Antonis A. Armoundas

Role of Sodium-Calcium Exchanger in Modulating the Action Potential of Ventricular Myocytes From Normal and Failing Hearts

Abstract— Increased Na+-Ca2+ exchange (NCX) activity in heart failure and hypertrophy may compensate for depressed sarcoplasmic reticular Ca2+ uptake, provide inotropic support through reverse-mode Ca2+ entry, and/or deplete intracellular Ca2+ stores. NCX is electrogenic and depends on Na+ and Ca2+ transmembrane gradients, making it difficult to predict its effect on the action potential (AP). Here, we examine the effect of [Na+]i on the AP in myocytes from normal and pacing-induced failing canine hearts and estimate the direction of the NCX driving force using simultaneously recorded APs and Ca2+ transients. AP duration shortened with increasing [Na+]i and was correlated with a shift in the reversal point of the NCX driving force. At [Na+]i ≥10 mmol/L, outward NCX current during the plateau facilitated repolarization, whereas at 5 mmol/L [Na+]i, NCX had a depolarizing effect, confirmed by partially inhibiting NCX with exchange inhibitory peptide. Exchange inhibitory peptide shortened the AP duration at 5 mmol/L [Na+]i and prolonged it at [Na+]i ≥10 mmol/L. With K+ currents blocked, total membrane current was outward during the late plateau of an AP clamp at 10 mmol/L [Na+]i and became inward close to the predicted reversal point for the NCX driving force. The results were reproduced using a computer model. These results indicate that NCX plays an important role in shaping the AP of the canine myocyte, helping it to repolarize at high [Na+]i, especially in the failing heart, but contributing a depolarizing, potentially arrhythmogenic, influence at low [Na+]i.

Pathophysiological basis and clinical application of T-wave alternans☆

We review the contemporary understanding of the pathophysiology of repolarization alternans and present a perspective on the use of T-wave alternans (TWA) as a risk stratification marker of malignant ventricular arrhythmias. Several studies have demonstrated a high correlation of susceptibility to ventricular arrhythmias and sudden cardiac death with the existence of TWA. We describe a number of cellular and molecular alterations in the diseased heart that may provide a link between electrical and mechanical alternans and arrhythmia susceptibility. Repolarization alternans is likely the result of distinct and diverse cellular and molecular alterations that are associated with exaggerated regional repolarization heterogeneity, which renders the heart susceptible to malignant arrhythmias.

Electrical and structural remodeling of the failing ventricle.

Heart failure (HF) is a complex disease that presents a major public health challenge to Western society. The prevalence of HF increases with age in the elderly population, and the societal disease burden will increase with prolongation of life expectancy. HF is initially characterized by an adaptive increase of neurohumoral activation to compensate for reduction of cardiac output. This leads to a combination of neurohumoral activation and mechanical stress in the failing heart that trigger a cascade of maladaptive electrical and structural events that impair both the systolic and diastolic function of the heart.

Post-Transcriptional Gene Silencing of KChIP2 and Navβ1 in Neonatal Rat Cardiac Myocytes Reveals a Functional Association between Na and Ito currents

The Ca(2+)-independent transient outward potassium current (I(to)) encoded by the Kv4 family of potassium channels, is central to normal repolarization of cardiac myocytes. KChIPs are a group of Ca(2+)-binding accessory subunits that modulate Kv4-encoded currents. However, the biophysical effects of KChIP2 on Kv4 currents raise questions about the role that KChIP2 plays in forming the native I(to). Previous heterologous expression studies demonstrated that the Na channel beta1 subunit modulates the gating properties of Kv4.3 to closely recapitulate native I(to) suggesting that Na(v)beta1 may modulate the function of Kv4-encoded channels in native cardiomyocytes. Therefore we hypothesized the existence of a structural or functional complex between subunits of I(to) and I(Na). In co-immunoprecipitation of proteins from neonatal rat ventricular myocardium (NRVM), Na(v)beta1 was pulled-down by Kv4.x antibodies suggesting a structural association between subunits that comprise I(to) and I(Na). Remarkably, post-transcriptional gene silencing of KChIP2 in NRVM, using small interfering RNAs specific to KChIP2, suppressed both cardiac I(to) and I(Na) consistent with a functional coupling of these channels. KChIP2 silencing suppressed Na channel alpha and beta1 subunit mRNA levels, leaving Kv4.x mRNAs unaltered, but reducing levels of immunoreactive proteins. Post-transcriptional gene silencing of Na(v)beta1 reduced its protein expression. Silencing of Na(v)beta1 also reduced mRNA and protein levels of its alpha-subunit, Na(v)1.5. Surprisingly, silencing of Na(v)beta1 also produced a reduction in KChIP2 mRNA and protein as well as Kv4.x proteins resulting in remarkably decreased I(Na) and I(to). These data are consistent with a novel structural and functional association of I(Na) and I(to) in NRVMs.

