Induction of persistent sodium current by exogenous and endogenous nitric oxide

Local delivery of gaseous signaling molecules for orthopedic disease therapy

Over the past decade, a proliferation of research has used nanoparticles to deliver gaseous signaling molecules for medical purposes. The discovery and revelation of the role of gaseous signaling molecules have been accompanied by nanoparticle therapies for their local delivery. While most of them have been applied in oncology, recent advances have demonstrated their considerable potential in diagnosing and treating orthopedic diseases. Three of the currently recognized gaseous signaling molecules, nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H₂S), are highlighted in this review along with their distinctive biological functions and roles in orthopedic diseases. Moreover, this review summarizes the progress in therapeutic development over the past ten years with a deeper discussion of unresolved issues and potential clinical applications.

Cardiotoxicity of pyrethroids: molecular mechanisms and therapeutic options for acute and long-term toxicity

Pyrethroids (PY) are synthetic pesticides used in many applications ranging from large-scale agriculture to household maintenance. Their classical mechanisms of action are associated with binding to the sodium channel of insect neurons, disrupting its inactivation, ensuring their use as insecticides. However, PY can also lead to toxicity in vertebrates, including humans. In most toxicological studies, the impact of PY on heart function is neglected. Acute exposure to a high dose of PY causes enhancement of the late sodium current (INaL), which impairs the action potential waveform and can cause severe cardiac arrhythmias. Moreover, long-term, low-dose exposure to PY displays oxidative stress in the heart, which could induce tissue remodeling and impairment. Isolated and preliminary evidence supports that, for acute exposure to PY, an antiarrhythmic therapy with ranolazine (an INaL blocker), can be a promising therapeutic approach. Besides, heart tissue remodeling associated with low doses and long-term exposure to PY seems to benefit from antioxidant therapy. Despite significant leaps in understanding the mechanical details of PY intoxication, currently, few studies are focusing on the heart. In this review, we present what is known and what are the gaps in the field of cardiotoxicity induced by PY.

Caveolin-3 and Arrhythmias: Insights into the Molecular Mechanisms

Caveolin-3 is a muscle-specific protein on the membrane of myocytes correlated with a variety of cardiovascular diseases. It is now clear that the caveolin-3 plays a critical role in the cardiovascular system and a significant role in cardiac protective signaling. Mutations in the gene encoding caveolin-3 cause a broad spectrum of clinical phenotypes, ranging from persistent elevations in the serum levels of creatine kinase in asymptomatic humans to cardiomyopathy. The influence of Caveolin-3(CAV-3) mutations on current density parallels the effect on channel trafficking. For example, mutations in the CAV-3 gene promote ventricular arrhythmogenesis in long QT syndrome 9 by a combined decrease in the loss of the inward rectifier current (I_K1) and gain of the late sodium current (I_Na-L). The functional significance of the caveolin-3 has proved that caveolin-3 overexpression or knockdown contributes to the occurrence and development of arrhythmias. Caveolin-3 overexpression could lead to reduced diastolic spontaneous Ca²⁺ waves, thus leading to the abnormal L-Type calcium channel current-induced ventricular arrhythmias. Moreover, CAV-3 knockdown resulted in a shift to more negative values in the hyperpolarization-activated cyclic nucleotide channel 4 current (I_HCN4) activation curve and a significant decrease in I_HCN4 whole-cell current density. Recent evidence indicates that caveolin-3 plays a significant role in adipose tissue and is related to obesity development. The role of caveolin-3 in glucose homeostasis has attracted increasing attention. This review highlights the underlining mechanisms of caveolin-3 in arrhythmia. Progress in this field may contribute to novel therapeutic approaches for patients prone to developing arrhythmia.

Late Sodium Current of the Heart: Where Do We Stand and Where Are We Going?

Late sodium current has long been linked to dysrhythmia and contractile malfunction in the heart. Despite the increasing body of accumulating information on the subject, our understanding of its role in normal or pathologic states is not complete. Even though the role of late sodium current in shaping action potential under physiologic circumstances is debated, it’s unquestioned role in arrhythmogenesis keeps it in the focus of research. Transgenic mouse models and isoform-specific pharmacological tools have proved useful in understanding the mechanism of late sodium current in health and disease. This review will outline the mechanism and function of cardiac late sodium current with special focus on the recent advances of the area.

Genomic and Non-Genomic Regulatory Mechanisms of the Cardiac Sodium Channel in Cardiac Arrhythmias

Nav1.5 is the predominant cardiac sodium channel subtype, encoded by the SCN5A gene, which is involved in the initiation and conduction of action potentials throughout the heart. Along its biosynthesis process, Nav1.5 undergoes strict genomic and non-genomic regulatory and quality control steps that allow only newly synthesized channels to reach their final membrane destination and carry out their electrophysiological role. These regulatory pathways are ensured by distinct interacting proteins that accompany the nascent Nav1.5 protein along with different subcellular organelles. Defects on a large number of these pathways have a tremendous impact on Nav1.5 functionality and are thus intimately linked to cardiac arrhythmias. In the present review, we provide current state-of-the-art information on the molecular events that regulate SCN5A/Nav1.5 and the cardiac channelopathies associated with defects in these pathways.

Attenuating persistent sodium current-induced atrial myopathy and fibrillation by preventing mitochondrial oxidative stress

Mechanistically driven therapies for atrial fibrillation (AF), the most common cardiac arrhythmia, are urgently needed, the development of which requires improved understanding of the cellular signaling pathways that facilitate the structural and electrophysiological remodeling that occurs in the atria. Similar to humans, increased persistent Na+ current leads to the development of an atrial myopathy and spontaneous and long-lasting episodes of AF in mice. How increased persistent Na+ current causes both structural and electrophysiological remodeling in the atria is unknown. We crossbred mice expressing human F1759A-NaV1.5 channels with mice expressing human mitochondrial catalase (mCAT). Increased expression of mCAT attenuated mitochondrial and cellular reactive oxygen species (ROS) and the structural remodeling that was induced by persistent F1759A-Na+ current. Despite the heterogeneously prolonged atrial action potential, which was unaffected by the reduction in ROS, the incidences of spontaneous AF, pacing-induced after-depolarizations, and AF were substantially reduced. Expression of mCAT markedly reduced persistent Na+ current-induced ryanodine receptor oxidation and dysfunction. In summary, increased persistent Na+ current in atrial cardiomyocytes, which is observed in patients with AF, induced atrial enlargement, fibrosis, mitochondrial dysmorphology, early after-depolarizations, and AF, all of which can be attenuated by resolving mitochondrial oxidative stress.

