Inhibition of If in the atrioventricular node as a mechanism for dronedarone’s reduction in ventricular rate during atrial fibrillation

PR Interval as a Novel Therapeutic Target of Ivabradine Therapy-Prognostic Impact of Ivabradine-Induced PR Prolongation in Heart Failure Patients

BACKGROUND

Ivabradine reduces heart rate by inhibiting the "funny current" expressed on the sinoatrial node and improves mortality and morbidity in patients with systolic heart failure and sinus tachycardia. The funny current is known to be expressed also on the atrioventricular node according to experimental studies. However, the impact of ivabradine on PR interval remained unknown.

METHODS

Patients with a left ventricular ejection fraction of less than 50% who received 1 month of ivabradine were screened. Electrocardiographic and echocardiographic data, particularly concerning heart rate, the PR interval, and trans-mitral flow pattern, were collected at baseline and 1-month follow-up. The primary endpoint was defined as the composite of cardiovascular death and hospital readmission for worsening heart failure following ivabradine administration.

RESULTS

In the cohort of 29 enrolled patients (median age: 66 years, 62% male), the median baseline heart rate was 86 beats per minute and the median PR interval was 168 milliseconds. Following ivabradine administration, a significant decrease of 20 beats per minute in the heart rate and a significant increase of 24 milliseconds in the PR interval were observed. The truncated interval of the A-wave, detected in the trans-mitral flow, consistently demonstrated a negative correlation with the PR interval both before and after the administration of ivabradine. During a median of 1.8 years of follow-up, six patients reached the primary endpoint. A combination of heart rate reduction and PR prolongation following ivabradine administration, both of which were independent factors associated with the primary endpoint (p < 0.05 for both), was associated with greater freedom from the primary endpoint compared with either/neither of them (p = 0.002).

CONCLUSIONS

Ivabradine seems to prolong PR interval, which is a novel surrogate marker of favorable clinical outcomes in patients with systolic heart failure. This effect may be associated with the dynamics of the trans-mitral flow pattern, in conjunction with heart rate and the PR interval. Clinical implications of PR interval-guided ivabradine therapy remains the future concern.

A pilot study evaluating the role of ivabradine for rate control in patients with rheumatic atrial fibrillation

OBJECTIVES

Ivabradine may have a role in rate control of atrial fibrillation (AF) due to effects on HCN channels in AV node. We studied role of Ivabradine in rate control of rheumatic AF.

METHODS

80 patients, rheumatic AF, HR > 100 bpm (age 47 ± 11 yrs, AF duration 6.8 ± 2.9 years, rate 131 ± 16 bpm) on maximally tolerated ββ or CCB's, randomized to Ivabradine or escalated ββ/CCB. Ivabradine started @ 2.5 mg BD; increased to 5 mg BD if inadequate response at 1 week (failure to decrease HR < 10% vs baseline). After Holter at 1 month, dose escalated to 7.5 mg BD if needed.

RESULTS

Ivabradine resulted in significantly lower HR (81 ± 10 vs 99 ± 9) at 3 months and 6 months (79 ± 8 vs 94 ± 8, p < 0.001). Absolute reduction in HR: 56 ± 15 vs 31 ± 14 bpm and % change in HR: 41 ± 7 vs 24 ± 9%, both p < 0.00001). At 6 months, Ivabradine group had. 1Significantly lower NT Pro BNP (1168 vs 1314 pg/ml), higher 6 min walk distance (410 ± 47 vs 349 ± 54 m, all p < 0.001) 2Better symptom class (EHRA score 1: asymptomatic 84% vs 40%), improvement >1 EHRA class; baseline 60% vs 17% 3Better LA Strain (22.8 ± 2.8% vs 20.6 ± 2.5%) Ivabradine was well tolerated and there was no drug withdrawal.

CONCLUSION

Our data suggest that Ivabradine can be an option for rate control in rheumatic AF.

Mitochondrial Dysfunction in Cardiac Arrhythmias

Electrophysiological and structural disruptions in cardiac arrhythmias are closely related to mitochondrial dysfunction. Mitochondria are an organelle generating ATP, thereby satisfying the energy demand of the incessant electrical activity in the heart. In arrhythmias, the homeostatic supply-demand relationship is impaired, which is often accompanied by progressive mitochondrial dysfunction leading to reduced ATP production and elevated reactive oxidative species generation. Furthermore, ion homeostasis, membrane excitability, and cardiac structure can be disrupted through pathological changes in gap junctions and inflammatory signaling, which results in impaired cardiac electrical homeostasis. Herein, we review the electrical and molecular mechanisms of cardiac arrhythmias, with a particular focus on mitochondrial dysfunction in ionic regulation and gap junction action. We provide an update on inherited and acquired mitochondrial dysfunction to explore the pathophysiology of different types of arrhythmias. In addition, we highlight the role of mitochondria in bradyarrhythmia, including sinus node dysfunction and atrioventricular node dysfunction. Finally, we discuss how confounding factors, such as aging, gut microbiome, cardiac reperfusion injury, and electrical stimulation, modulate mitochondrial function and cause tachyarrhythmia.

