Hyperpolarization-activated inward current in ventricular myocytes from normal and failing human hearts

Localization of hyperpolarization-activated cyclic nucleotide-gated channels in the vertebrate retinas across species and their physiological roles

Transmembrane proteins known as hyperpolarization-activated cyclic nucleotide-gated (HCN) channels control the movement of Na+ and K+ ions across cellular membranes. HCN channels are known to be involved in crucial physiological functions in regulating neuronal excitability and rhythmicity, and pacemaker activity in the heart. Although HCN channels have been relatively well investigated in the brain, their distribution and function in the retina have received less attention, remaining their physiological roles to be comprehensively understood. Also, because recent studies reported HCN channels have been somewhat linked with the dysfunction of photoreceptors which are affected by retinal diseases, investigating HCN channels in the retina may offer valuable insights into disease mechanisms and potentially contribute to identifying novel therapeutic targets for retinal degenerative disorders. This paper endeavors to summarize the existing literature on the distribution and function of HCN channels reported in the vertebrate retinas of various species and discuss the potential implications for the treatment of retinal diseases. Then, we recapitulate current knowledge regarding the function and regulation of HCN channels, as well as their relevance to various neurological disorders.

Prevalence and factors associated with initial and subsequent shockable cardiac arrest rhythms and their association with patient outcomes in dogs and cats undergoing cardiopulmonary resuscitation: A RECOVER registry study

OBJECTIVE

To report the prevalence of initial shockable cardiac arrest rhythms (I-SHKR), incidence of subsequent shockable cardiac arrest rhythms (S-SHKR), and factors associated with I-SHKRs and S-SHKRs and explore their association with return of spontaneous circulation (ROSC) rates in dogs and cats undergoing CPR.

DESIGN

Multi-institutional prospective case series from 2016 to 2021, retrospectively analyzed.

SETTING

Eight university and eight private practice veterinary hospitals.

ANIMALS

A total of 457 dogs and 170 cats with recorded cardiac arrest rhythm and event outcome reported in the Reassessment Campaign on Veterinary Resuscitation CPR registry.

MEASUREMENTS AND MAIN RESULTS

Logistic regression was used to evaluate association of animal, hospital, and arrest variables with I-SHKRs and S-SHKRs and with patient outcomes. Odds ratios (ORs) were generated, and significance was set at P < 0.05. Of 627 animals included, 28 (4%) had I-SHKRs. Odds for I-SHKRs were significantly higher in animals with a metabolic cause of arrest (OR 7.61) and that received lidocaine (OR 17.50) or amiodarone (OR 21.22) and significantly lower in animals experiencing arrest during daytime hours (OR 0.22), in the ICU (OR 0.27), in the emergency room (OR 0.13), and out of hospital (OR 0.18) and that received epinephrine (OR 0.19). Of 599 initial nonshockable rhythms, 74 (12%) developed S-SHKRs. Odds for S-SHKRs were significantly higher in animals with higher body weight (OR 1.03), hemorrhage (OR 2.85), or intracranial cause of arrest (OR 3.73) and that received epinephrine (OR 11.36) or lidocaine (OR 18.72) and significantly decreased in those arresting in ICU (OR 0.27), emergency room (OR 0.29), and out of hospital (OR 0.38). Overall, 171 (27%) animals achieved ROSC, 81 (13%) achieved sustained ROSC, and 15 (2%) survived. Neither I-SHKRs nor S-SHKRs were significantly associated with ROSC.

CONCLUSIONS

I-SHKRs and S-SHKRs occur infrequently in dogs and cats undergoing CPR and are not associated with increased ROSC rates.

Adapting to a new environment: postnatal maturation of the human cardiomyocyte

The postnatal mammalian heart undergoes remarkable developmental changes, which are stimulated by the transition from the intrauterine to extrauterine environment. With birth, increased oxygen levels promote metabolic, structural and biophysical maturation of cardiomyocytes, resulting in mature muscle with increased efficiency, contractility and electrical conduction. In this Topical Review article, we highlight key studies that inform our current understanding of human cardiomyocyte maturation. Collectively, these studies suggest that human atrial and ventricular myocytes evolve quickly within the first year but might not reach a fully mature adult phenotype until nearly the first decade of life. However, it is important to note that fetal, neonatal and paediatric cardiac physiology studies are hindered by a number of limitations, including the scarcity of human tissue, small sample size and a heavy reliance on diseased tissue samples, often without age-matched healthy controls. Future developmental studies are warranted to expand our understanding of normal cardiac physiology/pathophysiology and inform age-appropriate treatment strategies for cardiac disease.

Palmitoylation regulates the magnitude of HCN4-mediated currents in mammalian cells

The sinoatrial node (SAN) and subsidiary pacemakers in the cardiac conduction system generate spontaneous electrical activity which is indispensable for electrical and therefore contractile function of the heart. The hyperpolarisation-activated cyclic nucleotide-gated channel HCN4 is responsible for genesis of the pacemaker "funny" current during diastolic depolarisation. S-palmitoylation, the reversible conjugation of the fatty acid palmitate to protein cysteine sulfhydryls, regulates the activity of key cardiac Na+ and Ca2+ handling proteins, influencing their membrane microdomain localisation and function. We investigated HCN4 palmitoylation and its functional consequences in engineered human embryonic kidney 293T cells as well as endogenous HCN4 in neonatal rat ventricular myocytes. HCN4 was palmitoylated in all experimental systems investigated. We mapped the HCN4 palmitoylation sites to a pair of cysteines in the HCN4 intracellular amino terminus. A double cysteine-to-alanine mutation CC93A/179AA of full length HCN4 caused a ∼67% reduction in palmitoylation in comparison to wild type HCN4. We used whole-cell patch clamp to evaluate HCN4 current (IHCN4) in stably transfected 293T cells. Removal of the two N-terminal palmitoylation sites did not significantly alter half maximal activation voltage of IHCN4 or the activation slope factor. IHCN4 was significantly larger in cells expressing wild type compared to non-palmitoylated HCN4 across a range of voltages. Phylogenetic analysis revealed that although cysteine 93 is widely conserved across all classes of HCN4 vertebrate orthologs, conservation of cysteine 179 is restricted to placental mammals. Collectively, we provide evidence for functional regulation of HCN4 via palmitoylation of its amino terminus in vertebrates. We suggest that by recruiting the amino terminus to the bilayer, palmitoylation enhances the magnitude of HCN4-mediated currents, but does not significantly affect the kinetics.

