Abnormal electrical properties of myocytes from chronically infarcted canine heart. Alterations in Vmax and the transient outward current

Dynamic Changes in Ion Channels during Myocardial Infarction and Therapeutic Challenges

In different areas of the heart, action potential waveforms differ due to differences in the expressions of sodium, calcium, and potassium channels. One of the characteristics of myocardial infarction (MI) is an imbalance in oxygen supply and demand, leading to ion imbalance. After MI, the regulation and expression levels of K+, Ca2+, and Na+ ion channels in cardiomyocytes are altered, which affects the regularity of cardiac rhythm and leads to myocardial injury. Myocardial fibroblasts are the main effector cells in the process of MI repair. The ion channels of myocardial fibroblasts play an important role in the process of MI. At the same time, a large number of ion channels are expressed in immune cells, which play an important role by regulating the in- and outflow of ions to complete intracellular signal transduction. Ion channels are widely distributed in a variety of cells and are attractive targets for drug development. This article reviews the changes in different ion channels after MI and the therapeutic drugs for these channels. We analyze the complex molecular mechanisms behind myocardial ion channel regulation and the challenges in ion channel drug therapy.

Cardioprotective Effect against Ischemia-Reperfusion Injury of PAK-200, a Dihydropyridine Analog with an Inhibitory Effect on Cl- but Not Ca2+ Current

We examined the effects of a dihydropyridine analog, PAK-200, on guinea pig myocardium during experimental ischemia and reperfusion. In isolated ventricular cardiomyocytes, PAK-200 (1 μM) had no effect on the basal peak inward and steady-state currents but inhibited the isoprenaline-induced time-independent Cl- current. In the right atria, PAK-200 had no effect on the beating rate and the chronotropic response to isoprenaline. In an ischemia-reperfusion model with coronary-perfused right ventricular tissue, a decrease in contractile force and a rise in tension were observed during a period of 30-min no-flow ischemia. Upon reperfusion, contractile force returned to less than 50% of preischemic values. PAK-200 had no effect on the decline in contractile force during the no-flow ischemia but reduced the rise in resting tension. PAK-200 significantly improved the recovery of contractile force after reperfusion to about 70% of the preischemic value. PAK-200 was also shown to attenuate the decrease in tissue ATP during ischemia. Treatment of ventricular myocytes with an ischemia-mimetic solution resulted in depolarization of the mitochondrial membrane potential and an increase in cytoplasmic and mitochondrial Ca2+ concentrations. PAK-200 significantly delayed these changes. Thus, PAK-200 inhibits the cAMP-activated chloride current in cardiac muscle and may have protective effects against ischemia-reperfusion injury through novel mechanisms.

The derivative of tissue activation as a marker of arrhythmogenic myocardium

BACKGROUND

Mapping techniques to identify diseased myocardial substrate during ventricular tachycardia ablation procedures remain limited.

OBJECTIVE

We hypothesized that tissue derivative of the voltage with respect to time (dV/dt), the slope of the unipolar ventricular electrogram registered by local ventricular activation, represents a unique parameter for identifying potential arrhythmogenic tissue in the ischemic scar border zone.

METHODS

Using high-resolution electrical mapping, we examined dV/dt characteristics in the border zone of animals after chronic myocardial infarction (MI).

RESULTS

Minimum dV/dt (dV/dt_min) in MI animals was less than that in control animals (-344.7 ± 68.7 in controls vs -174.2 ± 104.5 in MI; P < .001) and related to ventricular fibrosis. In MI animals, dV/dt_min values were divided into high (≤-200 μV/ms) and low (>-200 μV/ms) dV/dt_min. Low dV/dt_min regions harbored arrhythmogenic substrates that were characterized by (1) high responsiveness to sympathetic stimulation, (2) presence of late potentials, and (3) lower unipolar and bipolar voltage amplitudes.

CONCLUSION

Our data indicate that dV/dt_min is a unique parameter for identifying arrhythmogenic myocardium and may add a useful metric to conventional mapping strategies.

The scar: the wind in the perfect storm-insights into the mysterious living tissue originating ventricular arrhythmias

BACKGROUND

Arrhythmic death is very common among patients with structural heart disease, and it is estimated that in European countries, 1 per 1000 inhabitants yearly dies for sudden cardiac death (SCD), mainly as a result of ventricular arrhythmias (VA). The scar is the result of cardiac remodelling process that occurs in several cardiomyopathies, both ischemic and non-ischemic, and is considered the perfect substrate for re-entrant and non-re-entrant arrhythmias.

METHODS

Our aim was to review published evidence on the histological and electrophysiological properties of myocardial scar and to review the central role of cardiac magnetic resonance (CMR) in assessing ventricular arrhythmias substrate and its potential implication in risk stratification of SCD.

RESULTS

Scarring process affects both structural and electrical myocardial properties and paves the background for enhanced arrhythmogenicity. Non-uniform anisotropic conduction, gap junctions remodelling, source to sink mismatch and refractoriness dispersion are some of the underlining mechanisms contributing to arrhythmic potential of the scar. All these mechanisms lead to the initiation and maintenance of VA. CMR has a crucial role in the evaluation of patients suffering from VA, as it is considered the gold standard imaging test for scar characterization. Mounting evidences support the use of CMR not only for the definition of gross scar features, as size, localization and transmurality, but also for the identification of possible conducting channels suitable of discrete ablation. Moreover, several studies call out the CMR-based scar characterization as a stratification tool useful in selecting patients at risk of SCD and amenable to implantable cardioverter-defibrillator (ICD) implantation.

CONCLUSIONS

Scar represents the substrate of ventricular arrhythmias. CMR, defining scar presence and its features, may be a useful tool for guiding ablation procedures and for identifying patients at risk of SCD amenable to ICD therapy.

Mechanisms of ventricular arrhythmias elicited by coexistence of multiple electrophysiological remodeling in ischemia: A simulation study

Myocardial ischemia, injury and infarction (MI) are the three stages of acute coronary syndrome (ACS). In the past two decades, a great number of studies focused on myocardial ischemia and MI individually, and showed that the occurrence of reentrant arrhythmias is often associated with myocardial ischemia or MI. However, arrhythmogenic mechanisms in the tissue with various degrees of remodeling in the ischemic heart have not been fully understood. In this study, biophysical detailed single-cell models of ischemia 1a, 1b, and MI were developed to mimic the electrophysiological remodeling at different stages of ACS. 2D tissue models with different distributions of ischemia and MI areas were constructed to investigate the mechanisms of the initiation of reentrant waves during the progression of ischemia. Simulation results in 2D tissues showed that the vulnerable windows (VWs) in simultaneous presence of multiple ischemic conditions were associated with the dynamics of wave propagation in the tissues with each single pathological condition. In the tissue with multiple pathological conditions, reentrant waves were mainly induced by two different mechanisms: one is the heterogeneity along the excitation wavefront, especially the abrupt variation in conduction velocity (CV) across the border of ischemia 1b and MI, and the other is the decreased safe factor (SF) for conduction at the edge of the tissue in MI region which is attributed to the increased excitation threshold of MI region. Finally, the reentrant wave was observed in a 3D model with a scar reconstructed from MRI images of a MI patient. These comprehensive findings provide novel insights for understanding the arrhythmic risk during the progression of myocardial ischemia and highlight the importance of the multiple pathological stages in designing medical therapies for arrhythmias in ischemia.

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.

3D printed PLA/copper bowtie antenna for biomedical imaging applications

This study aims to increase the performance of the microwave antenna by using 3D printed conductive substrates, which is mainly used in biomedical imaging applications. Conventional antennas such as Horn and Vivaldi have coarse dimensions to integrate into the microwave imaging systems. Therefore, 3D printed Bowtie antenna structures were developed, which yield low cost and smaller sizes. PLA, PLA/copper, and PLA/carbon substrates were produced with a 3D printer. These materials were tested in terms of their dielectric constants between 1 and 10 GHz. The conductive part of the antenna was copper, with a thickness of 0.8 mm, which was embedded in the substrate parts. The reflection coefficients of the antennas were tested within 0-3 GHz frequency range via miniVNA network analyzer. The results show that the 3D printed PLA/copper and PLA/carbon antenna are highly suitable for the usage in biomedical imaging systems.

