Role of L-type calcium channel window current in generating current-induced early afterdepolarizations

Multiple Dynamical Mechanisms of Phase-2 Early Afterdepolarizations in a Human Ventricular Myocyte Model: Involvement of Spontaneous SR Ca 2+ Release

Early afterdepolarization (EAD) is known to cause lethal ventricular arrhythmias in long QT syndrome (LQTS). In this study, dynamical mechanisms of EAD formation in human ventricular myocytes (HVMs) were investigated using the mathematical model developed by ten Tusscher and Panfilov (Am J Physiol Heart Circ Physiol 291, 2006). We explored how the rapid (IKr) and slow (IKs) components of delayed-rectifier K+ channel currents, L-type Ca2+ channel current (ICa L), Na+/Ca2+ exchanger current (INCX), and intracellular Ca2+ handling via the sarcoplasmic reticulum (SR) contribute to initiation, termination and modulation of phase-2 EADs during pacing in relation to bifurcation phenomena in non-paced model cells. Parameter-dependent dynamical behaviors of the non-paced model cell were determined by calculating stabilities of equilibrium points (EPs) and limit cycles, and bifurcation points to construct bifurcation diagrams. Action potentials (APs) and EADs during pacing were reproduced by numerical simulations for constructing phase diagrams of the paced model cell dynamics. Results are summarized as follows: (1) A modified version of the ten Tusscher-Panfilov model with accelerated ICaL inactivation could reproduce bradycardia-related EADs in LQTS type 2 and β-adrenergic stimulation-induced EADs in LQTS type 1. (2) Two types of EADs with different initiation mechanisms, ICaL reactivation-dependent and spontaneous SR Ca2+ release-mediated EADs, were detected. (3) Termination of EADs (AP repolarization) during pacing depended on the slow activation of IKs. (4) Spontaneous SR Ca2+ releases occurred at higher Ca2+ uptake rates, attributable to the instability of steady-state intracellular Ca2+ concentrations. Dynamical mechanisms of EAD formation and termination in the paced model cell are closely related to stability changes (bifurcations) in dynamical behaviors of the non-paced model cell, but they are model-dependent. Nevertheless, the modified ten Tusscher-Panfilov model would be useful for systematically investigating possible dynamical mechanisms of EAD-related arrhythmias in LQTS.

Computational Modeling of Electrophysiology and Pharmacotherapy of Atrial Fibrillation: Recent Advances and Future Challenges

The pathophysiology of atrial fibrillation (AF) is broad, with components related to the unique and diverse cellular electrophysiology of atrial myocytes, structural complexity, and heterogeneity of atrial tissue, and pronounced disease-associated remodeling of both cells and tissue. A major challenge for rational design of AF therapy, particularly pharmacotherapy, is integrating these multiscale characteristics to identify approaches that are both efficacious and independent of ventricular contraindications. Computational modeling has long been touted as a basis for achieving such integration in a rapid, economical, and scalable manner. However, computational pipelines for AF-specific drug screening are in their infancy, and while the field is progressing quite rapidly, major challenges remain before computational approaches can fill the role of workhorse in rational design of AF pharmacotherapies. In this review, we briefly detail the unique aspects of AF pathophysiology that determine requirements for compounds targeting AF rhythm control, with emphasis on delimiting mechanisms that promote AF triggers from those providing substrate or supporting reentry. We then describe modeling approaches that have been used to assess the outcomes of drugs acting on established AF targets, as well as on novel promising targets including the ultra-rapidly activating delayed rectifier potassium current, the acetylcholine-activated potassium current and the small conductance calcium-activated potassium channel. Finally, we describe how heterogeneity and variability are being incorporated into AF-specific models, and how these approaches are yielding novel insights into the basic physiology of disease, as well as aiding identification of the important molecular players in the complex AF etiology.