Mechanism-based targeting of cardiac arrhythmias by phytochemicals and medicinal herbs: A comprehensive review of preclinical and clinical evidence

Cardiac arrhythmias, characterized by an irregular heartbeat, are associated with high mortality and morbidity. Because of the narrow therapeutic window of antiarrhythmic drugs (AADs), the management of arrhythmia is still challenging. Therefore, searching for new safe, and effective therapeutic options is unavoidable. In this study, the antiarrhythmic effects of medicinal plants and their active constituents were systematically reviewed to introduce some possible candidates for mechanism-based targeting of cardiac arrhythmias. PubMed, Embase, and Cochrane library were searched from inception to June 2021 to find the plant extracts, phytochemicals, and multi-component herbal preparations with antiarrhythmic activities. From 7337 identified results, 57 original studies consisting of 49 preclinical and eight clinical studies were finally included. Three plant extracts, eight multi-component herbal preparations, and 26 phytochemicals were found to have antiarrhythmic effects mostly mediated by affecting K+ channels, followed by modulating Ca2+ channels, upstream target pathways, Nav channels, gap junction channels, and autonomic receptors. The most investigated medicinal plants were Rhodiola crenulata and Vitis vinifera. Resveratrol, Oxymatrine, and Curcumin were the most studied phytochemicals found to have multiple mechanisms of antiarrhythmic action. This review emphasized the importance of research on the cardioprotective effect of medicinal plants and their bioactive compounds to guide the future development of new AADs. The most prevalent limitation of the studies was their unqualified methodology. Thus, future well-designed experimental and clinical studies are necessary to provide more reliable evidence.

The Interplay Between Gut Microbiota and miRNAs in Cardiovascular Diseases

The human microbiota contains microorganisms found on the skin, mucosal surfaces and in other tissues. The major component, the gut microbiota, can be influenced by diet, genetics, and environmental factors. Any change in its composition results in pathophysiological changes that can further influence the evolution of different conditions, including cardiovascular diseases (CVDs). The microbiome is a complex ecosystem and can be considered the metagenome of the microbiota. MicroRNAs (miRNAs) are speculated to interact with the intestinal microbiota for modulating gene expressions of the host. miRNAs represent a category of small non-coding RNAs, consisting of approximately 22 nucleotides, which can regulate gene expression at post-transcriptional level, by influencing the degradation of mRNA and modifying protein amounts. miRNAs display a multitude of roles, being able to influence the pathogenesis and progression of various diseases. Circulating miRNAs are stable against degradation, due to their enclosure into extracellular vesicles (EVs). This review aims to assess the current knowledge of the possible interactions between gut microbiota, miRNAs, and CVDs. As more scientific research is conducted, it can be speculated that personalized patient care in the future may include the management of gut microbiota composition and the targeted treatment against certain expression of miRNAs.

Pleiotropic vascular effects of ivabradine in streptozotocin-induced diabetes

AIMS

Ivabradine (IVA) reduces heart rate (HR) by inhibiting hyperpolarization-activated cyclic nucleotide-gated (HCN) channels in sinoatrial node. Studies suggest that IVA has other beneficial effects on cardiovascular system that are not related to its effect on HR such as prevention of endothelial injury and the antioxidant effects. In addition to sinoatrial node, HCN channels exist in other tissues and their expression pattern differs in certain pathologies such as hypertension and hypertrophy. We investigated the mechanism of IVA effect in the setting of streptozotocin (STZ)-induced cardiovascular damage. Direct effects of IVA and their mechanism on thoracic aorta as well as possible prevention of vascular dysfunction in diabetes were investigated in this study.

METHODS AND RESULTS

The effects of IVA on vascular function were investigated in control and STZ-diabetic rats. Some control and diabetic rats were treated with IVA. IVA treatment prevented diabetes-induced increase in plasma p-selectin and vascular cell adhesion molecule-1 levels and the decrease in nitric oxide content in the aortas of diabetic animals. When added to isolated organ bath, IVA induced concentration-dependent relaxations in thoracic aorta. Pre-incubation with Nω-Nitro- L -arginine methyl ester reduced IVA-induced relaxations. Expression patterns of all isoforms of HCN proteins were affected by both diabetes and IVA treatment.

CONCLUSION

IVA improves vascular function in diabetes and HCN channels support vascular activity against damaging effects of diabetes. IVA may be added to prevent diabetic cardiovascular dysfunction with these beneficial effects that are unrelated to its primary mechanism of action.

Effect of Ivabradine on Cardiac Ventricular Arrhythmias: Friend or Foe?

Life-threatening ventricular arrhythmias, such as ventricular tachycardia and ventricular fibrillation remain an ongoing clinical problem and their prevention and treatment require optimization. Conventional antiarrhythmic drugs are associated with significant proarrhythmic effects that often outweigh their benefits. Another option, the implantable cardioverter defibrillator, though clearly the primary therapy for patients at high risk of ventricular arrhythmias, is costly, invasive, and requires regular monitoring. Thus there is a clear need for new antiarrhythmic treatment strategies. Ivabradine, a heartrate-reducing agent, an inhibitor of HCN channels, may be one of such options. In this review we discuss emerging data from experimental studies that indicate new mechanism of action of this drug and further areas of investigation and potential use of ivabradine as an antiarrhythmic agent. However, clinical evidence is limited, and the jury is still out on effects of ivabradine on cardiac ventricular arrhythmias in the clinical setting.