Prognostic significance of electrical alternans versus signal averaged electrocardiography in predicting the outcome of electrophysiological testing and arrhythmia-free survival

Objective To investigate the accuracy of signal averaged electrocardiography (SAECG) and measurement of microvolt level T wave alternans as predictors of susceptibility to ventricular arrhythmias. Design Analysis of new data from a previously published prospective investigation. Setting Electrophysiology laboratory of a major referral hospital. Patients and interventions 43 patients, not on class I or class III antiarrhythmic drug treatment, undergoing invasive electrophysiological testing had SAECG and T wave alternans measurements. The SAECG was considered positive in the presence of one (SAECG-I) or two (SAECG-II) of three standard criteria. T wave alternans was considered positive if the alternans ratio exceeded 3.0. Main outcome measures Inducibility of sustained ventricular tachycardia or fibrillation during electrophysiological testing, and 20 month arrhythmia-free survival. Results The accuracy of T wave alternans in predicting the outcome of electrophysiological testing was 84% (p < 0.0001). Neither SAECG-I (accuracy 60%; p < 0.29) nor SAECG-II (accuracy 71%; p < 0.10) was a statistically significant predictor of electrophysiological testing. SAECG, T wave alternans, electrophysiological testing, and follow up data were available in 36 patients while not on class I or III antiarrhythmic agents. The accuracy of T wave alternans in predicting the outcome of arrhythmia-free survival was 86% (p < 0.030). Neither SAECG-I (accuracy 65%; p < 0.21) nor SAECG-II (accuracy 71%; p < 0.48) was a statistically significant predictor of arrhythmia-free survival. Conclusions—T wave alternans was a highly significant predictor of the outcome of electrophysiological testing and arrhythmia-free survival, while SAECG was not a statistically significant predictor. Although these results need to be confirmed in prospective clinical studies, they suggest that T wave alternans may serve as a non-invasive probe for screening high risk populations for malignant ventricular arrhythmias.

Spatio-temporal oscillations of individual mitochondria in cardiac myocytes reveal modulation of synchronized mitochondrial clusters

Mitochondrial networks in cardiac myocytes under oxidative stress show collective (cluster) behavior through synchronization of their inner membrane potentials (ΔΨm). However, it is unclear whether the oscillation frequency and coupling strength between individual mitochondria affect the size of the cluster and vice versa. We used the wavelet transform and developed advanced signal processing tools that allowed us to capture individual mitochondrial ΔΨm oscillations in cardiac myocytes and examine their dynamic spatio-temporal properties. Heterogeneous frequency behavior prompted us to sort mitochondria according to their frequencies. Signal analysis of the mitochondrial network showed an inverse relationship between cluster size and cluster frequency as well as between cluster amplitude and cluster size. High cross-correlation coefficients between neighboring mitochondria clustered longitudinally along the myocyte striations, indicated anisotropic communication between mitochondria. Isochronal mapping of the onset of myocyte-wide ΔΨm depolarization further exemplified heterogeneous ΔΨm among mitochondria. Taken together, the results suggest that frequency and amplitude modulation of clusters of synchronized mitochondria arises by means of strong changes in local coupling between neighboring mitochondria.

