Induction Of Early Afterdepolarizations Research Articles

Dynamical mechanism for generation of arrhythmogenic early afterdepolarizations in cardiac myocytes: Insights from in silico electrophysiological models.

We analyze the dynamical mechanisms underlying the formation of arrhythmogenic early afterdepolarizations (EADs) in two mathematical models of cardiac cellular electrophysiology: the Sato etal. biophysically detailed model of a rabbit ventricular myocyte of dimension 27 and a reduced version of the Luo-Rudy mammalian myocyte model of dimension 3. Based on a comparison of the two models, with detailed bifurcation analysis using spike-counting techniques and continuation methods in the simple model and numerical explorations in the complex model, we locate the point where the first EAD originates in an unstable branch of periodic orbits. These results serve as a basis to propose a conjectured scheme involving a hysteresis mechanism with the creation of alternans and EADs in the unstable branch. This theoretical scheme fits well with electrophysiological experimental data on EAD generation and hysteresis phenomena. Our findings open the door to the development of novel methods for pro-arrhythmia risk prediction related to EAD generation without actual induction of EADs.

Optimization of an In silico Cardiac Cell Model for Proarrhythmia Risk Assessment.

Drug-induced Torsade-de-Pointes (TdP) has been responsible for the withdrawal of many drugs from the market and is therefore of major concern to global regulatory agencies and the pharmaceutical industry. The Comprehensive in vitro Proarrhythmia Assay (CiPA) was proposed to improve prediction of TdP risk, using in silico models and in vitro multi-channel pharmacology data as integral parts of this initiative. Previously, we reported that combining dynamic interactions between drugs and the rapid delayed rectifier potassium current (IKr) with multi-channel pharmacology is important for TdP risk classification, and we modified the original O'Hara Rudy ventricular cell mathematical model to include a Markov model of IKr to represent dynamic drug-IKr interactions (IKr-dynamic ORd model). We also developed a novel metric that could separate drugs with different TdP liabilities at high concentrations based on total electronic charge carried by the major inward ionic currents during the action potential. In this study, we further optimized the IKr-dynamic ORd model by refining model parameters using published human cardiomyocyte experimental data under control and drug block conditions. Using this optimized model and manual patch clamp data, we developed an updated version of the metric that quantifies the net electronic charge carried by major inward and outward ionic currents during the steady state action potential, which could classify the level of drug-induced TdP risk across a wide range of concentrations and pacing rates. We also established a framework to quantitatively evaluate a system's robustness against the induction of early afterdepolarizations (EADs), and demonstrated that the new metric is correlated with the cell's robustness to the pro-EAD perturbation of IKr conductance reduction. In summary, in this work we present an optimized model that is more consistent with experimental data, an improved metric that can classify drugs at concentrations both near and higher than clinical exposure, and a physiological framework to check the relationship between a metric and EAD. These findings provide a solid foundation for using in silico models for the regulatory assessment of TdP risk under the CiPA paradigm.

Divergent electrophysiologic profile of fluconazole and voriconazole in an experimental whole-heart model of proarrhythmia

In several case reports a prolongation of the QT-interval and even proarrhythmic effects of fluconazole and voriconazole were reported. The aim of the present study was to investigate if application of fluconazole or voriconazole has the potential to provoke polymorphic ventricular tachycardia in a sensitive model of proarrhythmia. In female rabbits, fluconazole (10, 30 and 50µM, n=6) and voriconazole (10, 30 and 50µM, n=6) were infused after obtaining baseline data. Eight endocardial and epicardial monophasic action potentials and a simultaneously recorded 12-lead ECG showed a significant QT prolongation after application of fluconazole as compared with baseline (10µM:+12ms, 30µM:+22ms, 50µM:+37ms; P<0.05) accompanied by an increase of action potential duration (APD90). Administration of voriconazole also induced QT prolongation (30µM:+10ms, 50µM:+20ms, P<0.05). Spatial dispersion of repolarization remained stable in voriconazole-treated hearts while fluconazole induced a significant increase (30µM:+15ms, 50µM:+16ms; P<0.05). Lowering of potassium concentration in bradycardic AV-blocked hearts did not provoke any early afterdepolarizations (EADs) or polymorphic ventricular tachycardia in voriconazole-treated hearts. Application of fluconazole led to the reproducible induction of EADs in 4 of 6 hearts and polymorphic ventricular tachycardia in 3 of 6 hearts (36 episodes). In the present study, voriconazole demonstrated a safe electrophysiologic profile despite significant QT prolongation. In contrast, fluconazole led to a more marked prolongation of myocardial repolarization combined with a more marked increase of dispersion of repolarization. These results imply that application of fluconazole might be torsadogenic and the QT-interval should be closely monitored.