Effect of ivabradine in heart failure: a meta-analysis of heart failure patients with reduced versus preserved ejection fraction

In clinical trials of heart failure reduced ejection fraction (HFrEF), ivabradine seemed to be an effective heart rate lowering agent associated with lower risk of cardiovascular death. In contrast, ivabradine failed to improve cardiovascular outcomes in heart failure preserved ejection fraction (HFpEF) despite the significant effect on heart rate. This meta-analysis is the first to compare the effects of ivabradine on heart rate and mortality parameters in HFpEF versus HFrEF. We screened three databases: PubMed, Embase, and Cochrane Library. The outcomes of these studies were mortality, reduction in heart rate, and left ventricular function improvement. We compared the efficacy of ivabradine treatment in HFpEF versus HFrEF. Heart rate analysis of pooled data showed decrease in both HFrEF (-17.646 beats/min) and HFpEF (-11.434 beats/min), and a tendency to have stronger bradycardic effect in HFrEF (p = 0.094) in randomized clinical trials. Left ventricular ejection fraction analysis revealed significant improvement in HFrEF (5.936, 95% CI: [4.199-7.672], p < 0.001) when compared with placebo (p < 0.001). We found that ivabradine significantly improves left ventricular performance in HFrEF, at the same time it exerts a tendency to have improved bradycardic effect in HFrEF. These disparate effects of ivabradine and the higher prevalence of non-cardiac comorbidities in HFpEF may explain the observed beneficial effects in HFrEF and the unchanged outcomes in HFpEF patients after ivabradine treatment.

Effect of ivabradine on cardiac arrhythmias: Antiarrhythmic or proarrhythmic?

Cardiac arrhythmias are a major source of mortality and morbidity. Unfortunately, their treatment remains suboptimal. Major classes of antiarrhythmic drugs pose a significant risk of proarrhythmia, and their side effects often outweigh their benefits. Therefore, implantable devices remain the only truly effective antiarrhythmic therapy, and new strategies of antiarrhythmic treatment are required. Ivabradine is a selective heart rate-reducing agent, an inhibitor of hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels, currently approved for treatment of coronary artery disease and chronic heart failure. In this review, we focus on the clinical and basic science evidence for the antiarrhythmic and proarrhythmic effects of ivabradine. We attempt to dissect the mechanisms behind the effects of ivabradine and indicate the focus of future studies.

Melatonin Prevents Early but Not Delayed Ventricular Fibrillation in the Experimental Porcine Model of Acute Ischemia

Antiarrhythmic effects of melatonin have been demonstrated ex vivo and in rodent models, but its action in a clinically relevant large mammalian model remains largely unknown. Objectives of the present study were to evaluate electrophysiological and antiarrhythmic effects of melatonin in a porcine model of acute myocardial infarction. Myocardial ischemia was induced by 40-min coronary occlusion in 25 anesthetized pigs. After ischemia onset, 12 animals received melatonin (4 mg/kg). 48 intramyocardial electrograms were recorded from left ventricular wall and interventricular septum (IVS). In each lead, activation time (AT) and repolarization time (RT) were determined. During ischemia, ATs and dispersion of repolarization (DOR = RTmax − RTmin) increased reaching maximal values by 3–5 and 20–25 min, respectively. Ventricular fibrillation (VF) incidence demonstrated no relations to redox state markers and was associated with increased DOR and delayed ATs (specifically, in an IVS base, an area adjacent to the ischemic zone) (p = 0.031). Melatonin prevented AT increase in the IVS base, (p < 0.001) precluding development of early VF (1–5 min, p = 0.016). VF occurrence in the delayed phase (17–40 min) where DOR was maximal was not modified by melatonin. Thus, melatonin-related enhancement of activation prevented development of early VF in the myocardial infarction model.

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

Disease-linked mutations alter the stoichiometries of HCN-KCNE2 complexes

The four hyperpolarization-activated cylic-nucleotide gated (HCN) channel isoforms and their auxiliary subunit KCNE2 are important in the regulation of peripheral and central neuronal firing and the heartbeat. Disruption of their normal function has been implicated in cardiac arrhythmias, peripheral pain, and epilepsy. However, molecular details of the HCN-KCNE2 complexes are unknown. Using single-molecule subunit counting, we determined that the number of KCNE2 subunits in complex with the pore-forming subunits of human HCN channels differs with each HCN isoform and is dynamic with respect to concentration. These interactions can be altered by KCNE2 gene-variants with functional implications. The results provide an additional consideration necessary to understand heart rhythm, pain, and epileptic disorders.

Human iPSC-Derived Cardiomyocytes for Investigation of Disease Mechanisms and Therapeutic Strategies in Inherited Arrhythmia Syndromes: Strengths and Limitations

During the last two decades, significant progress has been made in the identification of genetic defects underlying inherited arrhythmia syndromes, which has provided some clinical benefit through elucidation of gene-specific arrhythmia triggers and treatment. However, for most arrhythmia syndromes, clinical management is hindered by insufficient knowledge of the functional consequences of the mutation in question, the pro-arrhythmic mechanisms involved, and hence the most optimal treatment strategy. Moreover, disease expressivity and sensitivity to therapeutic interventions often varies between mutations and/or patients, underlining the need for more individualized strategies. The development of the induced pluripotent stem cell (iPSC) technology now provides the opportunity for generating iPSC-derived cardiomyocytes (CMs) from human material (hiPSC-CMs), enabling patient- and/or mutation-specific investigations. These hiPSC-CMs may furthermore be employed for identification and assessment of novel therapeutic strategies for arrhythmia syndromes. However, due to their relative immaturity, hiPSC-CMs also display a number of essential differences as compared to adult human CMs, and hence there are certain limitations in their use. We here review the electrophysiological characteristics of hiPSC-CMs, their use for investigating inherited arrhythmia syndromes, and their applicability for identification and assessment of (novel) anti-arrhythmic treatment strategies.

