Occurrence and properties of the hyperpolarization-activated current If in ventricular myocytes from normotensive and hypertensive rats during aging

Age-related increases in cardiac excitability, refractoriness and impulse conduction favor arrhythmogenesis in male rats

The effects of excitability, refractoriness, and impulse conduction have been independently related to enhanced arrhythmias in the aged myocardium in experimental and clinical studies. However, their combined arrhythmic effects in the elderly are not yet completely understood. Hence, the aim of the present work is to relate relevant cardiac electrophysiological parameters to enhanced arrhythmia vulnerability in the in vivo senescent heart. We used multiple-lead epicardial potential mapping in control (9-month-old) and aged (24-month-old) rat hearts. Cardiac excitability and refractoriness were evaluated at numerous epicardial test sites by means of the strength-duration curve and effective refractory period, respectively. During sinus rhythm, durations of electrogram intervals and waves were prolonged in the senescent heart, compared with control, demonstrating a latency in tissue activation and recovery. During ventricular pacing, cardiac excitability, effective refractory period, and dispersion of refractoriness increased in the aged animal. This scenario was accompanied by impairment of impulse propagation. Moreover, both spontaneous and induced arrhythmias were increased in senescent cardiac tissue. Histopathological evaluation of aged heart specimens revealed connective tissue deposition and perinuclear myocytolysis in the atria, while scattered microfoci of interstitial fibrosis were mostly present in the ventricular subendocardium. This work suggests that enhanced arrhythmogenesis in the elderly is a multifactorial process due to the joint increase in excitability and dispersion of refractoriness in association with enhanced conduction inhomogeneity. The knowledge of these electrophysiological changes will possibly contribute to improved prevention of the age-associated increase in cardiac arrhythmias.

A novel modular modeling approach for understanding different electromechanics between left and right heart in rat

While ion channels and transporters involved in excitation-contraction coupling have been linked and constructed as comprehensive computational models, validation of whether each individual component of a model can be reused has not been previously attempted. Here we address this issue while using a novel modular modeling approach to investigate the underlying mechanism for the differences between left ventricle (LV) and right ventricle (RV). Our model was developed from modules constructed using the module assembly principles of the CellML model markup language. The components of three existing separate models of cardiac function were disassembled as to create smaller modules, validated individually, and then the component parts were combined into a new integrative model of a rat ventricular myocyte. The model was implemented in OpenCOR using the CellML standard in order to ensure reproducibility. Simulated action potential (AP), Ca²⁺ transient, and tension were in close agreement with our experimental measurements: LV AP showed a prolonged duration and a more prominent plateau compared with RV AP; Ca²⁺ transient showed prolonged duration and slow decay in LV compared to RV; the peak value and relaxation of tension were larger and slower, respectively, in LV compared to RV. Our novel approach of module-based mathematical modeling has established that the ionic mechanisms underlying the APs and Ca²⁺ handling play a role in the variation in force production between ventricles. This simulation process also provides a useful way to reuse and elaborate upon existing models in order to develop a new model.

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.

The HCN channel as a pharmacological target: Why, where, and how to block it

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels, expressed in a variety of cell types and in all tissues, control excitation and rhythm. Since their discovery in neurons and cardiac pacemaker cells, they attracted the attention of medicinal chemistry and pharmacology as novel targets to shape (patho)physiological mechanisms. To date, ivabradine represents the first-in-class drug as specific bradycardic agent in cardiac diseases; however, new applications are emerging in parallel with the demonstration of the involvement of different HCN isoforms in central and peripheral nervous system. Hence, the possibility to target specific isoforms represents an attractive development in this field; indeed, HCN1, HCN2 or HCN4 specific blockers have shown promising features in vitro and in vivo, with remarkable pharmacological differences likely depending on the diverse functional role and tissue distribution. Here, we show a recently developed compound with high potency as HCN2-HCN4 blocker; because of its unique profile, this compound may deserve further investigation.

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.

The rationale for repurposing funny current inhibition for management of ventricular arrhythmia

Management of ventricular arrhythmia in structural heart disease is complicated by the toxicity of the limited antiarrhythmic options available. In others, proarrhythmia and deleterious hemodynamic and noncardiac effects prevent practical use. This necessitates new thinking in therapeutic agents for ventricular arrhythmia in structural heart disease. Ivabradine, a funny current (I_f) inhibitor, has proven safety in heart failure, angina, and inappropriate sinus tachycardia. Although it is commonly known that funny channels are primarily expressed in the sinoatrial node, atrioventricular node, and conducting system of the ventricle, ivabradine is known to exert effects on metabolism, ion homeostasis, and membrane electrophysiology of remodeled ventricular myocardium. This review considers novel concepts and evidence from clinical and experimental studies regarding this paradigm, with a potential role of ivabradine in ventricular arrhythmia.