Dual Mechanisms of Cardiac Action Potential Prolongation by 4-Oxo-Nonenal Increasing the Risk of Arrhythmia; Late Na+ Current Induction and hERG K+ Channel Inhibition

4-Oxo-nonenal (4-ONE) is an endogenous lipid peroxidation product that is more reactive than 4-hydroxy-nonenal (4-HNE). We previously reported the arrhythmic potential of 4-HNE by suppression of cardiac human Ether-a-go-go Related Gene (hERG) K⁺ channels with prolonged action potential duration (APD) in cardiomyocytes. Here, we illustrate the higher arrhythmic risk of 4-ONE by modulating the cardiac hNa_V1.5 channel currents (I_NaV). Although the peak amplitude of I_NaV was not significantly changed by 4-ONE up to 10 μM, the rate of I_NaV inactivation was slowed, and the late Na⁺ current (I_NaL) became larger by 10 μM 4-ONE. The chemical modification of specific residues in hNa_V1.5 by 4-ONE was identified using MS-fingerprinting analysis. In addition to the changes in I_NaV, 4-ONE decreased the delayed rectifier K⁺ channel currents including the hERG current. The L-type Ca²⁺ channel current was decreased, whereas its inactivation was slowed by 4-ONE. The APD prolongation by 10 μM of 4-ONE was more prominent than that by 100 μM of 4-HNE. In the computational in silico cardiomyocyte simulation analysis, the changes of I_NaL by 4-ONE significantly exacerbated the risk of arrhythmia exhibited by the TdP marker, qNet. Our study suggests an arrhythmogenic effect of 4-ONE on cardiac ion channels, especially hNa_V1.5.

Human induced pluripotent stem cell-derived atrial cardiomyocytes carrying an SCN5A mutation identify nitric oxide signaling as a mediator of atrial fibrillation

Mutations in SCN5A, encoding the cardiac sodium channel, are linked with familial atrial fibrillation (AF) but the underlying pathophysiologic mechanisms and implications for therapy remain unclear. To characterize the pathogenesis of AF-linked SCN5A mutations, we generated patient-specific induced pluripotent stem cell-derived atrial cardiomyocytes (iPSC-aCMs) from two kindreds carrying SCN5A mutations (E428K and N470K) and isogenic controls using CRISPR-Cas9 gene editing. We showed that mutant AF iPSC-aCMs exhibited spontaneous arrhythmogenic activity with beat-to-beat irregularity, prolonged action potential duration, and triggered-like beats. Single-cell recording revealed enhanced late sodium currents (INa,L) in AF iPSC-aCMs that were absent in a heterologous expression model. Gene expression profiling of AF iPSC-aCMs showed differential expression of the nitric oxide (NO)-mediated signaling pathway underlying enhanced INa,L. We showed that patient-specific AF iPSC-aCMs exhibited striking in vitro electrophysiological phenotype of AF-linked SCN5A mutations, and transcriptomic analyses supported that the NO signaling pathway modulated the INa,L and triggered AF.

Cardiac function dependence on carbon monoxide

Nitric oxide, studied to evaluate its role in cardiovascular physiology, has cardioprotective and therapeutic effects in cellular signaling, mitochondrial function, and in regulating inflammatory processes. Heme oxygenase (major role in catabolism of heme into biliverdin, carbon monoxide (CO), and iron) has similar effects as well. CO has been suggested as the molecule that is responsible for many of the above mentioned cytoprotective and therapeutic pathways as CO is a signaling molecule in the control of physiological functions. This is counterintuitive as toxic effects are related to its binding to hemoglobin. However, CO is normally produced in the body. Experimental evidence indicates that this toxic gas, CO, exerts cytoprotective properties related to cellular stress including the heart and is being assessed for its cytoprotective and cytotherapeutic properties. While survival of adult cardiomyocytes depends on oxidative phosphorylation (survival and resulting cardiac function is impaired by mitochondrial damage), mitochondrial biogenesis is modified by the heme oxygenase-1/CO system and can result in promotion of mitochondrial biogenesis by associating mitochondrial redox status to the redox-active transcription factors. It has been suggested that the heme oxygenase-1/CO system is important in differentiation of embryonic stem cells and maturation of cardiomyocytes which is thought to mitigate progression of degenerative cardiovascular diseases. Effects on other cardiac cells are being studied. Acute exposure to air pollution (and, therefore, CO) is associated with cardiovascular mortality, myocardial infarction, and heart failure, but changes in the endogenous heme oxygenase-1 system (and, thereby, CO) positively affect cardiovascular health. We will review the effect of CO on heart health and function in this article.

Nitric oxide down-regulates voltage-gated Na+ channel in cardiomyocytes possibly through S-nitrosylation-mediated signaling

Nitric oxide (NO) is produced from endothelial cells and cardiomyocytes composing the myocardium and benefits cardiac function through both vascular-dependent and—independent effects. This study was purposed to investigate the possible adverse effect of NO focusing on the voltage-gated Na⁺ channel in cardiomyocytes. We carried out patch-clamp experiments on rat neonatal cardiomyocytes demonstrating that NOC-18, an NO donor, significantly reduced Na⁺ channel current in a dose-dependent manner by a long-term application for 24 h, accompanied by a reduction of Nav1.5-mRNA and the protein, and an increase of a transcription factor forkhead box protein O1 (FOXO1) in the nucleus. The effect of NOC-18 on the Na⁺ channel was blocked by an inhibitor of thiol oxidation N-ethylmaleimide, a disulfide reducing agent disulfide 1,4-Dithioerythritol, or a FOXO1 activator paclitaxel, suggesting that NO is a negative regulator of the voltage-gated Na⁺ channel through thiols in regulatory protein(s) for the channel transcription.

