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

Background

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.

Methods and Results

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.

Conclusions

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.

Higher Sodium Channel Excitability in Cardiac Purkinje Fibers: Implications for Multifocal Ectopic Purkinje-Related Premature Contractions

BACKGROUND

Multifocal ectopic Purkinje-related premature contractions (MEPPCs) are associated with SCN5A variants. However, it is not well understood why Purkinje fibers, but not ventricular myocardium, play a predominant role in arrhythmogenesis.

OBJECTIVES

This study sought to explore the underlying mechanisms of MEPPC.

METHODS

Whole-cell patch-clamp and molecular biology techniques were used in the present study.

RESULTS

Clinical data from one patient with R814W variant showed MEPPC syndrome, which is well responsive to amiodarone. Compared with canine ventricular myocytes, Purkinje cells (PCs) had significantly larger sodium current (INa), leftward shift of INa activation and inactivation curves, suggesting higher sodium channel excitability in PCs. Real-time polymerase chain reaction and Western blot analysis showed that the mRNA and protein expression of NaVβ1 and NaVβ3 was higher in canine Purkinje fibers than in ventricular myocardium. INa in heterologous Chinese hamster ovary cell expression system co-expressing NaV1.5 and NaVβ1/NaVβ3 exhibited similar biophysical properties of INa in PCs. R814W variant shifted INa activation in a hyperdepolarized direction, caused a larger window current, and generated an outward-gating pore current at depolarized voltages. Coexpression of NaVβ1/NaVβ3 with Nav1.5-R814W further left-shifted INa activation and caused an even larger window current and gating pore current, suggesting higher susceptibility of Purkinje fibers to R814W variant. Amiodarone inhibited INa, shifted its inactivation to more negative voltages, and significantly decreased the window current.

CONCLUSIONS

A higher expression of β1 and β3 subunits contributes to higher sodium channel excitability in cardiac Purkinje fibers, making them more susceptible to MEPPC.

Eugenol delays the onset of ouabain-induced ventricular cardiac arrhythmias in guinea pigs

Eugenol is an aromatic compound used in the manufacture of medicines, perfumes, cosmetics and as an anaesthetic due to the ability of the drug to block the neuronal isoform of voltage-gated Na+ channels (NaV ). Some arrhythmias are associated with gain of function in the sodium current (INa ) found in cardiomyocytes, and antiarrhythmic sodium channel blockers are commonly used in the clinical practice. This study sought to elucidate the potential mechanisms of eugenol's protection in the arrhythmic model of ouabain-induced arrhythmias in guinea pig heart. Ex vivo arrhythmias were induced using 50 μM of ouabain. The antiarrhythmic properties of eugenol were evaluated in the ex vivo heart preparation and isolated ventricular cardiomyocytes. The compound's effects on cardiac sodium current and action potential using the patch-clamp technique were evaluated. In all, eugenol decreased the ex vivo cardiac arrhythmias induced by ouabain. Furthermore, eugenol showed concentration dependent effect upon peak INa , left-shifted the stationary inactivation curve and delayed the recovery from inactivation of the INa . All these aspects are considered to be antiarrhythmic. Our findings demonstrate that eugenol has antiarrhythmic activity, which may be partially explained by the ability of eugenol to change de biophysical properties of INa of cardiomyocytes.

The Potential Mechanisms behind Loperamide-Induced Cardiac Arrhythmias Associated with Human Abuse and Extreme Overdose

Loperamide has been a safe and effective treatment for diarrhea for many years. However, many cases of cardiotoxicity with intentional abuse of loperamide ingestion have recently been reported. We evaluated loperamide in in vitro and in vivo cardiac safety models to understand the mechanisms for this cardiotoxicity. Loperamide slowed conduction (QRS-duration) starting at 0.3 µM [~1200-fold (×) its human Free Therapeutic Plasma Concentration; FTPC] and reduced the QT-interval and caused cardiac arrhythmias starting at 3 µM (~12,000× FTPC) in an isolated rabbit ventricular-wedge model. Loperamide also slowed conduction and elicited Type II/III A-V block in anesthetized guinea pigs at overdose exposures of 879× and 3802× FTPC. In ion-channel studies, loperamide inhibited hERG (IKr), INa, and ICa currents with IC50 values of 0.390 µM, 0.526 µM, and 4.091 µM, respectively (i.e., >1560× FTPC). Additionally, in silico trials in human ventricular action potential models based on these IC50s confirmed that loperamide has large safety margins at therapeutic exposures (≤600× FTPC) and confirmed repolarization abnormalities in the case of extreme doses of loperamide. The studies confirmed the large safety margin for the therapeutic use of loperamide but revealed that at the extreme exposure levels observed in human overdose, loperamide can cause a combination of conduction slowing and alterations in repolarization time, resulting in cardiac proarrhythmia. Loperamide's inhibition of the INa channel and hERG-mediated IKr are the most likely basis for this cardiac electrophysiological toxicity at overdose exposures. The cardiac toxic effects of loperamide at the overdoses could be aggravated by co-medication with other drug(s) causing ion channel inhibition.

Blockade of Melatonin Receptors Abolishes Its Antiarrhythmic Effect and Slows Ventricular Conduction in Rat Hearts

Melatonin has been reported to cause myocardial electrophysiological changes and prevent ventricular tachycardia or fibrillation (VT/VF) in ischemia and reperfusion. We sought to identify electrophysiological targets responsible for the melatonin antiarrhythmic action and to explore whether melatonin receptor-dependent pathways or its antioxidative properties are essential for these effects. Ischemia was induced in anesthetized rats given a placebo, melatonin, and/or luzindole (MT1/MT2 melatonin receptor blocker), and epicardial mapping with reperfusion VT/VFs assessment was performed. The oxidative stress assessment and Western blotting analysis were performed in the explanted hearts. Transmembrane potentials and ionic currents were recorded in cardiomyocytes with melatonin and/or luzindole application. Melatonin reduced reperfusion VT/VF incidence associated with local activation time in logistic regression analysis. Melatonin prevented ischemia-related conduction slowing and did not change the total connexin43 (Cx43) level or oxidative stress markers, but it increased the content of a phosphorylated Cx43 variant (P-Cx43368). Luzindole abolished the melatonin antiarrhythmic effect, slowed conduction, decreased total Cx43, protein kinase Cε and P-Cx43368 levels, and the IK1 current, and caused resting membrane potential (RMP) depolarization. Neither melatonin nor luzindole modified INa current. Thus, the antiarrhythmic effect of melatonin was mediated by the receptor-dependent enhancement of impulse conduction, which was associated with Cx43 phosphorylation and maintaining the RMP level.

Silvio Weidmann: laying the foundations for unravelling the mechanism of heart rhythm

Silvio Weidmann laid the basis of cardiac electrophysiology and was the forerunner in the search for mechanisms governing the electrical activity of the heart in his legendary first studies of Purkinje fibres in the 1950s. His work was the cornerstone of research in this field for many generations, and countless cardiologists and electrophysiologists have based their studies on the knowledge generated by Weidmann's pioneering data. This review summarizes his key contributions from the first intracellular recordings of cardiac membrane potentials in 1949 to the publication of his monograph in 1956. That summary is followed by an imagined dialogue between the authors and Silvio Weidmann himself, in the format of a one-act play. Both of us have such good recollections of our real-life conversations with Silvio Weidmann that we decided we could achieve a better feel for the history and issues by using a dialogue format. We hope that, in that way, we may transmit the character of Silvio Weidmann better for those readers who will not have known him personally. Silvio Weidmann was an extraordinarily sensitive and conversational person as well as a great scientist, and we feel it is worth the effort to convey that fact here. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Functional Characterisation of the Rare SCN5A p.E1225K Variant, Segregating in a Brugada Syndrome Familial Case, in Human Cardiomyocytes from Pluripotent Stem Cells

Brugada syndrome (BrS) is an inherited autosomal dominant cardiac channelopathy. Pathogenic rare mutations in the SCN5A gene, encoding the alpha-subunit of the voltage-dependent cardiac Na⁺ channel protein (Nav1.5), are identified in 20% of BrS patients, affecting the correct function of the channel. To date, even though hundreds of SCN5A variants have been associated with BrS, the underlying pathogenic mechanisms are still unclear in most cases. Therefore, the functional characterization of the SCN5A BrS rare variants still represents a major hurdle and is fundamental to confirming their pathogenic effect. Human cardiomyocytes (CMs) differentiated from pluripotent stem cells (PSCs) have been extensively demonstrated to be reliable platforms for investigating cardiac diseases, being able to recapitulate specific traits of disease, including arrhythmic events and conduction abnormalities. Based on this, in this study, we performed a functional analysis of the BrS familial rare variant NM_198056.2:c.3673G>A (NP_932173.1:p.Glu1225Lys), which has been never functionally characterized before in a cardiac-relevant context, as the human cardiomyocyte. Using a specific lentiviral vector encoding a GFP-tagged SCN5A gene carrying the specific c.3673G>A variant and CMs differentiated from control PSCs (PSC-CMs), we demonstrated an impairment of the mutated Nav1.5, thus suggesting the pathogenicity of the rare BrS detected variant. More broadly, our work supports the application of PSC-CMs for the assessment of the pathogenicity of gene variants, the identification of which is increasing exponentially due to the advances in next-generation sequencing methods and their massive use in genetic testing.