Hyperpolarization-activated cyclic-nucleotide-gated channels: pathophysiological, developmental, and pharmacological insights into their function in cellular excitability

The hyperpolarization-activated cyclic-nucleotide-gated (HCN) proteins are voltage-dependent ion channels, conducting both Na⁺ and K⁺, blocked by millimolar concentrations of extracellular Cs⁺ and modulated by cyclic nucleotides (mainly cAMP) that contribute crucially to the pacemaker activity in cardiac nodal cells and subsidiary pacemakers. Over the last decades, much attention has focused on HCN current, I_f, in non-pacemaker cardiac cells and its potential role in triggering arrhythmias. In fact, in addition to pacemakers, HCN current is constitutively present in the human atria and has long been proposed to sustain atrial arrhythmias associated to different cardiac pathologies or triggered by various modulatory signals (catecholamines, serotonin, natriuretic peptides). An atypical I_f occurs in diseased ventricular cardiomyocytes, its amplitude being linearly related to the severity of cardiac hypertrophy. The properties of atrial and ventricular I_f and its modulation by pharmacological interventions has been object of intense study, including the synthesis and characterization of new compounds able to block preferentially HCN1, HCN2, or HCN4 isoforms. Altogether, clues emerge for opportunities of future pharmacological strategies exploiting the unique properties of this channel family: the prevalence of different HCN subtypes in organs and tissues, the possibility to target HCN gain- or loss-of-function associated with disease, the feasibility of novel isoform-selective drugs, as well as the discovery of HCN-mediated effects for old medicines.

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.

Age-dependent down-regulation of hyperpolarization-activated cyclic nucleotide-gated channel 4 causes deterioration of canine sinoatrial node function

The activity of pacemaker cells in the sinoatrial node (SAN) is an indicator of normal sinus rhythm. Clinical studies have revealed that the dysfunction of the SAN progressively increases with aging. In this study, we determined the changes in hyperpolarization-activated cyclic nucleotide-gated channel 4 (HCN4) expression and the relationship between aging and canine SAN dysfunction. The results of cardiac electrophysiological determination revealed that the intrinsic heart rate decreased from 168 ± 11 beats min-1 in young canines to 120 ± 9 beats min-1 in adults and to 88 ± 9 beats min-1 in aged canines. The sinus node recovery time (SNRT) increased from 412 ± 32 ms in young canines to 620 ± 56 ms in adults and to 838 ± 120 ms in aged canines. Corrected SNRT (CSNRT) increased from 55 ± 12 ms in young canines to 117 ± 27 ms in adults and to 171 ± 37 ms in aged canines. These results indicated that SAN function deteriorated with aging in the canine heart. However, histological staining illustrated that fibrosis was not significantly increased with aging in canine SAN. Real-time polymerase chain reaction indicated that the expression of HCN4 mRNA was downregulated in the elderly canine SAN. Similarly, we also verified that HCN4 protein expression within the SAN declined with aging via immunofluorescence staining and western blot analysis. Taken together, our data show that electrical remodeling, related to the down-regulation of HCN4, is responsible for the gradually increased incidence of SAN dysfunction with aging. Our results provide further evidence for explaining the mechanisms of age-related deterioration in the SAN.

Reduced expression of HCN channels in the sinoatrial node of streptozotocin-induced diabetic rats

Diabetes mellitus (DM) is associated with an electrical remodeling of the heart, increasing the risk of arrhythmias. However, knowledge of electrical remodeling in the sinoatrial node (SAN) by DM is limited. We investigated the expression of HCN channel isoforms, HCN1-HCN4, in SAN from streptozotocin (STZ)-induced diabetic rats and the age-matched controls. We found that the STZ-induced diabetic rats have a lower intrinsic heart rate, a lengthened sinoatrial conduction time, and rate-corrected maximal sinoatrial node recovery time in vivo as well as a longer cycle length (CL) in vitro, as compared with the control. Optical mapping of the SAN demonstrated an inferior leading pacemaker site, reduced SAN conduction velocity and diastolic depolarization slope, and a longer action potential duration in the STZ-induced diabetic rats than in the control. The transcripts and proteins of HCN2 and HCN4 in diabetic SAN were reduced. Specific blockade of HCN channels by 3 μmol/L ivabradine significantly prolonged the CL of a Langendorff heart by 18% in the diabetic rats and 26% in the control. The reduced expression of HCN channel isoforms in the SAN of the STZ-induced diabetic rat may be an important contributor to the reduced SAN function in DM.