NCX-Mediated Subcellular Ca2+ Dynamics Underlying Early Afterdepolarizations in LQT2 Cardiomyocytes

Long QT syndrome type 2 (LQT2) is a congenital disease characterized by loss of function mutations in hERG potassium channels (I_Kr). LQT2 is associated with fatal ventricular arrhythmias promoted by triggered activity in the form of early afterdepolarizations (EADs). We previously demonstrated that intracellular Ca²⁺ handling is remodeled in LQT2 myocytes. Remodeling leads to aberrant late RyR-mediated Ca²⁺ releases that drive forward-mode Na⁺-Ca²⁺ exchanger (NCX) current and slow repolarization to promote reopening of L-type calcium channels and EADs. Forward-mode NCX was found to be enhanced despite the fact that these late releases do not significantly alter the whole-cell cytosolic calcium concentration during a vulnerable period of phase 2 of the action potential corresponding to the onset of EADs. Here, we use a multiscale ventricular myocyte model to explain this finding. We show that because the local NCX current is a saturating nonlinear function of the local submembrane calcium concentration, a larger number of smaller-amplitude discrete Ca²⁺ release events can produce a large increase in whole-cell forward-mode NCX current without increasing significantly the whole-cell cytosolic calcium concentration. Furthermore, we develop novel insights, to our knowledge, into how alterations of stochastic RyR activity at the single-channel level cause late aberrant Ca²⁺ release events. Experimental measurements in transgenic LTQ2 rabbits confirm the critical arrhythmogenic role of NCX and identify this current as a potential target for antiarrhythmic therapies in LQT2.

Cardiac voltage-gated ion channels in safety pharmacology: Review of the landscape leading to the CiPA initiative

Voltage gated ion channels are central in defining the fundamental properties of the ventricular cardiac action potential (AP), and are also involved in the development of drug-induced arrhythmias. Many drugs can inhibit cardiac ion currents, including the Na⁺ current (I_Na), L-type Ca²⁺ current (Ica-L), and K⁺ currents (I_to, I_K1, I_Ks, and I_Kr), and thereby affect AP properties in a manner that can trigger or sustain cardiac arrhythmias. Since publication of ICH E14 and S7B over a decade ago, there has been a focus on drug effects on QT prolongation clinically, and on the rapidly activating delayed rectifier current (I_Kr), nonclinically, for evaluation of proarrhythmic risk. This focus on QT interval prolongation and a single ionic current likely impacted negatively some drugs that lack proarrhythmic liability in humans. To rectify this issue, the Comprehensive in vitro proarrhythmia assay (CiPA) initiative has been proposed to integrate drug effects on multiple cardiac ionic currents with in silico modelling of human ventricular action potentials, and in vitro data obtained from human stem cell-derived ventricular cardiomyocytes to estimate proarrhythmic risk of new drugs with improved accuracy. In this review, we present the physiological functions and the molecular basis of major cardiac ion channels that contribute to the ventricle AP, and discuss the CiPA paradigm in drug development.

Dynamical mechanisms of phase-2 early afterdepolarizations in human ventricular myocytes: insights from bifurcation analyses of two mathematical models

Early afterdepolarization (EAD) is known as a cause of ventricular arrhythmias in long QT syndromes. We theoretically investigated how the rapid (I_Kr) and slow (I_Ks) components of delayed-rectifier K⁺ channel currents, L-type Ca²⁺ channel current (I_CaL), Na⁺/Ca²⁺ exchanger current (I_NCX), Na⁺-K⁺ pump current (I_NaK), intracellular Ca²⁺ (Ca_i) handling via sarcoplasmic reticulum (SR), and intracellular Na⁺ concentration (Na_i) contribute to initiation, termination, and modulation of phase-2 EADs, using two human ventricular myocyte models. Bifurcation structures of dynamical behaviors in model cells were explored by calculating equilibrium points, limit cycles (LCs), and bifurcation points as functions of parameters. EADs were reproduced by numerical simulations. The results are summarized as follows: 1) decreasing I_Ks and/or I_Kr or increasing I_CaL led to EAD generation, to which mid-myocardial cell models were especially susceptible; the parameter regions of EADs overlapped the regions of stable LCs. 2) Two types of EADs (termination mechanisms), I_Ks activation-dependent and I_CaL inactivation-dependent EADs, were detected; I_Ks was not necessarily required for EAD formation. 3) Inhibiting I_NCX suppressed EADs via facilitating Ca²⁺-dependent I_CaL inactivation. 4) Ca_i dynamics (SR Ca²⁺ handling) and Na_i strongly affected bifurcations and EAD generation in model cells via modulating I_CaL, I_NCX, and I_NaK Parameter regions of EADs, often overlapping those of stable LCs, shifted depending on Ca_i and Na_i in stationary and dynamic states. 5) Bradycardia-related induction of EADs was mainly due to decreases in Na_i at lower pacing rates. This study demonstrates that bifurcation analysis allows us to understand the dynamical mechanisms of EAD formation more profoundly.

