Paul G.A. Volders

Progress in the understanding of cardiac early afterdepolarizations and torsades de pointes: time to revise current concepts

Time for primary review 19 days. Afterdepolarizations are oscillations of the transmembrane potential that depend on the preceding action potential (AP) for their generation and can give rise to new APs when they reach a critical threshold for activation of a depolarizing current. This form of abnormal impulse generation is called ‘triggered activity’ [1]. Two types of afterdepolarizations have been distinguished: delayed (DADs) and early afterdepolarizations (EADs). DADs have been defined as “oscillations in membrane potential that occur after repolarization of an action potential” [2]. EADs are generated during the AP and have been defined as “oscillations at the plateau level of membrane potential or later during phase 3 of repolarization” [2]. Depending on the level of the membrane potential at which they are generated, EADs can trigger new APs that may appear as ectopic beats on the ECG. EADs can also augment electrical heterogeneity in regions of neighboring myocardium, which can lead to the formation of new APs via electrotonic interaction between areas that are still inexcitable and those that have already recovered from refractoriness [3]. Although the latter mechanism is reentrant rather than triggered activity, the occurrence of EADs is of pivotal importance for arrhythmogenesis under these circumstances. The clinical significance of EADs lies in their capacity to provide both the trigger (premature ectopic beats) and the substrate (electrical heterogeneity with nonuniform repolarization and refractoriness) for the initiation and perpetuation of torsades de pointes. In this article, we discuss the evidence for a new concept of EAD formation, which includes an important role for cytoplasmic-[Ca2+]-dependent mechanisms, as schematically illustrated in Fig. 1. As a background, we will first review the recent literature on cellular Ca2+ homeostasis. Then, we introduce the classical view on EAD formation with a discussion of the … * Corresponding author. Tel.: +31-43-3875093; fax: +31-43-3875104 p.volders{at}cardio.azm.nl

Downregulation of Delayed Rectifier K+ Currents in Dogs With Chronic Complete Atrioventricular Block and Acquired Torsades de Pointes

BACKGROUND Acquired QT prolongation enhances the susceptibility to torsades de pointes (TdP). Clinical and experimental studies indicate ventricular action potential prolongation, increased regional dispersion of repolarization, and early afterdepolarizations as underlying factors. We examined whether K(+)-current alterations contribute to these proarrhythmic responses in an animal model of TdP: the dog with chronic complete atrioventricular block (AVB) and biventricular hypertrophy. METHODS AND RESULTS The whole-cell K(+) currents I(TO1), I(K1), I(Kr), and I(Ks) were recorded in left (LV) and right (RV) ventricular midmyocardial cells from dogs with 9+/-1 weeks of AVB and controls with sinus rhythm. I(TO1) density and kinetics and I(K1) outward current were not different between chronic AVB and control cells. I(Kr) had a similar voltage dependence of activation and time course of deactivation in chronic AVB and control. I(Kr) density was similar in LV myocytes but smaller in RV myocytes (-45%) of chronic AVB versus control. For I(Ks), voltage-dependence of activation and time course of deactivation were similar in chronic AVB and control. However, I(Ks) densities of LV (-50%) and RV (-55%) cells were significantly lower in chronic AVB than control. CONCLUSIONS Significant downregulation of delayed rectifier K(+) current occurs in both ventricles of the dog with chronic AVB. Acquired TdP in this animal model with biventricular hypertrophy is thus related to intrinsic repolarization defects.

Probing the Contribution of IKs to Canine Ventricular Repolarization: Key Role for β-Adrenergic Receptor Stimulation