Effects of ranolazine on torsades de pointes tachycardias in a healthy isolated rabbit heart model.

PurposeTorsades de pointes (TdP) tachycardias are triggered, polymorphic ventricular arrhythmias arising from early afterdepolarizations (EADs) and increased dispersion of repolarization. Ranolazine is a new agent which reduces pathologically elevated late INa but also IKr. Aim of this study was to evaluate the effects of ranolazine in a validated isolated Langendorff-perfused rabbit heart model.MethodsTdP was reproducibly induced with d-sotalol (10−4 mol/L) and low potassium (K) (1.0 mmol/L for 5 min, pacing at CL 1000 ms). In 10 hearts, ECG and 8 epi- and endocardial monophasic action potentials were recorded. Action potential duration (APD) was measured at 90% repolarization and dispersion defined as APD max–min.ResultsD-sotalol prolonged APD90 and increased dispersion of APD90, simultaneously causing EADs and induction of TdP. The combination of d-sotalol and two concentrations of ranolazine did not increase dispersion of ventricular APD90 as compared to vehicle. Ranolazine at 5 μmol/L did not cause additional induction of EADs and/or TdP but also did not significantly suppress arrhythmogenic triggers. The higher concentration of ranolazine (10 μmol/L) in combination with d-sotalol caused further prolongation of APD90, at the same time reduction in APD90 dispersion. In parallel, the incidence of EADs was reduced and an antitorsadogenic effect was seen.ConclusionsIn the healthy isolated rabbit heart (where late INa is not elevated), ranolazine does not cause proarrhythmia but exerts antiarrhythmic effects in a dose-dependent manner against d-sotalol/low K-induced TdP. This finding—despite additional APD prolongation—supports the safety of a combined use of both drugs and merits clinical investigation.

Effects of the Antiarrhythmic Drug Dofetilide on Transmural Dispersion of Repolarization in Ventriculum. A Computer Modeling Study

Dofetilide is a class-III drug that inhibits the rapid component of the delayed potassium current ( I(Kr)). Experimental studies have shown that the different layers of ventricular muscle present differences in action potential duration (APD) and different responses to class III agents. It has been suggested that it contributes to APD heterogeneity in the ventricles. However, in vivo studies suggest that the strong cellular coupling reduces APD dispersion in intact heart. The aim of this paper is to study the effect of dofetilide on the action potentials (APs) in isolated ventricular cells and on APD dispersion in a strand of ventricular tissue. A mathematical model of dofetilide effects on I(Kr) has been developed and incorporated into the Luo--Rudy dynamic model of ventricular AP. Our results show that dofetilide induces in midmyocardium cells a faster time-course inhibition of I(Kr) than in endocardial or epicardial cells, and periods of instability with beat-to-beat APs variability. This behavior could favor temporal dispersion of repolarization between the different cells. The results also indicate that although dofetilide increases, the transmural gradient of APD in the ventricular wall, early afterdepolarizations (EADs) did not appear even under strong uncoupling conditions. However, reduced repolarization reserve favors the induction of EADs, even under normal coupling conditions.