NEW & NOTEWORTHY

We investigated mechanisms of phase-2 early afterdepolarization (EAD) by bifurcation analyses of human ventricular myocyte (HVM) models. EAD formation in paced HVMs basically depended on bifurcation phenomena in non-paced HVMs, but was strongly affected by intracellular ion concentrations in stationary and dynamic states. EAD generation did not necessarily require I_Ks.

Atrial remodelling in atrial fibrillation: CaMKII as a nodal proarrhythmic signal

CaMKII is a serine-threonine protein kinase that is abundant in myocardium. Emergent evidence suggests that CaMKII may play an important role in promoting atrial fibrillation (AF) by targeting a diverse array of proteins involved in membrane excitability, cell survival, calcium homeostasis, matrix remodelling, inflammation, and metabolism. Furthermore, CaMKII inhibition appears to protect against AF in animal models and correct proarrhythmic, defective intracellular Ca(2+) homeostasis in fibrillating human atrial cells. This review considers current concepts and evidence from animal and human studies on the role of CaMKII in AF.

Endothelin-1 stimulates the expression of L-type Ca2+ channels in neonatal rat cardiomyocytes via the extracellular signal-regulated kinase 1/2 pathway

The cardiac L-type Ca(2+) channel current (I(Ca,L)) plays an important role in controlling both cardiac excitability and excitation-contraction coupling and is involved in the electrical remodeling during postnatal heart development and cardiac hypertrophy. However, the possible role of endothelin-1 (ET-1) in the electrical remodeling of postnatal and diseased hearts remains unclear. Therefore, the present study was designed to investigate the transcriptional regulation of I(Ca,L) mediated by ET-1 in neonatal rat ventricular myocytes using the whole-cell patch-clamp technique, quantitative RT-PCR and Western blotting. Furthermore, we determined whether the extracellular signal-regulated kinase 1/2 (ERK1/2) pathway is involved. ET-1 increased I(Ca,L) density without altering its voltage dependence of activation and inactivation. In line with the absence of functional changes, ET-1 increased L-type Ca(2+) channel pore-forming α1C-subunit mRNA and protein levels without affecting the mRNA expression of auxiliary β- and α2/δ-subunits. Furthermore, an actinomycin D chase experiment revealed that ET-1 did not alter α1C-subunit mRNA stability. These effects of ET-1 were inhibited by the ETA receptor antagonist BQ-123 but not the ETB receptor antagonist BQ-788. Moreover, the effects of ET-1 on I(Ca,L) and α1C-subunit expression were abolished by the ERK1/2 inhibitor (PD98059) but not by the p38 MAPK inhibitor (SB203580) or the c-Jun N-terminal kinase inhibitor (SP600125). These findings indicate that ET-1 increased the transcription of L-type Ca(2+) channel in cardiomyocytes via activation of ERK1/2 through the ETA receptor, which may contribute to the electrical remodeling of heart during postnatal development and cardiac hypertrophy.