The Anti-Epileptic Drugs Lamotrigine and Valproic Acid Reduce the Cardiac Sodium Current

Anti-epileptic drugs (AEDs) are associated with increased risk of sudden cardiac death. To establish whether gabapentin, lamotrigine, levetiracetam, pregabalin, and valproic acid reduce the Na_v1.5 current, we conducted whole-cell patch-clamp studies to study the effects of the five AEDs on currents of human cardiac Na_v1.5 channels stably expressed in HEK293 cells, and on action potential (AP) properties of freshly isolated rabbit ventricular cardiomyocytes. Lamotrigine and valproic acid exhibited inhibitory effects on the Na_v1.5 current in a concentration-dependent manner with an IC₅₀ of 142 ± 36 and 2022 ± 25 µM for lamotrigine and valproic acid, respectively. In addition, these drugs caused a hyperpolarizing shift of steady-state inactivation and a delay in recovery from inactivation. The changes on the Na_v1.5 properties were reflected by a reduction in AP upstroke velocity (43.0 ± 6.8% (lamotrigine) and 23.7 ± 10.6% (valproic acid) at 1 Hz) and AP amplitude; in contrast, AP duration was not changed. Gabapentin, levetiracetam, and pregabalin had no effect on the Na_v1.5 current. Lamotrigine and valproic acid reduce the Na_v1.5 current density and affect its gating properties, resulting in a decrease of the AP upstroke velocity. Gabapentin, levetiracetam, and pregabalin have no effects on the Na_v1.5 current.

The Antidepressant Paroxetine Reduces the Cardiac Sodium Current

A considerable amount of literature has been published on antidepressants and cardiac ion channel dysfunction. The antidepressant paroxetine has been associated with Brugada syndrome and long QT syndrome, albeit on the basis of conflicting findings. The cardiac voltage-gated sodium channel (Na_V1.5) is related to both of these syndromes, suggesting that paroxetine may have an effect on this channel. In the present study, we therefore carried out patch clamp experiments to examine the effect of paroxetine on human Na_V1.5 channels stably expressed in human embryonic kidney 293 (HEK-293) cells as well as on action potentials of isolated rabbit left ventricular cardiomyocytes. Additionally, computer simulations were conducted to test the functional effects of the experimentally observed paroxetine-induced changes in the Na_V1.5 current. We found that paroxetine led to a decrease in peak Na_V1.5 current in a concentration-dependent manner with an IC₅₀ of 6.8 ± 1.1 µM. In addition, paroxetine caused a significant hyperpolarizing shift in the steady-state inactivation of the Na_V1.5 current as well as a significant increase in its rate of inactivation. Paroxetine (3 µM) affected the action potential of the left ventricular cardiomyocytes, significantly decreasing its maximum upstroke velocity and amplitude, both of which are mainly regulated by the Na_V1.5 current. Our computer simulations demonstrated that paroxetine substantially reduces the fast sodium current of human left ventricular cardiomyocytes, thereby slowing conduction and reducing excitability in strands of cells, in particular if conduction and excitability are already inhibited by a loss-of-function mutation in the Na_V1.5 encoding SCN5A gene. In conclusion, paroxetine acts as an inhibitor of Na_V1.5 channels, which may enhance the effects of loss-of-function mutations in SCN5A.

Empagliflozin and Dapagliflozin Increase Na+ and Inward Rectifier K+ Current Densities in Human Cardiomyocytes Derived from Induced Pluripotent Stem Cells (hiPSC-CMs)

Dapagliflozin (dapa) and empagliflozin (empa) are sodium-glucose cotransporter-2 inhibitors (SGLT2is) that reduce morbidity and mortality in heart failure (HF) patients. Sodium and inward rectifier K+ currents (INa and IK1), carried by Nav1.5 and Kir2.1 channels, respectively, are responsible for cardiac excitability, conduction velocity, and refractoriness. In HF patients, Nav1.5 and Kir2.1 expression are reduced, enhancing risk of arrhythmia. Incubation with dapa or empa (24-h,1 µM) significantly increased INa and IK1 densities recorded in human-induced pluripotent stem cell-cardiomyocytes (hiPSC-CMs) using patch-clamp techniques. Dapa and empa, respectively, shifted to more hyperpolarized potentials the INa activation and inactivation curves. Identical effects were observed in Chinese hamster ovary (CHO) cells that were incubated with dapa or empa and transiently expressed human Nav1.5 channels. Conversely, empa but not dapa significantly increased human Kir2.1 currents in CHO cells. Dapa and empa effects on INa and IK1 were also apparent in Ca-calmodulin kinase II-silenced CHO cells. Cariporide, a Na+/H+ exchanger type 1 (NHE1) inhibitor, did not increase INa or IK1 in hiPSC-CMs. Dapa and empa at therapeutic concentrations increased INa and IK1 in healthy human cardiomyocytes. These SGLT2is could represent a new class of drugs with a novel and long-pursued antiarrhythmic mechanism of action.

Melatonin treatment improves ventricular conduction via upregulation of Nav1.5 channel proteins and sodium current in the normal rat heart

Melatonin treatment was reported to reduce the risk of cardiac arrhythmias, and crucial for this antiarrhythmic action was the effect of melatonin on activation spread. The aim of the present study was evaluation of the mechanisms of this activation enhancement. Experiments were performed in a total of 123 control and melatonin-treated (10 mg/kg, daily, for 7 days) male Wistar rats. In epicardial mapping studies (64 leads, interlead distance 0.5 mm) in the anesthetized animals, activation times (ATs) were determined in each lead as dV/dt minimum during QRS complex under sinus rhythm. Epicardial pacing was performed to measure conduction velocity (CV) across the mapped area. Average left ventricular ATs were shorter in the treated animals as compared to the controls, whereas the minimal epicardial ATs indicating the duration of activation propagation via the ventricular conduction system did not differ between the groups. CV was higher in the treated groups indicating that melatonin affected conduction via contractile myocardium The area of Cx43-derived fluorescence, as well as the expression of Cx43 protein, was similar in ventricles in the control and melatonin-treated groups. Expression of Gja1 gene transcripts encoding Cx43, was increased in the last group. An uncoupling agent octanol modified myocardial conduction properties (time of activation, action potential upstroke velocity, passive electrotonic phase duration) similarly in both groups. On the other hand, the expression of both Scn5a gene transcripts encoding Nav1.5 proteins, as well as peak density of transmembrane sodium current were increased in the ventricular myocytes from the melatonin-treated animals. Thus, a week-long melatonin treatment caused the increase of conduction velocity via enhancement of sodium channel proteins expression and increase of sodium current in the ventricular myocytes.

Knockout of interleukin-17A diminishes ventricular arrhythmia susceptibility in diabetic mice via inhibiting NF-κB-mediated electrical remodeling

Interleukin-17A (IL-17), a potent proinflammatory cytokine, has been shown to participate in cardiac electrical disorders. Diabetes mellitus is an independent risk factor for ventricular arrhythmia. In this study, we investigated the role of IL-17 in ventricular arrhythmia of diabetic mice. Diabetes was induced in both wild-type and IL-17 knockout mice by intraperitoneal injection of streptozotocin (STZ). High-frequency electrical stimuli were delivered into the right ventricle to induce ventricular arrhythmias. We showed that the occurrence rate of ventricular tachycardia was significantly increased in diabetic mice, which was attenuated by IL-17 knockout. We conducted optical mapping on perfused mouse hearts and found that cardiac conduction velocity (CV) was significantly decreased, and action potential duration (APD) was prolonged in diabetic mice, which were mitigated by IL-17 knockout. We performed whole-cell patch clamp recordings from isolated ventricular myocytes, and found that the densities of I_to, I_Na and I_Ca,L were reduced, the APDs at 50% and 90% repolarization were increased, and early afterdepolarization (EAD) was markedly increased in diabetic mice. These alterations were alleviated by the knockout of IL-17. Moreover, knockout of IL-17 alleviated the downregulation of Nav1.5 (the pore forming subunit of I_Na), Cav1.2 (the main component subunit of I_Ca,L) and KChIP2 (potassium voltage-gated channel interacting protein 2, the regulatory subunit of I_to) in the hearts of diabetic mice. The expression of NF-κB was significantly upregulated in the hearts of diabetic mice, which was suppressed by IL-17 knockout. In neonatal mouse ventricular myocytes, knockdown of NF-κB significantly increased the expression of Nav1.5, Cav1.2 and KChIP2. These results imply that IL-17 may represent a potential target for the development of agents against diabetes-related ventricular arrhythmias.

