25 mv. The parameters for half-maximal activation voltages (V1/2) and slope factors (k) showed no significant differences when comparing wild type, mutant, and combined expression groups (P = 0.98 and P = 0.65, respectively). Additionally, no significant disparities were evident in terms of the steady-state sodium channel inactivation parameters V1/2 and k (with P-values of 0.85 and 0.25, respectively), nor were there significant differences in the activation time constant τ (P = 0.59) and late sodium current density (P = 0.23) across the wild-type, mutant, and co-expressed groups. Confocal imaging and Western blot analysis demonstrated decreased plasma membrane localization of SCN3B and SCN5A in the P87l group. Computational simulations of cardiac action potentials suggested that SCN3B P87l can alter the morphology of the action potentials within the endocardium and epicardium while reducing the peak of depolarization. The pathogenic impact of the Scn3b P87l mutation predominantly originates from a reduction in peak INa activation current coupled with decreased cell surface expression of Nav1.5 and Navβ3. These alterations may influence cardiac action potential configurations and contribute to the risk of ventricular arrhythmias in individuals with BrS.

Intracellular fibroblast growth factors regulate the arrhythmogenic late sodium current in heart failure

Intracellular fibroblast growth factors regulate the arrhythmogenic late sodium current in heart failure