Pathophysiological mechanisms of atrial fibrillation: a translational appraisal

Atrial fibrillation (AF) is an arrhythmia that can occur as the result of numerous different pathophysiological processes in the atria. Some aspects of the morphological and electrophysiological alterations promoting AF have been studied extensively in animal models. Atrial tachycardia or AF itself shortens atrial refractoriness and causes loss of atrial contractility. Aging, neurohumoral activation, and chronic atrial stretch due to structural heart disease activate a variety of signaling pathways leading to histological changes in the atria including myocyte hypertrophy, fibroblast proliferation, and complex alterations of the extracellular matrix including tissue fibrosis. These changes in electrical, contractile, and structural properties of the atria have been called "atrial remodeling." The resulting electrophysiological substrate is characterized by shortening of atrial refractoriness and reentrant wavelength or by local conduction heterogeneities caused by disruption of electrical interconnections between muscle bundles. Under these conditions, ectopic activity originating from the pulmonary veins or other sites is more likely to occur and to trigger longer episodes of AF. Many of these alterations also occur in patients with or at risk for AF, although the direct demonstration of these mechanisms is sometimes challenging. The diversity of etiological factors and electrophysiological mechanisms promoting AF in humans hampers the development of more effective therapy of AF. This review aims to give a translational overview on the biological basis of atrial remodeling and the proarrhythmic mechanisms involved in the fibrillation process. We pay attention to translation of pathophysiological insights gained from in vitro experiments and animal models to patients. Also, suggestions for future research objectives and therapeutical implications are discussed.

Repolarization of the cardiac action potential. Does an increase in repolarization capacity constitute a new anti-arrhythmic principle?

The cardiac action potential can be divided into five distinct phases designated phases 0-4. The exact shape of the action potential comes about primarily as an orchestrated function of ion channels. The present review will give an overview of ion channels involved in generating the cardiac action potential with special emphasis on potassium channels involved in phase 3 repolarization. In humans, these channels are primarily K(v)11.1 (hERG1), K(v)7.1 (KCNQ1) and K(ir)2.1 (KCNJ2) being the responsible alpha-subunits for conducting I(Kr), I(Ks) and I(K1). An account will be given about molecular components, biophysical properties, regulation, interaction with other proteins and involvement in diseases. Both loss and gain of function of these currents are associated with different arrhythmogenic diseases. The second part of this review will therefore elucidate arrhythmias and subsequently focus on newly developed chemical entities having the ability to increase the activity of I(Kr), I(Ks) and I(K1). An evaluation will be given addressing the possibility that this novel class of compounds have the ability to constitute a new anti-arrhythmic principle. Experimental evidence from in vitro, ex vivo and in vivo settings will be included. Furthermore, conceptual differences between the short QT syndrome and I(Kr) activation will be accounted for.

Excito-oscillatory dynamics as a mechanism of ventricular fibrillation

BACKGROUND

The instabilities associated with reentrant spiral waves are of paramount importance to the initiation and maintenance of tachyarrhythmias, especially ventricular fibrillation (VF). In addition to tissue heterogeneities, there are only a few basic purported mechanisms of spiral wave breakup, most notably restitution.

OBJECTIVE

We test the hypothesis that oscillatory membrane properties act to destabilize spiral waves.

METHODS

We recorded transmembrane potential (V(m)) from isolated rabbit myocytes using a constant current stimulation protocol. We developed a mathematical model that included both the stable excitable equilibrium point at resting V(m) (-80 mV) and the unstable oscillatory equilibrium point at elevated V(m) (-10 mV). Spiral wave dynamics were studied in 2-dimensional grids using variants of the model.

RESULTS

All models showed restitution and reproduced the experimental values of transmembrane resistance at rest and during the action potential plateau. Stable spiral waves were observed when the model showed only 1 equilibrium point. However, spatio-temporal complexity was observed if the model showed both excitable and oscillatory equilibrium points (i.e., excito-oscillatory models). The initial wave breaks resulted from oscillatory waves expanding in all directions; after a few beats, the patterns were characterized by a combination of unstable spiral waves and target patterns consistent with the patterns observed on the heart surface during VF. In our model, this VF-like activity only occurred when the single cell period of V(m) oscillations was within a specific range.

CONCLUSION

The VF-like patterns observed in our excito-oscillatory models could not be explained by the existing proposed instability mechanisms. Our results introduce the important suggestion that membrane dynamics responsible for V(m) oscillations at elevated V(m) levels can destabilize spiral waves and thus may be a novel therapeutic target for preventing VF.

