Karin R. Sipido

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

Enhanced Ca(2+) release and Na/Ca exchange activity in hypertrophied canine ventricular myocytes: potential link between contractile adaptation and arrhythmogenesis.

BackgroundVentricular arrhythmias are a major cause of sudden death in patients with heart failure and hypertrophy. The dog with chronic complete atrioventricular block (CAVB) has biventricular hypertrophy and ventricular arrhythmias and is a useful model to study underlying cellular mechanisms. We investigated whether changes in Ca2+ homeostasis are part of the contractile adaptation to CAVB and might contribute to arrhythmogenesis. Methods and ResultsIn enzymatically isolated myocytes, cell shortening, Ca2+ release from the sarcoplasmic reticulum (SR), and SR Ca2+ content were enhanced at low stimulation frequencies. Ca2+ influx through L-type Ca2+ channels was unchanged, but Ca2+ influx via the Na/Ca exchanger was increased and contributed to Ca2+ loading of the SR. Inward Na/Ca exchange currents were also larger. Changes in Ca2+ fluxes were less pronounced in the right versus left ventricle. ConclusionsEnhanced Na/Ca exchange activity may improve contractile adaptation to CAVB but at the same time facilitate arrhythmias by (1) increasing the propensity to Ca2+ overload, (2) providing more inward current leading to (nonhomogeneous) action potential prolongation, and (3) enhancing (arrhythmogenic) currents during spontaneous Ca2+ release.

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.

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.

Remodeling of T-Tubules and Reduced Synchrony of Ca2+ Release in Myocytes From Chronically Ischemic Myocardium

In ventricular cardiac myocytes, T-tubule density is an important determinant of the synchrony of sarcoplasmic reticulum (SR) Ca2+ release and could be involved in the reduced SR Ca2+ release in ischemic cardiomyopathy. We therefore investigated T-tubule density and properties of SR Ca2+ release in pigs, 6 weeks after inducing severe stenosis of the circumflex coronary artery (91±3%, N=13) with myocardial infarction (8.8±2.0% of total left ventricular mass). Severe dysfunction in the infarct and adjacent myocardium was documented by magnetic resonance and Doppler myocardial velocity imaging. Myocytes isolated from the adjacent myocardium were compared with myocytes from the same region in weight-matched control pigs. T-tubule density quantified from the di-8-ANEPPS (di-8-butyl-amino-naphthyl-ethylene-pyridinium-propyl-sulfonate) sarcolemmal staining was decreased by 27±7% (P<0.05). Synchrony of SR Ca2+ release (confocal line scan images during whole-cell voltage clamp) was reduced in myocardium myocytes. Delayed release (ie, half-maximal [Ca2+]i occurring later than 20 ms) occurred at 35.5±6.4% of the scan line in myocardial infarction versus 22.7±2.5% in control pigs (P<0.05), prolonging the time to peak of the line-averaged [Ca2+]i transient (121±9 versus 102±5 ms in control pigs, P<0.05). Delayed release colocalized with regions of T-tubule rarefaction and could not be suppressed by activation of protein kinase A. The whole-cell averaged [Ca2+]i transient amplitude was reduced, whereas L-type Ca2+ current density was unchanged and SR content was increased, indicating a reduction in the gain of Ca2+-induced Ca2+ release. In conclusion, reduced T-tubule density during ischemic remodeling is associated with reduced synchrony of Ca2+ release and reduced efficiency of coupling Ca2+ influx to Ca2+ release.

Cellular Basis of Biventricular Hypertrophy and Arrhythmogenesis in Dogs With Chronic Complete Atrioventricular Block and Acquired Torsade de Pointes

BACKGROUND In the dog with chronic complete atrioventricular block (AVB), torsade de pointes arrhythmias (TdP) can be induced reproducibly by class III antiarrhythmic agents. In vivo studies reveal important electrophysiological alterations of the heart at 5 weeks of AVB, resulting in increased proarrhythmia. Autopsy studies indicate the presence of biventricular hypertrophy. In this study, the cellular basis of proarrhythmia and hypertrophy in chronic AVB was investigated. METHODS AND RESULTS From chronic-AVB dogs with increased heart weights and TdP, left midmyocardial and right ventricular myocytes were isolated by enzymatic dispersion. These myocytes were significantly larger than sinus rhythm (SR) controls. In chronic AVB, the action potential spike-and-dome configuration was preserved. However, the action potential duration (APD) at 95% and 50% of repolarization of the left midmyocardium was significantly larger in chronic AVB than in SR, with little change in the right ventricle, causing enhanced interventricular dispersion of repolarization at slow pacing rates. Treatment with the class III agent almokalant increased the APD to a much larger extent in chronic-AVB than in SR myocytes and resulted in a higher incidence of early afterdepolarizations (EADs). EADs had their takeoff potential between -35 and 0 mV. There was no evidence that spontaneous sarcoplasmic reticulum Ca2+ release underlies these EADs. CONCLUSIONS In the dog, chronic AVB leads to hypertrophy of both right and left ventricular myocytes. The repolarization abnormalities predisposing for class III-dependent TdP in vivo are the results of cellular electrophysiological remodeling.