Dysfunctional Hyperpolarization-Activated Cyclic Nucleotide-gated Ion Channels in Cardiac Diseases

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are reverse voltage-dependent, and their activation depends on the hyperpolarization of the membrane and may be directly or indirectly regulated by the cyclic adenosine monophosphate (cAMP) or other signal-transduction cascades. The distribution, quantity and activation states of HCN channels differ in tissues throughout the body. Evidence exhibits that HCN channels play critical roles in the generation and conduction of the electrical impulse and the physiopathological process of some cardiac diseases. They may constitute promising drug targets in the treatment of these cardiac diseases. Pharmacological treatment targeting HCN channels is of benefit to these cardiac conditions.

Long-QT syndrome-associated caveolin-3 mutations differentially regulate the hyperpolarization-activated cyclic nucleotide gated channel 4

Background

Caveolin-3 (cav-3) mutations are linked to the long-QT syndrome (LQTS) causing distinct clinical symptoms. Hyperpolarization-activated cyclic nucleotide channel 4 (HCN4) underlies the pacemaker current If. It associates with cav-3 and both form a macromolecular complex.

Methods

To examine the effects of human LQTS-associated cav-3 mutations on HCN4-channel function, HEK293-cells were cotransfected with HCN4 and wild-type (WT) cav-3 or a LQTS-associated cav-3 mutant (T78M, A85T, S141R, or F97C). HCN4 currents were recorded using the whole-cell patch-clamp technique.

Results

WT cav-3 significantly decreased HCN4 current density and shifted midpoint of activation into negative direction. HCN4 current properties were differentially modulated by LQTS-associated cav-3 mutations. When compared with WT cav-3, A85T, F97C, and T78M did not alter the specific effect of cav-3, but S141R significantly increased HCN4 current density. Compared with WT cav-3, no significant modifications of voltage dependence of steady-state activation curves were observed. However, while WT cav-3 alone had no significant effect on HCN4 current activation, all LQTS-associated cav-3 mutations significantly accelerated HCN4 activation kinetics.

Conclusions

Our results indicate that HCN4 channel function is modulated by cav-3. LQTS-associated mutations of cav-3 differentially influence pacemaker current properties indicating a pathophysiological role in clinical manifestations.

Antiarrhythmic Action ofβ-Blockers: Potential Mechanisms

Sympathetic nervous system overactivity has been linked to ventricular tachyarrhythmias and sudden death. It has been hypothesized that the extent and nature of the arrhythmogenic effect of sympathetic stimulation depends on the underlying myocardial substrate, the mechanism of the arrhythmia, and the integrated effects of sympathetic stimulation in the particular individual circumstance. Multiple direct and indirect mechanisms of adrenergic action on the heart may benefit from the known antiarrhythmic actions of β-blocker therapy and other interventions that decrease sympathetic tone. The antiarrhythmic mechanism of β-blockade (and possibly α-blockade) will depend on the specific mechanism of the individual arrhythmia and will differ for those arrhythmias caused by tachycardia and ischemia, those caused by reentry and promoted by decreased conduction velocity and shortened refractoriness, and those caused by early or delayed afterdepolarizations, usually in the context of prolonged action potential duration. Antagonism of cardiac adrenergic activity by β-blockade in particular is the best-established drug therapy to prevent ventricular arrhythmias.

Current Perspectives of Electrical Remodeling and Its Therapeutic Implications

Electrical remodeling involves alterations in the electrophysiologic milieu of myocardium in various disease states, such as ventricular hypertrophy, heart failure, atrial tachyarrhythmias, myocardial ischemia, and infarction that are associated with cardiac arrhythmias. Although research in this area dates back to early part of the 19th century, we still lack the exact knowledge of ionic remodeling, the role of various genes and channel proteins, and their relevance for the newer antiarrhythmic therapies. Structural remodeling may also have an impact on the electrical remodeling process, although differences in both structural and electrical remodeling are associated with different disease states. Various electrophysiologic, cellular, and structural alterations, including anisotropic conduction, increased intracellular calcium levels, and gap junction remodeling predispose to increased dispersion of action potential duration and refractoriness. This constitutes a favorable substrate for early and late afterdepolarizations and reentrant arrhythmias. Studying the role of ionic remodeling in the initiation and propagation of cardiac arrhythmias has significant relevance for developing newer antiarrhythmic therapies, for identifying patients at risk of developing fatal arrhythmias, and for implementing effective preventive measures. Further research is required to understand the specific effects of individual ion channel remodeling, to understand the signal transduction mechanisms, and to address whether detrimental effects of electrical remodeling can be altered.

HCN Channels Modulators: The Need for Selectivity

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels, the molecular correlate of the hyperpolarization-activated current (If/Ih), are membrane proteins which play an important role in several physiological processes and various pathological conditions. In the Sino Atrial Node (SAN) HCN4 is the target of ivabradine, a bradycardic agent that is, at the moment, the only drug which specifically blocks If. Nevertheless, several other pharmacological agents have been shown to modulate HCN channels, a property that may contribute to their therapeutic activity and/or to their side effects. HCN channels are considered potential targets for developing drugs to treat several important pathologies, but a major issue in this field is the discovery of isoform-selective compounds, owing to the wide distribution of these proteins into the central and peripheral nervous systems, heart and other peripheral tissues. This survey is focused on the compounds that have been shown, or have been designed, to interact with HCN channels and on their binding sites, with the aim to summarize current knowledge and possibly to unveil useful information to design new potent and selective modulators.

Ivabradine: Heart Failure and Beyond

Heart failure affects over 5 million people in the United States and carries a high rate of mortality. Ivabradine, a new agent has been added to the current medical options for managing heart failure. It is a selective funny current (If) inhibitor in sinoatrial node and slows its firing rate, prolonging diastolic depolarization without a negative inotropic effect. Ivabradine was only recently approved by Food and Drug administration after the results of Systolic Heart Failure Treatment with the If Inhibitor Ivabradine (SHIFT) trial, for a reduction in rehospitalizations from chronic heart failure. This trial assessed patients with stable heart failure with reduced ejection fraction and a heart rate of at least 70 beats per minute at rest on maximally tolerated beta-blocker therapy and demonstrated statistically significant reduction in heart failure hospitalization and deaths. Additionally, ivabradine has been associated with reduced cardiac remodeling, reduced heart rate variability, improvement in exercise tolerance, improved heart failure class of New York Heart Association, and better quality of life. It has also been tried in other conditions, such as inappropriate sinus tachycardia and cardiogenic shock, and is currently in phase II trial for patients with newly diagnosed multiple organ dysfunction syndrome.