Effects of L-type Ca2+ channel antagonism on ventricular arrhythmogenesis in murine hearts containing a modification in the Scn5a gene modelling human long QT syndrome 3

Ventricular arrhythmogenesis in long QT 3 syndrome (LQT3) involves both triggered activity and re-entrant excitation arising from delayed ventricular repolarization. Effects of specific L-type Ca2+ channel antagonism were explored in a gain-of-function murine LQT3 model produced by a DeltaKPQ 1505-1507 deletion in the SCN5A gene. Monophasic action potentials (MAPs) were recorded from epicardial and endocardial surfaces of intact, Langendorff-perfused Scn5a+/Delta hearts. In untreated Scn5a+/Delta hearts, epicardial action potential duration at 90% repolarization (APD90) was 60.0 +/- 0.9 ms compared with 46.9 +/- 1.6 ms in untreated wild-type (WT) hearts (P < 0.05; n = 5). The corresponding endocardial APD(90) values were 52.0 +/- 0.7 ms and 53.7 +/- 1.6 ms in Scn5a+/Delta and WT hearts, respectively (P > 0.05; n = 5). Epicardial early afterdepolarizations (EADs), often accompanied by spontaneous ventricular tachycardia (VT), occurred in 100% of MAPs from Scn5a+/Delta but not in any WT hearts (n = 10). However, EAD occurrence was reduced to 62 +/- 7.1%, 44 +/- 9.7%, 10 +/- 10% and 0% of MAPs following perfusion with 10 nm, 100 nm, 300 nm and 1 mum nifedipine, respectively (P < 0.05; n = 5), giving an effective IC50 concentration of 79.3 nm. Programmed electrical stimulation (PES) induced VT in all five Scn5a+/Delta hearts (n = 5) but not in any WT hearts (n = 5). However, repeat PES induced VT in 3, 2, 2 and 0 out of 5 Scn5a+/Delta hearts following perfusion with 10 nm, 100 nm, 300 nm and 1 mum nifedipine, respectively. Patch clamp studies in isolated ventricular myocytes from Scn5a+/Delta and WT hearts confirmed that nifedipine (300 nm) completely suppressed the inward Ca2+ current but had no effect on inward Na+ currents. No significant effects were seen on epicardial APD90, endocardial APD90 or ventricular effective refractory period in Scn5a+/Delta and WT hearts following perfusion with nifedipine at 1 nm, 10 nm, 100 nm, 300 nm and 1 microm nifedipine concentrations. We conclude that L-type Ca2+ channel antagonism thus exerts specific anti-arrhythmic effects in Scn5a+/Delta hearts through suppression of EADs.

Electrophysiological characterization of murine HL-5 atrial cardiomyocytes

HL-5 cells are cultured murine atrial cardiomyocytes and have been used in studies to address important cellular and molecular questions. However, electrophysiological features of HL-5 cells have not been characterized. In this study, we examined such properties using whole cell patch-clamp techniques. Membrane capacitance of the HL-5 cells was from 8 to 62 pF. The resting membrane potential was −57.8 ± 1.4 mV ( n = 51). Intracellular injection of depolarizing currents evoked action potentials (APs) with variable morphologies in 71% of the patched cells. Interestingly, the incidence of successful, current-induced APs positively correlated with the hyperpolarizing degrees of resting membrane potentials ( r = 0.99, P < 0.001). Only a few of the patched cells (4 of 51, 7.8%) exhibited spontaneous APs. The muscarinic agonist carbachol activated the acetylcholine-activated K+ current and significantly shortened the duration of APs. Immunostaining confirmed the presence of the muscarinic receptor type 2 in HL-5 cells. The hyperpolarization-activated cation current ( If) was detected in 39% of the patched cells. The voltage to activate 50% of If channels was −73.4 ± 1.2 mV ( n = 12). Voltage-gated Na+, Ca2+, and K+ currents were observed in the HL-5 cells with variable incidences. Compared with the adult mouse cardiomyocytes, the HL-5 cells had prolonged APs and small outward K+ currents. Our data indicate that HL-5 cells display significant electrophysiological heterogeneity of morphological appearance of APs and expression of functional ion channels. Compared with adult murine cardiomyocytes, HL-5 cells show an immature phenotype of cardiac AP morphology.