Background—In large mammals and humans, the contribution of IKs to ventricular repolarization is still incompletely understood. Methods and Results—In vivo and cellular electrophysiological experiments were conducted to study IKs in canine ventricular repolarization. In conscious dogs, administration of the selective IKs blocker HMR 1556 (3, 10, or 30 mg/kg PO) caused substantial dose-dependent QT prolongations with broad-based T waves. In isolated ventricular myocytes under baseline conditions, however, IKs block (chromanols HMR 1556 and 293B) did not significantly prolong action potential duration (APD) at fast or slow steady-state pacing rates. This was because of the limited activation of IKs in the voltage and time domains of the AP, although at seconds-long depolarizations, the current was substantial. Isoproterenol increased and accelerated IKs activation to promote APD95 shortening. This shortening was importantly reversed by HMR 1556 and 293B. Quantitatively similar effects were obtained in ventricular-tissue preparations. Finally, when cellular repolarization was impaired by IKr block, IKs block exaggerated repolarization instability with further prolongation of APD. Conclusions—Ventricular repolarization in conscious dogs is importantly dependent on IKs. IKs function becomes prominent during &bgr;-adrenergic receptor stimulation, when it promotes AP shortening by increased activation, and during IKr block, when it limits repolarization instability by time-dependent activation. Unstimulated IKs does not contribute to cellular APD at baseline. These data highlight the importance of the synergism between an intact basal IKs and the sympathetic nervous system in vivo.

Repolarizing K+ Currents ITO1 and IKs Are Larger in Right Than Left Canine Ventricular Midmyocardium

BACKGROUND The ventricular action potential exhibits regional heterogeneity in configuration and duration (APD). Across the left ventricular (LV) free wall, this is explained by differences in repolarizing K+ currents. However, the ionic basis of electrical nonuniformity in the right ventricle (RV) versus the LV is poorly investigated. We examined transient outward (ITO1), delayed (IKs and IKr), and inward rectifier K+ currents (IK1) in relation to action potential characteristics of RV and LV midmyocardial (M) cells of the same adult canine hearts. METHODS AND RESULTS Single RV and LV M cells were used for microelectrode recordings and whole-cell voltage clamping. Action potentials showed deeper notches, shorter APDs at 50% and 95% of repolarization, and less prolongation on slowing of the pacing rate in RV than LV. ITO1 density was significantly larger in RV than LV, whereas steady-state inactivation and rate of recovery were similar. IKs tail currents, measured at -25 mV and insensitive to almokalant (2 micromol/L), were considerably larger in RV than LV. IKr, measured as almokalant-sensitive tail currents at -50 mV, and IK1 were not different in the 2 ventricles. CONCLUSIONS Differences in K+ currents may well explain the interventricular heterogeneity of action potentials in M layers of the canine heart. These results contribute to a further phenotyping of the ventricular action potential under physiological conditions.

Altered Na/Ca exchange activity in cardiac hypertrophy and heart failure: a new target for therapy?

Increased Na/Ca exchange (NCX) expression may be part of the genetic reprogramming in cardiac remodeling. In this review we address the following questions: (1) Is increased NCX activity a general feature of cardiac remodeling in hypertrophy and heart failure? (2) How does this contribute to the contractile and electrical phenotype of hypertrophy and heart failure? (3) Should be consider NCX a potential therapeutic target? From a review of the literature we found that NCX activity can be increased, unchanged, or even downregulated during cardiac remodeling. When NCX activity is increased, it can be considered compensatory for contractile function, but with negative side-effects, including an increased risk of arrhythmias. Changes in activity do not necessarily reflect changes in gene expression. Altered NCX activity can also be a consequence of changes in other Ca(2+) fluxes or in [Na(+)](i) homeostasis. The role of NCX in contractile alterations and arrhythmogenesis varies with the different stimuli or stages of cardiac remodeling. Pharmacological block of NCX in heart failure or hypertrophy may thus be useful, but most likely only in specific conditions, perhaps as part of a combined approach. Development of drugs that target only a specific mode of the exchanger may offer a further advantage.