Carbon monoxide activates large-conductance calcium-activated potassium channels of human cardiac fibroblasts through various mechanisms

Carbon monoxide (CO) is a cardioprotectant and potential cardiovascular therapeutic agent. Human cardiac fibroblasts (HCFs) are important determinants of myocardial structure and function. Large-conductance Ca²⁺-activated K⁺ (BK) channel is a potential therapeutic target for cardiovascular disease. We investigated whether CO modulates BK channels and the signaling pathways in HCFs using whole-cell mode patch-clamp recordings. CO-releasing molecules (CORMs; CORM-2 and CORM-3) significantly increased the amplitudes of BK currents (IBK). The CO-induced stimulating effects on IBK were blocked by pre-treatment with specific nitric oxide synthase (NOS) blockers (L-N^G-monomethyl arginine citrate and L-N^G-nitroarginine methyl ester). 8-bromo-cyclic GMP increased IBK. KT5823 (inhibits PKG) or ODQ (inhibits soluble guanylate cyclase) blocked the CO-stimulating effect on IBK. Moreover, 8-bromo-cyclic AMP also increased IBK, and pre-treatment with KT5720 (inhibits PKA) or SQ22536 (inhibits adenylate cyclase) blocked the CO effect. Pre-treatment with N-ethylmaleimide (a thiol-alkylating reagent) also blocked the CO effect on IBK, and DL-dithiothreitol (a reducing agent) reversed the CO effect. These data suggest that CO activates IBK through NO via the NOS and through the PKG, PKA, and S-nitrosylation pathways.

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.

Nitric Oxide and Mechano-Electrical Transduction in Cardiomyocytes

The ability§ of the heart to adapt to changes in the mechanical environment is critical for normal cardiac physiology. The role of nitric oxide is increasingly recognized as a mediator of mechanical signaling. Produced in the heart by nitric oxide synthases, nitric oxide affects almost all mechano-transduction pathways within the cardiomyocyte, with roles mediating mechano-sensing, mechano-electric feedback (via modulation of ion channel activity), and calcium handling. As more precise experimental techniques for applying mechanical stresses to cells are developed, the role of these forces in cardiomyocyte function can be further understood. Furthermore, specific inhibitors of different nitric oxide synthase isoforms are now available to elucidate the role of these enzymes in mediating mechano-electrical signaling. Understanding of the links between nitric oxide production and mechano-electrical signaling is incomplete, particularly whether mechanically sensitive ion channels are regulated by nitric oxide, and how this affects the cardiac action potential. This is of particular relevance to conditions such as atrial fibrillation and heart failure, in which nitric oxide production is reduced. Dysfunction of the nitric oxide/mechano-electrical signaling pathways are likely to be a feature of cardiac pathology (e.g., atrial fibrillation, cardiomyopathy, and heart failure) and a better understanding of the importance of nitric oxide signaling and its links to mechanical regulation of heart function may advance our understanding of these conditions.

Molecular mechanisms by which iNOS uncoupling can induce cardiovascular dysfunction during sepsis: Role of posttranslational modifications (PTMs)

Human sepsis is the result of a multifaceted pathological process causing marked dysregulation of cardiovascular responses. A more sophisticated understanding of the pathogenesis of sepsis is certainly prerequisite. Evidence from studies provide further insight into the role of inducible nitric oxide synthase (iNOS) isoform. Results on inhibition of iNOS in sepsis models remain inconclusive. Concern has been devoted to improving our knowledge and understanding of the role of iNOS. The aim of this review is to define the role of iNOS in redox homeostasis disturbance, the detailed mechanisms linking iNOS and posttranslational modifications (PTMs) to cardiovascular dysfunctions, and their future implications in sepsis settings. Many questions related to the iNOS and PTMs still remain open, and much more work is needed on this.

Late Sodium Current Inhibitors as Potential Antiarrhythmic Agents

Based on recent findings, an increased late sodium current (INa,late) plays an important pathophysiological role in cardiac diseases, including rhythm disorders. The article first describes what is INa,late and how it functions under physiological circumstances. Next, it shows the wide range of cellular mechanisms that can contribute to an increased INa,late in heart diseases, and also discusses how the upregulated INa,late can play a role in the generation of cardiac arrhythmias. The last part of the article is about INa,late inhibiting drugs as potential antiarrhythmic agents, based on experimental and preclinical data as well as in the light of clinical trials.

A small-molecule LF3 abrogates β-catenin/TCF4-mediated suppression of NaV1.5 expression in HL-1 cardiomyocytes

Increased nuclear β-catenin interacting with T-cell factor 4 (TCF4) affects the expression of target genes including SCN5A in ischemic heart disease, which is characterized by frequent ventricular tachycardia/fibrillation. A complex of β-catenin and TCF4 inhibits cardiac Na+ channel activity by reducing NaV1.5 expression through suppressing SCN5A promoter activity in HL-1 cardiomyocytes. LF3, a 4-thioureido-benzenesulfonamide derivative and an inhibitor of β-catenin/TCF4 interaction, has been shown to block the self-renewal capacity of cancer stem cells. We performed studies to determine if LF3 can reverse suppressive effects of β-catenin/TCF4 signaling on the expression of NaV1.5 in HL-1 cardiomyocytes. Western blotting and real-time qRT-PCR analyses showed that 10 μM LF3 significantly increased the expression of NaV1.5 but it did not alter β-catenin and TCF4 expression. Subcellular fractionation analysis demonstrated that LF3 significantly increased the levels of NaV1.5 in both membrane and cytoplasm. Whole-cell patch-clamp recordings revealed that Na+ currents were significantly increased with no changes in the steady-state parameters, activation and inactivation time constants and recovery from inactivation of Na+ channel in HL-1 cells treated with LF3. Immunoprecipitation exhibited that LF3 blocked the interaction of β-catenin and TCF4. Luciferase reporter assays performed in HEK 293 cells and HL-1 revealed that LF3 increased the SCN5A promoter activity in HL-1 cells and prevented β-catenin suppressive effect on SCN5A promoter activity in HEK 293 cells. Taken together, we conclude that LF3, an inhibitor of β-catenin/TCF4 interaction, elevates NaV1.5 expression, leading to increase Na+ channel activity in HL-1 cardiomyocytes.