Inhibition of cardiac Kv4.3 (Ito) channel isoforms by class I antiarrhythmic drugs lidocaine and mexiletine

Transient outward K⁺ current, I_to, contributes to cardiac action potential generation and is primarily carried by K_v4.3 (KCND3) channels. Two K_v4.3 isoforms are expressed in human ventricle and show differential remodeling in heart failure (HF). Lidocaine and mexiletine may be applied in selected patients to suppress ventricular arrhythmias, without effects on sudden cardiac death or mortality. Isoform-dependent effects of antiarrhythmic drugs on K_v4.3 channels and potential implications for remodeling-based antiarrhythmic management have not been assessed to date. We sought to test the hypotheses that K_v4.3 channels are targeted by lidocaine and mexiletine, and that drug sensitivity is determined in isoform-specific manner. Expression of KCND3 isoforms was quantified using qRT-PCR in left ventricular samples of patients with HF due to either ischemic or dilated cardiomyopathies (ICM or DCM). Long (K_v4.3-L) and short (K_v4.3-S) isoforms were heterologously expressed in Xenopus laevis oocytes to study drug sensitivity and effects on biophysical characteristics activation, deactivation, inactivation, and recovery from inactivation. In the present HF patient cohort KCND3 isoform expression did not differ between ICM and DCM. In vitro, lidocaine (IC₅₀-K_v4.3-L: 0.8 mM; IC₅₀-K_v4.3-S: 1.2 mM) and mexiletine (IC₅₀-K_v4.3-L: 146 μM; IC₅₀-K_v4.3-S: 160 μM) inhibited K_v4.3 with different sensitivity. Biophysical analyses identified accelerated and enhanced inactivation combined with delayed recovery from inactivation as primary biophysical mechanisms underlying K_v4.3 current reduction. In conclusion, differential effects on K_v4.3 isoforms extend the electropharmacological profile of lidocaine and mexiletine. Patient-specific remodeling of K_v4.3 isoforms may determine individual drug responses and requires consideration during clinical application of compounds targeting K_v4.3.

Altered adrenergic response in myocytes bordering a chronic myocardial infarction underlies in vivo triggered activity and repolarization instability

Key points

Ventricular arrhythmias are a major complication after myocardial infarction (MI), associated with sympathetic activation. The structurally heterogeneous peri‐infarct zone is a known substrate, but the functional role of the myocytes is less well known.

Recordings of monophasic action potentials in vivo reveal that the peri‐infarct zone is a source of delayed afterdepolarizations (DADs) and has a high beat‐to‐beat variability of repolarization (BVR) during adrenergic stimulation (isoproterenol, ISO).

Myocytes isolated from the peri‐infarct region have more DADs and spontaneous action potentials, with spontaneous Ca²⁺ release, under ISO. These myocytes also have reduced repolarization reserve and increased BVR. Other properties of post‐MI remodelling are present in both peri‐infarct and remote myocytes.

These data highlight the importance of altered myocyte adrenergic responses in the peri‐infarct region as source and substrate of post‐MI arrhythmias.

Abstract

Ventricular arrhythmias are a major early complication after myocardial infarction (MI). The heterogeneous peri‐infarct zone forms a substrate for re‐entry while arrhythmia initiation is often associated with sympathetic activation. We studied the mechanisms triggering these post‐MI arrhythmias in vivo and their relation to regional myocyte remodelling. In pigs with chronic MI (6 weeks), in vivo monophasic action potentials were simultaneously recorded in the peri‐infarct and remote regions during adrenergic stimulation with isoproterenol (isoprenaline; ISO). Sham animals served as controls. During infusion of ISO in vivo, the incidence of delayed afterdepolarizations (DADs) and beat‐to‐beat variability of repolarization (BVR) was higher in the peri‐infarct than in the remote region. Myocytes isolated from the peri‐infarct region, in comparison to myocytes from the remote region, had more DADs, associated with spontaneous Ca²⁺ release, and a higher incidence of spontaneous action potentials (APs) when exposed to ISO (9.99 ± 4.2 vs. 0.16 ± 0.05 APs/min, p = 0.004); these were suppressed by CaMKII inhibition. Peri‐infarct myocytes also had reduced repolarization reserve and increased BVR (26 ± 10 ms vs. 9 ± 7 ms, P < 0.001), correlating with DAD activity. In contrast to these regional distinctions under ISO, alterations in Ca²⁺ handling at baseline and myocyte hypertrophy were present throughout the left ventricle (LV). Expression of some of the related genes was, however, different between the regions. In conclusion, altered myocyte adrenergic responses in the peri‐infarct but not the remote region provide a source of triggered activity in vivo and of repolarization instability amplifying the substrate for re‐entry. These findings stimulate further exploration of region‐specific therapies targeting myocytes and autonomic modulation.

Ventricular arrhythmias are a major complication after myocardial infarction (MI), associated with sympathetic activation. The structurally heterogeneous peri‐infarct zone is a known substrate, but the functional role of the myocytes is less well known.

Recordings of monophasic action potentials in vivo reveal that the peri‐infarct zone is a source of delayed afterdepolarizations (DADs) and has a high beat‐to‐beat variability of repolarization (BVR) during adrenergic stimulation (isoproterenol, ISO).

Myocytes isolated from the peri‐infarct region have more DADs and spontaneous action potentials, with spontaneous Ca²⁺ release, under ISO. These myocytes also have reduced repolarization reserve and increased BVR. Other properties of post‐MI remodelling are present in both peri‐infarct and remote myocytes.

These data highlight the importance of altered myocyte adrenergic responses in the peri‐infarct region as source and substrate of post‐MI arrhythmias.

A model for human action potential dynamics in vivo

Computational modeling based on experimental data remains an important component in cardiac electrophysiological research, especially because clinical data such as human action potential (AP) dynamics are scarce or limited by practical or ethical concerns. Such modeling has been used to develop and test a variety of mechanistic hypotheses, with the majority of these studies involving the rate dependence of AP duration (APD) including APD restitution and conduction velocity (CV). However, there is very little information regarding the complex dynamics at the boundary of repolarization (or refractoriness) and reexcitability. Here, we developed a "minimal" ionic model of the human AP, based on in vivo human monophasic AP (MAP) recordings obtained during clinical programmed electrical stimulation (PES) to address the progressive decrease in AP take-off potential (TOP) and associated CV slowing seen during three tightly spaced extrastimuli. Recent voltage-clamp data demonstrating the effect of intracellular calcium on sodium current availability were incorporated and were required to reproduce large (>15 mV) elevations in take-off potential and progressive encroachment. Introducing clinically observed APD gradients into the model enabled us to replicate the dynamic response to PES in patients leading to conduction block and reentry formation for the positive, but not the negative, APD gradient. Finally, we modeled the dynamics of reentry and show that spiral waves follow a meandering trajectory with a period of ~180 ms. We conclude that our model reproduces a variety of electrophysiological behavior including the response to sequential premature stimuli and provides a basis for studies of the initiation of reentry in human ventricular tissue.NEW & NOTEWORTHY This work presents a new model of the action potential of the human which reproduces the complex dynamics during premature stimulation in patients.

Extracellular Vesicles Derived from Hypoxic Human Mesenchymal Stem Cells Attenuate GSK3β Expression via miRNA-26a in an Ischemia-Reperfusion Injury Model

PURPOSE

Bioactive molecules critical to intracellular signaling are contained in extracellular vesicles (EVs) and have cardioprotective effects in ischemia/reperfusion (IR) injured hearts. This study investigated the mechanism of the cardioprotective effects of EVs derived from hypoxia-preconditioned human mesenchymal stem cells (MSCs).