PDK1-AKT signaling pathway regulates the expression and function of cardiac hyperpolarization-activated cyclic nucleotide-modulated channels

AIM

The enzyme 3-phosphoinositide-dependent protein kinase-1 (PDK1) is associated with cardiac and pathological remodeling and ion channel function regulation. However, whether it regulates hyperpolarization-activated cyclic nucleotide-modulated channels (HCNs) remains unclear.

MAIN METHODS

In the atrial myocytes of heart-specific PDK1 "knockout" mouse model and neonatal mice, protein kinase B (AKT)-related inhibitors or agonists as well as knockdown or overexpression plasmids were used to study the relationship between PDK1 and HCNs.

KEY FINDINGS

HCN1 expression and AKT phosphorylation at the Thr308 site were significantly decreased in atrial myocytes after PDK1 knockout or inhibition; in contrast, HCN2 and HCN4 levels were significantly increased. Also, a similar trend of HCNs expression has been observed in cultured atrial myocytes after PDK1 inhibition, as further demonstrated via immunofluorescence and patch-clamp experiments. Moreover, these results of PDK1 overexpression indicate an opposite trend compared with the previous experimental results. However, the results of PDK1 inhibition or overexpression could be reversed by activating or inhibiting AKT, respectively.

SIGNIFICANCE

These results indicate that the PDK1-AKT signaling pathway is involved in the regulation of HCN mRNA transcription, protein expression, HCN current density, and cell membrane location.

The Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels: from Biophysics to Pharmacology of a Unique Family of Ion Channels

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels are important members of the voltage-gated pore loop channels family. They show unique features: they open at hyperpolarizing potential, carry a mixed Na/K current, and are regulated by cyclic nucleotides. Four different isoforms have been cloned (HCN1-4) that can assemble to form homo- or heterotetramers, characterized by different biophysical properties. These proteins are widely distributed throughout the body and involved in different physiologic processes, the most important being the generation of spontaneous electrical activity in the heart and the regulation of synaptic transmission in the brain. Their role in heart rate, neuronal pacemaking, dendritic integration, learning and memory, and visual and pain perceptions has been extensively studied; these channels have been found also in some peripheral tissues, where their functions still need to be fully elucidated. Genetic defects and altered expression of HCN channels are linked to several pathologies, which makes these proteins attractive targets for translational research; at the moment only one drug (ivabradine), which specifically blocks the hyperpolarization-activated current, is clinically available. This review discusses current knowledge about HCN channels, starting from their biophysical properties, origin, and developmental features, to (patho)physiologic role in different tissues and pharmacological modulation, ending with their present and future relevance as drug targets.

Atrioventricular Node Dysfunction and Ion Channel Transcriptome in Pulmonary Hypertension

BACKGROUND

Heart block is associated with pulmonary hypertension, and the aim of the study was to test the hypothesis that the heart block is the result of a change in the ion channel transcriptome of the atrioventricular (AV) node.

METHODS AND RESULTS

The most commonly used animal model of pulmonary hypertension, the monocrotaline-injected rat, was used. The functional consequences of monocrotaline injection were determined by echocardiography, ECG recording, and electrophysiological experiments on the Langendorff-perfused heart and isolated AV node. The ion channel transcriptome was measured by quantitative PCR, and biophysically detailed computer modeling was used to explore the changes observed. After monocrotaline injection, echocardiography revealed the pattern of pulmonary artery blood flow characteristic of pulmonary hypertension and right-sided hypertrophy and failure; the Langendorff-perfused heart and isolated AV node revealed dysfunction of the AV node (eg, 50% incidence of heart block in isolated AV node); and quantitative PCR revealed a widespread downregulation of ion channel and related genes in the AV node (eg, >50% downregulation of Ca_v1.2/3 and HCN1/2/4 channels). Computer modeling predicted that the changes in the transcriptome if translated into protein and function would result in heart block.

CONCLUSIONS

Pulmonary hypertension results in a derangement of the ion channel transcriptome in the AV node, and this is the likely cause of AV node dysfunction in this disease.

Dronedarone: Basic Pharmacology and Clinical Use

Dronedarone is the newest antiarrhythmic drug approved for the maintenance of sinus rhythm in patients with nonpermanent atrial fibrillation (AF). It is a multi-channel blocker with diverse electrophysiologic properties. Dronedarone decreases the incidence of AF recurrence and the ventricular rate during recurrence. Dronedarone decreases rates of cardiovascular hospitalizations in patients with paroxysmal and persistent AF. Dronedarone increases mortality in patients with permanent AF and those with moderate-severe heart failure, and should thus be avoided in these populations. Dronedarone is less effective than amiodarone but also has less toxicity. Direct comparison with other antiarrhythmic drugs is not available.

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.