Spironolactone Regulates HCN Protein Expression Through Micro-RNA-1 in Rats With Myocardial Infarction

Emerging evidence has shown that aldosterone blockers reduced the incidence of ventricular arrhythmias in patients with myocardial infarction (MI). However, the mechanism remains unknown. In this study, we investigated the mechanism by which spironolactone, a classic aldosterone blocker, regulates hyperpolarization-activated cyclic nucleotide-gated channel (HCN) protein expression in ischemic rat myocardium after MI. Eighteen rats surviving 24 hours after MI were randomly assigned into 3 groups: MI, spironolactone, and spironolactone + antagomir-1. Six sham-operated rats had a suture loosely tied around the left coronary artery, without ligation. The border zone of the myocardial infarct was collected from each rat at 1 week after MI. HCN2 and HCN4 protein and messenger RNA (mRNA) level were measured in addition to miRNA-1 levels. Spironolactone significantly increased miRNA-1 levels and downregulated HCN2 and HCN4 protein and mRNA levels. miRNA-1 suppression with antagomir-1 increased HCN2 and HCN4 protein levels; however, HCN2 and HCN4 mRNA levels were not affected. These results suggested that spironolactone could increase miRNA-1 expression in ischemic rat myocardium after MI and that the upregulation of miRNA-1 expression partially contributed to the posttranscriptional repression of HCN protein expression, which may contribute to the effect of spironolactone to reduce the incidence of MI-associated ventricular arrhythmias.

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.

Altered Left Ventricular Ion Channel Transcriptome in a High-Fat-Fed Rat Model of Obesity: Insight into Obesity-Induced Arrhythmogenesis

Introduction. Obesity is increasingly common and is associated with an increased prevalence of cardiac arrhythmias. The aim of this study was to see whether in obesity there is proarrhythmic gene expression of ventricular ion channels and related molecules. Methods and Results. Rats were fed on a high-fat diet and compared to control rats on a normal diet (n = 8). After 8 weeks, rats on the high-fat diet showed significantly greater weight gain and higher adiposity. Left ventricle samples were removed at 8 weeks and mRNA expression of ion channels and other molecules was measured using qPCR. Obese rats had significant upregulation of Ca_v1.2, HCN4, K_ir2.1, RYR2, NCX1, SERCA2a, and RYR2 mRNA and downregulation of ERG mRNA. In the case of HCN4, it was confirmed that there was a significant increase in protein expression. The potential effects of the mRNA changes on the ventricular action potential and intracellular Ca²⁺ transient were predicted using computer modelling. Modelling predicted prolongation of the ventricular action potential and an increase in the intracellular Ca²⁺ transient, both of which would be expected to be arrhythmogenic. Conclusion. High-fat diet causing obesity results in arrhythmogenic cardiac gene expression of ion channels and related molecules.

Advantage of four-electrode over two-electrode defibrillators

Defibrillation is the standard clinical treatment used to stop ventricular fibrillation. An electrical device delivers a controlled amount of electrical energy via a pair of electrodes in order to reestablish a normal heart rate. We propose a technique that is a combination of biphasic shocks applied with a four-electrode system rather than the standard two-electrode system. We use a numerical model of a one-dimensional ring of cardiac tissue in order to test and evaluate the benefit of this technique. We compare three different shock protocols, namely a monophasic and two types of biphasic shocks. The results obtained by using a four-electrode system are compared quantitatively with those obtained with the standard two-electrode system. We find that a huge reduction in defibrillation threshold is achieved with the four-electrode system. For the most efficient protocol (asymmetric biphasic), we obtain a reduction in excess of 80% in the energy required for a defibrillation success rate of 90%. The mechanisms of successful defibrillation are also analyzed. This reveals that the advantage of asymmetric biphasic shocks with four electrodes lies in the duration of the cathodal and anodal phase of the shock.

TRPM4 in cardiac electrical activity

TRPM4 forms a non-selective cation channel activated by internal Ca(2+). Its functional expression was demonstrated in cardiomyocytes of several mammalian species including humans, but the channel is also present in many other tissues. The recent characterization of the TRPM4 inhibitor 9-phenanthrol, and the availability of transgenic mice have helped to clarify the role of TRPM4 in cardiac electrical activity, including diastolic depolarization from the sino-atrial node cells in mouse, rat, and rabbit, as well as action potential duration in mouse cardiomyocytes. In rat and mouse, pharmacological inhibition of TRPM4 prevents cardiac ischaemia-reperfusion injuries and decreases the occurrence of arrhythmias. Several studies have identified TRPM4 mutations in patients with inherited cardiac diseases including conduction blocks and Brugada syndrome. This review identifies TRPM4 as a significant actor in cardiac electrophysiology.

Inhibition of type 5 phosphodiesterase counteracts β2-adrenergic signalling in beating cardiomyocytes

AIMS

Compartmentalization of cAMP and PKA activity in cardiac muscle cells plays a key role in maintaining basal and enhanced contractility stimulated by sympathetic nerve activity. In cardiomyocytes, activation of adrenergic receptor increases cAMP production, which is countered by the hydrolytic activity of selective phosphodiesterases (PDEs). The intracellular regional dynamics of cAMP production and hydrolysis modulate downstream signals resulting in different biological responses. The interplay between beta receptors (βARs) signalling and phosphodiesterase 5 (PDE5) activity remains to be addressed.