How can we improve our understanding of cardiovascular safety liabilities to develop safer medicines

Given that cardiovascular safety liabilities remain a major cause of drug attrition during preclinical and clinical development, adverse drug reactions, and post‐approval withdrawal of medicines, the Medical Research Council Centre for Drug Safety Science hosted a workshop to discuss current challenges in determining, understanding and addressing ‘Cardiovascular Toxicity of Medicines’. This article summarizes the key discussions from the workshop that aimed to address three major questions: (i) what are the key cardiovascular safety liabilities in drug discovery, drug development and clinical practice? (ii) how good are preclinical and clinical strategies for detecting cardiovascular liabilities? and (iii) do we have a mechanistic understanding of these liabilities? It was concluded that in order to understand, address and ultimately reduce cardiovascular safety liabilities of new therapeutic agents there is an urgent need to:

Arrhythmogenic Right Ventricular Dysplasia/Cardiomyopathy Pathogenic Desmosome Mutations in Index-Patients Predict Outcome of Family Screening: Dutch Arrhythmogenic Right Ventricular Dysplasia/Cardiomyopathy Genotype-Phenotype Follow-Up Study

Background— Arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C) is an autosomal dominant inherited disease with incomplete penetrance and variable expression. Causative mutations in genes encoding 5 desmosomal proteins are found in ≈50% of ARVD/C index patients. Previous genotype-phenotype relation studies involved mainly overt ARVD/C index patients, so follow-up data on relatives are scarce. Methods and Results— One hundred forty-nine ARVD/C index patients (111 male patients; age, 49±13 years) according to 2010 Task Force criteria and 302 relatives from 93 families (282 asymptomatic; 135 male patients; age, 44±13 years) were clinically and genetically characterized. DNA analysis comprised sequencing of plakophilin-2 ( PKP2 ), desmocollin-2, desmoglein-2, desmoplakin, and plakoglobin and multiplex ligation-dependent probe amplification to identify large deletions in PKP2. Pathogenic mutations were found in 87 index patients (58%), mainly truncating PKP2 mutations, including 3 cases with multiple mutations. Multiplex ligation-dependent probe amplification revealed 3 PKP2 exon deletions. ARVD/C was diagnosed in 31% of initially asymptomatic mutation-carrying relatives and 5% of initially asymptomatic relatives of index patients without mutation. Prolonged terminal activation duration was observed more than negative T waves in V1 to V3, especially in mutation-carrying relatives <20 years of age. In 45% of screened families, ≥1 affected relatives were identified (90% with mutations). Conclusions— Pathogenic desmosomal gene mutations, mainly truncating PKP2 mutations, underlie ARVD/C in the majority (58%) of Dutch index patients and even 90% of familial cases. Additional multiplex ligation-dependent probe amplification analysis contributed to discovering pathogenic mutations underlying ARVD/C. Discovering pathogenic mutations in index patients enables those relatives who have a 6-fold increased risk of ARVD/C diagnosis to be identified. Prolonged terminal activation duration seems to be a first sign of ARVD/C in young asymptomatic relatives. # Clinical Perspective {#article-title-38}

Similarities between early and delayed afterdepolarizations induced by isoproterenol in canine ventricular myocytes

OBJECTIVES This study aims at clarifying the role of cellular Ca2+ overload and spontaneous sarcoplasmic reticulum (SR) Ca2+ release in the generation of early afterdepolarizations (EAD) by isoproterenol. The involvement of a Ca(2+)-activated membrane current in isoproterenol-induced EAD is investigated. METHODS Membrane potential and contraction (an indicator of SR Ca2+ release) were recorded in canine left ventricular myocytes at pacing cycle lengths (CL) of 300-4000 ms. Threshold concentration for EAD was 20-50 mmol/l isoproterenol. Ni2+ (2.0-5.0 mmol/l) was used at normal and high (5.4 mmol/l) [Ca2+]o to examine the role of Ca2+ current and/or Na(+)-Ca2+ exchange (1Na-Ca) in EAD. RESULTS In all cells delayed afterdepolarizations (DAD) appeared during isoproterenol. In most (approximately equal to 70%) cells EAD were also generated, which were fast-pacing dependent, occurring only at CL of 400-1000 ms. EAD were always initiated by a delay in repolarization. Early aftercontractions preceded the EAD upstrokes, often occurring without them. They coincided with the initial delays in repolarization. During treatment with isoproterenol, Ni2+ and high [Ca2+]o, EAD and DAD were suppressed despite the continued presence of early and delayed aftercontractions. CONCLUSIONS Our data indicate that beta-adrenergic EAD share a common ionic mechanism with DAD in terms of cellular Ca2+ overload and spontaneous SR Ca2+ release. beta-Adrenergic EAD consist of two phases: (1) a conditional phase coinciding with the onset of an early aftercontraction, often followed by (2) an EAD upstroke. A Ca2(+)-activated membrane current, probably I Na-Ca, is necessary at least for the initiation of these EAD.