MATERIALS AND METHODS

EV solutions (0.4 μg/μL) derived from normoxia-preconditioned MSCs (EV(NM)) and hypoxia-preconditioned MSCs (EV(HM)) were delivered in a rat IR injury model. Successful EV delivery was confirmed by the detection of PKH26 staining in hearts from EV-treated rats.

RESULTS

EV(HM) significantly reduced infarct size (24±2% vs. 8±1%, p<0.001), and diminished arrhythmias by recovering electrical conduction, I(Na) current, and Cx43 expression. EV(HM) also reversed reductions in Wnt1 and β-catenin levels and increases in GSK3β induced after IR injury. miRNA-26a was significantly increased in EV(HM), compared with EV(NM), in real-time PCR. Finally, in in vitro experiments, hypoxia-induced increases in GSK3β expression were significantly reduced by the overexpression of miRNA-26a.

CONCLUSION

EV(HM) reduced IR injury by suppressing GSK3β expression via miRNA-26a and increased Cx43 expression. These findings suggest that the beneficial effect of EVHM is related with Wnt signaling pathway.

Modeling the Electrophysiological Properties of the Infarct Border Zone

Ventricular arrhythmias (VA) in patients with myocardial infarction (MI) are thought to be associated with structural and electrophysiological remodeling within the infarct border zone (BZ). Personalized computational models have been used to investigate the potential role of the infarct BZ in arrhythmogenesis, which still remains incompletely understood. Most recent models have relied on experimental data to assign BZ properties. However, experimental measurements vary significantly resulting in different computational representations of this region. Here, we review experimental data available in the literature to determine the most prominent properties of the infarct BZ. Computational models are then used to investigate the effect of different representations of the BZ on activation and repolarization properties, which may be associated with VA. Experimental data obtained from several animal species and patients with infarct show that BZ properties vary significantly depending on disease's stage, with the early disease stage dominated by ionic remodeling and the chronic stage by structural remodeling. In addition, our simulations show that ionic remodeling in the BZ leads to large repolarization gradients in the vicinity of the scar, which may have a significant impact on arrhythmia simulations, while structural remodeling plays a secondary role. We conclude that it is imperative to faithfully represent the properties of regions of infarction within computational models specific to the disease stage under investigation in order to conduct in silico mechanistic investigations.

Microwave Sensors for Breast Cancer Detection

Breast cancer is the leading cause of death among females, early diagnostic methods with suitable treatments improve the 5-year survival rates significantly. Microwave breast imaging has been reported as the most potential to become the alternative or additional tool to the current gold standard X-ray mammography for detecting breast cancer. The microwave breast image quality is affected by the microwave sensor, sensor array, the number of sensors in the array and the size of the sensor. In fact, microwave sensor array and sensor play an important role in the microwave breast imaging system. Numerous microwave biosensors have been developed for biomedical applications, with particular focus on breast tumor detection. Compared to the conventional medical imaging and biosensor techniques, these microwave sensors not only enable better cancer detection and improve the image resolution, but also provide attractive features such as label-free detection. This paper aims to provide an overview of recent important achievements in microwave sensors for biomedical imaging applications, with particular focus on breast cancer detection. The electric properties of biological tissues at microwave spectrum, microwave imaging approaches, microwave biosensors, current challenges and future works are also discussed in the manuscript.

Biological therapies targeting arrhythmias: are cells and genes the answer?

INTRODUCTION

Arrhythmias can cause symptoms ranging from simple dizziness to life-threatening circulatory collapse. Current management includes medical therapy and procedures such as catheter ablation or device implantation. However, these strategies still pose a risk of serious side effects, and some patients remain symptomatic. Advancement in our understanding of how arrhythmias develop on the cellular level has made more targeted approaches possible. In addition, contemporary studies have found that several genes are involved in the pathogenesis of arrhythmias.

AREAS COVERED

In the present review, the authors explore the cellular and genetic mechanisms leading to arrhythmias as well as the progress that has been made in using both gene and cell therapy to treat tachy- and bradyarrhythmias. They also consider why gene and cell therapy has resulted into a few clinical trials with promising results, however still not applicable in routine clinical practice.

EXPERT OPINION

The question currently is whether such biological therapies could replace current established approaches. The contemporary evidence suggests that despite recent advances in this field, it will need more work in experimental models before this is applied into clinical practice. Gene and cell studies targeting conduction and repolarization are promising, but still not ready for use in the clinical setting.

Association of Serum Magnesium on Mortality in Patients Admitted to the Intensive Cardiac Care Unit

BACKGROUND

Although electrolyte disturbances may affect cardiac action potential, little is known about the association between serum magnesium and corrected QT (QTc) interval as well as clinical outcomes.

METHODS

A consecutive 8498 patients admitted to the Mayo Clinic Hospital-Rochester cardiac care unit (CCU) from January 1, 2004 through December 31, 2013 with 2 or more documented serum magnesium levels, were studied to test the hypothesis that serum magnesium levels are associated with in-hospital mortality, sudden cardiac death, and QTc interval.

RESULTS

Patients were 67 ± 15 years; 62.2% were male. The primary diagnoses for CCU admissions were acute myocardial infarction (50.7%) and acute decompensated heart failure (42.5%), respectively. Patients with higher magnesium levels were older, more likely male, and had lower glomerular filtration rates. After multivariate analyses adjusted for clinical characteristics including kidney disease and serum potassium, admission serum magnesium levels were not associated with QTc interval or sudden cardiac death. However, the admission magnesium levels ≥2.4 mg/dL were independently associated with an increase in mortality when compared with the reference level (2.0 to <2.2 mg/dL), having an adjusted odds ratio of 1.80 and a 95% confidence interval of 1.25-2.59. The sensitivity analysis examining the association between postadmission magnesium and analysis that excluded patients with kidney failure and those with abnormal serum potassium yielded similar results.

CONCLUSION

This retrospective study unexpectedly observed no association between serum magnesium levels and QTc interval or sudden cardiac death. However, serum magnesium ≥2.4 mg/dL was an independent predictor of increased hospital morality among CCU patients.

Kvβ1.1 (AKR6A8) senses pyridine nucleotide changes in the mouse heart and modulates cardiac electrical activity

The present study investigates the physiological role of Kvβ1 subunit for sensing pyridine nucleotide (NADH/NAD+) changes in the heart. We used Kvβ1.1 knockout (KO) or wild-type (WT) mice and established that Kvβ1.1 preferentially binds with Kv4.2 and senses the pyridine nucleotide changes in the heart. The cellular action potential duration (APD) obtained from WT cardiomyocytes showed longer APDs with lactate perfusion, which increases intracellular NADH levels, while the APDs remained unaltered in the Kvβ1.1 KO. Ex vivo monophasic action potentials showed a similar response, in which the APDs were prolonged in WT mouse hearts with lactate perfusion; however, the Kvβ1.1 KO mouse hearts did not show APD changes upon lactate perfusion. COS-7 cells coexpressing Kv4.2 and Kvβ1.1 were used for whole cell patch-clamp recordings to evaluate changes caused by NADH (lactate). These data reveal that Kvβ1.1 is required in the mediated inactivation of Kv4.2 currents, when NADH (lactate) levels are increased. In vivo, isoproterenol infusion led to increased NADH in the heart along with QTc prolongation in wild-type mice; regardless of the approach, our data show that Kvβ1.1 recognizes NADH changes and modulates Kv4.2 currents affecting AP and QTc durations. Overall, this study uses multiple levels of investigation, including the heterologous overexpression system, cardiomyocyte, ex vivo, and ECG, and clearly depicts that Kvβ1.1 is an obligatory sensor of NADH/NAD changes in vivo, with a physiological role in the heart.NEW & NOTEWORTHY Cardiac electrical activity is mediated by ion channels, and Kv4.2 plays a significant role, along with its binding partner, the Kvβ1.1 subunit. In the present study, we identify Kvβ1.1 as a sensor of pyridine nucleotide changes and as a modulator of Kv4.2 gating, action potential duration, and ECG in the mouse heart.