Zfhx3 Transcription Factor Represses the Expression of SCN5A Gene and Decreases Sodium Current Density (INa)

The ZFHX3 and SCN5A genes encode the zinc finger homeobox 3 (Zfhx3) transcription factor (TF) and the human cardiac Na⁺ channel (Nav1.5), respectively. The effects of Zfhx3 on the expression of the Nav1.5 channel, and in cardiac excitability, are currently unknown. Additionally, we identified three Zfhx3 variants in probands diagnosed with familial atrial fibrillation (p.M1260T) and Brugada Syndrome (p.V949I and p.Q2564R). Here, we analyzed the effects of native (WT) and mutated Zfhx3 on Na⁺ current (I_Na) recorded in HL-1 cardiomyocytes. ZFHX3 mRNA can be detected in human atrial and ventricular samples. In HL-1 cardiomyocytes, transfection of Zfhx3 strongly reduced peak I_Na density, while the silencing of endogenous expression augmented it (from −65.9 ± 8.9 to −104.6 ± 10.8 pA/pF; n ≥ 8, p < 0.05). Zfhx3 significantly reduced the transcriptional activity of human SCN5A, PITX2, TBX5, and NKX25 minimal promoters. Consequently, the mRNA and/or protein expression levels of Nav1.5 and Tbx5 were diminished (n ≥ 6, p < 0.05). Zfhx3 also increased the expression of Nedd4-2 ubiquitin-protein ligase, enhancing Nav1.5 proteasomal degradation. p.V949I, p.M1260T, and p.Q2564R Zfhx3 produced similar effects on I_Na density and time- and voltage-dependent properties in WT. WT Zfhx3 inhibits I_Na as a result of a direct repressor effect on the SCN5A promoter, the modulation of Tbx5 increasing on the I_Na, and the increased expression of Nedd4-2. We propose that this TF participates in the control of cardiac excitability in human adult cardiac tissue.

Neural excitability increases with axonal resistance between soma and axon initial segment

The position of the axon initial segment (AIS) is thought to play a critical role in neuronal excitability. Previous experimental studies have found that a distal shift in AIS position correlates with a reduction in excitability. Yet theoretical work has suggested the opposite, because of increased electrical isolation. A distal shift in AIS position corresponds to an elevation of axial resistance R _a We therefore examined how changes in R _a at the axon hillock impact the voltage threshold (V_th) of the somatic action potential in L5 pyramidal neurons. Increasing R _a by mechanically pinching the axon between the soma and the AIS was found to lower V_th by ∼6 mV. Conversely, decreasing R _a by substituting internal ions with higher mobility elevated V_th All R _a -dependent changes in V_th could be reproduced in a Hodgkin-Huxley compartmental model. We conclude that in L5 pyramidal neurons, excitability increases with axial resistance and therefore with a distal shift of the AIS.

Overlap Arrhythmia Syndromes Resulting from Multiple Genetic Variations Studied in Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) are used for genetic models of cardiac diseases. We report an arrhythmia syndrome consisting of Early Repolarization Syndrome (ERS) and Short QT Syndrome (SQTS). The index patient (MMRL1215) developed arrhythmia-mediated syncope after electrocution and was found to carry six mutations. Functional alterations resulting from these mutations were examined in patient-derived hiPSC-CMs. Electrophysiological recordings were made in hiPSC-CMs from MMRL1215 and healthy controls. ECG analysis of the index patient showed slurring of the QRS complex and QTc = 326 ms. Action potential (AP) recordings from MMRL1215 myocytes showed slower spontaneous activity and AP duration was shorter. Field potential recordings from MMRL1215 hiPSC-CMs lack a "pseudo" QRS complex suggesting reduced inward current(s). Voltage clamp analysis of ICa showed no difference in the magnitude of current. Measurements of INa reveal a 60% reduction in INa density in MMRL1215 hiPSC-CMs. Steady inactivation and recovery of INa was unaffected. mRNA analysis revealed ANK2 and SCN5A are significantly reduced in hiPSC-CM derived from MMRL1215, consistent with electrophysiological recordings. The polygenic cause of ERS/SQTS phenotype is likely due to a loss of INa due to a mutation in PKP2 coupled with and a gain of function in IK,ATP due to a mutation in ABCC9.

Methylene Blue Application to Lessen Pain: Its Analgesic Effect and Mechanism

Methylene blue (MB) is a cationic thiazine dye, widely used as a biological stain and chemical indicator. Growing evidence have revealed that MB functions to restore abnormal vasodilation and notably it is implicated even in pain relief. Physicians began to inject MB into degenerated disks to relieve pain in patients with chronic discogenic low back pain (CDLBP), and some of them achieved remarkable outcomes. For osteoarthritis and colitis, MB abates inflammation by suppressing nitric oxide production, and ultimately relieves pain. However, despite this clinical efficacy, MB has not attracted much public attention in terms of pain relief. Accordingly, this review focuses on how MB lessens pain, noting three major actions of this dye: anti-inflammation, sodium current reduction, and denervation. Moreover, we showed controversies over the efficacy of MB on CDLBP and raised also toxicity issues to look into the limitation of MB application. This analysis is the first attempt to illustrate its analgesic effects, which may offer a novel insight into MB as a pain-relief dye.

Late sodium current and calcium homeostasis in arrhythmogenesis

The cardiac late sodium current (I_Na,late) is the small sustained component of the sodium current active during the plateau phase of the action potential. Several studies demonstrated that augmentation of the current can lead to cardiac arrhythmias; therefore, I_Na,late is considered as a promising antiarrhythmic target. Fundamentally, enlarged I_Na,late increases Na⁺ influx into the cell, which, in turn, is converted to elevated intracellular Ca²⁺ concentration through the Na⁺/Ca²⁺ exchanger. The excessive Ca²⁺ load is known to be proarrhythmic. This review describes the behavior of the voltage-gated Na⁺ channels generating I_Na,late in health and disease and aims to discuss the physiology and pathophysiology of Na⁺ and Ca²⁺ homeostasis in context with the enhanced I_Na,late demonstrating also the currently accessible antiarrhythmic choices.

Mechanistic Insights Into the Reduced Pacemaking Rate of the Rabbit Sinoatrial Node During Postnatal Development: A Simulation Study

Marked age- and development- related differences have been observed in morphology and characteristics of action potentials (AP) of neonatal and adult sinoatrial node (SAN) cells. These may be attributable to a different set of ion channel interactions between the different ages. However, the underlying mechanism(s) have yet to be elucidated. The objective of this study was to determine the mechanisms underlying different spontaneous APs and heart rate between neonatal and adult SAN cells of the rabbit heart by biophysical modeling approaches. A mathematical model of neonatal rabbit SAN cells was developed by modifying the current densities and/or kinetics of ion channels and transporters in an adult cell model based on available experimental data obtained from neonatal SAN cells. The single cell models were then incorporated into a multi-cellular, two-dimensional model of the intact SAN-atrium to investigate the functional impact of altered ion channels during maturation on pacemaking electrical activities and their conduction at the tissue level. Effects of the neurotransmitter acetylcholine on the pacemaking activities in neonatal cells were also investigated and compared to those in the adult. Our results showed: (1) the differences in ion channel properties between neonatal and adult SAN cells are able to account for differences in their APs and the heart rate, providing mechanistic insight into understanding the reduced pacemaking rate of the rabbit sinoatrial node during postnatal development; (2) in the 2D model of the intact SAN-atria, it was shown that cellular changes during postnatal development impaired pacemaking activity through increasing the activation time and reducing the conduction velocity across the SAN; (3) the neonatal SAN model, with its faster beating rates, showed a greater sensitivity to parasympathetic modulation in response to acetylcholine than did the adult model. These results provide novel insights into the understanding of the cellular mechanisms underlying the differences in the cardiac pacemaking activities of the neonatal and adult SAN.

Relationship between sodium channel function and clinical phenotype in SCN5A variants associated with Brugada syndrome

The identification of a pathogenic SCN5A variant confers an increased risk of conduction defects and ventricular arrhythmias (VA) in Brugada syndrome (BrS). However, specific aspects of sodium channel function that influence clinical phenotype have not been defined. A systematic literature search identified SCN5A variants associated with BrS. Sodium current (I_Na) functional parameters (peak current, decay, steady‐state activation and inactivation, and recovery from inactivation) and clinical features (conduction abnormalities [CA], spontaneous VA or family history of sudden cardiac death [SCD], and spontaneous BrS electrocardiogram [ECG]) were extracted. A total of 561 SCN5A variants associated with BrS were identified, for which data on channel function and clinical phenotype were available in 142. In the primary analysis, no relationship was found between any aspect of channel function and CA, VA/SCD, or spontaneous BrS ECG pattern. Sensitivity analyses including only variants graded pathogenic or likely pathogenic suggested that reduction in peak current and positive shift in steady‐state activation were weakly associated with CA and VA/SCD, although sensitivity and specificity remained low. The relationship between in vitro assessment of channel function and BrS clinical phenotype is weak. The assessment of channel function does not enhance risk stratification. Caution is needed when extrapolating functional testing to the likelihood of variant pathogenicity.

Modeling the Interactions Between Sodium Channels Provides Insight Into the Negative Dominance of Certain Channel Mutations

BACKGROUND

Nav1.5 cardiac Na+ channel mutations can cause arrhythmogenic syndromes. Some of these mutations exert a dominant negative effect on wild-type channels. Recent studies showed that Na+ channels can dimerize, allowing coupled gating. This leads to the hypothesis that allosteric interactions between Na+ channels modulate their function and that these interactions may contribute to the negative dominance of certain mutations.

METHODS

To investigate how allosteric interactions affect microscopic and macroscopic channel function, we developed a modeling paradigm in which Markovian models of two channels are combined. Allosteric interactions are incorporated by modifying the free energies of the composite states and/or barriers between states.

RESULTS

Simulations using two generic 2-state models (C-O, closed-open) revealed that increasing the free energy of the composite states CO/OC leads to coupled gating. Simulations using two 3-state models (closed-open-inactivated) revealed that coupled closings must also involve interactions between further composite states. Using two 6-state cardiac Na+ channel models, we replicated previous experimental results mainly by increasing the energies of the CO/OC states and lowering the energy barriers between the CO/OC and the CO/OO states. The channel model was then modified to simulate a negative dominant mutation (Nav1.5 p.L325R). Simulations of homodimers and heterodimers in the presence and absence of interactions showed that the interactions with the variant channel impair the opening of the wild-type channel and thus contribute to negative dominance.