Ectopic expression of KCNE3 accelerates cardiac repolarization and abbreviates the QT interval

Regulatory subunit KCNE3 (E3) interacts with KCNQ1 (Q1) in epithelia, regulating its activation kinetics and augmenting current density. Since E3 is expressed weakly in the heart, we hypothesized that ectopic expression of E3 in cardiac myocytes might abbreviate action potential duration (APD) by interacting with Q1 and augmenting the delayed rectifier current (I(K)). Thus, we transiently coexpressed E3 with Q1 and KCNE1 (E1) in Chinese hamster ovary cells and found that E3 coexpression increased outward current at potentials by > or = -80 mV and accelerated activation. We then examined the changes in cardiac electrophysiology following injection of adenovirus-expressed E3 into the left ventricular cavity of guinea pigs. After 72 hours, the corrected QT interval of the electrocardiogram was reduced by approximately 10%. APD was reduced by >3-fold in E3-transduced cells relative to controls, while E-4031-insensitive I(K) and activation kinetics were significantly augmented. Based on quantitative modeling of a transmural cardiac segment, we demonstrate that the degree of QT interval abbreviation observed results from electrotonic interactions in the face of limited transduction efficiency and that heterogeneous transduction of E3 may actually potentiate arrhythmias. Provided that fairly homogeneous ectopic ventricular expression of regulatory subunits can be achieved, this approach may be useful in enhancing repolarization and in treating long QT syndrome.

T-wave alternans and dispersion of the QT interval as risk stratification markers in patients susceptible to sustained ventricular arrhythmias ☆

T-wave alternans and QT dispersion were compared as predictors of the outcome of electrophysiologic study and arrhythmia-free survival in patients undergoing electrophysiologic evaluation. T-wave alternans was a highly significant predictor of these 2 outcome variables, whereas QT dispersion was not.

Can microvolt T-wave alternans testing reduce unnecessary defibrillator implantation?

The Multicenter Automatic Defibrillator Implantation Trial II (MADIT II) and the Sudden Cardiac Death in Heart Failure Trial (SCD-HeFT) have established that patients with a reduced ejection fraction gain an overall mortality benefit from prophylactic implantable cardioverter-defibrillator therapy. Only a small proportion of the patients in these studies, however, have received life-saving therapy from the defibrillator. Because defibrillator therapy is invasive and expensive, patients with a low ejection fraction would benefit from effective risk stratification so that defibrillator therapy was used only in those at significant risk. In this review, we analyze prospective clinical trials that have evaluated microvolt T-wave alternans (MTWA) testing as a predictor of ventricular tachyarrhythmic events in populations of patients similar to those studied in MADIT II or SCD-HeFT; that is, patients with a reduced ejection fraction who were not selected on the basis of a history of ventricular tachyarrhythmias. In these studies, the average annual rate of fatal and nonfatal ventricular tachyarrhythmic events among the patients who tested negative for MTWA was around 1%. This rate is so low that it is unlikely that such patients would benefit from implantable cardioverter-defibrillator therapy. The mortality, moreover, was lower among MTWA-negative patients who did not receive implantable defibrillators than that observed in the MADIT II and SCD-HeFT patients who received implantable cardioverter-defibrillators. In response, patients with a low ejection fraction who are being considered for implantable cardioverter-defibrillator therapy should undergo MTWA testing as part of their evaluation.

A single equivalent moving dipole model: an efficient approach for localizing sites of origin of ventricular electrical activation.

AbstractWe propose a new method for guiding catheter ablation procedures to abolish sites of origin of arrhythmias. This method models both cardiac electrical activity and current pulses delivered from the tip of the ablation catheter with a single equivalent moving dipole (SEMD). The SEMD parameters are obtained from analysis of body surface potentials. In this paper we examine the feasibility of this method by evaluating the performance of an inverse algorithm we developed to localize the SEMD from the surface potentials. In computer simulations realistic levels of measurement noise led to uncertainties in SEMD location ∼0.005 cm. Dipole orientation randomization contributed to increased uncertainty (0.04 cm) in SEMD location only when boundary effects were included. In ventricular pacing swine studies, we found that the SEMD model accurately accounted for electrocardiographic wave forms and that measurement noise led to an uncertainty of approximately 0.04 cm in the SEMD at 15 ms after the pacing spike. We have also found that the algorithm we developed to identify the SEMD parameters yielded positions for two spatially separated pacing sites that maintained their direction and were very close to their physical separation. These results suggest that the SEMD method may potentially be used to guide radio-frequency ablation procedures.