Antiarrhythmic gene therapy - will biologics replace catheters, drugs and devices?

The clinical management of heart rhythm disorders still constitutes a major challenge. The development of alternatives to current approaches is of significant interest in order to establish more effective therapies that increase quality of life and reduce symptoms and hospitalizations. Over the past two decades the mechanistic understanding of pathophysiological pathways underlying cardiac arrhythmias has advanced profoundly, opening up novel avenues for mechanism-based therapeutic approaches. In particular, gene therapy offers greater selectivity than small molecule-based or interventional treatment. The gene of interest is packaged into viral or non-viral carriers and delivered to the target area via direct injection or using catheter-based techniques, providing the advantage of site-restricted action in contrast to systemic application of drugs. This work summarizes the current knowledge on mechanistic background, application strategies, and preclinical outcome of antiarrhythmic gene therapy for atrial fibrillation, ventricular tachycardia, and modulation of sinus node function.

Can heart function lost to disease be regenerated by therapeutic targeting of cardiac scar tissue?

Myocardial infarction results in scar tissue that cannot actively contribute to heart mechanical function and frequently causes lethal arrhythmias. The healing response after infarction involves inflammation, biochemical signaling, changes in cellular phenotype, activity, and organization, and alterations in electrical conduction due to variations in cell and tissue geometry and alterations in protein expression, organization, and function - particularly in membrane channels. The intensive research focus on regeneration of myocardial tissues has, as of yet, only met with modest success, with no near-term prospect of improving standard-of-care for patients with heart disease. An alternative concept for novel therapeutic approach is the rejuvenation of cardiac electrical and mechanical properties through the modification of scar tissue. Several peptide therapeutics, locally applied genetic therapies, or delivery of genetically modified cells have shown promise in improving the characteristics of the fibrous scar and post-myocardial infarction prognosis in experimental models. This review highlights several factors that contribute to arrhythmogenesis in scar formation and how these might be targeted to regenerate some of the electrical and mechanical function of the post-MI scar.

The crossroads of inflammation, fibrosis, and arrhythmia following myocardial infarction

Optimal healing of damaged tissue following myocardial infarction (MI) requires a coordinated cellular response that can be divided into three phases: inflammatory, proliferative/reparative, and maturation. The inflammatory phase, characterized by rapid influx of cytokines, chemokines, and immune cells, is critical to the removal of damaged tissue. The onset of the proliferative/reparative phase is marked by increased proliferation of myofibroblasts and secretion of collagen to replace dead tissue. Lastly, crosslinking of collagen fibers and apoptosis of immune cells marks the maturation phase. Excessive inflammation or fibrosis has been linked to increased incidence of arrhythmia and other MI-related pathologies. This review describes the roles of inflammation and fibrosis in arrhythmogenesis and prospective therapies for anti-arrhythmic treatment.

Cardiac Sympathetic Nerve Sprouting and Susceptibility to Ventricular Arrhythmias after Myocardial Infarction

Ventricular arrhythmogenesis is thought to be a common cause of sudden cardiac death following myocardial infarction (MI). Nerve remodeling as a result of MI is known to be an important genesis of life-threatening arrhythmias. It is hypothesized that neural modulation might serve as a therapeutic option of malignant arrhythmias. In fact, left stellectomy or β-blocker therapy is shown to be effective in the prevention of ventricular tachyarrhythmias (VT), ventricular fibrillation (VF), and sudden cardiac death (SCD) after MI both in patients and in animal models. Results from decades of research already evidenced a positive relationship between abnormal nerve density and ventricular arrhythmias after MI. In this review, we summarized the molecular mechanisms involved in cardiac sympathetic rejuvenation and mechanisms related to sympathetic hyperinnervation and arrhythmogenesis after MI and analyzed the potential therapeutic implications of nerve sprouting modification for ventricular arrhythmias and SCD control.

Deranged sodium to sudden death

In February 2014, a group of scientists convened as part of the University of California Davis Cardiovascular Symposium to bring together experimental and mathematical modelling perspectives and discuss points of consensus and controversy on the topic of sodium in the heart. This paper summarizes the topics of presentation and discussion from the symposium, with a focus on the role of aberrant sodium channels and abnormal sodium homeostasis in cardiac arrhythmias and pharmacotherapy from the subcellular scale to the whole heart. Two following papers focus on Na(+) channel structure, function and regulation, and Na(+)/Ca(2+) exchange and Na(+)/K(+) ATPase. The UC Davis Cardiovascular Symposium is a biannual event that aims to bring together leading experts in subfields of cardiovascular biomedicine to focus on topics of importance to the field. The focus on Na(+) in the 2014 symposium stemmed from the multitude of recent studies that point to the importance of maintaining Na(+) homeostasis in the heart, as disruption of homeostatic processes are increasingly identified in cardiac disease states. Understanding how disruption in cardiac Na(+)-based processes leads to derangement in multiple cardiac components at the level of the cell and to then connect these perturbations to emergent behaviour in the heart to cause disease is a critical area of research. The ubiquity of disruption of Na(+) channels and Na(+) homeostasis in cardiac disorders of excitability and mechanics emphasizes the importance of a fundamental understanding of the associated mechanisms and disease processes to ultimately reveal new targets for human therapy.

Making better scar: Emerging approaches for modifying mechanical and electrical properties following infarction and ablation

Following myocardial infarction (MI), damaged myocytes are replaced by collagenous scar tissue, which serves an important mechanical function - maintaining integrity of the heart wall against enormous mechanical forces - but also disrupts electrical function as structural and electrical remodeling in the infarct and borderzone predispose to re-entry and ventricular tachycardia. Novel emerging regenerative approaches aim to replace this scar tissue with viable myocytes. Yet an alternative strategy of therapeutically modifying selected scar properties may also prove important, and in some cases may offer similar benefits with lower risk or regulatory complexity. Here, we review potential goals for such modifications as well as recent proof-of-concept studies employing specific modifications, including gene therapy to locally increase conduction velocity or prolong the refractory period in and around the infarct scar, and modification of scar anisotropy to improve regional mechanics and pump function. Another advantage of scar modification techniques is that they have applications well beyond MI. In particular, ablation treats electrical abnormalities of the heart by intentionally generating scar to block aberrant conduction pathways. Yet in diseases such as atrial fibrillation (AF) where ablation can be extensive, treating the electrical disorder can significantly impair mechanical function. Creating smaller, denser scars that more effectively block conduction, and choosing the location of those lesions by balancing their electrical and mechanical impacts, could significantly improve outcomes for AF patients. We review some recent advances in this area, including the use of computational models to predict the mechanical effects of specific lesion sets and gene therapy for functional ablation. Overall, emerging techniques for modifying scar properties represents a potentially important set of tools for improving patient outcomes across a range of heart diseases, whether used in place of or as an adjunct to regenerative approaches.

Increased Short-Term Beat-To-Beat Variability of QT Interval in Patients with Acromegaly

Cardiovascular diseases, including ventricular arrhythmias are responsible for increased mortality in patients with acromegaly. Acromegaly may cause repolarization abnormalities such as QT prolongation and impairment of repolarization reserve enhancing liability to arrhythmia. The aim of this study was to determine the short-term beat-to-beat QT variability in patients with acromegaly. Thirty acromegalic patients (23 women and 7 men, mean age±SD: 55.7±10.4 years) were compared with age- and sex-matched volunteers (mean age 51.3±7.6 years). Cardiac repolarization parameters including frequency corrected QT interval, PQ and QRS intervals, duration of terminal part of T waves (T_peak-T_end) and short-term variability of QT interval were evaluated. All acromegalic patients and controls underwent transthoracic echocardiographic examination. Autonomic function was assessed by means of five standard cardiovascular reflex tests. Comparison of the two groups revealed no significant differences in the conventional ECG parameters of repolarization (QT: 401.1±30.6 ms vs 389.3±16.5 ms, corrected QT interval: 430.1±18.6 ms vs 425.6±17.3 ms, QT dispersion: 38.2±13.2 ms vs 36.6±10.2 ms; acromegaly vs control, respectively). However, short-term beat-to-beat QT variability was significantly increased in acromegalic patients (4.23±1.03 ms vs 3.02±0.80, P<0.0001). There were significant differences between the two groups in the echocardiographic dimensions (left ventricular end diastolic diameter: 52.6±5.4 mm vs 48.0±3.9 mm, left ventricular end systolic diameter: 32.3±5.2 mm vs 29.1±4.4 mm, interventricular septum: 11.1±2.2 mm vs 8.8±0.7 mm, posterior wall of left ventricle: 10.8±1.4 mm vs 8.9±0.7 mm, P<0.05, respectively). Short-term beat-to-beat QT variability was elevated in patients with acromegaly in spite of unchanged conventional parameters of ventricular repolarization. This enhanced temporal QT variability may be an early indicator of increased liability to arrhythmia.