CONCLUSION

Our new modeling framework recapitulates qualitatively previous experimental observations and helps identifying possible interaction mechanisms between ion channels.

The sodium channel NaV 1.5 impacts on early murine embryonic cardiac development, structure and function in a non-electrogenic manner

AIM

The voltage-gated sodium channel Na_V 1.5, encoded by SCN5A, is essential for cardiac excitability and ensures proper electrical conduction. Early embryonic death has been observed in several murine models carrying homozygous Scn5amutations. We investigated when sodium current (I_Na ) becomes functionally relevant in the murine embryonic heart and how Scn5a/Na_V 1.5 dysfunction impacts on cardiac development.

METHODS

Involvement of Na_V 1.5-generated I_Na in murine cardiac electrical function was assessed by optical mapping in wild type (WT) embryos (embryonic day (E)9.5 and E10.5) in the absence and presence of the sodium channel blocker tetrodotoxin (30 µmol/L). I_Na was assessed by patch-clamp analysis in cardiomyocytes isolated from WT embryos (E9.5-17.5). In addition, cardiac morphology and electrical function was assessed in Scn5a-1798insD^-/- embryos (E9.5-10.5) and their WT littermates.

RESULTS

In WT embryos, tetrodotoxin did not affect cardiac activation at E9.5, but slowed activation at E10.5. Accordingly, patch-clamp measurements revealed that I_Na was virtually absent at E9.5 but robustly present at E10.5. Scn5a-1798insD^-/- embryos died in utero around E10.5, displaying severely affected cardiac activation and morphology. Strikingly, altered ventricular activation was observed in Scn5a-1798insD^-/- E9.5 embryos before the onset of I_Na , in addition to reduced cardiac tissue volume compared to WT littermates.

CONCLUSION

We here demonstrate that Na_V 1.5 is involved in cardiac electrical function from E10.5 onwards. Scn5a-1798insD^-/- embryos displayed cardiac structural abnormalities at E9.5, indicating that Na_V 1.5 dysfunction impacts on embryonic cardiac development in a non-electrogenic manner. These findings are potentially relevant for understanding structural defects observed in relation to Na_V 1.5 dysfunction.

Rhythm dynamics of the aging heart: an experimental study using conscious, restrained mice

Heart rate variability (HRV) is a measure of variation in time interval between heartbeats and reflects the influence of autonomic nervous system and circulating/locally released factors on sinoatrial node discharge. Here, we tested whether electrocardiograms (ECGs) obtained in conscious, restrained mice, a condition that affects sympathovagal balance, reveal alterations of heart rhythm dynamics with aging. Moreover, based on emergence of sodium channels as modulators of pacemaker activity, we addressed consequences of altered sodium channels on heart rhythm. C57Bl/6 mice and mice with enhanced late sodium current due to Nav1.5 mutation at Ser571 (S571E) at ~4 to ~24 mo of age, were studied. HRV was assessed using time- and frequency-domain and nonlinear parameters. For C57Bl/6 and S571E mice, standard deviation of RR intervals (SDRR), total power of RR interval variation, and nonlinear standard deviation 2 (SD2) were maximal at ~4 mo and decreased at ~18 and ~24 mo, together with attenuation of indexes of sympathovagal balance. Modulation of sympathetic and/or parasympathetic divisions revealed attenuation of autonomic tone at ~24 mo. At ~4 mo, S571E mice presented lower heart rate and higher SDRR, total power, and SD2 with respect to C57Bl/6, properties reversed by late sodium current inhibition. At ~24 mo, heart rate decreased in C57Bl/6 but increased in S571E, a condition preserved after autonomic blockade. Collectively, our data indicate that aging is associated with reduced HRV. Moreover, sodium channel function conditions heart rate and its age-related adaptations, but does not interfere with HRV decline occurring with age.NEW & NOTEWORTHY We have investigated age-associated alterations of heart rate properties in mice using conscious electrocardiographic recordings. Our findings support the notion that aging is coupled with altered sympathovagal balance with consequences on heart rate variability. Moreover, by using a genetically engineered mouse line, we provide evidence that sodium channels modulate heart rate and its age-related adaptations.

Cardiac protection of Bauhinia championii against reperfusion injury

This study aims to investigate the protective effects of the Bauhinia championii (BC) against ischemia/reperfusion (I/R)-induced injury in an isolated heart model. Langendorff-perfused C57BL/6JNarl mice hearts were performed with 30 minutes ischemia and 60 minutes reperfusion by left anterior descending artery ligation. Before reperfusion, boiling water extracts of BC (10 mg/L) was pretreated for 15 minutes. During reperfusion, BC significantly decreased the occurrence of ventricular arrhythmias by lead II electrocardiogram (ECG). Electrophysiological effect of BC was further determined in isolated ventricular myocytes by whole-cell patch clamp technique. The underlying mechanism may result from its Na⁺ channel blocking activity characterized with reduced rise slope of action potential and Na⁺ current density. Moreover, BC dramatically reduced I/R-caused infarct size, which was accessed by 2,3,5-triphenyltetrazolium chloride (TTC) assay. Since BC decreased I/R-induced myoglobin release and oxidation of Ca²⁺ -calmodulin-dependent protein kinase, inhibition of myocardial necroptosis may account for the protective effects of BC on myocytes lose. This study indicated that BC may prevent I/R induced ventricular arrhythmias and myocyte death by blocking Na⁺ channels and decreasing necroptosis, respectively. Since most of the available antiarrhythmic remedies have unwanted adverse actions, BC could be a novel candidate for the treatment of myocardial infarction and ventricular arrhythmia.

Electrophysiological Alterations of Pyramidal Cells and Interneurons of the CA1 Region of the Hippocampus in a Novel Mouse Model of Dravet Syndrome

Dravet syndrome is a developmental epileptic encephalopathy caused by pathogenic variation in SCN1A To characterize the pathogenic substitution (p.H939R) of a local individual with Dravet syndrome, fibroblast cells from the individual were reprogrammed to pluripotent stem cells and differentiated into neurons. Sodium currents of these neurons were compared with healthy control induced neurons. A novel Scn1a ^H939R/+ mouse model was generated with the p.H939R substitution. Immunohistochemistry and electrophysiological experiments were performed on hippocampal slices of Scn1a ^H939R/+ mice. We found that the sodium currents recorded in the proband-induced neurons were significantly smaller and slower compared to wild type (WT). The resting membrane potential and spike amplitude were significantly depolarized in the proband-induced neurons. Similar differences in resting membrane potential and spike amplitude were observed in the interneurons of the hippocampus of Scn1a ^H939R/+ mice. The Scn1a ^H939R/+ mice showed the characteristic features of a Dravet-like phenotype: increased mortality and both spontaneous and heat-induced seizures. Immunohistochemistry showed a reduction in amount of parvalbumin and vesicular acetylcholine transporter in the hippocampus of Scn1a ^H939R/+ compared to WT mice. Overall, these results underline hyper-excitability of the hippocampal CA1 circuit of this novel mouse model of Dravet syndrome which, under certain conditions, such as temperature, can trigger seizure activity. This hyper-excitability is due to the altered electrophysiological properties of pyramidal neurons and interneurons which are caused by the dysfunction of the sodium channel bearing the p.H939R substitution. This novel Dravet syndrome model also highlights the reduction in acetylcholine and the contribution of pyramidal cells, in addition to interneurons, to network hyper-excitability.

Functional Consequences of the SCN5A-p.Y1977N Mutation within the PY Ubiquitylation Motif: Discrepancy between HEK293 Cells and Transgenic Mice

Dysfunction of the cardiac sodium channel Nav1.5 (encoded by the SCN5A gene) is associated with arrhythmias and sudden cardiac death. SCN5A mutations associated with long QT syndrome type 3 (LQT3) lead to enhanced late sodium current and consequent action potential (AP) prolongation. Internalization and degradation of Na_v1.5 is regulated by ubiquitylation, a post-translational mechanism that involves binding of the ubiquitin ligase Nedd4-2 to a proline-proline-serine-tyrosine sequence of Na_v1.5, designated the PY-motif. We investigated the biophysical properties of the LQT3-associated SCN5A-p.Y1977N mutation located in the Na_v1.5 PY-motif, both in HEK293 cells as well as in newly generated mice harboring the mouse homolog mutation Scn5a-p.Y1981N. We found that in HEK293 cells, the SCN5A-p.Y1977N mutation abolished the interaction between Na_v1.5 and Nedd4-2, suppressed PY-motif-dependent ubiquitylation of Na_v1.5, and consequently abrogated Nedd4-2 induced sodium current (I_Na) decrease. Nevertheless, homozygous mice harboring the Scn5a-p.Y1981N mutation showed no electrophysiological alterations nor changes in AP or (late) I_Na properties, questioning the in vivo relevance of the PY-motif. Our findings suggest the presence of compensatory mechanisms, with additional, as yet unknown, factors likely required to reduce the “ubiquitylation reserve” of Na_v1.5. Future identification of such modulatory factors may identify potential triggers for arrhythmias and sudden cardiac death in the setting of LQT3 mutations.