Arrhythmogenic Mechanisms in Heart Failure: Linking β-Adrenergic Stimulation, Stretch, and Calcium

Heart failure (HF) is associated with elevated sympathetic tone and mechanical load. Both systems activate signaling transduction pathways that increase cardiac output, but eventually become part of the disease process itself leading to further worsening of cardiac function. These alterations can adversely contribute to electrical instability, at least in part due to the modulation of Ca²⁺ handling at the level of the single cardiac myocyte. The major aim of this review is to provide a definitive overview of the links and cross talk between β-adrenergic stimulation, mechanical load, and arrhythmogenesis in the setting of HF. We will initially review the role of Ca²⁺ in the induction of both early and delayed afterdepolarizations, the role that β-adrenergic stimulation plays in the initiation of these and how the propensity for these may be altered in HF. We will then go onto reviewing the current data with regards to the link between mechanical load and afterdepolarizations, the associated mechano-sensitivity of the ryanodine receptor and other stretch activated channels that may be associated with HF-associated arrhythmias. Furthermore, we will discuss how alterations in local Ca²⁺ microdomains during the remodeling process associated the HF may contribute to the increased disposition for β-adrenergic or stretch induced arrhythmogenic triggers. Finally, the potential mechanisms linking β-adrenergic stimulation and mechanical stretch will be clarified, with the aim of finding common modalities of arrhythmogenesis that could be targeted by novel therapeutic agents in the setting of HF.

β-adrenergic regulation of late Na+ current during cardiac action potential is mediated by both PKA and CaMKII

Late Na⁺ current (I_NaL) significantly contributes to shaping cardiac action potentials (APs) and increased I_NaL is associated with cardiac arrhythmias. β-adrenergic receptor (βAR) stimulation and its downstream signaling via protein kinase A (PKA) and Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) pathways are known to regulate I_NaL. However, it remains unclear how each of these pathways regulates I_NaL during the AP under physiological conditions. Here we performed AP-clamp experiments in rabbit ventricular myocytes to delineate the impact of each signaling pathway on I_NaL at different AP phases to understand the arrhythmogenic potential. During the physiological AP (2 Hz, 37 °C) we found that I_NaL had a basal level current independent of PKA, but partially dependent on CaMKII. βAR activation (10 nM isoproterenol, ISO) further enhanced I_NaL via both PKA and CaMKII pathways. However, PKA predominantly increased I_NaL early during the AP plateau, whereas CaMKII mainly increased I_NaL later in the plateau and during rapid repolarization. We also tested the role of key signaling pathways through exchange protein activated by cAMP (Epac), nitric oxide synthase (NOS) and reactive oxygen species (ROS). Direct Epac stimulation enhanced I_NaL similar to the βAR-induced CaMKII effect, while NOS inhibition prevented the βAR-induced CaMKII-dependent I_NaL enhancement. ROS generated by NADPH oxidase 2 (NOX2) also contributed to the ISO-induced I_NaL activation early in the AP. Taken together, our data reveal differential modulations of I_NaL by PKA and CaMKII signaling pathways at different AP phases. This nuanced and comprehensive view on the changes in I_NaL during AP deepens our understanding of the important role of I_NaL in reshaping the cardiac AP and arrhythmogenic potential under elevated sympathetic stimulation, which is relevant for designing therapeutic treatment of arrhythmias under pathological conditions.

Nitric oxide augments single persistent Na+ channel currents via the cGMP/PKG signaling pathway in Kenyon cells isolated from cricket mushroom bodies

The nitric oxide (NO)/cyclic GMP signaling pathway has been suggested to be important in the formation of olfactory memory in insects. However, the molecular targets of the NO signaling cascade in the central neurons associated with olfactory learning and memory have not been fully analyzed. In this study, we investigated the effects of NO donors on single voltage-dependent Na⁺ channels in intrinsic neurons, called Kenyon cells, in the mushroom bodies in the brain of the cricket. Step depolarization on cell-attached patch membranes induces single-channel currents with fast-activating and -inactivating brief openings at the beginning of the voltage steps followed by more persistently recurring brief openings all along the 150-ms pulses. Application of the NO donor S-nitrosoglutathione (GSNO) increased the number of channel openings of both types of single Na⁺ channels. This excitatory effect of GSNO on the activity of these Na⁺ channels was diminished by KT5823, an inhibitor of protein kinase G (PKG), indicating an involvement of PKG in the downstream pathway of NO. Application of KT5823 alone decreased the activity of the persistent Na⁺ channels without significant effects on the fast-inactivating Na⁺ channels. The membrane-permeable cGMP analog 8Br-cGMP increased the number of channel openings of both types of single Na⁺ channels, similar to the action of NO. Taken together, these results indicate that NO acts as a critical modulator of both fast-inactivating and persistent Na⁺ channels and that persistent Na⁺ channels are constantly upregulated by the endogenous cGMP/PKG signaling cascade. NEW & NOTEWORTHY This study clarified that nitric oxide (NO) increases the activity of both fast-inactivating and persistent Na⁺ channels via the cGMP/PKG signaling cascade in cricket Kenyon cells. The persistent Na⁺ channels are also found to be upregulated constantly by endogenous cGMP/PKG signaling. On the basis of the present results and the results of previous studies, we propose a hypothetical model explaining NO production and NO-dependent memory formation in cricket large Kenyon cells.

THE PROFILE OF THE LEVEL OF ENDOGENOUS NO IN MIGRAINE SUFFERERS

Migraine is a health problem which had big impact. A neurochemistry change in the dura, that is the increase of the amount of nitrite oxide, originated from an excessive NOS (inducible NOS) activation during ictal. Some researchers assume NO as a main cause of the pain intensity in ictal. Migraine diagnosis is guided by the International Headache Society. Hopefully, this examination can be used as a more objective diagnosis in the future. Methode of this research is descriptive analytic. This study to find the level of endogenous exhaled NO on 91 subject, consisting of  30 normal people, 31 interictal, 30 ictal. An exclusion was done previously. A validation equipment and measurement had been done in accordance with American Thoracic Association guidance. The result is level of endogenous exhaled NO on the normal group (median=5), interictal group (median=11), ictal group (median=14). Coefficient correlation between VAS and level endogenous exhaled NO was 0,815. So, there is an increase of the NO endogen level in the normal people, migraine interictal, and ictal. Strong correlation between intensity of pain (VAS) and level endogenous exhaled NO during ictal.