Contributions of Ion Channel Currents to Ventricular Action Potential Changes and Induction of Early Afterdepolarizations During Acute Hypoxia

Variability in delivery of oxygen can lead to electric instability in the myocardium and the generation of arrhythmias. In addition ischemic heart disease and angina are associated with an increase in circulating catecholamines that further increases the risk of developing ventricular tachyarrhythmias. We investigated the net effects of acute hypoxia and catecholamines on the cardiac action potential. We incorporated all published data on the effects of hypoxia on the late Na(+) current (I(Na-L)), the fast Na(+) current (I(Na)), the basal L-type Ca(2+) channel current (I(Ca-L)), and the slow (I(Ks)) and rapid components of the delayed rectifier K(+)-current (I(Kr)) in the absence and presence of beta-adrenergic receptor (beta-AR) stimulation into the Luo-Rudy model of the action potential. Hypoxia alone had little effect on the action potential configuration or action potential duration. However in the presence of beta-AR stimulation, hypoxia caused a prolongation of the action potential and early afterdepolarizations (EADs) and spontaneous tachycardia were induced. Experiments performed in guinea pig ventricular myocytes confirmed the modeling results. EADs occur predominantly because of the increased sensitivity of I(Ca-L) to beta-AR stimulation during hypoxia. beta-AR stimulation is necessary to induce EADs as EADs are never observed during hypoxia in the absence of beta-AR stimulation.

Contributions of Ion Currents to Action Potential Changes and Induction of Early Afterdepolarizations and Sustained Triggered Activity During Acute Hypoxia in Ventricular Myocytes

Contributions of Ion Currents to Action Potential Changes and Induction of Early Afterdepolarizations and Sustained Triggered Activity During Acute Hypoxia in Ventricular Myocytes

The Role Of Depolarizing And Repolarizing Currents In The Induction Of Early Afterdepolarizations During Acute Hypoxia In Ventricular Myocytes

Coronary occlusion is associated with acute hypoxia and increased catecholamine levels. Acute hypoxia (decreasing PO2 from 150 to 17mmHg) is not energy limiting but can alter the function of ion channels. Hypoxia decreases the transient Na+-current (INa-T), the basal L-type Ca2+ channel current (ICa-L) and the slow component of the delayed rectifier K+-current (IKs), without effecting the rapid component (IKr). Hypoxia also increases the persistent Na+-current (INa-P) and the sensitivity of ICa-L and IKs to the β-adrenergic receptor agonist isoproterenol (Iso). The net effects of acute hypoxia and catecholamines on the cardiac action potential are not known.We incorporated all published data reporting the effects of acute hypoxia on cardiac ion channels into the Luo-Rudy model. Results from the model were compared with experimental data we obtained from myocytes.Our modelled data predicts acute hypoxia, in the absence of Iso, has little effect on resting membrane potential (RMP), action potential peak (APP) or action potential duration (APD). Hypoxia alone did not trigger EADs. When we added 0.6nM Iso to ICa-L during hypoxic conditions, APD was prolonged and EADs were generated. Repeating this for IKs did not alter APD or generate EADs. When we modelled the effects of hypoxia on ICa-L and IKs, APD was prolonged and EADs were generated, suggesting any anti-arrhythmic effect of IKs is small and that ICa-L effects predominate.Experimental data confirmed that acute hypoxia alone did not markedly alter RMP, APP or APD. In the presence of 3nM Iso, hypoxia significantly increased APD by 20% and induced EADs and spontaneous tachycardia. We conclude that during acute hypoxia, EADs are induced predominantly as a result of increased sensitivity of the L-type Ca2+ channel to β-adrenergic receptor stimulation.

Long QT 1 Mutation KCNQ1A344VIncreases Local Anesthetic Sensitivity of the Slowly Activating Delayed Rectifier Potassium Current