Ca2+ currents in cardiac myocytes: Old story, new insights

Calcium is a ubiquitous second messenger which plays key roles in numerous physiological functions. In cardiac myocytes, Ca2+ crosses the plasma membrane via specialized voltage-gated Ca2+ channels which have two main functions: (i) carrying depolarizing current by allowing positively charged Ca2+ ions to move into the cell; (ii) triggering Ca2+ release from the sarcoplasmic reticulum. Recently, it has been suggested than Ca2+ channels also participate in excitation-transcription coupling. The purpose of this review is to discuss the physiological roles of Ca2+ currents in cardiac myocytes. Next, we describe local regulation of Ca2+ channels by cyclic nucleotides. We also provide an overview of recent studies investigating the structure-function relationship of Ca2+ channels in cardiac myocytes using heterologous system expression and transgenic mice, with descriptions of the recently discovered Ca2+ channels alpha(1D) and alpha(1E). We finally discuss the potential involvement of Ca2+ currents in cardiac pathologies, such as diseases with autoimmune components, and cardiac remodeling.

The role of interleukin-1 in the failing heart

The prevalence of congestive heart failure and its continued poor prognosis despite presently available therapeutic options emphasize the importance of pursuing the observations suggesting an important role for an immunomodulatory approach to decompensated cardiac failure. Furthermore, there are several pieces of background information that suggest that cytokines like IL-1 may play a significant role in the pathogenesis of several forms of myocardial dysfunction. Although it seems clear that IL-1 is not acting alone under circumstances of myocardial injury, but in concert with other pro-inflammatory molecules and their effectors, we believe that continued investigations into the cytokine hypothesis will ultimately increase the understanding of how pro-inflammatory molecules influence myocardial function and how the modulation of such factors may improve the myocardial response to injury. The specific observations that emphasize the importance of pursuing a substantive role for IL-1 in this process are: (1) IL-1 is elevated in several cardiac disease states, (2) IL-1 is produced by myocardial cells themselves in response to injury, (3)The alterations in gene expression seen in response IL-1 resembles in many ways the phenotype of the failing heart, and (4) The co-localization of the IL-1 response with that of several previously described negative transcriptional regulators (making them potential targets for therapeutic manipulation).

Adaptation to high altitude hypoxia protects the rat heart against ischemia-induced arrhythmias. Involvement of mitochondrial K(ATP) channel

The aim was to determine whether adaptation to chronic hypoxia protects the heart against ischemic arrhythmias and whether ATP-dependent potassium channels (K(ATP)) play a role in the antiarrhythmic mechanism. Adult male rats were adapted to intermittent high altitude hypoxia (5000 m, 4 h/day) and susceptibility to ischemia-induced ventricular arrhythmias was evaluated in the Langendorff-perfused hearts subjected to either an occlusion of the coronary artery for 30 min or pre-conditioning by brief occlusion of the same artery prior to 30-min reocclusion. In separate groups, either a K(ATP) blocker, glibenclamide (10 micromol/l), or a mitochondrial K(ATP) opener, diazoxide (50 micromol/l), were added to a perfusion medium 20 min before the occlusion. Adaptation to hypoxia reduced the total number of ventricular arrhythmias by 64% as compared with normoxic controls. Preconditioning by a single 3-min coronary artery occlusion was antiarrhythmic only in the normoxic group, while two occlusion periods of 5 min each were needed to pre-condition the hypoxic hearts. Glibenclamide increased the number of arrhythmias in the normoxic hearts from 1316+/-215 to 2091+/-187 (by 59%) and in the hypoxic group from 636+/-103 to 1777+/-186 (by 179%). In contrast, diazoxide decreased the number of arrhythmias only in the normoxic group from 1374+/-96 to 582+/-149 (by 58%), while its effect in the hypoxic group was not significant. It is concluded that long-term adaptation of rats to high altitude hypoxia decreases the susceptibility of their hearts to ischemic arrhythmias and increases an antiarrhythmic threshold of pre-conditioning. The mitochondrial K(ATP) channel, rather than the sarcolemmal K(ATP) channel, appears to be involved in the protective mechanism afforded by adaptation.