Isoform dependent regulation of human HCN channels by cholesterol

Cholesterol has been shown to regulate numerous ion channels. HCN channels represent the molecular correlate of I_f or I_h in sinoatrial node (SAN) and neuronal cells. Previous studies have implicated a role for cholesterol in the regulation of rabbit HCN4 channels with effects on pacing in the rabbit SAN. Using electrophysiological and biochemical approaches, we examined the effect of cholesterol modulation on human HCN1, HCN2 and HCN4 isoforms. Patch-clamp experiments uncovered isoform specific differences in the effect of cholesterol on gating kinetics upon depletion by MβCD or mevastatin or enrichment using MβCD/cholesterol. Most dramatically cholesterol had isoform specific effects on mode-shifting, which has been suggested to play a key role in stabilizing firing rate and preventing arrhythmic firing in SAN cells and neurons. Mode-shifting in HCN1 channels was insensitive to cholesterol manipulation, while HCN2 and HCN4 were strongly affected. Trafficking of each isoform to the plasma membrane was also affected by cholesterol modulation differentially between isoforms, however, each isoform remained localized in lipid raft domains after cholesterol depletion. These effects may contribute to the side effects of cholesterol reducing therapies including disrupted heart rhythm and neuropathic pain, as well as the susceptibility of sinus dysfunction in patients with elevated cholesterol.

HCN channels--modulators of cardiac and neuronal excitability

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels comprise a family of cation channels activated by hyperpolarized membrane potentials and stimulated by intracellular cyclic nucleotides. The four members of this family, HCN1-4, show distinct biophysical properties which are most evident in the kinetics of activation and deactivation, the sensitivity towards cyclic nucleotides and the modulation by tyrosine phosphorylation. The four isoforms are differentially expressed in various excitable tissues. This review will mainly focus on recent insights into the functional role of the channels apart from their classic role as pacemakers. The importance of HCN channels in the cardiac ventricle and ventricular hypertrophy will be discussed. In addition, their functional significance in the peripheral nervous system and nociception will be examined. The data, which are mainly derived from studies using transgenic mice, suggest that HCN channels contribute significantly to cellular excitability in these tissues. Remarkably, the impact of the channels is clearly more pronounced in pathophysiological states including ventricular hypertrophy as well as neural inflammation and neuropathy suggesting that HCN channels may constitute promising drug targets in the treatment of these conditions. This perspective as well as the current therapeutic use of HCN blockers will also be addressed.

miRNA in the regulation of ion channel/transporter expression

Ion channels and transporters are expressed in every living cell, where they participate in controlling a plethora of biological processes and physiological functions, such as excitation of cells in response to stimulation, electrical activities of cells, excitation-contraction coupling, cellular osmolarity, and even cell growth and death. Alterations of ion channels/transporters can have profound impacts on the cellular physiology associated with these proteins. Expression of ion channels/transporters is tightly regulated and expression deregulation can trigger abnormal processes, leading to pathogenesis, the channelopathies. While transcription factors play a critical role in controlling the transcriptome of ion channels/transporters at the transcriptional level by acting on the 5'-flanking region of the genes, microribonucleic acids (miRNAs), a newly discovered class of regulators in the gene network, are also crucial for expression regulation at the posttranscriptional level through binding to the 3'untranslated region of the genes. These small noncoding RNAs fine tune expression of genes involved in a wide variety of cellular processes. Recent studies revealed the role of miRNAs in regulating expression of ion channels/transporters and the associated physiological functions. miRNAs can target ion channel genes to alter cardiac excitability (conduction, repolarization, and automaticity) and affect arrhythmogenic potential of heart. They can modulate circadian rhythm, pain threshold, neuroadaptation to alcohol, brain edema, etc., through targeting ion channel genes in the neuronal systems. miRNAs can also control cell growth and tumorigenesis by acting on the relevant ion channel genes. Future studies are expected to rapidly increase to unravel a new repertoire of ion channels/transporters for miRNA regulation.

Roles of I(f) and intracellular Ca2+ release in spontaneous activity of ventricular cardiomyocytes during murine embryonic development

In recent years, the contribution of I(f), an important pacemaker current, and intracellular Ca(2+) release (ICR) from sarcoplasmic reticulum to pacemaking and arrhythmia has been intensively studied. However, their functional roles in embryonic heart remain uncertain. Using patch clamp, Ca(2+) imaging, and RT-PCR, we found that I(f) regulated the firing rate in early and late stage embryonic ventricular cells, as ivabradine (30 µM), a specific blocker of I(f), slowed down action potential (AP) frequency. This inhibitory effect was even stronger in late stage cells, though I(f) was down-regulated. In contrast to I(f), ICR was found to be indispensable for the occurrence of APs in ventricular cells of different stages, because abolishment of ICR with ryanodine and 2-aminoethoxydiphenyl borate (2-APB), specific blockers of ryanodine receptors (RyRs) and inositol trisphosphate receptors (IP3Rs), completely abolished APs. In addition, we noticed that RyR- and IP3R-mediated ICR coexisted in early-stage ventricular cells and RyRs functionally dominated. While at late stage RyRs, but not IP3Rs, mediated ICR. In both early and late stage ventricular cells, Na-Ca exchanger current (I(Na/Ca)) mediated ICR-triggered depolarization of membrane potential and resulted in the initiation of APs. We also observed that different from I(f), which presented as the substantial component of the earlier diastolic depolarization current, application of ryanodine, and/or 2-APB slowed the late phase of diastolic depolarization. Thus, we conclude that in murine embryonic ventricular cells I(f) regulates firing rate, while RyRs and IP3Rs (early stage) or RyRs (late stage)-mediated ICR determines the occurrence of APs.