Repolarization reserve evolves dynamically during the cardiac action potential: effects of transient outward currents on early afterdepolarizations

BACKGROUND

Transient outward K currents (Ito) have been reported both to suppress and to facilitate early afterdepolarizations (EADs) when repolarization reserve is reduced. Here, we used the dynamic clamp technique to analyze how Ito accounts for these paradoxical effects on EADs by influencing the dynamic evolution of repolarization reserve during the action potential.

METHODS AND RESULTS

Isolated patch-clamped rabbit ventricular myocytes were exposed to either oxidative stress (H2O2) or hypokalemia to induce bradycardia-dependent EADs at a long pacing cycle length of 6 s, when native rabbit Ito is substantial. EADs disappeared when the pacing cycle length was shortened to 1 s, when Ito becomes negligible because of incomplete recovery from inactivation. During 6-s pacing cycle length, EADs were blocked by the Ito blocker 4-aminopyridine, but reappeared when a virtual current with appropriate Ito-like properties was reintroduced using the dynamic clamp (n=141 trials). During 1-s pacing cycle length in the absence of 4-aminopyridine, adding a virtual Ito-like current (n=1113 trials) caused EADs to reappear over a wide range of Ito conductance (0.005-0.15 nS/pF), particularly when inactivation kinetics were slow (τinact≥20 ms) and the pedestal (noninactivating component) was small (<25% of peak Ito). Faster inactivation or larger pedestals tended to suppress EADs.

CONCLUSIONS

Repolarization reserve evolves dynamically during the cardiac action potential. Whereas sufficiently large Ito can suppress EADs, a wide range of intermediate Ito properties can promote EADs by influencing the temporal evolution of other currents affecting late repolarization reserve. These findings raise caution in targeting Ito as an antiarrhythmic strategy.

Targeting protein tyrosine phosphatase σ after myocardial infarction restores cardiac sympathetic innervation and prevents arrhythmias

Millions of people suffer a myocardial infarction (MI) every year, and those who survive have increased risk of arrhythmias and sudden cardiac death. Recent clinical studies have identified sympathetic denervation as a predictor of increased arrhythmia susceptibility. Chondroitin sulfate proteoglycans present in the cardiac scar after MI prevent sympathetic reinnervation by binding the neuronal protein tyrosine phosphatase receptor σ (PTPσ). Here we show that the absence of PTPσ, or pharmacologic modulation of PTPσ by the novel intracellular sigma peptide (ISP) beginning 3 days after injury, restores sympathetic innervation to the scar and markedly reduces arrhythmia susceptibility. Using optical mapping we observe increased dispersion of action potential duration, supersensitivity to β-adrenergic receptor stimulation and Ca(2+) mishandling following MI. Sympathetic reinnervation prevents these changes and renders hearts remarkably resistant to induced arrhythmias.

Multiscale computational analysis of the bioelectric consequences of myocardial ischaemia and infarction

Ischaemic heart disease is considered as the single most frequent cause of death, provoking more than 7 000 000 deaths every year worldwide. A high percentage of patients experience sudden cardiac death, caused in most cases by tachyarrhythmic mechanisms associated to myocardial ischaemia and infarction. These diseases are difficult to study using solely experimental means due to their complex dynamics and unstable nature. In the past decades, integrative computational simulation techniques have become a powerful tool to complement experimental and clinical research when trying to elucidate the intimate mechanisms of ischaemic electrophysiological processes and to aid the clinician in the improvement and optimization of therapeutic procedures. The purpose of this paper is to briefly review some of the multiscale computational models of myocardial ischaemia and infarction developed in the past 20 years, ranging from the cellular level to whole-heart simulations.

Ischemia-related subcellular redistribution of sodium channels enhances the proarrhythmic effect of class I antiarrhythmic drugs: a simulation study

BACKGROUND

Cardiomyocytes located at the ischemic border zone of infarcted ventricle are accompanied by redistribution of gap junctions, which mediate electrical transmission between cardiomyocytes. This ischemic border zone provides an arrhythmogenic substrate. It was also shown that sodium (Na+) channels are redistributed within myocytes located in the ischemic border zone. However, the roles of the subcellular redistribution of Na+ channels in the arrhythmogenicity under ischemia remain unclear.

METHODS

Computer simulations of excitation conduction were performed in a myofiber model incorporating both subcellular Na+ channel redistribution and the electric field mechanism, taking into account the intercellular cleft potentials.

RESULTS

We found in the myofiber model that the subcellular redistribution of the Na+ channels under myocardial ischemia, decreasing in Na+ channel expression of the lateral cell membrane of each myocyte, decreased the tissue excitability, resulting in conduction slowing even without any ischemia-related electrophysiological change. The conventional model (i.e., without the electric field mechanism) did not reproduce the conduction slowing caused by the subcellular Na+ channel redistribution. Furthermore, Na+ channel blockade with the coexistence of a non-ischemic zone with an ischemic border zone expanded the vulnerable period for reentrant tachyarrhythmias compared to the model without the ischemic border zone. Na+ channel blockade tended to cause unidirectional conduction block at sites near the ischemic border zone. Thus, such a unidirectional conduction block induced by a premature stimulus at sites near the ischemic border zone is associated with the initiation of reentrant tachyarrhythmias.

CONCLUSIONS

Proarrhythmia of Na+ channel blockade in patients with old myocardial infarction might be partly attributable to the ischemia-related subcellular Na+ channel redistribution.

Apamin-sensitive calcium-activated potassium currents in rabbit ventricles with chronic myocardial infarction

INTRODUCTION

The apamin-sensitive small-conductance calcium-activated potassium current (IKAS ) is increased in heart failure. It is unknown if myocardial infarction (MI) is also associated with an increase of IKAS .

METHODS AND RESULTS

We performed Langendorff perfusion and optical mapping in 6 normal hearts and 10 hearts with chronic (5 weeks) MI. An additional 6 normal and 10 MI hearts were used for patch clamp studies. The infarct size was 25% (95% confidence interval, 20-31) and the left ventricular ejection fraction was 50 (46-54). The rabbits did not have symptoms of heart failure. The action potential duration measured to 80% repolarization (APD80 ) in the peri-infarct zone (PZ) was 150 (142-159) milliseconds, significantly (P = 0.01) shorter than that in the normal ventricles (167 [158-177] milliseconds. The intracellular Ca transient duration was also shorter in the PZ (148 [139-157] milliseconds) than that in normal ventricles (168 [157-180] milliseconds; P = 0.017). Apamin prolonged the APD80 in PZ by 9.8 (5.5-14.1)%, which is greater than that in normal ventricles (2.8 [1.3-4.3]%, P = 0.006). Significant shortening of APD80 was observed at the cessation of rapid pacing in MI but not in normal ventricles. Apamin prevented postpacing APD80 shortening. Patch clamp studies showed that IKAS was significantly higher in the PZ cells (2.51 [1.55-3.47] pA/pF, N = 17) than in the normal cells (1.08 [0.36-1.80] pA/pF, N = 15, P = 0.019).

CONCLUSION

We conclude that IKAS is increased in MI ventricles and contributes significantly to ventricular repolarization especially during tachycardia.