Redox modification of caveolar proteins in the cardiovascular system- role in cellular signalling and disease

Rapid and coordinated release of a variety of reactive oxygen species (ROS) such as superoxide (O₂^.-), hydrogen peroxide (H₂O₂) and peroxynitrite, in specific microdomains, play a crucial role in cell signalling in the cardiovascular system. These reactions are mediated by reversible and functional modifications of a wide variety of key proteins. Dysregulation of this oxidative signalling occurs in almost all forms of cardiovascular disease (CVD), including at the very early phases. Despite the heavily publicized failure of "antioxidants" to improve CVD progression, pharmacotherapies such as those targeting the renin-angiotensin system, or statins, exert at least part of their large clinical benefit via modulating cellular redox signalling. Over 250 proteins, including receptors, ion channels and pumps, and signalling proteins are found in the caveolae. An increasing proportion of these are being recognized as redox regulated-proteins, that reside in the immediate vicinity of the two major cellular sources of ROS, nicotinamide adenine dinucleotide phosphate oxidase (Nox) and uncoupled endothelial nitric oxide synthase (eNOS). This review focuses on what is known about redox signalling within the caveolae, as well as endogenous protective mechanisms utilized by the cell, and new approaches to targeting dysregulated redox signalling in the caveolae as a therapeutic strategy in CVD.

Posttranslational Modification of Sodium Channels

Voltage-gated sodium channels (VGSCs) are critical determinants of excitability. The properties of VGSCs are thought to be tightly controlled. However, VGSCs are also subjected to extensive modifications. Multiple posttranslational modifications that covalently modify VGSCs in neurons and muscle have been identified. These include, but are not limited to, phosphorylation, ubiquitination, palmitoylation, nitrosylation, glycosylation, and SUMOylation. Posttranslational modifications of VGSCs can have profound impact on cellular excitability, contributing to normal and abnormal physiology. Despite four decades of research, the complexity of VGSC modulation is still being determined. While some modifications have similar effects on the various VGSC isoforms, others have isoform-specific interactions. In addition, while much has been learned about how individual modifications can impact VGSC function, there is still more to be learned about how different modifications can interact. Here we review what is known about VGSC posttranslational modifications with a focus on the breadth and complexity of the regulatory mechanisms that impact VGSC properties.

Uniform low-level dystrophin expression in the heart partially preserved cardiac function in an aged mouse model of Duchenne cardiomyopathy

Dystrophin deficiency results in Duchenne cardiomyopathy, a primary cause of death in Duchenne muscular dystrophy (DMD). Gene therapy has shown great promise in ameliorating the cardiac phenotype in mouse models of DMD. However, it is not completely clear how much dystrophin is required to treat dystrophic heart disease. We and others have shown that mosaic dystrophin expression at the wild-type level, depending on the percentage of dystrophin positive cardiomyocytes, can either delay the onset of or fully prevent cardiomyopathy in dystrophin-null mdx mice. Many gene therapy strategies will unlikely restore dystrophin to the wild-type level in a cardiomyocyte. To determine whether low-level dystrophin expression can reduce the cardiac manifestations in DMD, we examined heart histology, ECG and hemodynamics in 21-m-old normal BL6 and two strains of BL6-background dystrophin-deficient mice. Mdx3cv mice show uniform low-level expression of a near full-length dystrophin protein in every myofiber while mdx4cv mice have no dystrophin expression. Immunostaining and western blot confirmed marginal level dystrophin expression in the heart of mdx3cv mice. Although low-level expression did not reduce myocardial histopathology, it significantly ameliorated QRS prolongation and normalized diastolic hemodynamic deficiencies. Our study demonstrates for the first time that low-level dystrophin can partially preserve heart function.

Tolterodine reduces veratridine-augmented late INa, reverse-INCX and early afterdepolarizations in isolated rabbit ventricular myocytes

AIM

The augmentation of late sodium current (I_Na.L) not only causes intracellular Na⁺ accumulation, which results in intracellular Ca²⁺ overload via the reverse mode of the Na⁺/Ca²⁺ exchange current (reverse-I_NCX), but also prolongs APD and induces early afterdepolarizations (EAD), which can lead to arrhythmia and cardiac dysfunction. Thus, the inhibition of I_Na.L is considered to be a potential way for therapeutic intervention in ischemia and heart failure. In this study we investigated the effects of tolterodine (Tol), a competitive muscarinic receptor antagonist, on normal and veratridine (Ver)-augmented I_Na.L, reverse-I_NCX and APD in isolated rabbit ventricular myocytes, which might contribute to its cardioprotective activity.

METHODS

Rabbit ventricular myocytes were prepared. The I_Na.L and reverse-I_NCX were recorded in voltage clamp mode, whereas action potentials and Ver-induced early afterdepolarizations (EADs) were recorded in current clamp mode. Drugs were applied via superfusion.

RESULTS

Tol (3-120 nmol/L) concentration-dependently inhibited the normal and Ver-augmented I_Na.L with IC₅₀ values of 32.08 nmol/L and 42.47 nmol/L, respectively. Atropine (100 μmol/L) did not affect the inhibitory effects of Tol (30 nmol/L) on Ver-augmented I_Na.L. In contrast, much high concentrations of Tol was needed to inhibit the transient sodium current (I_Na.T) with an IC₅₀ value of 183.03 μmol/L. In addition, Tol (30 nmol/L) significantly shifted the inactivation curve of I_Na.T toward a more depolarizing membrane potential without affecting its activation characteristics. Moreover, Tol (30 nmol/L) significantly decreased Ver-augmented reverse-I_NCX. Tol (30 nmol/L) increased the action potential duration (APD) by 16% under the basal conditions. Ver (20 μmol/L) considerably extended the APD and evoked EADs in 18/24 cells (75%). In the presence of Ver, Tol (30 nmol/L) markedly decreased the APD and eliminated EADs (0/24 cells).

CONCLUSION

Tol inhibits normal and Ver-augmented I_NaL and decreases Ver-augmented reverse-I_NCX. In addition, Tol reverses the prolongation of the APD and eliminates the EADs induced by Ver, thus prevents Ver-induced arrhythmia.

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.

Cardiac Na Channels: Structure to Function

Heart rhythms arise from electrical activity generated by precisely timed opening and closing of ion channels in individual cardiac myocytes. Opening of the primary cardiac voltage-gated sodium (NaV1.5) channel initiates cellular depolarization and the propagation of an electrical action potential that promotes coordinated contraction of the heart. The regularity of these contractile waves is critically important since it drives the primary function of the heart: to act as a pump that delivers blood to the brain and vital organs. When electrical activity goes awry during a cardiac arrhythmia, the pump does not function, the brain does not receive oxygenated blood, and death ensues. Perturbations to NaV1.5 may alter the structure, and hence the function, of the ion channel and are associated downstream with a wide variety of cardiac conduction pathologies, such as arrhythmias.