Electrical homogenization of ventricular scar by application of collagenase: a novel strategy for arrhythmia therapy

BACKGROUND

Radiofrequency ablation for ventricular tachycardia is an established therapy. Use of chemical agents for scar homogenization represents an alternative approach. The purpose of this study was to characterize the efficacy of collagenase (CLG) application on epicardial ventricular scar homogenization.

METHODS AND RESULTS

Myocardial infarcts were created in Yorkshire pigs (n=6) by intracoronary microsphere injection. After 46.6±4.3 days, CLG type 2, type 4, and purified CLG were applied in vitro (n=1) to myocardial tissue blocks containing normal myocardium, border zone, and dense scar. Histopathologic studies were performed to identify the optimal CLG subtype. In vivo high-density electroanatomic mapping of the epicardium was also performed, and border zone and dense scar surface area and late potentials were quantified before and after CLG-4 application (n=5). Of the CLG subtypes tested in vitro, CLG-4 provided the best scar modification and least damage to normal myocardium. During in vivo testing, CLG-4 application decreased border zone area (21.3±14.3 to 17.1±11.1 mm(2), P=0.043) and increased dense scar area (9.1±10.3 to 22.0±20.6 mm(2), P=0.043). The total scar area before and after CLG application was 30.4±23.4 and 39.2±29.5 mm(2), respectively (P=0.08). Late potentials were reduced by CLG-4 application (28.8±21.8 to 13.8±13.1, P=0.043). During CLG-4 application (50.0±15.5 minutes), systolic blood pressure and heart rate were not significantly changed (68.0±7.7 versus 61.8±5.3 mmHg, P=0.08; 77.4±7.3 versus 78.8±6.0 beats per minute, P=0.50, respectively).

CONCLUSIONS

Ventricular epicardial scar homogenization by CLG-4 application is feasible and effective. This represents the first report on bioenzymatic ablation of arrhythmogenic tissue as an alternative strategy for lesion formation.

The role of HCN channels in ventricular repolarization

Hyperpolarization-activated cyclic nucleotide gated (HCN) channels pass a cationic current (I(h)/I(f)) that crucially contributes to the slow diastolic depolarization (SDD) of sinoatrial pacemaker cells and, hence, is a key determinant of cardiac automaticity and the generation of the heartbeat. However, there is growing evidence that HCN channels are not restricted to the spontaneously active cells of the sinoatrial node and the conduction system but are also present in ventricular cardiomyocytes that produce an action potential lacking SDD. This observation raises the question of the principal function(s) of HCN channels in working myocardium. Our recent analysis of an HCN3-deficient (HCN3-/-) mouse line has shed new light on this central question. We propose that HCN channels contribute to the ventricular action potential waveform, specifically during late repolarization. In this review, we outline this new concept.

HCN channels in the heart: lessons from mouse mutants

Hyperpolarization-activated cation channels generate the I(f) current in the heart. In the sino-atrial node (SAN), I(f) is thought to play an essential role in setting the heart rate and mediating its autonomic control. This review focuses on the role of I(f) in pacemaking and non-pacemaking cardiomyocytes and the resulting therapeutic implications. HCN4 represents the principal isoform underlying sino-atrial I(f) , but other isoforms may also be of importance. To examine the functional role of cardiac channels, several mouse mutants, most of them targeting HCN4, have been generated by different groups. Unexpectedly, these lines display greatly different and as yet unexplained phenotypes. We provide an overview about these HCN mutants and suggest an interpretation of the functional significance of I(f) in the SAN in light of these studies. HCN channels are also present in ventricular myocytes, and an up-regulation of I(f) in the hypertrophic and failing heart may contribute to arrhythmogenesis. Inhibition of I(f) by HCN channel blockers is a novel approach in the treatment of cardiac disorders, and ivabradine is approved for treatment of stable angina pectoris. Remarkably, a recent clinical trial assessing this substance in heart failure showed a significantly improved outcome. The mechanism underlying this beneficial effect is not yet clear and might lie beyond heart rate slowing. Thus, the growing knowledge about cardiac HCN channels will undoubtedly promote the development of the promising class of HCN channel blockers.

Induced pluripotent stem cell derived cardiomyocytes as models for cardiac arrhythmias

Cardiac arrhythmias are a major cause of morbidity and mortality. In younger patients, the majority of sudden cardiac deaths have an underlying Mendelian genetic cause. Over the last 15 years, enormous progress has been made in identifying the distinct clinical phenotypes and in studying the basic cellular and genetic mechanisms associated with the primary Mendelian (monogenic) arrhythmia syndromes. Investigation of the electrophysiological consequences of an ion channel mutation is ideally done in the native cardiomyocyte (CM) environment. However, the majority of such studies so far have relied on heterologous expression systems in which single ion channel genes are expressed in non-cardiac cells. In some cases, transgenic mouse models have been generated, but these also have significant shortcomings, primarily related to species differences. The discovery that somatic cells can be reprogrammed to pluripotency as induced pluripotent stem cells (iPSC) has generated much interest since it presents an opportunity to generate patient- and disease-specific cell lines from which normal and diseased human CMs can be obtained These genetically diverse human model systems can be studied in vitro and used to decipher mechanisms of disease and identify strategies and reagents for new therapies. Here, we review the present state of the art with respect to cardiac disease models already generated using IPSC technology and which have been (partially) characterized. Human iPSC (hiPSC) models have been described for the cardiac arrhythmia syndromes, including LQT1, LQT2, LQT3-Brugada Syndrome, LQT8/Timothy syndrome and catecholaminergic polymorphic ventricular tachycardia (CPVT). In most cases, the hiPSC-derived cardiomyoctes recapitulate the disease phenotype and have already provided opportunities for novel insight into cardiac pathophysiology. It is expected that the lines will be useful in the development of pharmacological agents for the management of these disorders.