METHODS AND RESULTS

Using combined strategies with pharmacological inhibitors and genetic deletion of PDEs and βAR isoforms, we revealed a specific pool of cAMP that is under dual regulation by PDE2 and, indirectly, PDE5 activity. Inhibition of PDE5 with sildenafil produces a cGMP-dependent activation of PDE2 that attenuates cAMP generation induced by βAR agonists, with concomitant modulation of stimulated contraction rate and calcium transients. PDE2 haploinsufficiency abolished the effects of sildenafil. The negative chronotropic effect of PDE5 inhibition through PDE2 activation was also observed in sinoatrial node tissue from adult mice. PDE5 inhibition selectively lowered contraction rate stimulated by β2AR, but not β1AR activation, supporting a compartmentalization of the cGMP-modulated pool of cAMP.

CONCLUSION

These data identify a new effect of PDE5 inhibitors on the modulation of cardiomyocyte response to adrenergic stimulation via PDE5-PDE2-mediated cross-talk.

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.

Structural and functional plasticity in long-term cultures of adult ventricular myocytes

Cultured heart cells have long been valuable for characterizing biological mechanism and disease pathogenesis. However, these preparations have limitations, relating to immaturity in key properties like excitation-contraction coupling and β-adrenergic stimulation. Progressive attenuation of the latter is intimately related to pathogenesis and therapy in heart failure. Highly valuable would be a long-term culture system that emulates the structural and functional changes that accompany disease and development, while concurrently permitting ready access to underlying molecular events. Accordingly, we here produce functional monolayers of adult guinea-pig ventricular myocytes (aGPVMs) that can be maintained in long-term culture for several weeks. At baseline, these monolayers exhibit considerable myofibrillar organization and a significant contribution of sarcoplasmic reticular (SR) Ca(2+) release to global Ca(2+) transients. In terms of electrical signaling, these monolayers support propagated electrical activity and manifest monophasic restitution of action-potential duration and conduction velocity. Intriguingly, β-adrenergic stimulation increases chronotropy but not inotropy, indicating selective maintenance of β-adrenergic signaling. It is interesting that this overall phenotypic profile is not fixed, but can be readily enhanced by chronic electrical stimulation of cultures. This simple environmental cue significantly enhances myofibrillar organization as well as β-adrenergic sensitivity. In particular, the chronotropic response increases, and an inotropic effect now emerges, mimicking a reversal of the progression seen in heart failure. Thus, these aGPVM monolayer cultures offer a valuable platform for clarifying long elusive features of β-adrenergic signaling and its plasticity.

Structure, function and clinical relevance of the cardiac conduction system, including the atrioventricular ring and outflow tract tissues

It is now over 100years since the discovery of the cardiac conduction system, consisting of three main parts, the sinus node, the atrioventricular node and the His-Purkinje system. The system is vital for the initiation and coordination of the heartbeat. Over the last decade, immense strides have been made in our understanding of the cardiac conduction system and these recent developments are reviewed here. It has been shown that the system has a unique embryological origin, distinct from that of the working myocardium, and is more extensive than originally thought with additional structures: atrioventricular rings, a third node (so called retroaortic node) and pulmonary and aortic sleeves. It has been shown that the expression of ion channels, intracellular Ca(2+)-handling proteins and gap junction channels in the system is specialised (different from that in the ordinary working myocardium), but appropriate to explain the functioning of the system, although there is continued debate concerning the ionic basis of pacemaking. We are beginning to understand the mechanisms (fibrosis and remodelling of ion channels and related proteins) responsible for dysfunction of the system (bradycardia, heart block and bundle branch block) associated with atrial fibrillation and heart failure and even athletic training. Equally, we are beginning to appreciate how naturally occurring mutations in ion channels cause congenital cardiac conduction system dysfunction. Finally, current therapies, the status of a new therapeutic strategy (use of a specific heart rate lowering drug) and a potential new therapeutic strategy (biopacemaking) are reviewed.

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.

Long-term treatment with ivabradine in post-myocardial infarcted rats counteracts f-channel overexpression

BACKGROUND AND PURPOSE

Recent clinical data suggest beneficial effects of ivabradine, a specific heart rate (HR)-lowering drug, in heart failure patients. However, the mechanisms responsible for these effects have not been completely clarified. Thus, we investigated functional/molecular changes in I(f), the specific target of ivabradine, in the failing atrial and ventricular myocytes where this current is up-regulated as a consequence of maladaptive remodelling.

EXPERIMENTAL APPROACH

We investigated the effects of ivabradine (IVA; 10 mg·kg(-1) ·day(-1) for 90 days) on electrophysiological remodelling in left atrial (LA), left ventricular (LV) and right ventricular (RV) myocytes from post-mycardial infarcted (MI) rats, with sham-operated (sham or sham + IVA) rats as controls. I(f) current was measured by patch-clamp; hyperpolarization-activated cyclic nucleotide-gated (HCN) channel isoforms and microRNA (miRNA-1 and miR-133) expression were evaluated by reverse transcription quantitative PCR.