Anesthesia in patients with long QT syndrome (LQTS) is a matter of concern. Congenital LQTS is most frequently caused by mutations in KCNQ1 (Kv7.1), whereas drug-induced LQTS is a consequence of HERG (human ether-a-go-go-related gene) channel inhibition. The aim of this study was to investigate whether the LQT1 mutation A344V in the S6 region of KCNQ1, at a position corresponding to the local anesthetic binding site in HERG, may render drug insensitive KCNQ1 channels into a toxicologically relevant target of these pharmacologic agents. This may suggest that LQTS constitutes not only a nonspecific but also a specific pharmacogenetic risk factor for anesthesia. The authors examined electrophysiologic and pharmacologic properties of wild-type and mutant KCNQ1 channels. The effects of bupivacaine, ropivacaine, and mepivacaine were investigated using two-electrode voltage clamp and whole cell patch clamp recordings. The mutation A344V induced voltage-dependent inactivation in homomeric KCNQ1 channels and shifted the voltage dependence of KCNQ1/KCNE1 channel activation by +30 mV. The mutation furthermore increased the sensitivity of KCNQ1/KCNE1 channels for bupivacaine 22-fold (KCNQ1wt/KCNE1: IC50 = 2,431 +/- 582 microM, n = 20; KCNQ1A344V/KCNE1: IC50 = 110 +/- 9 microM, n = 24). Pharmacologic effects of the mutant channels were dominant when mutant and wild-type channels were coexpressed. Simulation of cardiac action potentials with the Luo-Rudy model yielded a prolongation of the cardiac action potential duration and induction of early afterdepolarizations by the mutation A344V that were aggravated by local anesthetic intoxication. The results indicate that certain forms of the LQTS may constitute a specific pharmacogenetic risk factor for regional anesthesia.

Calmodulin Antagonist W‐7 Prevents Sparfloxacin‐Induced Early Afterdepolarizations (EADs) in Isolated Rabbit Purkinje Fibers: Importance of Beat‐to‐Beat Instability of the Repolarization

The occurrence of early afterdepolarizations (EADs) has been related to the incidence of torsades de pointes in drug-induced long QT (LQT). The generation of EADs may be facilitated by Ca(2+)/calmodulin-dependent protein kinase II (CaM kinase). In the present study, we investigated a possible involvement of Ca(2+)/Calmodulin dependent protein kinase in the generation of sparfloxacin-induced EADs in isolated rabbit Purkinje fibers by means of a calmodulin antagonist W-7. EADs were evident in 8 of the 10 preparations perfused with sparfloxacin at 1 x 10(-4) M and stimulated at 0.2 Hz. The induction of EADs by sparfloxacin was associated with a large prolongation of the duration of the action potential (APD), an increase in the triangulation, and the short-term instability of the repolarization. CaM kinase blockade with the calmodulin antagonist W-7 inhibited sparfloxacin-induced EADs in a concentration-dependent manner (EADs were induced in 3 of 10, 1 of 10, and 0 of 8 preparations in the presence of W-7 at 5 x 10(-7) M, 5 x 10(-6) M, and 5 x 10(-5) M, respectively; P < 0.01 at 5 x 10(-6) M and 5 x 10(-5) M). The inhibition of sparfloxacin-induced EADs by W-7 at 5 x 10(-7) M and 5 x 10(-6) M was associated with a significant decrease in the beat-to-beat instability but not associated with a significant shortening of the APD and reduction of V(max). The present findings support the hypothesis that CaM kinase may be a proarrhythmic signaling molecule and demonstrate that CaM kinase may be involved in the generation of EADs in drug-induced LQT and enhanced beat-to-beat instability of repolarization is essential for the genesis of EADs in rabbit in vitro.

Drug-induced long QT in isolated rabbit Purkinje fibers: importance of action potential duration, triangulation and early afterdepolarizations

In the present study, we investigated three drug-induced long-QT syndromes in isolated rabbit Purkinje fibers in order to identify the relationship of action potential duration (APD), triangulation of action potentials (APD 90–APD 40) and early afterdepolarizations. Isolated rabbit Purkinje fibers were superperfused in Tyrode solution with solvent, indapamide (1×10 −4 M, an I ks blocker mimicking long QT1), dofetilide (1×10 −9, 1×10 −8 or 1×10 −7 M, an I kr blocker mimicking long QT2) or anthopleurin (1×10 −8 M, an inhibitor of the inactivation of the I Na + current mimicking long QT3) ( n=8 per group) for 25 min, and stimulated at 1 Hz for 20 min and at 0.2 Hz for another 5 min. Indapamide did not change APD and triangulation or elicit early afterdepolarizations even in the presence of β-adrenergic stimulation with isoproterenol. Dofetilide concentration-dependently prolonged APD 90, increased triangulation and elicited early afterdepolarizations. Anthopleurin markedly increased APD 90 as well as triangulation and elicited early afterdepolarizations. The induction of early afterdepolarizations by dofetilide and anthopleurin was associated with a prolongation of APD 90 or an increase in triangulation, but not with a change in APD 40. Moreover, the degree of the increase in the triangulation was larger than that of APD 90 in long QT2 (dofetilide-induced) and long QT3 (anthopleurin-induced) models in isolated rabbit Purkinje fibers. Our present study indicates that rabbit Purkinje fibers can be used as long QT2 (dofetilide-mimicking) and LQT3 (anthopleurin-mimicking) syndrome models, and confirms that drug-induced long QT1 (indapamide-mimicking) is absent. Our present study also shows the relationship between a prolongation of APD 90 or increase in triangulation and the induction of early afterdepolarizations with dofetilide ( I kr blocker) and anthopleurin ( I Na modulator) in isolated rabbit Purkinje fibers.