Inter-individual variability in the pre-clinical drug cardiotoxic safety assessment--analysis of the age-cardiomyocytes electric capacitance dependence

Electrical phenomena located within the plasma membrane of the mammalian cardiac cells are connected with the cells' main physiological functions--signals processing and contractility. They were extensively studied and described mathematically in so-called Hodgkin-Huxley paradigm. One of the physiological parameters, namely cell electric capacitance, has not been analyzed in-depth. The aim of the study was to validate the mechanistic model describing the capacitive properties of cells, based on a collected experimental dataset which describes the electric capacitance of human ventricular myocytes. The gathered data was further utilized for developing an empirical correlation between a healthy individual's age and cardiomyocyte electric capacitance.

Ventricular HCN channels decrease the repolarization reserve in the hypertrophic heart

AIMS

Cardiac hypertrophy is accompanied by reprogramming of gene expression, where the altered expression of ion channels decreases electrical stability and increases the risk of life-threatening arrhythmias. However, the underlying mechanisms are not fully understood. Here, we analysed the role of the depolarizing current I(f) which has been hypothesized to contribute to arrhythmogenesis in the hypertrophied ventricle.

METHODS AND RESULTS

We used transverse aortic constriction in mice to induce ventricular hypertrophy. This resulted in an increased number of I(f) positive ventricular myocytes as well as a strongly enhanced and accelerated I(f) when compared with controls. Of the four HCN (hyperpolarization-activated cyclic nucleotide-gated channels) isoforms mediating I(f), HCN2 and HCN4 were the predominantly expressed subunits in healthy as well as hypertrophied hearts. Unexpectedly, only the HCN1 transcript was significantly upregulated in response to hypertrophy. However, the combined deletion of HCN2 and HCN4 disrupted ventricular I(f) completely. The lack of I(f) in hypertrophic double-knockouts resulted in a strong attenuation of pro-arrhythmogenic parameters characteristically observed in hypertrophic hearts. In particular, prolongation of the action potential was significantly decreased and lengthening of the QT interval was reduced.

CONCLUSIONS

We suggest that the strongly increased HCN channel activity in hypertrophied myocytes prolongs the repolarization of the ventricular action potential and thereby may increase the arrhythmogenic potential. Our results provide for the first time a direct link between an upregulation of ventricular I(f) and a diminished repolarization reserve in cardiac hypertrophy.

Antiapolipoprotein A-1 IgG chronotropic effects require nongenomic action of aldosterone on L-type calcium channels

Autoantibodies to apolipoprotein A-1 (antiapoA-1 IgG) have been shown to be associated with higher resting heart rate and morbidity in myocardial infarction patients and to behave as a chronotropic agent in the presence of aldosterone on isolated neonatal rat ventricular cardiomyocytes (NRVC). We aimed at identifying the pathways accounting for this aldosterone-dependent antiapoA-1 IgG-positive chronotropic effect on NRVC. The rate of regular spontaneous contractions was determined on NRVC in the presence of different steroid hormones and antagonists. AntiapoA-1 IgG chronotropic response was maximal within 20 min and observed only in aldosterone-pretreated cells but not in those exposed to other steroids. The positive antiapoA-1 IgG chronotropic effect was already significant after 5 min aldosterone preincubation, was dependent on 3-kinase and protein kinase A activities, was not inhibited by actinomycin D, and was fully abrogated by eplerenone (but not by spironolactone), demonstrating the dependence on a nongenomic action of aldosterone elicited through the mineralocorticoid receptor (MR). Under oxidative conditions (but not under normal redox state), corticosterone mimicked the permissive action of aldosterone on the antiapoA-1 IgG chronotropic response. Pharmacological and patch-clamp studies identified L-type calcium channels as crucial effectors of antiapoA-1 IgG chronotropic action, involving two converging pathways that increase the channel activity. The first one involves the rapid, nongenomic activation of the phosphatidylinositol 3-kinase enzyme by MR, and the second one requires a constitutive basal protein kinase A activity. In conclusion, our results indicate that, on NRVC, the aldosterone-dependent chronotropic effects of antiapoA-1 IgG involve the nongenomic activation of L-type calcium channels.

Effects of a novel amiodarone-like compound SAR114646A on the pig atrium and susceptibility to ventricular fibrillation in dogs and pigs

Amiodarone is one of the most effective antiarrhythmic drugs. However, poor solubility of this compound has limited its intravenous application. SAR11464A is a water-soluble amiodarone-like drug that lacks iodine and inhibits multiple cardiac ion channels in vitro. This study evaluated the antiarrhythmic efficacy of this drug in vivo. In porcine studies, atrial effective refractory period (AERP) was measured in pentobarbital-anesthetized thoracotomized pigs and atrial fibrillation (AF) was induced by a premature beat. Ventricular fibrillation (VF) was induced via either burst pacing or programmed electrical stimulation (a series of progressively shorter beats, S1-S5). In canine studies, VF was induced by a 2-min occlusion of the left circumflex coronary artery during the last minute of exercise in dogs with healed myocardial infarctions (n = 8). One week later, this test was repeated after pretreatment with SAR114646A (3.0 mg/kg, i.v., slow bolus). SAR114646A produced a significant dose-dependent prolongation of AERP, inhibited AF induced by a premature stimulus, and electrically induced VF in anesthetized pigs. At 1.0 and 3.0 mg/kg, i.v., it was superior to amiodarone, dofetilide, and flecainide. In dogs, SAR114646A did not alter any ECG parameter including QTc (control, 236.9 ± 8.5 ms vs. SAR, 237.2 ± 3.5 ms) but significantly reduced the incidence of VF, protecting six of eight animals (Fisher's exact test, P = 0.01). SAR114646A was effective against both atrial and ventricular arrhythmias without altering ventricular repolarization. These data suggest that the amiodarone-like drug SAR114646A may be an effective antiarrhythmic intervention that does not adversely prolong ventricular repolarization.