Mutations in NaV1.5 Reveal Calcium-Calmodulin Regulation of Sodium Channel

Mutations in the SCN5A gene, encoding the cardiac voltage-gated sodium channel Na_V1.5, are associated with inherited cardiac arrhythmia and conduction disease. Ca²⁺-dependent mechanisms and the involvement of β-subunit (Na_Vβ) in Na_V1.5 regulation are not fully understood. A patient with severe sinus-bradycardia and cardiac conduction-disease was genetically evaluated and compound heterozygosity in the SCN5A gene was found. Mutations were identified in the cytoplasmic DIII-IV linker (K1493del) and the C-terminus (A1924T) of Na_V1.5, both are putative CaM-binding domains. These mutants were functionally studied in human embryonic kidney (HEK) cells and HL-1 cells using whole-cell patch clamp technique. Calmodulin (CaM) interaction and cell-surface expression of heterologously expressed Na_V1.5 mutants were studied by pull-down and biotinylation assays. The mutation K1493del rendered Na_V1.5 non-conductive. Na_V1.5_K1493del altered the gating properties of co-expressed functional Na_V1.5, in a Ca²⁺ and Na_Vβ1-dependent manner. Na_V1.5_A1924T impaired Na_Vβ1-dependent gating regulation. Ca²⁺-dependent CaM-interaction with Na_V1.5 was blunted in Na_V1.5_K1493del. Electrical charge substitution at position 1493 did not affect CaM-interaction and channel functionality. Arrhythmia and conduction-disease -associated mutations revealed Ca²⁺-dependent gating regulation of Na_V1.5 channels. Our results highlight the role of Na_V1.5 DIII-IV linker in the CaM-binding complex and channel function, and suggest that the Ca²⁺-sensing machinery of Na_V1.5 involves Na_Vβ1.

Cellular Mechanisms of Sinus Node Dysfunction in Carriers of the SCN5A-E161K Mutation and Role of the H558R Polymorphism

Background: Carriers of the E161K mutation in the SCN5A gene, encoding the Na_V1.5 pore-forming α-subunit of the ion channel carrying the fast sodium current (I_Na), show sinus bradycardia and occasional exit block. Voltage clamp experiments in mammalian expression systems revealed a mutation-induced 2.5- to 4-fold reduction in I_Na peak current density as well as a +19 mV shift and reduced steepness of the steady-state activation curve. The highly common H558R polymorphism in Na_V1.5 limits this shift to +13 mV, but also introduces a -10 mV shift in steady-state inactivation. Aim: We assessed the cellular mechanism by which the E161K mutation causes sinus node dysfunction in heterozygous mutation carriers as well as the potential role of the H558R polymorphism. Methods: We incorporated the mutation-induced changes in I_Na into the Fabbri-Severi model of a single human sinoatrial node cell and the Maleckar et al. human atrial cell model, and carried out simulations under control conditions and over a wide range of acetylcholine levels. Results: In absence of the H558R polymorphism, the E161K mutation increased the basic cycle length of the sinoatrial node cell from 813 to 866 ms. In the simulated presence of 10 and 25 nM acetylcholine, basic cycle length increased from 1027 to 1131 and from 1448 to 1795 ms, respectively. The increase in cycle length was the result of a significant slowing of diastolic depolarization. The mutation-induced reduction in I_Na window current had reduced the contribution of the mutant component of I_Na to the net membrane current during diastolic depolarization to effectively zero. Highly similar results were obtained in presence of the H558R polymorphism. Atrial excitability was reduced, both in absence and presence of the H558R polymorphism, as reflected by an increase in threshold stimulus current and a concomitant decrease in capacitive current of the atrial cell. Conclusion: We conclude that the experimentally identified mutation-induced changes in I_Na can explain the clinically observed sinus bradycardia and potentially the occasional exit block. Furthermore, we conclude that the common H558R polymorphism does not significantly alter the effects of the E161K mutation and can thus not explain the reduced penetrance of the E161K mutation.

Irbesartan prevents sodium channel remodeling in a canine model of atrial fibrillation

Introduction:

Activation of the renin-angiotensin system (RAS) plays an important role in atrial electrical remodeling (AER). The purpose of the present study was to evaluate the effects of irbesartan on cardiac sodium current (I_Na) in a canine model of atrial fibrillation.

Materials and methods:

Eighteen dogs were randomized into sham, pacing or pacing+irbesartan groups (n = 6 in each group). The dogs in the pacing and irbesartan group were paced at 500 bpm for two weeks. Irbesartan (60 mg·kg⁻¹·d⁻¹) was administered orally in the pacing+irbesartan groups. I_Na was recorded using the whole-cell patch clamp technique from canine atrial myocytes. The expressions of cardiac Na⁺ channels (Nav1.5) mRNA were semi-quantified by reverse transcription-polymerase chain reaction.

Results:

Our results showed that I_Na density and Nav1.5 mRNA expression in the pacing group decreased significantly (p < 0.05 vs. sham). However, rapid atrial pacing had no effects on the half-activation voltage (V_1/2act) and half-inactivation voltage (V_1/2inact) of I_Na (p > 0.05 vs. sham). Irbesartan significantly increased I_Na densities and gene expression and hyperpolarized V_1/2act without concomitant changes in V_1/2inact.

Conclusions:

Irbesartan significantly increased I_Na densities, which contributed to improving intra-atrial conduction and prevented the induction and promotion of AF in atrial pacing dogs.

Stimulatory Action of Telmisartan, an Antagonist of Angiotensin II Receptor, on Voltage-Gated Na⁺ Current: Experimental and Theoretical Studies

Telmisartan (Tel) is recognized as a non-peptide blocker of AT1R. Whether this agent has any direct effects on ion currents remains unexplored. In whole-cell current recordings, addition of Tel increased the peak amplitude of voltage-gated Na⁺ (NaV) current (INa) accompanied by the increased time constant of INa inactivation in differentiated NSC-34 motor neuron-like cells. Tel-stimulated INa in these cells is unlinked to either blockade of AT1R or activation of peroxisome proliferator-activated receptor gamma (PPAR-γ). In order to explore how this compound affects the amplitude and kinetics of INa in neurons, a Hodgkin-Huxley-based (HH-based) model designed to mimic effect of Tel on the functional activities of neurons was computationally created in this study. In this framework, the parameter for h inactivation gating variable, which was changed in a stepwise fashion, was implemented to predict changes in membrane potentials (V) as a function of maximal Na⁺ (GNa), K⁺ conductance (GK), or both. As inactivation time course of INa was increased, the bifurcation point of V versus GNa became lower, and the range between subcritical and supercritical values at the bifurcation of V versus GK correspondingly became larger. During a slowing in INa inactivation, the critical boundary between GNa and GK was shifted towards the left. Simulation studies demonstrated that progressive slowing in the inactivation time course of INa resulted in unanticipated increase of neuronal excitability by mimicking the effect of Tel in neuronal cells. Collectively, Tel can directly interact with the NaV channel to increase peak INa as well as to slow INa inactivation. It is thus highly likely that the effects of Tel or its structurally similar drugs could be another intriguing mechanism underlying their pharmacological actions in neurons or neuroendocrine cells occurring in vivo.

Interventricular differences in sodium current and its potential role in Brugada syndrome

Brugada syndrome (BrS) is an inherited disease associated with ST elevation in the right precordial leads, polymorphic ventricular tachycardia (PVT), and sudden cardiac death in adults. Mutations in the cardiac sodium channel account for a large fraction of BrS cases. BrS manifests in the right ventricle (RV), which led us to examine the biophysical and molecular properties of sodium channel in myocytes isolated from the left (LV) and right ventricle. Patch clamp was used to record sodium current (INa ) in single canine RV and LV epicardial (epi) and endocardial (endo) myocytes. Action potentials were recorded from multicellular preparations and single cells. mRNA and proteins were determined using quantitative RT-PCR and Western blot. Although LV wedge preparations were thicker than RV wedges, transmural ECG recordings showed no difference in the width of the QRS complex or transmural conduction time. Action potential characteristics showed RV epi and endo had a lower Vmax compared with LV epi and endo cells. Peak INa density was significantly lower in epi and endo RV cells compared with epi and endo LV cells. Recovery from inactivation of INa in RV cells was slightly faster and half maximal steady-state inactivation was more positive. β2 and β4 mRNA was detected at very low levels in both ventricles, which was confirmed at the protein level. Our observations demonstrate that Vmax and Na+ current are smaller in RV, presumably due to differential Nav 1.5/β subunit expression. These results provide a potential mechanism for the right ventricular manifestation of BrS.