KEY RESULTS

Maximal specific conductance of I(f) was increased in MI, versus sham, in LV (P < 0.01) and LA myocytes (P < 0.05). Ivabradine reduced HR in both MI and sham rats (P < 0.05). In MI + IVA, I(f) overexpression was attenuated and HCN4 transcription reduced by 66% and 54% in LV and RV tissue, respectively, versus MI rats (all P < 0.05). miR-1 and miR-133, which modulate post-transcriptional expression of HCN2 and HCN4 genes, were significantly increased in myocytes from MI + IVA.

CONCLUSION AND IMPLICATION

The beneficial effects of ivabradine may be due to the reversal of electrophysiological cardiac remodelling in post-MI rats by reduction of functional overexpression of HCN channels. This is attributable to transcriptional and post-transcriptional mechanisms.

Blocking effects of acehytisine on pacemaker currents (I(f)) in sinoatrial node cells and human HCN4 channels expressed in Xenopus laevis oocytes

ETHNOPHARMACOLOGICAL RELEVANCE

The root of Aconitum coreanum (Levl.) Raipaics has been extensively used to treat various kinds of disorders including cardiovascular disease in China for a long time. According to recent studies, its antiarrhythmic actions are attributable to the active component, acehytisine. However, the underlying mechanism remains poorly understood.

AIM OF THE STUDY

The effects of acehytisine on the spontaneous activity in sinoatrial nodes and the electropharmacological action of this drug on I(f) in pacemaker cells and hHCN4 channels in oocytes were to be investigated.

MATERIALS AND METHODS

Sinoatrial nodes were cut from rabbit heart, and transmembrane potentials were recorded by standard microelectrode technique. A whole-cell patch clamp technique was employed to record I(f) isolated enzymatically from rabbit sinoatrial node pacemaker cells. Human HCN4 channels were heterologously expressed in Xenopus oocytes and studied using the two-electrode voltage clamp technique.

RESULTS

Acehytisine decreased the pacemaker rate of firing and slope of diastolic depolarization, modified the action potential configurations and blocked I(f) in rabbit sinoatrial node cells and hHCN4 channels expressed in Xenopus oocytes in a concentration-dependent, voltage-independent and non-use-dependent manner. Its electropharmacological properties were consistent with those of a close-state blocker.

CONCLUSION

Our findings are likely to shed light on the clinical application of acehytisine in the treatment of cardiovascular disorders.

MECHANISM OF ANODE BREAK EXCITATION IN THE HEART: THE RELATIVE INFLUENCE OF MEMBRANE AND ELECTROTONIC FACTORS

Two hypotheses for the mechanism of anode break excitation in cardiac tissue are electrotonic interaction between adjacent regions of depolarization and hyperpolarization, and a hyperpolarization-activated membrane current, if. We incorporate membrane kinetics proposed for if into the bidomain model with unequal anisotropy ratios. During unipolar stimulation, we find that:

(1) The mechanisms of cathode make, cathode break, and anode make excitation are insensitive to if.

(2) Both electrotonic interactions and if contribute to anode break excitation. In our simulations, if makes the dominant contribution.

(3) Electrotonic interactions cause the "dip" in the anodal strength-interval curve.

(4) Following anode break excitation, the wave front propagates in the direction perpendicular to the fibers.

(5) if improves the agreement between the measured and calculated strength-interval curves.

We suggest three experiments to determine the mechanism of anode break excitation: measure the site and timing of initial excitation, or use drugs to suppress if.

Sialic acids attached to O-glycans modulate voltage-gated potassium channel gating

Neuronal, cardiac, and skeletal muscle action potentials are produced and conducted through the highly regulated activity of several types of voltage-gated ion channels. Voltage-gated potassium (K(v)) channels are responsible for action potential repolarization. Glycans can be attached to glycoproteins through N- and O-linkages. Previous reports described the impact of N-glycans on voltage-gated ion channel function. Here, we show that sialic acids attached through O-linkages modulate gating of K(v)2.1, K(v)4.2, and K(v)4.3. The conductance-voltage (G-V) relationships for each isoform were shifted uniquely by a depolarizing 8-16 mV under conditions of reduced sialylation. The data indicate that sialic acids modulate K(v) channel activation through apparent electrostatic mechanisms that promote channel activity. Voltage-dependent steady-state inactivation was unaffected by changes in sialylation. N-Linked sialic acids cannot be responsible for the G-V shifts because K(v)4.2 and K(v)4.3 cannot be N-glycosylated, and immunoblot analysis confirmed K(v)2.1 is not N-glycosylated. Glycosidase gel shift analysis suggested that K(v)2.1, K(v)4.2, and K(v)4.3 were O-glycosylated and sialylated. To confirm this, azide-modified sugar residues involved specifically in O-glycan and sialic acid biosynthesis were shown to incorporate into all three K(v) channel isoforms using Cu(I)-catalyzed cycloaddition chemistry. Together, the data indicate that sialic acids attached to O-glycans uniquely modulate gating of three K(v) channel isoforms that are not N-glycosylated. These data provide the first evidence that external O-glycans, with core structures distinct from N-glycans in type and number of sugar residues, can modulate K(v) channel function and thereby contribute to changes in electrical signaling that result from regulated ion channel expression and/or O-glycosylation.