Drug therapy for ventricular tachyarrhythmia in heart failure

Sudden cardiac death (SCD) accounts for approximately one-third of all deaths in patients with heart failure, and is generally the result of ventricular tachycardia (VT) and/or ventricular fibrillation (VF). The mechanisms of VT/VF associated with heart failure are complex and heterogeneous; they include functional and structural remodeling, as well as neurohormonal activation. The implantable cardioverter-defibrillator is very useful for preventing SCD, but the improvement of outcome is limited in patients with cardiac dysfunction and advanced heart failure. This article reviews the current status of drug therapy for the treatment of VT/VF in patients with heart failure. Chronic beta-blocker therapy reduces SCD and improves survival. Angiotensin-converting enzyme inhibitors, angiotensin-receptor blockers and aldosterone antagonists are thought to reduce SCD by preventing ventricular remodeling. Amiodarone is potentially effective for preventing VT/VF in patients at high risk, especially those with nonischemic heart failure. This may be a result of the complex pharmacodynamics of amiodarone, which affects many kinds of ion channels/transporters, as well as thyroid function. The pure class III antiarrhythmic drug, nifekalant, is useful in the emergency treatment of VT/VF.

The increase in rat ventricular automaticity induced by salbutamol is mediated through β(1)- but not β(2)-adrenoceptors: role of phosphodiesterases

AIMS

While β(2)-adrenoceptor (AR) agonists are useful bronchodilators, they also produce cardiac arrhythmias. These agents are not fully selective and also activate β(1)-AR, but the involvement of β(1)-AR and β(2)-AR in the observed pro-arrhythmic effect has not been established. We studied the effect of β(1)-AR and β(2)-AR activation on ventricular automaticity and the role of phosphodiesterases (PDE) in regulating this effect.

MAIN METHODS

Experiments were performed in the spontaneously beating isolated right ventricle of the rat heart. We also measured cAMP production in this tissue.

KEY FINDINGS

The β(2)-AR agonist salbutamol (1-100 μM) produced a concentration-dependent increase in ventricular automaticity that was not affected by 50nM of the β(2)-AR antagonist ICI 118551. This effect was enhanced by the non-selective PDE inhibitor theophylline (100 μM) and by the selective PDE4 inhibitors rolipram (1 μM) and Ro 201724 (2 μM), but not modified by the selective PDE3 inhibitors cilostamide (0.3 μM) or milrinone (0.2 μM). The effects of salbutamol alone and in the presence of either theophylline or rolipram were virtually abolished by 0.1 μM β(1)-AR antagonist CGP 20712A. Salbutamol (10 μM) increased the cAMP concentration, and this effect was abolished by CGP 20712A (0.1 μM) but enhanced by theophylline (100 μM) or rolipram (1 μM). Cilostamide (0.3 μM) failed to modify the effect of salbutamol on cAMP concentration.

SIGNIFICANCE

These results indicate that the increase of ventricular automaticity elicited by salbutamol was exclusively mediated through β(1)-AR and enhanced by non-selective PDE inhibition with theophylline or selective PDE4 inhibition. However, PDE3 did not appear to regulate this effect.

Local anesthetic inhibits hyperpolarization-activated cationic currents

Systemic administration of local anesthetics has beneficial perioperative properties and an anesthetic-sparing and antiarrhythmic effect, although the detailed mechanisms of these actions remain unclear. In the present study, we investigated the effects of a local anesthetic, lidocaine, on hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels that contribute to the pacemaker currents in rhythmically oscillating cells of the heart and brain. Voltage-clamp recordings were used to examine the properties of cloned HCN subunit currents expressed in Xenopus laevis oocytes and human embryonic kidney (HEK) 293 cells under control condition and lidocaine administration. Lidocaine inhibited HCN1, HCN2, HCN1-HCN2, and HCN4 channel currents at 100 μM in both oocytes and/or HEK 293 cells; it caused a decrease in both tonic and maximal current (∼30-50% inhibition) and slowed current activation kinetics for all subunits. In addition, lidocaine evoked a hyperpolarizing shift in half-activation voltage (ΔV(1/2) of ∼-10 to -14 mV), but only for HCN1 and HCN1-HCN2 channels. By fitting concentration-response data to logistic functions, we estimated half-maximal (EC(50)) concentrations of lidocaine of ∼30 to 40 μM for the shift in V(1/2) observed with HCN1 and HCN1-HCN2; for inhibition of current amplitude, calculated EC(50) values were ∼50 to 70 μM for HCN1, HCN2, and HCN1-HCN2 channels. A lidocaine metabolite, monoethylglycinexylidide (100 μM), had similar inhibitory actions on HCN channels. These results indicate that lidocaine potently inhibits HCN channel subunits in dose-dependent manner over a concentration range relevant for systemic application. The ability of local anesthetics to modulate I(h) in central neurons may contribute to central nervous system depression, whereas effects on I(f) in cardiac pacemaker cells may contribute to the antiarrhythmic and/or cardiovascular toxic action.

MicroRNAs and atrial fibrillation: new fundamentals

Atrial fibrillation (AF) is the most commonly encountered clinical arrhythmia associated with pronounced morbidity, mortality, and socio-economic burden. This pathological entity is associated with an altered expression profile of genes that are important for atrial function. MicroRNAs (miRNAs), a new class of non-coding mRNAs of around 22 nucleotides in length, have rapidly emerged as one of the key players in the gene expression regulatory network. The potential roles of miRNAs in controlling AF have recently been investigated. The studies have provided some promising results for our better understanding of the molecular mechanisms of AF. In this review article, we provide a synopsis of the studies linking miRNAs to cardiac excitability and other processes pertinent to AF. To introduce the main topic, we discuss basic knowledge about miRNA biology and our current understanding of mechanisms for AF. The most up-to-date research data on the possible roles of miRNAs in AF initiation and maintenance are presented, and the available experimental results on miRNA and AF are discussed. Some speculations pertinent to the subject are made. Finally, perspectives on future directions of research on miRNAs in AF are provided.