KV4.3 Expression Modulates NaV1.5 Sodium Current

In cardiomyocytes, the voltage-gated transient outward potassium current (I_to) is responsible for the phase-1 repolarization of the action potential (AP). Gain-of-function mutations in KCND3, the gene encoding the I_to carrying K_V4.3 channel, have been associated with Brugada syndrome (BrS). While the role of I_to in the pro-arrhythmic mechanism of BrS has been debated, recent studies have suggested that an increased I_to may directly affect cardiac conduction. However, the effects of an increased I_to on AP upstroke velocity or sodium current at the cellular level remain unknown. We here investigated the consequences of K_V4.3 overexpression on Na_V1.5 current and consequent sodium channel availability. We found that overexpression of K_V4.3 protein in HEK293 cells stably expressing Na_V1.5 (HEK293-Na_V1.5 cells) significantly reduced Na_V1.5 current density without affecting its kinetic properties. In addition, K_V4.3 overexpression decreased AP upstroke velocity in HEK293-Na_V1.5 cells, as measured with the alternating voltage/current clamp technique. These effects of K_V4.3 could not be explained by alterations in total Na_V1.5 protein expression. Using computer simulations employing a multicellular in silico model, we furthermore demonstrate that the experimentally observed increase in K_V4.3 current and concurrent decrease in Na_V1.5 current may result in a loss of conduction, underlining the potential functional relevance of our findings. This study gives the first proof of concept that K_V4.3 directly impacts on Na_V1.5 current. Future studies employing appropriate disease models should explore the potential electrophysiological implications in (patho)physiological conditions, including BrS associated with KCND3 gain-of-function mutations.

Sinus Bradycardia in Carriers of the SCN5A-1795insD Mutation: Unraveling the Mechanism through Computer Simulations

The SCN5A gene encodes the pore-forming α-subunit of the ion channel that carries the cardiac fast sodium current (I_Na). The 1795insD mutation in SCN5A causes sinus bradycardia, with a mean heart rate of 70 beats/min in mutation carriers vs. 77 beats/min in non-carriers from the same family (lowest heart rate 41 vs. 47 beats/min). To unravel the underlying mechanism, we incorporated the mutation-induced changes in I_Na into a recently developed comprehensive computational model of a single human sinoatrial node cell (Fabbri–Severi model). The 1795insD mutation reduced the beating rate of the model cell from 74 to 69 beats/min (from 49 to 43 beats/min in the simulated presence of 20 nmol/L acetylcholine). The mutation-induced persistent I_Na per se resulted in a substantial increase in beating rate. This gain-of-function effect was almost completely counteracted by the loss-of-function effect of the reduction in I_Na conductance. The further loss-of-function effect of the shifts in steady-state activation and inactivation resulted in an overall loss-of-function effect of the 1795insD mutation. We conclude that the experimentally identified mutation-induced changes in I_Na can explain the clinically observed sinus bradycardia. Furthermore, we conclude that the Fabbri–Severi model may prove a useful tool in understanding cardiac pacemaker activity in humans.

Distribution of cardiac sodium channels in clusters potentiates ephaptic interactions in the intercalated disc

KEY POINTS

It has been proposed that ephaptic conduction, relying on interactions between the sodium (Na⁺ ) current and the extracellular potential in intercalated discs, might contribute to cardiac conduction when gap junctional coupling is reduced, but this mechanism is still controversial. In intercalated discs, Na⁺ channels form clusters near gap junction plaques, but the functional significance of these clusters has never been evaluated. In HEK cells expressing cardiac Na⁺ channels, we show that restricting the extracellular space modulates the Na⁺ current, as predicted by corresponding simulations accounting for ephaptic effects. In a high-resolution model of the intercalated disc, clusters of Na⁺ channels that face each other across the intercellular cleft facilitate ephaptic impulse transmission when gap junctional coupling is reduced. Thus, our simulations reveal a functional role for the clustering of Na⁺ channels in intercalated discs, and suggest that rearrangement of these clusters in disease may influence cardiac conduction.

ABSTRACT

It has been proposed that ephaptic interactions in intercalated discs, mediated by extracellular potentials, contribute to cardiac impulse propagation when gap junctional coupling is reduced. However, experiments demonstrating ephaptic effects on the cardiac Na⁺ current (I_Na ) are scarce. Furthermore, Na⁺ channels form clusters around gap junction plaques, but the electrophysiological significance of these clusters has never been investigated. In patch clamp experiments with HEK cells stably expressing human Na_v 1.5 channels, we examined how restricting the extracellular space modulates I_Na elicited by an activation protocol. In parallel, we developed a high-resolution computer model of the intercalated disc to investigate how the distribution of Na⁺ channels influences ephaptic interactions. Approaching the HEK cells to a non-conducting obstacle always increased peak I_Na at step potentials near the threshold of I_Na activation and decreased peak I_Na at step potentials far above threshold (7 cells, P = 0.0156, Wilcoxon signed rank test). These effects were consistent with corresponding control simulations with a uniform Na⁺ channel distribution. In the intercalated disc computer model, redistributing the Na⁺ channels into a central cluster of the disc potentiated ephaptic effects. Moreover, ephaptic impulse transmission from one cell to another was facilitated by clusters of Na⁺ channels facing each other across the intercellular cleft when gap junctional coupling was reduced. In conclusion, our proof-of-principle experiments demonstrate that confining the extracellular space modulates cardiac I_Na , and our simulations reveal the functional role of the aggregation of Na⁺ channels in the perinexus. These findings highlight novel concepts in the physiology of cardiac excitation.

Andrographolide inhibits arrhythmias and is cardioprotective in rabbits

Andrographolide has a protective effect on the cardiovascular system. To study its cardic-electrophysiological effects, action potentials and voltage-gated Na⁺ (I_Na), Ca²⁺ (I_CaL), and K⁺ (I_K1, I_Kr, I_to and I_Kur) currents were recorded using whole-cell patch clamp and current clamp techniques. Additionally, the effects of andrographolide on aconitine-induced arrhythmias were assessed on electrocardiograms in vivo. We found that andrographolide shortened action potential duration and reduced maximum upstroke velocity in rabbit left ventricular and left atrial myocytes. Andrographolide attenuated rate-dependence of action potential duration, and reduced or abolished delayed afterdepolarizations and triggered activities induced by isoproterenol (1 μM) and high calcium ([Ca²⁺]_o=3.6 mM) in left ventricular myocytes. Andrographolide also concentration-dependently inhibited I_Na and I_CaL, but had no effect on I_to, I_Kur, I_K1, or I_Kr in rabbit left ventricular and left atrial myocytes. Andrographolide treatment increased the time and dosage thresholds of aconitine-induced arrhythmias, and reduced arrhythmia incidence and mortality in rabbits. Our results indicate that andrographolide inhibits cellular arrhythmias (delayed afterdepolarizations and triggered activities) and aconitine-induced arrhythmias in vivo, and these effects result from I_Na and I_CaL inhibition. Andrographolide may be useful as a class I and IV antiarrhythmic therapeutic.

Acquired long QT syndrome and phosphoinositide 3-kinase

While it is well known that mutation of several different ion channels can cause congenital long QT syndrome, block of I_Kr is widely thought to be responsible for most cases of drug-induced acquired long QT syndrome (aLQTS). In this article, we review evidence supporting another cause of aLQTS due to inhibition of phosphoinositide 3-kinase (PI3K) signaling. Inhibition of PI3K affects multiple plateau currents, reducing I_Kr, I_Ks, and I_CaL while increasing the persistent sodium current (I_NaP). The effects of PI3K inhibitors develop slowly, requiring hours to days to reach steady state. Dofetilide and terfenadine, an antihistamine on which much of the original I_Kr hypothesis was based, are among the many drugs that inhibit the PI3K pathway. Reduced PI3K signaling may also play a role in aLQTS associated with diabetes. Drug safety testing to identify aLQTS risk may be improved by examining PI3K-dependent effects that develop over time.

Changes in cardiac Nav1.5 expression, function, and acetylation by pan-histone deacetylase inhibitors

Histone deacetylase (HDAC) inhibitors are small molecule anticancer therapeutics that exhibit limiting cardiotoxicities including QT interval prolongation and life-threatening cardiac arrhythmias. Because the molecular mechanisms for HDAC inhibitor-induced cardiotoxicity are poorly understood, we performed whole cell patch voltage-clamp experiments to measure cardiac sodium currents (I_Na) from wild-type neonatal mouse ventricular or human-induced pluripotent stem cell-derived cardiomyocytes treated with trichostatin A (TSA), vorinostat (VOR), or romidepsin (FK228). All three pan-HDAC inhibitors dose dependently decreased peak I_Na density and shifted the voltage activation curve 3- to 8-mV positive. Increases in late I_Na were not observed despite a moderate slowing of the inactivation rate at low activating potentials (<-40 mV). Scn5a mRNA levels were not significantly altered but Na_V1.5 protein levels were significantly reduced. Immunoprecipitation with anti-Na_V1.5 and Western blotting with anti-acetyl-lysine antibodies indicated that Na_V1.5 acetylation is increased in vivo after HDAC inhibition. FK228 inhibited total cardiac HDAC activity with two apparent IC₅₀s of 5 nM and 1.75 μM, consistent with previous findings with TSA and VOR. FK228 also decreased ventricular gap junction conductance (g_j), again consistent with previous findings. We conclude that pan-HDAC inhibition reduces cardiac I_Na density and Na_V1.5 protein levels without affecting late I_Na amplitude and, thus, probably does not contribute to the reported QT interval prolongation and arrhythmias associated with pan-HDAC inhibitor therapies. Conversely, reductions in g_j may enhance the occurrence of triggered activity by limiting electrotonic inhibition and, combined with reduced I_Na, slow myocardial conduction and increase vulnerability to reentrant arrhythmias.

Reduced Sodium Current in the Lateral Ventricular Wall Induces Inferolateral J-Waves

BACKGROUND

J-waves in inferolateral leads are associated with a higher risk for idiopathic ventricular fibrillation. We aimed to test potential mechanisms (depolarization or repolarization dependent) responsible for inferolateral J-waves. We hypothesized that inferolateral J-waves can be caused by regional delayed activation of myocardium that is activated late during normal conditions.