Mesenchymal stem cell transplantation mitigates electrophysiological remodeling in a rat model of myocardial infarction

INTRODUCTION

Transplantation of mesenchymal stem cells (MSCs) has shown therapeutic potential for cardiovascular diseases, but the electrophysiological implications are not understood. The purpose of this study was to evaluate the impact of MSC transplantation on adverse electrophysiological remodeling in the heart following myocardial infarction (MI).

METHODS AND RESULTS

Three weeks after coronary ligation to induce MI in rats, MSCs or culture medium were directly injected into each infarct. One to two weeks later, hearts were excised, Langendorff-perfused, and optically mapped using the potentiometric fluorescent dye Di-4-ANEPPS. Quantitative real-time PCR was also performed to assess gene expression. Optical mapping showed that post-MI reduction in conduction velocity (from 0.70 ± 0.04 m/s in 12 normal controls to 0.47 ± 0.02 m/s in 11 infarcted hearts, P < 0.05) was attenuated with MSC transplantation (0.65 ± 0.04 m/s, n = 18, P < 0.05). Electrophysiological changes correlated with higher vascular density and better-preserved ventricular geometry in MSC-transplanted hearts. A number of ion channel genes showed changes in RNA expression following infarction. In particular, the expression of Kir2.1, which mediates the inward rectifier potassium current, I(K1), was reduced in infarcted tissues (n = 7) to 13.8 ± 3.7% of normal controls, and this post-MI reduction was attenuated with MSC transplantation (44.4 ± 11.2%, n = 7, P < 0.05).

CONCLUSION

In addition to promoting angiogenesis and limiting adverse structural remodeling in infarcted hearts, MSC transplantation also alters ion channel expression and mitigates electrophysiological remodeling. Further understanding of the electrophysiological impact of MSC transplantation to the heart may lead to the development of cell-based therapies for post-MI arrhythmias.

Connexin43 gene transfer reduces ventricular tachycardia susceptibility after myocardial infarction

OBJECTIVES

The aim of this study was to evaluate the links between connexin43 (Cx43) expression, myocardial conduction velocity, and ventricular tachycardia in a model of healed myocardial infarction.

BACKGROUND

Post-infarction ventricular arrhythmias frequently cause sudden death. Impaired myocardial conduction has previously been linked to ventricular arrhythmias. Altered connexin expression is a potential source of conduction slowing identified in healed scar border tissues. The functional effect of increasing border-zone Cx43 has not been previously evaluated.

METHODS

Twenty-five Yorkshire pigs underwent anterior infarction by transient left anterior descending coronary artery occlusion, followed by weekly testing for arrhythmia inducibility. Twenty animals with reproducibly inducible sustained monomorphic ventricular tachycardia were randomized 2:1:1 to receive AdCx43, Adβgal, or no gene transfer. One week later, animals underwent follow-up electrophysiologic study and tissue assessment for several functional and molecular measures.

RESULTS

Animals receiving AdCx43 had less electrogram fractionation and faster conduction velocity in the anterior-septal border zone. Only 40% of AdCx43 animals remained inducible for ventricular tachycardia, while 100% of controls were inducible after gene transfer. AdCx43 animals had 2-fold higher Cx43 protein levels in the anterior-septal infarct border, with similar percents of phosphorylated and intercalated disk-localized Cx43 compared with controls.

CONCLUSIONS

These data mechanistically link Cx43 expression to slow conduction and arrhythmia susceptibility in the healed scar border zone. Targeted manipulation of Cx43 levels improved conduction velocity and reduced ventricular tachycardia susceptibility. Cx43 gene transfer represents a novel treatment strategy for post-infarction arrhythmias.

Effect of skeletal muscle Na(+) channel delivered via a cell platform on cardiac conduction and arrhythmia induction

BACKGROUND

In depolarized myocardial infarct epicardial border zones, the cardiac sodium channel is largely inactivated, contributing to slow conduction and reentry. We have demonstrated that adenoviral delivery of the skeletal muscle Na(+) channel (SkM1) to epicardial border zones normalizes conduction and reduces induction of ventricular tachycardia/ventricular fibrillation. We now studied the impact of canine mesenchymal stem cells (cMSCs) in delivering SkM1.

METHODS AND RESULTS

cMSCs were isolated and transfected with SkM1. Coculture experiments showed cMSC/SkM1 but not cMSC alone and maintained fast conduction at depolarized potentials. We studied 3 groups in the canine 7d infarct: sham, cMSC, and cMSC/SkM1. In vivo epicardial border zones electrograms were broad and fragmented in sham, narrower in cMSCs, and narrow and unfragmented in cMSC/SkM1 (P<0.05). During programmed electrical stimulation of epicardial border zones, QRS duration in cMSC/SkM1 was shorter than in cMSC and sham (P<0.05). Programmed electrical stimulation-induced ventricular tachycardia/ventricular fibrillation was equivalent in all groups (P>0.05).

CONCLUSION

cMSCs provide efficient delivery of SkM1 current. The interventions performed (cMSCs or cMSC/SkM1) were neither antiarrhythmic nor proarrhythmic. Comparing outcomes with cMSC/SkM1 and viral gene delivery highlights the criticality of the delivery platform to SkM1 antiarrhythmic efficacy.

Gene therapy for ventricular tachyarrhythmias

Cardiac arrest is the leading cause of death in the United States and other developed countries. Ventricular tachyarrhythmias are the most prominent cause of cardiac arrest, and patients with structural heart disease are at increased risk for these abnormal heart rhythms. Drug and device therapies have important limitations that make them inadequate to meet this challenge. We and others have proposed development of arrhythmia gene therapy as an alternative to current treatment methods. In this review, I discuss the basic mechanisms of ventricular arrhythmias and summarize the literature on the use of gene therapy for ventricular tachyarrhythmias.

Susceptibility to arrhythmia in the infarcted heart depends on myofibroblast density

Fibroblasts are electrophysiologically quiescent in the healthy heart. Evidence suggests that remodeling following myocardial infarction may include coupling of myofibroblasts (Mfbs) among themselves and with myocytes via gap junctions. We use a magnetic resonance imaging-based, three-dimensional computational model of the chronically infarcted rabbit ventricles to characterize the arrhythmogenic substrate resulting from Mfb infiltration as a function of Mfb density. Mfbs forming gap junctions were incorporated into both infarct regions, the periinfarct zone (PZ) and the scar; six scenarios were modeled: 0%, 10%, and 30% Mfbs in the PZ, with either 80% or 0% Mfbs in the scar. Ionic current remodeling in PZ was also included. All preparations exhibited elevated resting membrane potential within and near the PZ and action potential duration shortening throughout the ventricles. The unique combination of PZ ionic current remodeling and different degrees of Mfb infiltration in the infarcted ventricles determines susceptibility to arrhythmia. At low densities, Mfbs do not alter arrhythmia propensity; the latter arises predominantly from ionic current remodeling in PZ. At intermediate densities, Mfbs cause additional action potential shortening and exacerbate arrhythmia propensity. At high densities, Mfbs protect against arrhythmia by causing resting depolarization and blocking propagation, thus overcoming the arrhythmogenic effects of PZ ionic current remodeling.