Effects of nicardipine and bupivacaine on early after depolarization in rabbit sinoatrial node cells: a possible mechanism of bupivacaine-induced arrhythmias

The effects of nicardipine and bupivacaine on early afterdepolarizations (EADs) were investigated in rabbit sinoatrial (SA) nodes using the conventional microelectrode technique. In a nominally Ca2+ -free, Mg2+ -free solution, addition of 0.5 mM Sr2+ produced EADs following prolongation of action potential duration. Nicardipine (10 microM) as well as Mg2+ (1 mM), both of which block the L-type Ca2+ channel current (iCa,L), abolished Sr2+ -induced EADs. Bupivacaine (5 microM), blocking the delayed rectifier K+ current (iK), facilitated the generation of EADs in the Sr2+ solution containing 1 mM Mg2+. The EADs in Sr2+ solution and the effect of bupivacaine were well simulated by the mathematical model when enhancement of slowly inactivating iCa,L and suppression of iK were assumed. Bupivacaine may cause sinus arrhythmias by facilitating EAD generation in SA node cells.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

Acceleration-induced action potential prolongation and early afterdepolarizations

INTRODUCTION

Precipitation of torsades de pointes (TdP) has been shown to be associated with acceleration of heart rate in both experimental and clinical studies. To gain insight into the cellular mechanism(s) responsible for the initiation of acceleration-induced TdP, we studied the effect of acceleration of pacing rate in canine left ventricular epicardial, M region, endocardial, and Purkinje fiber preparations pretreated with E-4031, an IKr blocker known to induce the long QT syndrome and TdP.

METHODS AND RESULTS

Standard microelectrode techniques were used. E-4031 (1 to 2 microM) induced early after depolarization (EAD) activity in 31 of 36 M cell, 0 of 10 epicardial, 0 of 10 endocardial, and 9 of 12 Purkinje fiber preparations at basic cycle lengths (BCLs) > or = 800 msec. In 30 of 36 M cells, sudden acceleration from a BCL range of 900 to 4,000 msec to a range of 500 to 1,500 msec induced transient EAD activity if none existed before or increased the amplitude of EADs if already present. Acceleration-induced augmentation of EAD activity was far less impressive and less readily demonstrable in Purkinje fibers (4/12). In M cells, appearance of EAD activity during acceleration usually was accompanied by an abbreviation of action potential duration (APD). Within discrete ranges of rates in the physiologic range, acceleration caused a transient prolongation of APD in 38% of M cells, whether or not a distinct EAD was generated. Acceleration produced still more dramatic APD prolongation and EADs in M cells after the BCL was returned to the original slow rate. Epicardium and endocardium APD showed little change immediately after acceleration. A decrease of BCL as small as 10% and, in some cases, a single premature beat could promote EAD activity and APD prolongation in some M cells. Ryanodine (1 microM, 10/10), flunarizine (10 microM, 3/6), and low Na (97 vs 129 mM, 5/5) abolished the acceleration-induced EAD activity and APD prolongation as well as the EAD activity observed at slow rates in M cells pretreated with E-4031.

CONCLUSION

Our results suggest that acceleration from an initially slow rate or a single premature beat can induce or facilitate transient EAD activity and APD prolongation in canine ventricular M cell preparations pretreated with an IKr blocker via a mechanism linked to intracellular calcium loading. Our data provide evidence in support of an important contribution of electrogenic Na/Ca exchange current to this process. These acceleration-induced changes can result in the development of triggered activity as well as a marked dispersion of repolarization in ventricular myocardium and, thus, may contribute to the precipitation of TdP in patients with the congenital (HERG defect) and acquired (drug-induced) long QT syndrome.

Lidocaine and nisoldipine attenuate almokalant-induced dispersion of repolarization and early afterdepolarizations in vitro

INTRODUCTION

Treatment with Class III antiarrhythmic agents may lead to increased dispersion of repolarization and early afterdepolarizations (EADs), which are both likely substrates for torsades de pointes. Recent studies in vivo have shown that the prevalence of proarrhythmias induced by Class III agents may be reduced by Na(+)- or Ca(2+)-blocking agents. In the present study, tentative mechanisms for this protective effect were investigated in vitro.