A sodium afterdepolarization in rat superior colliculus neurons and its contribution to population activity

The mammalian superior colliculus (SC) is a midbrain structure that integrates multimodal sensory inputs and computes commands to initiate rapid eye movements. SC neurons burst with the sudden onset of a visual stimulus, followed by persistent activity that may underlie shifts of attention and decision making. Experiments in vitro suggest that circuit reverberations play a role in the burst activity in the SC, but the origin of persistent activity is unclear. In the present study we characterized an afterdepolarization (ADP) that follows action potentials in slices of rat SC. Population responses seen with voltage-sensitive dye imaging consisted of rapid spikes followed immediately by a second distinct depolarization of lower amplitude and longer duration. Patch-clamp recordings showed qualitatively similar behavior: in nearly all neurons throughout the SC, rapid spikes were followed by an ADP. Ionic and pharmacological manipulations along with experiments with current and voltage steps indicated that the ADP of SC neurons arises from Na(+) current that either persists or resurges following Na(+) channel inactivation at the end of an action potential. Comparisons of pharmacological properties and frequency dependence revealed a clear parallel between patch-clamp recordings and voltage imaging experiments, indicating a common underlying membrane mechanism for the ADP in both single neurons and populations. The ADP can initiate repetitive spiking at intervals consistent with the frequency of persistent activity in the SC. These results indicate that SC neurons have intrinsic membrane properties that can contribute to electrical activity that underlies shifts of attention and decision making.

α1-Syntrophin Variant Identified in Drug-Induced Long QT Syndrome Increases Late Sodium Current

Drug-induced long-QT syndrome (diLQTS) is often due to drug block of I_Kr, especially in genetically susceptible patients with subclinical mutations in the I_Kr-encoding KCHN2. Few variants in the cardiac Na_V1.5 Na⁺ channel complex have been associated with diLQTS. We tested whether a novel SNTA1 (α1-syntrophin) variant (p.E409Q) found in a patient with diLQTS increases late sodium current (I_Na-L), thereby providing a disease mechanism. Electrophysiological studies were performed in HEK293T cells co-expressing human Na_V1.5/nNOS/PMCA4b with either wild type (WT) or SNTA1 variants (A390V-previously reported in congenital LQTS; and E409Q); and in adult rat ventricular cardiomyocytes infected with SNTA1 expressing adenoviruses (WT or one of the two SNTA1 variants). In HEK293T cells and in cardiomyocytes, there was no significant difference in the peak I_Na densities among the SNTA1 WT and variants. However, both variants increased I_Na-L (% of peak current) in HEK293T cells (0.58±0.10 in WT vs. 0.90±0.11 in A390V, p = 0.048; vs. 0.88±0.07 in E409Q, p = 0.023). In cardiomyocytes, I_Na-L was significantly increased by E409Q, but not by A390V compared to WT (0.49±0.14 in WT vs.0.94±0.23 in A390V, p = 0.099; vs. 1.12±0.24 in E409Q, p = 0.019). We demonstrated that a novel SNTA1 variant is likely causative for diLQTS by augmenting I_Na-L. These data suggest that variants within the Na_V1.5-interacting α1-syntrophin are a potential mechanism for diLQTS, thereby expanding the concept that variants within congenital LQTS loci can cause diLQTS.

Ketamine attenuates the Na+-dependent Ca2+ overload in rabbit ventricular myocytes in vitro by inhibiting late Na+ and L-type Ca2+ currents

AIM

Intracellular Ca(2+) ([Ca(2+)]i) overload occurs in myocardial ischemia. An increase in the late sodium current (INaL) causes intracellular Na(+) overload and subsequently [Ca(2+)]i overload via the reverse-mode sodium-calcium exchanger (NCX). Thus, inhibition of INaL is a potential therapeutic target for cardiac diseases associated with [Ca(2+)]i overload. The aim of this study was to investigate the effects of ketamine on Na(+)-dependent Ca(2+) overload in ventricular myocytes in vitro.

METHODS

Ventricular myocytes were enzymatically isolated from hearts of rabbits. INaL, NCX current (INCX) and L-type Ca(2+) current (ICaL) were recorded using whole-cell patch-clamp technique. Myocyte shortening and [Ca(2+)]i transients were measured simultaneously using a video-based edge detection and dual excitation fluorescence photomultiplier system.

RESULTS

Ketamine (20, 40, 80 μmol/L) inhibited INaL in a concentration-dependent manner. In the presence of sea anemone toxin II (ATX, 30 nmol/L), INaL was augmented by more than 3-fold, while ketamine concentration-dependently suppressed the ATX-augmented INaL. Ketamine (40 μmol/L) also significantly suppressed hypoxia or H2O2-induced enhancement of INaL. Furthermore, ketamine concentration-dependently attenuated ATX-induced enhancement of reverse-mode INCX. In addition, ketamine (40 μmol/L) inhibited ICaL by 33.4%. In the presence of ATX (3 nmol/L), the rate and amplitude of cell shortening and relaxation, the diastolic [Ca(2+)]i, and the rate and amplitude of [Ca(2+)]i rise and decay were significantly increased, which were reverted to control levels by tetrodotoxin (TTX, 2 μmol/L) or by ketamine (40 μmol/L).

CONCLUSION

Ketamine protects isolated rabbit ventricular myocytes against [Ca(2+)]i overload by inhibiting INaL and ICaL.

Role of sodium and calcium dysregulation in tachyarrhythmias in sudden cardiac death

Despite improvements in the therapy of underlying heart disease, sudden cardiac death is a major cause of death worldwide. Disturbed Na and Ca handling is known to be a major predisposing factor for life-threatening tachyarrhythmias. In cardiomyocytes, many ion channels and transporters, including voltage-gated Na and Ca channels, cardiac ryanodine receptors, Na/Ca-exchanger, and SR Ca-ATPase are involved in this regulation. We have learned a lot about the pathophysiological relevance of disturbed ion channel function from monogenetic disorders. Changes in the gating of a single ion channel and the activity of an ion pump suffice to dramatically increase the propensity for arrhythmias even in structurally normal hearts. Nevertheless, patients with heart failure with acquired dysfunction in many ion channels and transporters exhibit profound dysregulation of Na and Ca handling and Ca/calmodulin-dependent protein kinase and are especially prone to arrhythmias. A deeper understanding of the underlying arrhythmic principles is mandatory if we are to improve their outcome. This review addresses basic tachyarrhythmic mechanisms, the underlying ionic mechanisms and the consequences for ion homeostasis, and the situation in complex diseases like heart failure.