Species plays an important role in drug-induced prolongation of action potential duration and early afterdepolarizations in isolated Purkinje fibers.

Although isolated Purkinje fibers (PFs) often are used to evaluate the electrophysiologic effects of new drugs in terms of prolongation of action potential duration (APD) and induction of early afterdepolarizations (EADs), species differences in this respect remain elusive. We evaluated potential species-specific differences in drug-induced prolongation of APD and EADs in isolated PF from various species. Using a microelectrode technique, PFs (n = 7 to 11 per species) were isolated from hearts of rabbits, guinea pigs, dogs, swine, goats, or sheep, superperfused in Tyrode's solution with dofetilide (1 x 10(-8) M) or quinidine (1 x 10(-5) M) for 25 minutes, and stimulated at 1 Hz for 20 minutes and at 0.2 Hz for another 5 minutes. Dofetilide increased APD at 90% repolarization (APD90) at 1 Hz by 83% (rabbit), 24% (guinea pig), 65% (dogs), 18% (swine), 61% (goat), and 30% (sheep), and prolonged APD90 at 0.2 Hz by 187% (rabbit), 31% (guinea pig), 154% (dog), 17% (swine), 61% (goat), and 8% (sheep). Similarly, quinidine changed APD90 by 93% (rabbit), 0% (guinea pig), 16% (dog), -3% (swine), 0% (goat), and -24% (sheep) at 1 Hz, and by 124% (rabbit), 15% (guinea pig), 53% (dog), 17% (swine), 11% (goat), and -39% (sheep) at 0.2 Hz in PF. During superfusion of dofetilide or quinidine, EADs occurred in most preparations in rabbit PFs at 0.2 Hz, but not in any of the PFs from other species at 0.2 Hz. Our study demonstrates that species plays an important role in the response of PF to drug-induced prolongation of APD and EADs. Rabbit PFs constitute the most sensitive model for detecting drug-induced, potential long APD and proarrhythmogenic effects in vitro.

Enhancement of delayed afterdepolarizations and triggered activity by class III antiarrhythmic drugs: Multiple effects of E-4031 and dofetilide

The effects of class III antiarrhythmic agents E-4031 and dofetilide were studied on action potentials and subthreshold delayed afterdepolarizations (DADs) induced by the cardiac glycoside acetylstrophanthidin (AS) in isolated cardiac Purkinje fibers. Action potentials were recorded from cardiac Purkinje fibers using microelectrode techniques. E-4031 and dofetilide consistently increased DAD amplitude, occasionally caused triggered action potentials and shortened action potential duration. The application of E-4031 without prior AS exposure, resulted in the typical class III antiarrhythmic effects of action potential lengthening and the induction of early afterdepolarizations. These findings suggest that under our conditions of AS-induced cell Ca2+ overload, the effects of the "pure" class III antiarrhythmic drugs, E-4031 and dofetilide, are markedly different from those found in non-Ca2+ loaded cells. This may represent an additional electrophysiological mechanism for class III antiarrhythmic drug toxicity.

Experimental models of torsade de pointes.