Clinical Presentation, Long-Term Follow-Up, and Outcomes of 1001 Arrhythmogenic Right Ventricular Dysplasia/Cardiomyopathy Patients and Family Members

Background—Arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C) is a progressive cardiomyopathy. We aimed to define long-term outcome in a transatlantic cohort of 1001 individuals. Methods and Results—Clinical and genetic characteristics and follow-up data of ARVD/C index-patients (n=439, fulfilling of 2010 criteria in all) and family members (n=562) were assessed. Mutations were identified in 276 index-patients (63%). Index-patients presented predominantly with sustained ventricular arrhythmias (268; 61%). During a median follow-up of 7 years, 301 of the 416 index-patients presenting alive (72%) experienced sustained ventricular arrhythmias. Sudden cardiac death during follow-up occurred more frequently among index-patients without an implantable cardioverter-defibrillator (10/63, 16% versus 2/335, 0.6%). Overall, cardiac mortality and the need for cardiac transplantation were low (6% and 4%, respectively). Clinical characteristics and outcomes were similar in index-patients with and without mutations, as well as in those with familial and nonfamilial ARVD/C. ARVD/C was diagnosed in 207 family members (37%). Symptoms at first evaluation correlated with disease expression. Family members with mutations were more likely to meet Task Force Criteria for ARVD/C (40% versus 18%), experience sustained ventricular arrhythmias (11% versus 1%), and die from a cardiac cause (2% versus 0%) than family members without mutations. Conclusions—Long-term outcome was favorable in diagnosed and treated ARVD/C index-patients and family members. Outcome in index-patients was modulated by implantable cardioverter-defibrillator implantation, but not by mutation status and familial background of disease. One third of family members developed ARVD/C. Outcome in family members was determined by symptoms at first evaluation and mutations.

Accumulation of slowly activating delayed rectifier potassium current (IKs) in canine ventricular myocytes

In guinea‐pig ventricular myocytes, in which the deactivation of slowly activating delayed rectifier potassium current (IKs) is slow, IKs can be increased by rapid pacing as a result of incomplete deactivation and subsequent current accumulation. Whether accumulation of IKs occurs in dogs, in which the deactivation is much faster, is still unclear. In this study the conditions under which accumulation occurs in canine ventricular myocytes were studied with regard to its physiological relevance in controlling action potential duration (APD). At baseline, square pulse voltage clamp experiments revealed that the accumulation of canine IKs could occur, but only at rather short interpulse intervals (< 100 ms). With action potential (AP) clamp commands of constant duration (originally recorded at rate of 2 Hz), an accumulation was only found at interpulse intervals close to 0 ms. Transmembrane potential recordings with high‐resistance microelectrodes revealed, however, that at the fastest stimulation rates with normally captured APs (5 Hz) the interpulse interval exceeded 50 ms. This suggested that no IKs accumulation occurs, which was supported by the lack of effect of an IKs blocker, HMR 1556 (500 nM), on APD. In the presence of the β‐adrenergic receptor agonist isoproterenol (isoprenaline, 100 nM) the accumulation with AP clamp commands of constant duration was much more pronounced and a significant accumulating current was found at a relevant interpulse interval of 100 ms. HMR 1556 prolonged APD, but this lengthening was reverse rate dependent. AP clamp experiments in a physiologically relevant setting (short, high rate APs delivered at a corresponding rate) revealed a limited accumulation of IKs in the presence of isoproterenol. In conclusion, a physiologically relevant accumulation of IKs was only observed in the presence of isoproterenol. Block of IKs, however, led to a reverse rate‐dependent prolongation of APD indicating that IKs does not have a dominant role at short cycle lengths.