Mechanisms of sudden cardiac death: oxidants and metabolism

Ventricular arrhythmia is the leading cause of sudden cardiac death (SCD). Deranged cardiac metabolism and abnormal redox state during cardiac diseases foment arrhythmogenic substrates through direct or indirect modulation of cardiac ion channel/transporter function. This review presents current evidence on the mechanisms linking metabolic derangement and excessive oxidative stress to ion channel/transporter dysfunction that predisposes to ventricular arrhythmias and SCD. Because conventional antiarrhythmic agents aiming at ion channels have proven challenging to use, targeting arrhythmogenic metabolic changes and redox imbalance may provide novel therapeutics to treat or prevent life-threatening arrhythmias and SCD.

Late sodium current: A mechanism for angina, heart failure, and arrhythmia

The peak sodium current underlies excitability and conduction in heart muscle, but a late sodium current flowing after the peak contributes to maintaining and prolonging the action potential plateau, and also to intracellular sodium loading, which in turn increases intracellular calcium with consequent effects on arrhythmia and diastolic function. Late sodium current is pathologically increased in both genetic and acquired heart disease, making it an attractive target for therapy to treat arrhythmia, heart failure, and angina. This review provides an overview of the underlying bases for the clinical implications of late sodium current block.

Regulation of the cardiac Na+ channel NaV1.5 by post-translational modifications

The cardiac voltage-gated Na(+) channel, Na(V)1.5, is responsible for the upstroke of the action potential in cardiomyocytes and for efficient propagation of the electrical impulse in the myocardium. Even subtle alterations of Na(V)1.5 function, as caused by mutations in its gene SCN5A, may lead to many different arrhythmic phenotypes in carrier patients. In addition, acquired malfunctions of Na(V)1.5 that are secondary to cardiac disorders such as heart failure and cardiomyopathies, may also play significant roles in arrhythmogenesis. While it is clear that the regulation of Na(V)1.5 protein expression and function tightly depends on genetic mechanisms, recent studies have demonstrated that Na(V)1.5 is the target of various post-translational modifications that are pivotal not only in physiological conditions, but also in disease. In this review, we examine the recent literature demonstrating glycosylation, phosphorylation by Protein Kinases A and C, Ca(2+)/Calmodulin-dependent protein Kinase II, Phosphatidylinositol 3-Kinase, Serum- and Glucocorticoid-inducible Kinases, Fyn and Adenosine Monophosphate-activated Protein Kinase, methylation, acetylation, redox modifications, and ubiquitylation of Na(V)1.5. Modern and sensitive mass spectrometry approaches, applied directly to channel proteins that were purified from native cardiac tissues, have enabled the determination of the precise location of post-translational modification sites, thus providing essential information for understanding the mechanistic details of these regulations. The current challenge is first, to understand the roles of these modifications on the expression and the function of Na(V)1.5, and second, to further identify other chemical modifications. It is postulated that the diversity of phenotypes observed with Na(V)1.5-dependent disorders may partially arise from the complex post-translational modifications of channel protein components.

Redox control of cardiac remodeling in atrial fibrillation

BACKGROUND

Atrial fibrillation (AF) is the most common arrhythmia in clinical practice and is a potential cause of thromboembolic events. AF induces significant changes in the electrophysiological properties of atrial myocytes and causes alterations in the structure, metabolism, and function of the atrial tissue. The molecular basis for the development of structural atrial remodeling of fibrillating human atria is still not fully understood. However, increased production of reactive oxygen or nitrogen species (ROS/RNS) and the activation of specific redox-sensitive signaling pathways observed both in patients with and animal models of AF are supposed to contribute to development, progression and self-perpetuation of AF.

SCOPE OF REVIEW

The present review summarizes the sources and targets of ROS/RNS in the setting of AF and focuses on key redox-sensitive signaling pathways that are implicated in the pathogenesis of AF and function either to aggravate or protect from disease.

MAJOR CONCLUSIONS

NADPH oxidases and various mitochondrial monooxygenases are major sources of ROS during AF. Besides direct oxidative modification of e.g. ion channels and ion handling proteins that are crucially involved in action potential generation and duration, AF leads to the reversible activation of redox-sensitive signaling pathways mediated by activation of redox-regulated proteins including Nrf2, NF-κB, and CaMKII. Both processes are recognized to contribute to the formation of a substrate for AF and, thus, to increase AF inducibility and duration.

GENERAL SIGNIFICANCE

AF is a prevalent disease and due to the current demographic developments its socio-economic relevance will further increase. Improving our understanding of the role that ROS and redox-related (patho)-mechanisms play in the development and progression of AF may allow the development of a targeted therapy for AF that surpasses the efficacy of previous general anti-oxidative strategies. This article is part of a Special Issue entitled Redox regulation of differentiation and de-differentiation.

The late sodium current in heart failure: pathophysiology and clinical relevance

Large and growing body of data suggest that an increased late sodium current (I_Na,late ) can have a significant pathophysiological role in heart failure and other heart diseases. The first goal of this article is to describe how I_Na,late functions under physiological circumstances. The second goal is to show the wide range of cellular mechanisms that can increase I_Na,late in cardiac disease, and also to describe how the up-regulated I_Na,late contributes to the pathophysiology of heart failure. The final section of the article discusses the possible use of I_Na,late -modifying drugs in heart failure, on the basis of experimental and preclinical data.

Drugs targeting nitric oxide synthase for migraine treatment

INTRODUCTION

Ample evidence that nitric oxide (NO) is a causative molecule in migraine has encouraged research to develop drugs that target the NO-cGMP cascade for migraine treatment. NO synthase (NOS) inhibition is an innovative therapeutic principle.

AREAS COVERED

This paper reviews the rationale underlying NOS inhibition in migraine treatment. It also provides a review on the efficacy and safety data for NOS inhibitors (nonselective NOS inhibitor L-N(G)-methyl-arginine hydrochloride [L-NMMA], selective inducible NOS [iNOS] inhibitors GW273629 and GW274150, combined neuronal NOS [nNOS] inhibitor and 5-HT1B/1D receptor agonist NXN-188) in acute or preventive migraine treatment.