Expression of hyperpolarization-activated cyclic nucleotide-gated cation channel (HCN4) is increased in hypertrophic cardiomyopathy

OBJECTIVE

Hypertrophic cardiomyopathy (HCM) is a disease of the myocardium with uncertain etiology and often leads to sudden death as the result of arrhythmia. Pacemaker hyperpolarization-activated current I(f) was altered in hypertrophic hearts and was probably responsible for arrhythmia. I(f) channels are compose\d of four hyperpolarization-activated cyclic nucleotide-gated cation subunits (HCN1-4). A previous study found significantly high levels of HCN2 and HCN4 mRNA in hypertrophic hearts compared to control hearts in septum and left ventricles in rats. No studies, however, have investigated the HCN gene expression in the myocardium from human HCM heart.

METHODS

The left ventricular tissue from four patients who died of HCM and six healthy patients who died of motor vehicle accidents was included in this study. The fluorescent quantitative reverse transcription-polymerase chain reaction (RT-PCR) assay was used to detect HCN4 mRNA. The expression of HCN4 mRNA of the two groups was detected on the assay.

RESULTS

In the HCM hearts, disorganization of the hypertrophic myofibers and interstitial fibrosis were observed in all four patients, although absent in healthy control hearts. By quantitative polymerase chain reaction, the mean copy number of HCN4 mRNA was 2.2×10(7) (range, 6.8×10(6) to 4.55×10(7)) in HCM hearts and 8.17×10(3) (range, 8.76×10(1) to 3.5×10(4)) in control hearts (P=.0318).

CONCLUSION

Higher HCN4 mRNA levels in the HCM hearts suggest that up-regulation of HCN4 gene expression might be responsible for ventricular arrhythmia that leads to sudden death.

I(f) channels as a therapeutic target in heart disease

In the normal heart, impulses are generated from the sinoatrial node. It is generally accepted that the pacemaker current, I(f), plays a major role in the spontaneous rhythmic activity. Recently, several electrophysiological and molecular data demonstrate that I(f) channels are present in embryonic and post-natal ventricular myocytes and undergo a downregulation during maturation. Interestingly, the I(f) current is re-expressed in some pathological conditions such as cardiac hypertrophy and heart failure. In these conditions, the overexpression of f-channels is a consequence of electrophysiological remodeling and may represent an arrhythmogenic mechanism in heart failure, a condition associated with high risk for sudden cardiac death. For its physiological and pathophysiological role and the availability of selective f-channel blockers, I(f) may be a suitable therapeutic target in heart failure.

The Contribution of HCN4 to normal sinus node function in humans and animal models

Although sinus node bradycardia is a very common clinical condition, the cellular mechanisms contributing to abnormal sinus node function are not clearly delineated. In recent publications, mutations in the hyperpolarization-activated, cyclic nucleotide-gated (HCN) 4 channels have been associated with sinus bradycardia. These channels are thought to be crucial in generating the spontaneous sinus node action potential, in accelerating the heart rate during sympathetic drive, and decelerating heart rate during vagal stimulation. Humans carrying HCN4 mutations indeed display significant bradycardia. Recent studies generating HCN4 knock out mice suggested that although HCN4 is crucial in early development, other mechanisms may also play a role in the accelerated heat rate achieved during sympathetic drive. In this review, we focus on genotype-phenotype correlation of these mutations and discuss the relative contribution of various ion channels to sinus node function. We also discuss the importance of HCN in treating clinical conditions such as brady- and tachycardia.

Hyperpolarization-activated cation channels: from genes to function

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels comprise a small subfamily of proteins within the superfamily of pore-loop cation channels. In mammals, the HCN channel family comprises four members (HCN1-4) that are expressed in heart and nervous system. The current produced by HCN channels has been known as I(h) (or I(f) or I(q)). I(h) has also been designated as pacemaker current, because it plays a key role in controlling rhythmic activity of cardiac pacemaker cells and spontaneously firing neurons. Extensive studies over the last decade have provided convincing evidence that I(h) is also involved in a number of basic physiological processes that are not directly associated with rhythmicity. Examples for these non-pacemaking functions of I(h) are the determination of the resting membrane potential, dendritic integration, synaptic transmission, and learning. In this review we summarize recent insights into the structure, function, and cellular regulation of HCN channels. We also discuss in detail the different aspects of HCN channel physiology in the heart and nervous system. To this end, evidence on the role of individual HCN channel types arising from the analysis of HCN knockout mouse models is discussed. Finally, we provide an overview of the impact of HCN channels on the pathogenesis of several diseases and discuss recent attempts to establish HCN channels as drug targets.