Inhibition and Rapid Recovery of Ca2+ Current During Ca2+ Release From Sarcoplasmic Reticulum in Guinea Pig Ventricular Myocytes

We have investigated the modulation of the L-type Ca2+ channel by Ca2+ released from the sarcoplasmic reticulum (SR) in single guinea pig ventricular myocytes under whole-cell voltage clamp. [Ca2+]i was monitored by fura 2. By use of impermeant monovalent cations in intracellular and extracellular solutions, the current through Na+ channels, K+ channels, nonspecific cation channels, and the Na+-Ca2+ exchanger was effectively blocked. By altering the amount of Ca2+ loading of the SR, the time course of the Ca2+ current (ICa) could be studied during various amplitudes of Ca2+ release. In the presence of a large Ca2+ release, fast inhibition of ICa occurred, whereas on relaxation of [Ca2+]i, fast recovery was observed. The time course of this transient inhibition of ICa reflected the time course of [Ca2+]i. However, the inhibition seen in the first 50 ms, ie, the time of net Ca2+ release from the SR, exceeded the inhibition observed later during the pulse, suggesting the existence of a higher [Ca2+] near the channel during this time. Transient inhibition of ICa during Ca2+ release was observed to a similar degree at all potentials. It could still be observed in the presence of intracellular ATP-gamma-S and of cAMP. Therefore, we conclude that the modulation of ICa by Ca2+ release from the SR is not related to dephosphorylation. It could be related to a reduction in the driving force and to a direct inhibition of the channel by [Ca2+]i. The observation that the degree of inhibition does not depend on membrane potential suggests that the Ca2+ binding site for this modulation is located outside the pore.(ABSTRACT TRUNCATED AT 250 WORDS)

Ventricular Phosphodiesterase-5 Expression Is Increased in Patients With Advanced Heart Failure and Contributes to Adverse Ventricular Remodeling After Myocardial Infarction in Mice

Background— Ventricular expression of phosphodiesterase-5 (PDE5), an enzyme responsible for cGMP catabolism, is increased in human right ventricular hypertrophy, but its role in left ventricular (LV) failure remains incompletely understood. We therefore measured LV PDE5 expression in patients with advanced systolic heart failure and characterized LV remodeling after myocardial infarction in transgenic mice with cardiomyocyte-specific overexpression of PDE5 (PDE5-TG). Methods and Results— Immunoblot and immunohistochemistry techniques revealed that PDE5 expression was greater in explanted LVs from patients with dilated and ischemic cardiomyopathy than in control hearts. To evaluate the impact of increased ventricular PDE5 levels on cardiac function, PDE5-TG mice were generated. Confocal and immunoelectron microscopy revealed increased PDE5 expression in cardiomyocytes, predominantly localized to Z-bands. At baseline, myocardial cGMP levels, cell shortening, and calcium handling in isolated cardiomyocytes and LV hemodynamic measurements were similar in PDE5-TG and wild-type littermates. Ten days after myocardial infarction, LV cGMP levels had increased to a greater extent in wild-type mice than in PDE5-TG mice (P<0.05). Ten weeks after myocardial infarction, LV end-systolic and end-diastolic volumes were larger in PDE5-TG than in wild-type mice (57±5 versus 39±4 and 65±6 versus 48±4 &mgr;L, respectively; P<0.01 for both). LV systolic dysfunction and diastolic dysfunction were more marked in PDE5-TG than in wild-type mice, associated with enhanced hypertrophy and reduced contractile function in isolated cardiomyocytes from remote myocardium. Conclusions— Increased PDE5 expression predisposes mice to adverse LV remodeling after myocardial infarction. Increased myocardial PDE5 expression in patients with advanced cardiomyopathy may contribute to the development of heart failure and represents an important therapeutic target.

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.