EXPERT OPINION

The data highlighted herein, from four placebo-controlled trials and 1 open-labeled clinical trial using 4 different NOS inhibitors on a total of 705 patients, provide convincing efficacy data only for the nonselective NOS inhibitor L-NMMA. Unfortunately, this NOS inhibitor raises cardiovascular safety concerns and has an unfavorable pharmacokinetic profile. As experimental studies predicted, iNOS inhibitors are ineffective in migraine. Still, upcoming selective nNOS inhibitors are a hope for migraine treatment, with the nNOS isoform being most clearly involved in trigeminovascular transmission and central sensitization. Future studies should help to clarify whether NOS inhibition is equally fruitful in acute and preventive treatment. It should also clarify if nNOS inhibition holds promise as a therapeutic tool for the treatment of chronic migraine and other forms of headache.

Pharmacological insights and quirks of bacterial sodium channels

The pedigree of voltage-gated sodium channels spans the millennia from eukaryotic members that initiate the action potential firing in excitable tissues to primordial ancestors that act as enviro-protective complexes in bacterial extremophiles. Eukaryotic sodium channels (eNavs) are central to electrical signaling throughout the cardiovascular and nervous systems in animals and are established clinical targets for the therapeutic management of epilepsy, cardiac arrhythmia, and painful syndromes as they are inhibited by local anesthetic compounds. Alternatively, bacterial voltage-gated sodium channels (bNavs) likely regulate the survival response against extreme pH conditions, electrophiles, and hypo-osmotic shock and may represent a founder of the voltage-gated cation channel family. Despite apparent differences between eNav and bNav channel physiology, gating, and gene structure, the discovery that bNavs are amenable to crystallographic study opens the door for the possibility of structure-guided rational design of the next generation of therapeutics that target eNavs. Here we summarize the gating behavior of these disparate channel members and discuss mechanisms of local anesthetic inhibition in light of the growing number of bNav structures.

The role of late I Na in development of cardiac arrhythmias

Late I Na is an integral part of the sodium current, which persists long after the fast-inactivating component. The magnitude of the late I Na is relatively small in all species and in all types of cardiomyocytes as compared with the amplitude of the fast sodium current, but it contributes significantly to the shape and duration of the action potential. This late component had been shown to increase in several acquired or congenital conditions, including hypoxia, oxidative stress, and heart failure, or due to mutations in SCN5A, which encodes the α-subunit of the sodium channel, as well as in channel-interacting proteins, including multiple β subunits and anchoring proteins. Patients with enhanced late I Na exhibit the type-3 long QT syndrome (LQT3) characterized by high propensity for the life-threatening ventricular arrhythmias, such as Torsade de Pointes (TdP), as well as for atrial fibrillation. There are several distinct mechanisms of arrhythmogenesis due to abnormal late I Na, including abnormal automaticity, early and delayed after depolarization-induced triggered activity, and dramatic increase of ventricular dispersion of repolarization. Many local anesthetic and antiarrhythmic agents have a higher potency to block late I Na as compared with fast I Na. Several novel compounds, including ranolazine, GS-458967, and F15845, appear to be the most selective inhibitors of cardiac late I Na reported to date. Selective inhibition of late I Na is expected to be an effective strategy for correcting these acquired and congenital channelopathies.

Carbon monoxide effects on human ventricle action potential assessed by mathematical simulations

Carbon monoxide (CO) that is produced in a number of different mammalian tissues is now known to have significant effects on the cardiovascular system. These include: (i) vasodilation, (ii) changes in heart rate and strength of contractions, and (iii) modulation of autonomic nervous system input to both the primary pacemaker and the working myocardium. Excessive CO in the environment is toxic and can initiate or mediate life threatening cardiac rhythm disturbances. Recent reports link these ventricular arrhythmias to an increase in the slowly inactivating, or “late” component of the Na⁺ current in the mammalian heart. The main goal of this paper is to explore the basis of this pro-arrhythmic capability of CO by incorporating changes in CO-induced ion channel activity with intracellular signaling pathways in the mammalian heart. To do this, a quite well-documented mathematical model of the action potential and intracellular calcium transient in the human ventricular myocyte has been employed. In silico iterations based on this model provide a useful first step in illustrating the cellular electrophysiological consequences of CO that have been reported from mammalian heart experiments. Specifically, when the Grandi et al. model of the human ventricular action potential is utilized, and after the Na⁺ and Ca²⁺ currents in a single myocyte are modified based on the experimental literature, early after-depolarization (EAD) rhythm disturbances appear, and important elements of the underlying causes of these EADs are revealed/illustrated. Our modified mathematical model of the human ventricular action potential also provides a convenient digital platform for designing future experimental work and relating these changes in cellular cardiac electrophysiology to emerging clinical and epidemiological data on CO toxicity.

Caveolin-3 suppresses late sodium current by inhibiting nNOS-dependent S-nitrosylation of SCN5A

AIMS

Mutations in CAV3-encoding caveolin-3 (Cav3) have been implicated in type 9 long QT syndrome (LQT9) and sudden infant death syndrome (SIDS). When co-expressed with SCN5A-encoded cardiac sodium channels these mutations increased late sodium current (INa) but the mechanism was unclear. The present study was designed to address the mechanism by which the LQT9-causing mutant Cav3-F97C affects the function of caveolar SCN5A.

METHODS AND RESULTS

HEK-293 cells expressing SCN5A and LQT9 mutation Cav3-F97C resulted in a 2-fold increase in late INa compared to Cav3-WT. This increase was reversed by the neural nitric oxide synthase (nNOS) inhibitor L-NMMA. Based on these findings, we hypothesized that an nNOS complex mediated the effect of Cav3 on SCN5A. A SCN5A macromolecular complex was established in HEK-293 cells by transiently expressing SCN5A, α1-syntrophin (SNTA1), nNOS, and Cav3. Compared with Cav3-WT, Cav3-F97C produced significantly larger peak INa amplitudes, and showed 3.3-fold increase in the late INa associated with increased S-nitrosylation of SCN5A. L-NMMA reversed both the Cav3-F97C induced increase in late and peak INa and decreased S-nitrosylation of SCN5A. Overexpression of Cav3-F97C in adult rat cardiomyocytes caused a significant increase in late INa compared to Cav3-WT, and prolonged the action potential duration (APD90) in a nNOS-dependent manner.

CONCLUSIONS

Cav3 is identified as an important negative regulator for cardiac late INa via nNOS dependent direct S-nitrosylation of SCN5A. This provides a molecular mechanism for how Cav3 mutations increase late INa to cause LQT9. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".