The cardiac pacemaker current

In mammals cardiac rate is determined by the duration of the diastolic depolarization of sinoatrial node (SAN) cells which is mainly determined by the pacemaker I(f) current. f-channels are encoded by four members of the hyperpolarization-activated cyclic nucleotide-gated gene (HCN1-4) family. HCN4 is the most abundant isoform in the SAN, and its relevance to pacemaking has been further supported by the discovery of four loss-of-function mutations in patients with mild or severe forms of cardiac rate disturbances. Due to its selective contribution to pacemaking, the I(f) current is also the pharmacological target of a selective heart rate-reducing agent (ivabradine) currently used in the clinical practice. Albeit to a minor extent, the I(f) current is also present in other spontaneously active myocytes of the cardiac conduction system (atrioventricular node and Purkinje fibres). In working atrial and ventricular myocytes f-channels are expressed at a very low level and do not play any physiological role; however in certain pathological conditions over-expression of HCN proteins may represent an arrhythmogenic mechanism. In this review some of the most recent findings on f/HCN channels contribution to pacemaking are described.

Proteolytic processing of HCN2 and co-assembly with HCN4 in the generation of cardiac pacemaker channels

In sino-atrial and atrio-ventricular nodal cells, hyperpolarization-activated cyclic nucleotide-gated (HCN) inward current carrying cationic channels, I(f), are expressed that contribute importantly to the diastolic depolarization critical for cardiac pacemaker activity. Although previous studies have demonstrated myocardial expression of both the HCN2 and HCN4 subunits, the specific roles of these subunits in the generation of functional myocardial I(f) channels remain unclear. To explore the molecular compositions of functional cardiac I(f) channels, antibodies targeted against specific C- and N-terminal sequences in HCN2 and HCN4 were exploited to examine HCN2 and HCN4 subunit expression in adult (mouse) heart and to immunoprecipitate endogenous HCN-encoded cardiac I(f) channel complexes. Western blot experiments revealed that although the full-length HCN2 (105 kDa) and HCN4 (160 kDa) proteins are readily detected in transiently transfected HEK-293 cells and in adult (mouse) brain, the molecular mass of the HCN2 protein in the myocardium is approximately 60 kDa. In addition, the myocardial 60-kDa HCN2 protein lacks the C terminus, which contains the cAMP binding domain. In heterologous cells, the C-terminal-truncated HCN2 protein co-assembles with HCN4 to form functional heteromeric HCN channels, which activate faster than homomeric HCN2 or homomeric HCN4 channels, and display properties similar to endogenous myocardial I(f) channels Taken together, these results suggest that functional myocardial I(f) channels reflect the heteromeric assembly of HCN2 and HCN4 subunits and further that the HCN4 subunit underlies the cAMP-mediated regulation of cardiac I(f) channels.

Electrophysiologic changes in heart failure: focus on pacemaker channels

Heart failure is a common clinical syndrome occurring as a result of cardiac overload, injury, and a complex interplay among genetic, neurohormonal, inflammatory, and biochemical factors. Occurrence of arrhythmias in heart failure is largely a consequence of disease-induced electrical remodeling of cardiac myocytes, a phenomenon consisting of alterations of ion channels and the ion-transport function that predispose patients to develop lethal arrhythmias. In most cases, the mechanism is the rapid onset of a ventricular tachyarrhythmia progressing to ventricular fibrillation and hemodynamic compromise. This paper highlights some of the important changes in ion channel expression and function that underlie electrical remodeling of the failing heart. Particular attention will be focused on the presence, features, and pharmacologic modulation of f channels expressed in ventricular cardiac myocytes.

Novel mechanism for suppression of hyperpolarization-activated cyclic nucleotide-gated pacemaker channels by receptor-like tyrosine phosphatase-alpha

We have previously reported an important role of increased tyrosine phosphorylation activity by Src in the modulation of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. Here we provide evidence showing a novel mechanism of decreased tyrosine phosphorylation on HCN channel properties. We found that the receptor-like protein-tyrosine phosphatase-alpha (RPTPalpha) significantly inhibited or eliminated HCN2 channel expression in HEK293 cells. Biochemical evidence showed that the surface expression of HCN2 was remarkably reduced by RPTPalpha, which was in parallel to the decreased tyrosine phosphorylation of the channel protein. Confocal imaging confirmed that the membrane surface distribution of the HCN2 channel was inhibited by RPTPalpha. Moreover, we detected the presence of RPTPalpha proteins in cardiac ventricles with expression levels changed during development. Inhibition of tyrosine phosphatase activity by phenylarsine oxide or sodium orthovanadate shifted ventricular hyperpolarization-activated current (I(f), generated by HCN channels) activation from nonphysiological voltages into physiological voltages associated with accelerated activation kinetics. In conclusion, we showed a critical role RPTPalpha plays in HCN channel function via tyrosine dephosphorylation. These findings are also important to neurons where HCN and RPTPalpha are richly expressed.