Ionic mechanisms of electrophysiological heterogeneity and conduction block in the infarct border zone

The increased incidence of arrhythmia in the healing phase after infarction has been linked to remodeling in the epicardial border zone (EBZ). Ionic models of normal zone (NZ) and EBZ myocytes were incorporated into one-dimensional models of propagation to gain mechanistic insights into how ion channel remodeling affects action potential (AP) duration (APD) and refractoriness, vulnerability to conduction block, and conduction safety postinfarction. We found that EBZ tissue exhibited abnormal APD restitution. The remodeled Na(+) current (I(Na)) and L-type Ca(2+) current (I(Ca,L)) promoted increased effective refractory period and prolonged APD at a short diastolic interval. While postrepolarization refractoriness due to remodeled EBZ I(Na) was the primary determinant of the vulnerable window for conduction block at the NZ-to-EBZ transition in response to premature S2 stimuli, altered EBZ restitution also promoted APD dispersion and increased the vulnerable window at fast S1 pacing rates. Abnormal EBZ APD restitution and refractoriness also led to abnormal periodic conduction block patterns for a range of fast S1 pacing rates. In addition, we found that I(Na) remodeling decreased conduction safety in the EBZ but that inward rectifier K(+) current remodeling partially offset this decrease. EBZ conduction was characterized by a weakened AP upstroke and short intercellular delays, which prevented I(Ca,L) and transient outward K(+) current remodeling from playing a role in EBZ conduction in uncoupled tissue. Simulations of a skeletal muscle Na(+) channel SkM1-I(Na) injection into the EBZ suggested that this recently proposed antiarrhythmic therapy has several desirable effects, including normalization of EBZ effective refractory period and APD restitution, elimination of vulnerability to conduction block, and normalization of conduction in tissue with reduced intercellular coupling.

EH domain proteins regulate cardiac membrane protein targeting

RATIONALE

Cardiac membrane excitability is tightly regulated by an integrated network of membrane-associated ion channels, transporters, receptors, and signaling molecules. Membrane protein dynamics in health and disease are maintained by a complex ensemble of intracellular targeting, scaffolding, recycling, and degradation pathways. Surprisingly, despite decades of research linking dysfunction in membrane protein trafficking with human cardiovascular disease, essentially nothing is known regarding the molecular identity or function of these intracellular targeting pathways in excitable cardiomyocytes.

OBJECTIVE

We sought to discover novel pathways for membrane protein targeting in primary cardiomyocytes.

METHODS AND RESULTS

We report the initial characterization of a large family of membrane trafficking proteins in human heart. We used a tissue-wide screen for novel ankyrin-associated trafficking proteins and identified 4 members of a unique Eps15 homology (EH) domain-containing protein family (EHD1, EHD2, EHD3, EHD4) that serve critical roles in endosome-based membrane protein targeting in other cell types. We show that EHD1-4 directly associate with ankyrin, provide the first information on the expression and localization of these molecules in primary cardiomyocytes, and demonstrate that EHD1-4 are coexpressed with ankyrin-B in the myocyte perinuclear region. Notably, the expression of multiple EHD proteins is increased in animal models lacking ankyrin-B, and EHD3-deficient cardiomyocytes display aberrant ankyrin-B localization and selective loss of Na/Ca exchanger expression and function. Finally, we report significant modulation of EHD expression following myocardial infarction, suggesting that these proteins may play a key role in regulating membrane excitability in normal and diseased heart.

CONCLUSIONS

Our findings identify and characterize a new class of cardiac trafficking proteins, define the first group of proteins associated with the ankyrin-based targeting network, and identify potential new targets to modulate membrane excitability in disease. Notably, these data provide the first link between EHD proteins and a human disease model.

Oxidized calmodulin kinase II regulates conduction following myocardial infarction: a computational analysis

Calmodulin kinase II (CaMKII) mediates critical signaling pathways responsible for divergent functions in the heart including calcium cycling, hypertrophy and apoptosis. Dysfunction in the CaMKII signaling pathway occurs in heart disease and is associated with increased susceptibility to life-threatening arrhythmia. Furthermore, CaMKII inhibition prevents cardiac arrhythmia and improves heart function following myocardial infarction. Recently, a novel mechanism for oxidative CaMKII activation was discovered in the heart. Here, we provide the first report of CaMKII oxidation state in a well-validated, large-animal model of heart disease. Specifically, we observe increased levels of oxidized CaMKII in the infarct border zone (BZ). These unexpected new data identify an alternative activation pathway for CaMKII in common cardiovascular disease. To study the role of oxidation-dependent CaMKII activation in creating a pro-arrhythmia substrate following myocardial infarction, we developed a new mathematical model of CaMKII activity including both oxidative and autophosphorylation activation pathways. Computer simulations using a multicellular mathematical model of the cardiac fiber demonstrate that enhanced CaMKII activity in the infarct BZ, due primarily to increased oxidation, is associated with reduced conduction velocity, increased effective refractory period, and increased susceptibility to formation of conduction block at the BZ margin, a prerequisite for reentry. Furthermore, our model predicts that CaMKII inhibition improves conduction and reduces refractoriness in the BZ, thereby reducing vulnerability to conduction block and reentry. These results identify a novel oxidation-dependent pathway for CaMKII activation in the infarct BZ that may be an effective therapeutic target for improving conduction and reducing heterogeneity in the infarcted heart.

Calmodulin kinase II (CaMKII) is a multifunctional serine/threonine kinase that regulates diverse functions in heart. Recently, a novel pathway for CaMKII activation was discovered where oxidation of the kinase at specific methionine residues produces persistent activity. This alternative oxidation-dependent pathway has important implications for heart disease where oxidative stress is increased (e.g., heart failure and following myocardial infarction). We hypothesized that myocardial infarction caused by occlusion of a coronary artery would increase levels of oxidized CaMKII. Moreover, we hypothesized that oxidative CaMKII activation represents an important mechanistic link between increased oxidative stress and life-threatening heart rhythm disturbances (arrhythmias) in heart disease. We report a dramatic increase in levels of oxidized CaMKII following myocardial infarction in the canine. Based on these experimental data, we developed a novel mathematical model of CaMKII activity to study the role of oxidation-dependent CaMKII activation in regulating cardiac cell excitability. Our findings identify a novel role for oxidation-dependent CaMKII activation following myocardial infarction and provide a mechanistic link between oxidative stress and lethal cardiac arrhythmias in heart disease.

Selective gamma-ketoaldehyde scavengers protect Nav1.5 from oxidant-induced inactivation

The cardiac sodium channel (SCN5A, Na(V)1.5) is a key determinant of electrical impulse conduction in cardiac tissue. Acute myocardial infarction leads to diminished sodium channel availability, both because of decreased channel expression and because of greater inactivation of channels already present. Myocardial infarction leads to significant increases in reactive oxygen species and their downstream effectors including lipoxidation products. The effects of reactive oxygen species on Na(V)1.5 function in whole hearts can be modeled in cultured myocytes, where oxidants shift the availability curve of I(Na) to hyperpolarized potentials, decreasing cardiac sodium current at the normal activation threshold. We recently examined potential mediators of the oxidant-induced inactivation and found that one specific lipoxidation product, the isoketals, recapitulated the effects of oxidant on sodium currents. Isoketals are highly reactive gamma-ketoaldehydes formed by the peroxidation of arachidonic acid that covalently modify the lysine residues of proteins. We now confirm that exposure to oxidants induces lipoxidative modification of Na(V)1.5 and that the selective isoketal scavengers block voltage-dependent changes in sodium current by the oxidant tert-butylhydroperoxide, both in cells heterologously expressing Na(V)1.5 and in a mouse cardiac myocyte cell line (HL-1). Thus, inhibition of this lipoxidative modification pathway is sufficient to protect the sodium channel from oxidant induced inactivation and suggests the potential use of isoketal scavengers as novel therapeutics to prevent arrhythmogenesis during myocardial infarction.