METHODS AND RESULTS

Transmembrane action potentials were recorded simultaneously from rabbit isolated ventricular muscle (VM) and Purkinje fibers (PF). At a basic cycle length (BCL) of 500 msec, the Class III agent almokalant (0.1 microM) increased the dispersion by prolonging the action potential duration (APD) significantly more in the PF (33% +/- 4.2%, n = 18) than in the VM (17% +/- 5.9%, n = 18, P < 0.05). In six of the preparations, addition of 1, 5, and 25 microM lidocaine reduced the almokalant-induced prolongation in a concentration-dependent manner mainly in the PF, thereby decreasing the dispersion. At 5 microM lidocaine, the remaining prolongation was 7% +/- 12.2% (P < 0.05 vs time controls) in the PF and 14% +/- 6.4% in the VM, respectively. In six other preparations, the addition of 0.01, 0.05, and 0.25 microM nisoldipine did not reduce the almokalant-induced prolongation in the PF and VM, but attenuated the spike-and-dome appearance of the action potential in the PF. In separate experiments performed at a BCL of 1000 msec, EADs developed in 2 of 6 and 5 of 6 PF during superfusion with almokalant (0.3 and 1 microM, respectively) at an APD of 828 +/- 41.4 msec. In six separate preparations pretreated with lidocaine (5 microM), the almokalant-induced prolongation in the PF was less pronounced and EADs were not observed. Pretreatment with nisoldipine (0.05 microM) did not influence the response to almokalant, and in 4 of 6 preparations the APD exceeded 1000 msec. Despite this extensive prolongation, EADs did not appear.

CONCLUSION

At concentrations that did not affect the APD in the VM but reduced the APD in the PF, lidocaine suppressed almokalant-induced dispersion and the development of EADs. Nisoldipine, on the other hand, inhibited almokalant-induced EADs directly. Hence, the primary APD-prolonging effect of a Class III agent may be preserved, but the risk of proarrhythmias reduced, during concomitant treatment with low concentrations of a Na(+)- or Ca(2+)-blocking agent.

Ionic mechanisms of ischemia-related ventricular arrhythmias

The aim of this review is the utmost simplification of the cellular electrophysiologic background of ischemia-related arrhythmias. In the acute and subacute phase of myocardial infarction, arrhythmias can be caused by an abnormal impulse generation, abnormal automaticity or triggered activity caused by early or delayed afterdepolarizations (EAD and DAD), or by abnormalities of impulse conduction (i.e., reentry). This paper addresses therapeutic intervention aimed at preventing the depolarization of "pathologic" slow fibers, counteracting the inward calcium (Ca) influx that takes place through the L-type channels (Ca antagonists), or hyperpolarizing the diastolic membrane action potential, increasing potassium (K) efflux (K-channel openers) in arrhythmias generated by an abnormal automaticity (ectopic tachycardias or accelerated idioventricular rhythms). If the cause enhanced impulse generation is related to triggered activity, and since both EAD and DAD are dependent on calcium currents that can appear during a delayed repolarization, the therapeutic options are to shorten the repolarization phase through K-channel openers or Ca antagonists, or to suppress the inward currents directly responsible for the afterdepolarization with Ca blockers. Magnesium seems to represent a reasonable choice, as it is able to shorten the action potential duration and to function as a Ca antagonist. Abnormalities of impulse conduction (re-entry) account for the remainder of arrhythmias that occur in the acute and subacute phase of ischemia and for most dysrhythmias that develop during the chronic phase. Reentrant circuits due to ischemia are usually Na channel-dependent. Drug choice will depend on the length of the excitable gap: in case of a short gap (ventricular fibrillation, polymorphic ventricular tachycardia, etc.), the refractory period has been identified as the most vulnerable parameter, and therefore a correct therapeutic approach will be based on drugs able to prolong the effective refractory period (K-channel blockers, such as class III antiarrhythmic drugs); on the other hand, for those arrhythmias characterized by a long excitable gap (most of the monomorphic ventricular tachycardias), the most appropriate therapeutic intervention consists of depressing ventricular excit-ability and conduction by use of sodium-channel blockers such as mexiletine and lidocaine. Compared with other class I antiarrhythmic agents, these drugs minimally affect refractoriness and exhibit a use-dependent effect and a voltage dependent action (i.e., more pronounced on the ischemic tissue because of its partial depolarization).