METHODS

Computer simulations were performed to evaluate how J-point elevation is influenced by reducing sodium current conductivity (GNa), increasing transient outward current conductivity (Gto), or cellular uncoupling in three predefined ventricular regions (lateral, anterior, or septal). Two pig hearts were Langendorff-perfused with selective perfusion with a sodium channel blocker of lateral or anterior/septal regions. Volume-conducted pseudo-electrocardiograms (ECG) were recorded to detect the presence of J-waves. Epicardial unipolar electrograms were simultaneously recorded to obtain activation times (AT).

RESULTS

Simulation data showed that conduction slowing, caused by reduced sodium current, in lateral, but not in other regions induced inferolateral J-waves. An increase in transient outward potassium current or cellular uncoupling in the lateral zone elicited slight J-point elevations which did not meet J-wave criteria. Additional conduction slowing in the entire heart attenuated J-waves and J-point elevations on the ECG, because of masking by the QRS. Experimental data confirmed that conduction slowing attributed to sodium channel blockade in the left lateral but not in the anterior/septal ventricular region induced inferolateral J-waves. J-waves coincided with the delayed activation.

CONCLUSION

Reduced sodium current in the left lateral ventricular myocardium can cause inferolateral J-waves on the ECG.

Contrasting roles of Ih and the persistent sodium current at subthreshold voltages during naturalistic stimuli

The subthreshold activity of hippocampal CA1 pyramidal neurons is regulated by the persistent sodium current (I_NaP) and the h-current (I_h), carried by tetrodotoxin-sensitive sodium channels and hyperpolarization-activated cyclic-nucleotide-gated channels, respectively. Recently, Yamada-Hanff and Bean (J Neurophysiol 114: 2376-2389, 2015) used pharmacological methods to discern the roles of I_h and I_NaP at subthreshold voltages during naturalistic stimuli. We discuss these findings in the context of dorsoventral heterogeneity in the hippocampus and suggest further applications of the method.

[Functional analysis of a novel SCN5A mutation G1712C identified in Brugada syndrome]

OBJECTIVE

To elucidate the molecular and electrophysiological mechanisms of Brugada syndrome through functional analysis of a novel SCN5A gene mutation G1712C.

METHODS

A recombinant plasmid pRc<CMV-hH1 containing the mutant human cardiac sodium channel α subunit (hH1) cDNA was constructed using in vitro PCR-based site-directed mutagenesis technique. LipofectamineTM 3000 was used to transfect the plasmid DNA into HEK293 cell line to induce stable expression of Na⁺ channel β1-subunit, and the positive colonies were selected by screening with G418.The standard liposome method was used to transiently transfect HEK293 cells with either the wild-type or mutant Na⁺ channel subunits (hH1 and mhH1, respectively), and the macroscopic Na⁺ currents were recorded using whole-cell patch-clamp technique. Data acquisition and analysis, generation of voltage commands and curve fitting were accomplished with EPC-10, PatchMaster and IGOR Pro 6.0.

RESULTS

An HEK293 cell line that stably expressed Na⁺ channel β1-subunit was successfully established. After transient transfection with the WT subunit, large Na⁺ currents were recorded from the stable β1-cell line. Transient transfection with the G1712C subunit, however, did not elicit a Na⁺ current in the cells.

CONCLUSION

Compared with normal Na⁺ channel, the wild-type channel exhibits a similar sodium current. The characteristic kinetics of sodium channel of WT-hH1 was identical to that in normal cardiac muscle cell, and the missense mutation (G1712C) in the P-loop region of the domain IV may have caused the failure of sodium channel expression.

Tetrodotoxin-resistant sodium channels in sensory neurons generate slow resurgent currents that are enhanced by inflammatory mediators

Resurgent sodium currents contribute to the regeneration of action potentials and enhanced neuronal excitability. Tetrodotoxin-sensitive (TTX-S) resurgent currents have been described in many different neuron populations, including cerebellar and dorsal root ganglia (DRG) neurons. In most cases, sodium channel Nav1.6 is the major contributor to these TTX-S resurgent currents. Here we report a novel TTX-resistant (TTX-R) resurgent current recorded from rat DRG neurons. The TTX-R resurgent currents are similar to classic TTX-S resurgent currents in many respects, but not all. As with TTX-S resurgent currents, they are activated by membrane repolarization, inhibited by lidocaine, and enhanced by a peptide-mimetic of the β4 sodium channel subunit intracellular domain. However, the TTX-R resurgent currents exhibit much slower kinetics, occur at more depolarized voltages, and are sensitive to the Nav1.8 blocker A803467. Moreover, coimmunoprecipitation experiments from rat DRG lysates indicate the endogenous sodium channel β4 subunits associate with Nav1.8 in DRG neurons. These results suggest that slow TTX-R resurgent currents in DRG neurons are mediated by Nav1.8 and are generated by the same mechanism underlying TTX-S resurgent currents. We also show that both TTX-S and TTX-R resurgent currents in DRG neurons are enhanced by inflammatory mediators. Furthermore, the β4 peptide increased excitability of small DRG neurons in the presence of TTX. We propose that these slow TTX-R resurgent currents contribute to the membrane excitability of nociceptive DRG neurons under normal conditions and that enhancement of both types of resurgent currents by inflammatory mediators could contribute to sensory neuronal hyperexcitability associated with inflammatory pain.

Angiotensin-(1-7) prevent atrial tachycardia induced sodium channel remodeling

BACKGROUND

Activation of the renin-angiotensin system plays an important role in atrial electrical remodeling; angiotensin-(1-7) (Ang-(1-7)) counterbalances the actions of angiotensin II. The aim of this study was to determine the effects of Ang-(1-7) on cardiac sodium current (INa ) in a canine model of atrial tachycardia.

METHODS

Eighteen dogs were randomly assigned to sham, pacing, or pacing + Ang-(1-7) groups (n = 6 in each group). Rapid atrial pacing (500 beats/min) was maintained for 2 weeks, while the dogs in the sham group were not paced. Ang-(1-7) (6 μg/kg/h) was administered intravenously during pacing. Whole-cell patch clamp techniques were utilized to record INa from canine atrial myocytes. Reverse transcription-polymerase chain reaction was used to assess possible underlying changes in cardiac Na(+) channels (Nav1.5).

RESULTS

Our results showed that INa density and expression of the Nav1.5 mRNA significantly decreased following pacing (P < 0.05 vs sham); however, the half-activation voltage (V1/2act ) and half-inactivation voltage (V1/2inact ) of INa were not significantly altered (P > 0.05 vs sham). Ang-(1-7) treatment significantly increased INa densities and hyperpolarized V1/2act without concomitant changes in V1/2inact but have no effect on the expression of the Nav1.5 gene.

CONCLUSIONS

Ang-(1-7) significantly increased INa densities, which contributed to improving intraatrial conduction and decreasing the likelihood of atrial fibrillation maintenance.

Investigations of the Navβ1b sodium channel subunit in human ventricle; functional characterization of the H162P Brugada syndrome mutant

Brugada syndrome (BrS) is a rare inherited disease that can give rise to ventricular arrhythmia and ultimately sudden cardiac death. Numerous loss-of-function mutations in the cardiac sodium channel Nav1.5 have been associated with BrS. However, few mutations in the auxiliary Navβ1-4 subunits have been linked to this disease. Here we investigated differences in expression and function between Navβ1 and Navβ1b and whether the H162P/Navβ1b mutation found in a BrS patient is likely to be the underlying cause of disease. The impact of Navβ subunits was investigated by patch-clamp electrophysiology, and the obtained in vitro values were used for subsequent in silico modeling. We found that Navβ1b transcripts were expressed at higher levels than Navβ1 transcripts in the human heart. Navβ1 and Navβ1b coexpressed with Nav1.5 induced a negative shift on steady state of activation and inactivation compared with Nav1.5 alone. Furthermore, Navβ1b was found to increase the current level when coexpressed with Nav1.5, Navβ1b/H162P mutated subunit peak current density was reduced by 48% (-645 ± 151 vs. -334 ± 71 pA/pF), V1/2 steady-state inactivation shifted by -6.7 mV (-70.3 ± 1.5 vs. -77.0 ± 2.8 mV), and time-dependent recovery from inactivation slowed by >50% compared with coexpression with Navβ1b wild type. Computer simulations revealed that these electrophysiological changes resulted in a reduction in both action potential amplitude and maximum upstroke velocity. The experimental data thereby indicate that Navβ1b/H162P results in reduced sodium channel activity functionally affecting the ventricular action potential. This result is an important replication to support the notion that BrS can be linked to the function of Navβ1b and is associated with loss-of-function of the cardiac sodium channel.

Lessons from the heart: mirroring electrophysiological characteristics during cardiac development to in vitro differentiation of stem cell derived cardiomyocytes

The ability of human pluripotent stem cells (hPSCs) to differentiate into any cell type of the three germ layers makes them a very promising cell source for multiple purposes, including regenerative medicine, drug discovery, and as a model to study disease mechanisms and progression. One of the first specialized cell types to be generated from hPSC was cardiomyocytes (CM), and differentiation protocols have evolved over the years and now allow for robust and large-scale production of hPSC-CM. Still, scientists are struggling to achieve the same, mainly ventricular, phenotype of the hPSC-CM in vitro as their adult counterpart in vivo. In vitro generated cardiomyocytes are generally described as fetal-like rather than adult. In this review, we compare the in vivo development of cardiomyocytes to the in vitro differentiation of hPSC into CM with focus on electrophysiology, structure and contractility. Furthermore, known epigenetic changes underlying the differences between adult human CM and CM differentiated from pluripotent stem cells are described. This should provide the reader with an extensive overview of the current status of human stem cell-derived cardiomyocyte phenotype and function. Additionally, the reader will gain insight into the underlying signaling pathways and mechanisms responsible for cardiomyocyte development.