Acute effects of intravenous dronedarone on electrocardiograms, hemodynamics and cardiac functions in anesthetized dogs

Dronedarone is a class III antiarrhythmic that has been used for management of atrial fibrillation in humans, but limited information was found in dogs. The objective of this study was to determine the acute effects of escalating concentrations of dronedarone on electrocardiograms (ECG), hemodynamics and cardiac mechanics in healthy dogs. A total of 7 beagle dogs were anesthetized with isoflurane and instrumented to obtain lead II ECG, pressures at ascending aorta, right atrium, pulmonary artery and left ventricle, and left ventricular pressure-volume relationship. Five dogs were given vehicle and followed by escalating doses of dronedarone (0.5, 1.0 and 2.5 mg/kg, 15 min for each dose), and two dogs were used as a vehicle-treated control. All parameters were measured at 15 min after the end of each dose. The results showed that all parameters in vehicle-treated dogs were unaltered. Dronedarone at 2.5 mg/kg significantly lengthened PQ interval (P<0.01), reduced cardiac output (P<0.01) and increased systemic vascular resistance (P<0.01). Dronedarone produced negative inotropy assessed by significantly lowered end-systolic pressure-volume relationship, preload recruitable stroke work, contractility index and dP/dtmax. It also impaired diastolic function by significantly increased end-diastolic pressure-volume relationship, tau and dP/dtmin. These results suggested that acute effects of dronedarone produced negative dromotropy, inotropy and lusitropy in anesthetized dogs. Care should be taken when given dronedarone to dogs, especially when the patients have impaired cardiac function.

Hydroxychloroquine reduces heart rate by modulating the hyperpolarization-activated current If: Novel electrophysiological insights and therapeutic potential

BACKGROUND

Bradycardic agents are of interest for the treatment of ischemic heart disease and heart failure, as heart rate is an important determinant of myocardial oxygen consumption.

OBJECTIVES

The purpose of this study was to investigate the propensity of hydroxychloroquine (HCQ) to cause bradycardia.

METHODS

We assessed the effects of HCQ on (1) cardiac beating rate in vitro (mice); (2) the "funny" current (If) in isolated guinea pig sinoatrial node (SAN) myocytes (1, 3, 10 µM); (3) heart rate and blood pressure in vivo by acute bolus injection (rat, dose range 1-30 mg/kg), (4) blood pressure and ventricular function during feeding (mouse, 100 mg/kg/d for 2 wk, tail cuff plethysmography, anesthetized echocardiography).

RESULTS

In mouse atria, spontaneous beating rate was significantly (P < .05) reduced (by 9% ± 3% and 15% ± 2% at 3 and 10 µM HCQ, n = 7). In guinea pig isolated SAN cells, HCQ conferred a significant reduction in spontaneous action potential firing rate (17% ± 6%, 1 μM dose) and a dose-dependent reduction in If (13% ± 3% at 1 µM; 19% ± 2% at 3 µM). Effects were also observed on L-type calcium ion current (ICaL) (12% ± 4% reduction) and rapid delayed rectifier potassium current (IKr) (35% ± 4%) at 3 µM. Intravenous HCQ decreased heart rate in anesthetized rats (14.3% ± 1.1% at 15mg/kg; n = 6) without significantly reducing mean arterial blood pressure. In vivo feeding studies in mice showed no significant change in systolic blood pressure nor left ventricular function.

CONCLUSIONS

We have shown that HCQ acts as a bradycardic agent in SAN cells, in atrial preparations, and in vivo. HCQ slows the rate of spontaneous action potential firing in the SAN through multichannel inhibition, including that of If.

Pharmacokinetic and pharmacodynamic profile of dronedarone , a new antiarrhythmic agent for the treatment of atrial fibrillation

INTRODUCTION

Atrial fibrillation (AF) is the most common arrhythmia and is associated with increased morbidity and mortality. Dronedarone is a recent antiarrhythmic drug that has been developed for treatment of AF, with electrophysiological properties similar to amiodarone but with a lower incidence of side effects.

AREAS COVERED

This review evaluates the efficacy, safety, tolerability and side effects of dronedarone in the treatment of AF. In particular, the review includes studies comparing: dronedarone and placebo (ANDROMEDA, ATHENA, DAFNE, ERATO, EURIDIS/ADONIS, HESTIA, PALLAS trials), dronedarone and amiodarone (DIONYSOS trial), ranolazine and dronedarone given alone and in combination (HARMONY trial).

EXPERT OPINION

Dronedarone is an interesting antiarrhythmic agent in well-selected groups of patients. It also has several other pleiotropic effects that may potentially be beneficial in clinical practice, such as the reduction of the risk of stroke and acute coronary syndromes. In addition, combination therapies such as those with dronedarone and ranolazine, currently being investigated in the HARMONY trial, may provide another interesting approach to increase the antiarrhythmic efficacy and further reduce the incidence of side effects. A better understanding of the mechanisms underlying dronedarone's pleiotropic actions is expected to facilitate the selection of patients benefiting from dronedarone, as well as the development of novel antiarrhythmic drugs for AF.