Molecular basis of funny current (If) in normal and failing human heart

I(f) overexpression has been functionally demonstrated in ventricular myocytes from failing human hearts. Altered expression of I(f)-channels as a consequence of electrophysiological remodeling may represent an arrhythmogenic mechanism in heart failure; however, the molecular basis of I(f) overexpression in human cardiac disease is unknown. HCN1, 2 and 4 subtypes, which encode I(f)-channels, have been identified in the heart. The present study was designed to characterize HCN isoform expression in failing and non-failing hearts. Ventricular and atrial samples were obtained from normal or failing hearts explanted from patients with end-stage ischemic cardiomyopathy. I(f) was recorded in patch-clamped left ventricular myocytes. mRNA and protein expression of HCN subunits were measured in both atria and ventricles of control and diseased hearts. HCN2 and HCN4 were detected in human myocardium. Both mRNA and protein levels of HCN2/4 were significantly augmented in failing ventricles (p<0.01 for mRNA, p<0.05 for protein). These results are consistent with the electrophysiological data showing that, in failing ventricular myocytes, I(f) is of larger amplitude and activates at less negative potential. Changes in mRNA and protein expression of both HCN2/4 isoforms in atrial specimens from patients with heart failure mirrored those observed in ventricles (p<0.001 for mRNA, p<0.05 for protein). No disease-dependent alteration was detected for MiRP1, the putative beta-subunit of the I(f)-channel. In conclusion, HCN4 is the predominant channel subtype in normal human heart, and its expression is further amplified by disease. HCN upregulation likely contributes to increased I(f) and may play a role in ventricular and atrial arrhythmogenesis in heart failure.

Down-regulation of miR-1/miR-133 contributes to re-expression of pacemaker channel genes HCN2 and HCN4 in hypertrophic heart

Cardiac hypertrophy is characterized by electrical remolding with increased risk of arrhythmogenesis. Enhanced abnormal automaticity of ventricular cells contributes critically to hypertrophic arrhythmias. The pacemaker current I(f), carried by the hyperpolarization-activated channels encoded mainly by the HCN2 and HCN4 genes in the heart, plays an important role in determining cardiac automaticity. Their expressions reportedly increase in hypertrophic and failing hearts, contributing to arrhythmogenesis under these conditions. We performed a study on post-transcriptional regulation of expression of HCN2 and HCN4 genes by microRNAs. We experimentally established HCN2 as a target for repression by the muscle-specific microRNAs miR-1 and miR-133 and established HCN4 as a target for miR-1 only. We unraveled robust increases in HCN2 and HCN4 protein levels in a rat model of left ventricular hypertrophy and in angiotensin II-induced neonatal ventricular hypertrophy. The up-regulation of HCN2/HCN4 was accompanied by pronounced reduction of miR-1/miR-133 levels. Forced expression of miR-1/miR-133 by transfection prevented overexpression of HCN2/HCN4 in hypertrophic cardiomyocytes. The serum-responsive factor protein level was found significantly decreased in hypertrophic hearts, and silencing of this protein by RNA interference resulted in increased levels of miR-1/miR-133 and concomitant increases in HCN2 and HCN4 protein levels. We conclude that down-regulation of miR-1 and miR-133 expression contributes to re-expression of HCN2/HCN4 and thereby the electrical remodeling process in hypertrophic hearts. Our study also sheds new light on the cellular function and pathological role of miR-1/miR-133 in the heart.

Aldosterone modulatesIfcurrent through gene expression in cultured neonatal rat ventricular myocytes

Mineralocorticoid receptor (MR) antagonists decrease the incidence of sudden cardiac death in patients with heart failure, as has been reported in two clinical trials (Randomized Aldactone Evaluation Study and Eplerenone Post-Acute Myocardial Infarction Heart Failure Efficacy and Survival Study). Aldosterone has been shown to increase the propensity to arrhythmias by changing the expression or function of various ion channels. In this study, we investigate the effect of aldosterone on the expression of hyperpolarization-activated current ( If) channels in cultured neonatal rat ventricular myocytes, using the whole cell patch-clamp technique, real-time PCR, and Western blotting. Incubation with 10 nM aldosterone for 17–24 h significantly accelerates the rate of spontaneous beating by increasing diastolic depolarization. Ifcurrent elicited by hyperpolarization from −50 to −130 mV significantly increases aldosterone by 10 nM (by 1.9-fold). Exposure to aldosterone for 1.5 h increases hyperpolarization-activated cyclic nucleotide-gated (HCN) 2 mRNA by 26.3% and HCN4 mRNA by 47.2%, whereas HCN1 mRNA expression remains unaffected. Aldosterone (24-h incubation) increases the expression of HCN2 protein (by 60.0%) and HCN4 protein (by 84.8%), but not HCN1 protein. MR antagonists (1 μM eplerenone or 0.1 μM spironolactone) abolish the increase of Ifchannel expression (currents, mRNA, and protein levels) by 10 nM aldosterone. In contrast, 1 μM aldosterone downregulated Ifchannel gene expression. Glucocorticoid receptor antagonist (100 nM RU-38486) did not affect the increase of Ifcurrent by 10 nM aldosterone. These findings suggest that aldosterone in physiological concentrations upregulates Ifchannel gene expression by MR activation in cardiac myocytes and may increase excitability, which may have a potential proarrhythmic bearing under pathophysiological conditions.