Electrophysiological properties of neurosensory progenitors derived from human embryonic stem cells

In severe cases of sensorineural hearing loss where the numbers of auditory neurons are significantly depleted, stem cell-derived neurons may provide a potential source of replacement cells. The success of such a therapy relies upon producing a population of functional neurons from stem cells, to enable precise encoding of sound information to the brainstem. Using our established differentiation assay to produce sensory neurons from human stem cells, patch-clamp recordings indicated that all neurons examined generated action potentials and displayed both transient sodium and sustained potassium currents. Stem cell-derived neurons reliably entrained to stimuli up to 20 pulses per second (pps), with 50% entrainment at 50 pps. A comparison with cultured primary auditory neurons indicated similar firing precision during low-frequency stimuli, but significant differences after 50 pps due to differences in action potential latency and width. The firing properties of stem cell-derived neurons were also considered relative to time in culture (31-56 days) and revealed no change in resting membrane potential, threshold or firing latency over time. Thus, while stem cell-derived neurons did not entrain to high frequency stimulation as effectively as mammalian auditory neurons, their electrical phenotype was stable in culture and consistent with that reported for embryonic auditory neurons.

Regulation of intracellular Na(+) in health and disease: pathophysiological mechanisms and implications for treatment

Transmembrane sodium (Na⁺) fluxes and intracellular sodium homeostasis are central players in the physiology of the cardiac myocyte, since they are crucial for both cell excitability and for the regulation of the intracellular calcium concentration. Furthermore, Na⁺ fluxes across the membrane of mitochondria affect the concentration of protons and calcium in the matrix, regulating mitochondrial function. In this review we first analyze the main molecular determinants of sodium fluxes across the sarcolemma and the mitochondrial membrane and describe their role in the physiology of the healthy myocyte. In particular we focus on the interplay between intracellular Ca²⁺ and Na⁺. A large part of the review is dedicated to discuss the changes of Na⁺ fluxes and intracellular Na⁺ concentration([Na⁺]_i) occurring in cardiac disease; we specifically focus on heart failure and hypertrophic cardiomyopathy, where increased intracellular [Na⁺]_i is an established determinant of myocardial dysfunction. We review experimental evidence attributing the increase of [Na⁺]_i to either decreased Na⁺ efflux (e.g. via the Na⁺/K⁺ pump) or increased Na⁺ influx into the myocyte (e.g. via Na⁺ channels). In particular, we focus on the role of the “late sodium current” (I_NaL), a sustained component of the fast Na⁺ current of cardiac myocytes, which is abnormally enhanced in cardiac diseases and contributes to both electrical and contractile dysfunction. We analyze the pathophysiological role of I_NaL enhancement in heart failure and hypertrophic cardiomyopathy and the consequences of its pharmacological modulation, highlighting the clinical implications.

The central role of Na⁺ fluxes and intracellular Na⁺ physiology and pathophysiology of cardiac myocytes has been highlighted by a large number of recent works. The possibility of modulating Na⁺ inward fluxes and [Na⁺]_i with specific I_NaL inhibitors, such as ranolazine, has made Na⁺a novel suitable target for cardiac therapy, potentially capable of addressing arrhythmogenesis and diastolic dysfunction in severe conditions such as heart failure and hypertrophic cardiomyopathy.

Electronic "expression" of the inward rectifier in cardiocytes derived from human-induced pluripotent stem cells

BACKGROUND

Human-induced pluripotent stem cell (h-iPSC)-derived cardiac myocytes are a unique model in which human myocyte function and dysfunction are studied, especially those from patients with genetic disorders. They are also considered a major advance for drug safety testing. However, these cells have considerable unexplored potential limitations when applied to quantitative action potential (AP) analysis. One major factor is spontaneous activity and resulting variability and potentially anomalous behavior of AP parameters.

OBJECTIVE

To demonstrate the effect of using an in silico interface on electronically expressed I(K1), a major component lacking in h-iPSC-derived cardiac myocytes.

METHODS

An in silico interface was developed to express synthetic I(K1) in cells under whole-cell voltage clamp.

RESULTS

Electronic I(K1) expression established a physiological resting potential, eliminated spontaneous activity, reduced spontaneous early and delayed afterdepolarizations, and decreased AP variability. The initiated APs had the classic rapid upstroke and spike and dome morphology consistent with data obtained with freshly isolated human myocytes as well as the readily recognizable repolarization attributes of ventricular and atrial cells. The application of 1 µM of BayK-8644 resulted in anomalous AP shortening in h-iPSC-derived cardiac myocytes. When I(K1) was electronically expressed, BayK-8644 prolonged the AP, which is consistent with the existing results on native cardiac myocytes.

CONCLUSIONS

The electronic expression of I(K1) is a simple and robust method to significantly improve the physiological behavior of the AP and electrical profile of h-iPSC-derived cardiac myocytes. Increased stability enables the use of this preparation for a controlled quantitative analysis of AP parameters, for example, drug responsiveness, genetic disorders, and dynamic behavior restitution profiles.

The cardiac sodium current Na(v)1.5 is functionally expressed in rabbit bronchial smooth muscle cells

A collagenase-proteinase mixture was used to isolate airway smooth muscle cells (ASMC) from rabbit bronchi, and membrane currents were recorded using the whole cell patch-clamp technique. Stepping from -100 mV to a test potential of -40 mV evoked a fast voltage-dependent Na(+) current, sometimes with an amplitude of several nanoamperes. The current disappeared within 15 min of exposure to papain + DTT (n = 6). Comparison of the current in ASMC with current mediated by NaV1.5 α-subunits expressed in human embryonic kidney cells revealed similar voltage dependences of activation (V1/2 = -42 mV for NaV1.5) and sensitivities to TTX (IC50 = 1.1 and 1.2 μM for ASMC and NaV1.5, respectively). The current in ASMC was also blocked by lidocaine (IC50 = 160 μM). Although veratridine, an agonist of voltage-gated Na(+) channels, reduced the peak current by 33%, it slowed inactivation, resulting in a fourfold increase in sustained current (measured at 25 ms after onset). In current-clamp mode, veratridine prolonged evoked action potentials from 37 ± 9 to 1,053 ± 410 ms (n = 8). Primers for NaV1.2-1.9 were used to amplify mRNA from groups of ∼20 isolated ASMC and from whole bronchial tissue by RT-PCR. Transcripts for NaV1.2, NaV1.3, and NaV1.5-1.9 were detected in whole tissue, but only NaV1.2 and NaV1.5 were detected in single cells. We conclude that freshly dispersed rabbit ASMC express a fast voltage-gated Na(+) current that is mediated mainly by the NaV1.5 subtype.

Role of G-proteins in the effects of leptin on pedunculopontine nucleus neurons

The pedunculopontine nucleus (PPN), the cholinergic arm of the reticular activating system, regulates waking and rapid eye movement sleep. Here, we demonstrate immunohistochemical labeling of the leptin receptor signaling isoform in PPN neurons, and investigated the effects of G-protein modulation and the leptin triple antagonist (TA) on the action of leptin in the PPN. Whole-cell patch clamp recordings were performed in rat brainstem slices from 9 to 17 day old pups. Previous results showed that leptin caused a partial blockade of sodium (I(Na)) and h-current (I(H)) in PPN neurons. TA (100 nM) reduced the blockade of I(Na) (~ 50% reduction) and I(H) (~ 93% reduction) caused by leptin. Intracellular guanosine 5'-[β-thio]diphosphate trilithium salt (a G-protein inhibitor) significantly reduced the effect of leptin on I(Na) (~ 60% reduction) but not on I(H) (~ 25% reduction). Intracellular GTPγS (a G-protein activator) reduced the effect of leptin on both I(Na) (~ 80% reduction) and I(H) (~ 90% reduction). These results suggest that the effects of leptin on the intrinsic properties of PPN neurons are leptin receptor- and G-protein dependent. We also found that leptin enhanced NMDA receptor-mediated responses in single neurons and in the PPN population as a whole, an effect blocked by TA. These experiments further strengthen the association between leptin dysregulation and sleep disturbances. Beck et al. investigated the effects of leptin on the intrinsic properties of neurons from the pedunculopontine nucleus (PPN). Leptin reduced the amplitude of voltage-gated sodium (I(Na)) and hyperpolarization-activated cyclic nucleotide-gated HCN (I(H)) channels. These effects were antagonized by a leptin receptor (OB-R) antagonist and by the G-protein antagonist GDPβ.

Mechanisms of ranolazine's dual protection against atrial and ventricular fibrillation

Coronary artery disease and heart failure carry concurrent risk for atrial fibrillation and life-threatening ventricular arrhythmias. We review evidence indicating that at therapeutic concentrations, ranolazine has potential for dual suppression of these arrhythmias. Mechanisms and clinical implications are discussed.