Cardiac autonomic neural remodeling and susceptibility to sudden cardiac death: effect of endurance exercise training

Sudden cardiac death resulting from ventricular tachyarrhythmias remains the leading cause of death in industrially developed countries, accounting for between 300,000 and 500,000 deaths each year in the United States. Yet, despite the enormity of this problem, both the identification of factors contributing to ventricular fibrillation as well as the development of safe and effective antiarrhythmic agents remain elusive. Subnormal cardiac parasympathetic regulation coupled with an elevated cardiac sympathetic activation may allow for the formation of malignant ventricular arrhythmias. In particular, myocardial infarction can reduce cardiac parasympathetic regulation and alter beta-adrenoceptor subtype expression enhancing beta(2)-adrenoceptor sensitivity that can lead to intracellular calcium dysregulation and arrhythmias. As such, myocardial infarction can induce a remodeling of cardiac autonomic regulation that may be required to maintain cardiac pump function. If alterations in cardiac autonomic regulation play an important role in the genesis of life-threatening arrhythmias, then one would predict that interventions designed to either augment parasympathetic activity and/or reduce cardiac adrenergic activity would also protect against ventricular fibrillation. Recently, studies using a canine model of sudden death demonstrate that endurance exercise training (treadmill running) enhanced cardiac parasympathetic regulation (increased heart rate variability), restored a more normal beta-adrenoceptor balance (i.e., reduced beta(2)-adrenoceptor sensitivity and expression), and protected against ventricular fibrillation induced by acute myocardial ischemia. Thus exercise training may reverse the autonomic neural remodeling induced by myocardial infarction and thereby enhance the electrical stability of the heart in individuals shown to be at an increased risk for sudden cardiac death.

Extracellular space attenuates the effect of gap junctional remodeling on wave propagation: a computational study

Ionic channels and gap junctions are remodeled in cells from the 5-day epicardial border zone (EBZ) of the healing canine infarct. The main objective of the study was to determine the effect of gap junctional conductance (Gj) remodeling and Cx43 redistribution to the lateral membrane on conduction velocity (theta) and anisotropic ratio, and how gap junctional remodeling is modulated by the extracellular space. We first implemented subcellular monodomain and two-domain computer models of normal epicardium (NZ) to understand how extracellular space modulates the relationship between Gj and theta in NZ. We found that the extracellular space flattens the Gj-theta relationship, thus theta becomes less sensitive to changes in Gj. We then investigated the functional consequences of Gj remodeling and Cx43 distribution in subcellular computer models of cells of the outer pathway (IZo) and central pathway (IZc) of reentrant circuits. In IZo cells, side-to-side (transverse) Gj is 10% the value in NZ cells. Such Gj remodeling causes a 45% decrease in transverse theta (theta(T)). Inclusion of an extracellular space reduces the decrease in theta(T) to 31%. In IZc cells, Cx43 redistribution along the lateral membrane results in a 29% increase in theta(T). That increase in theta(T) is a consequence of the decrease in access resistance to the Cx43 plaques that occur with the Cx43 redistribution. Extracellular space reduces the increase in theta(T) to 10%.

IN CONCLUSION

1), The extracellular space included in normal epicardial simulations flattens the Gj-theta relationship with theta becoming less sensitive to changes in Gj. 2), The extracellular space attenuates the effects of gap junction epicardial border zone remodeling (i.e., Gj reduction and Cx43 lateralization) on theta(T).

Properties and ionic mechanisms of action potential adaptation, restitution, and accommodation in canine epicardium

Computational models of cardiac myocytes are important tools for understanding ionic mechanisms of arrhythmia. This work presents a new model of the canine epicardial myocyte that reproduces a wide range of experimentally observed rate-dependent behaviors in cardiac cell and tissue, including action potential (AP) duration (APD) adaptation, restitution, and accommodation. Model behavior depends on updated formulations for the 4-aminopyridine-sensitive transient outward current (I(to1)), the slow component of the delayed rectifier K(+) current (I(Ks)), the L-type Ca(2+) channel current (I(Ca,L)), and the Na(+)-K(+) pump current (I(NaK)) fit to data from canine ventricular myocytes. We found that I(to1) plays a limited role in potentiating peak I(Ca,L) and sarcoplasmic reticulum Ca(2+) release for propagated APs but modulates the time course of APD restitution. I(Ks) plays an important role in APD shortening at short diastolic intervals, despite a limited role in AP repolarization at longer cycle lengths. In addition, we found that I(Ca,L) plays a critical role in APD accommodation and rate dependence of APD restitution. Ca(2+) entry via I(Ca,L) at fast rate drives increased Na(+)-Ca(2+) exchanger Ca(2+) extrusion and Na(+) entry, which in turn increases Na(+) extrusion via outward I(NaK). APD accommodation results from this increased outward I(NaK). Our simulation results provide valuable insight into the mechanistic basis of rate-dependent phenomena important for determining the heart's response to rapid and irregular pacing rates (e.g., arrhythmia). Accurate simulation of rate-dependent phenomena and increased understanding of their mechanistic basis will lead to more realistic multicellular simulations of arrhythmia and identification of molecular therapeutic targets.

Regulation of the ankyrin-B-based targeting pathway following myocardial infarction

AIMS

Ion channel reorganization is a critical step in the pro-arrhythmogenic remodelling process that occurs in heart disease. Ankyrin-B (AnkB) is required for targeting and stabilizing ion channels, exchangers, and pumps. Despite a wealth of knowledge implicating the importance of AnkB in human cardiovascular physiology, nothing is known regarding the role of AnkB in common forms of acquired human disease.

METHODS AND RESULTS

We present the first report of AnkB regulation following myocardial infarction (MI). AnkB protein levels were reduced in the infarct border zone 5 days following coronary artery occlusion in the canine. We also observed a dramatic increase in AnkB mRNA levels 5 days post-occlusion. Surprisingly, the expression of the upstream AnkB cytoskeletal component beta2-spectrin was unchanged in post-infarct tissues. However, protein levels and/or membrane expression of downstream AnkB-associated ion channels and transporters Na+/K+ ATPase, Na+/Ca2+ exchanger, and IP3 receptor were altered 5 days post-occlusion. Interestingly, protein levels of the protein phosphatase 2A, an AnkB-associated signalling protein, were significantly affected 5 days post-occlusion. AnkB and PP2A protein levels recovered by 14 days post-occlusion, whereas Na+/K+ ATPase levels recovered by 2 months post-occlusion.

CONCLUSION

These findings reveal the first evidence of ankyrin remodelling following MI and suggest an unexpected divergence point for regulation between ankyrin and the underlying cytoskeletal network. These findings suggest a logical, but unexpected, molecular mechanism underlying ion channel and transporter remodelling following MI.

Deleting the accessory subunit KChIP2 results in loss of I(to,f) and increased I(K,slow) that maintains normal action potential configuration

BACKGROUND

Four voltage-gated potassium currents, I(to,f) (K(V)4.2), I(to,s) (K(V)1.4), I(K,slow) (K(V)1.5+K(V)2.1), and I(SS) (TASK1), govern murine ventricular repolarization. Although the accessory subunit KChIP2 influences I(to,f) expression, in preliminary experiments we found that action potential duration (APD) is maintained in KChIP2 knockout mice.

OBJECTIVE

We tested the role of KChIP2 in regulating APD and studied the underlying ionic currents.

METHODS

We used microelectrode techniques, whole-cell patch clamp studies, and real-time polymerase chain reaction amplification to characterize ventricular repolarization and its determinants in wild-type and KChIP2(-/-) mice.

RESULTS

Despite comparable baseline action potentials, APD was more markedly prolonged by 4-aminopyridine (4-AP) in KChIP2(-/-) preparations. Peak K(+) current densities were similar in wild-type and KChIP2(-/-) cells (mean +/- SEM I(P): 28.3 +/- 2 (n = 27) vs. 29.2 +/- 2 pA/pF (n = 24), respectively; P > .05). Heteropodatoxin-2 (HpTx-2, 1 microM) had no effect on current amplitude in KChIP2(-/-) myocytes. The current fractions sensitive to 4-AP (50 microM and 1 mM) were larger in KChIP2(-/-) than wild-type (P < .05). Real-time polymerase chain reaction showed absence of KChIP2 and increased K(V)1.5 expression in KChIP2(-/-) ventricular myocardium.

CONCLUSION

KChIP2 deficiency eliminated HpTx-2-sensitive I(to,f), but had little impact on total APD, secondary to upregulation of 4-AP-sensitive I(K,slow) in association with increased K(V)1.5 expression. There is increased sensitivity to 4-AP-mediated APD prolongation in KChIP2(-/-). Thus, KChIP2 seems important for murine repolarization in circumstances of reduced repolarization reserve.