Torsade de pointes is a potentially life threatening form of polymorphic ventricular tachycardia typically seen in the setting of congenital and acquired abnormal QT-prolongation. Numerous in vitro studies have investigated basic ionic mechanisms underlying delayed repolarization. The role of different ion channels and the induction of early afterdepolarizations have been studied in various cardiac cells including M cells. In addition, isolated heart models with and without electrical stimulation and/or the use of drugs which prolong repolarization have been developed in recent years. Some of these models have simulated conditions likely to exist in the clinical setting of torsade de pointes, such as bradycardia and hypokalemia. In in vivo canine and rabbit models, torsade-like polymorphic ventricular tachyarrhythmias have been induced by the administration of different agents such as cesium, neurotoxins, e.g., anthopleurin or various class III drugs under conditions designed to mimic the clinical situation. In the context of recent advances in the molecular genetics of long QT syndrome, those models which have used sodium or potassium channel blockers have gained particular interest. Based on all experimental studies it seems probable that the first beats of torsade occur due to early afterdepolarizations and triggered activity. The development of subsequent beats is less clear. Reentry based on inhomogeneity of refractoriness has been suggested as the underlying mechanism.

Differential Electrophysiologic Effects of Chronically Administered Amiodarone on Canine Purkinje Fibers versus Ventricular Muscle.

BACKGROUND: Acute and chronic treatment with amiodarone has been reported to cause different electrocardiographic changes in patients. The cellular electrophysiologic effects of chronic administration (50 mg/kg/day orally for 6 weeks) and acute superfusion (5 µM in the tissue bath) of amiodarone were therefore studied in dog cardiac ventricular muscle and Purkinje fibers using conventional microelectrode techniques. METHODS AND RESULTS: During stimulation at 1 Hz, chronic amiodarone treatment lengthened the ventricular muscle action potential duration (APD) from 227.8 +/- 6.3 ms (n = 20) to 262.3 +/- 5.2 ms (n = 21; P <.01), but shortened that of Purkinje fibers from 337.6 +/- 9.2 (n = 21) to 308.3 +/- 7.1 (n = 19; P <.05). Acute superfusion of 5 µM amiodarone in cardiac tissue obtained from chronically treated dogs did not change ventricular muscle APD but shortened Purkinje fiber AP from 309.7 +/- 13.6 ms to 281.9 +/- 11.9 ms (n = 8; P <.05). Neither the chronic nor the acute amiodarone exposure prevented the APD shortening in ventricular muscle evoked by 10 µM pinacidil, suggesting that amiodarone does not interfere with the ATP-dependent potassium channels. The normal difference in APD between ventricular muscle and Purkinje fibers in untreated, control preparations was 110 ms but decreased to 46 ms in fibers obtained from dogs chronically treated with amiodarone and increased to 185 ms in fibers obtained from dogs chronically treated with amiodarone and increased to 185 ms in the presence of 30 µM sotalol, a class III antiarrhythmic drug used for comparison. Amiodarone (5 µM) applied directly abolished early afterdepolarizations (EADs) (induced by 1 µM dofetilide + 20 µM BaCl(2) + 2 mM CsCl) in 5 of 6 experiments and caused strong use-dependent V(max) block with relatively fast onset kinetics (rate constant = 1.23 +/- 0.13 AP(-1), n = 5) and offset (time constant = 364 +/- 62.5 ms, n = 5). After chronic amiodarone treatment, in contrast with acute sotalol application (30 µM), no reverse use-dependent effect was observed on the APD in Purkinje fibers. CONCLUSIONS: These results provide further evidence that amiodarone differs from other recognized class III antiarrhythmic drugs (ie, it is a mixed type agent with acute fast kinetic class I [type B] and a unique class III antiarrhythmic action characterized by decreased dispersion of APDs between ventricular muscle and Purkinje fibers). Amiodarone can abolish EADs unlike other class III agents that are usually associated to induction of EADs. These features might be responsible not only for the antiarrhythmic efficacy, but also for the relative safety (low incidence of torsade de pointes) of amiodarone in clinical settings.

Alpha1-adrenergic stimulation induces early afterdepolarizations in ferret Purkinje fibers.

The aim of this study was to evaluate whether alpha1-adrenergic stimulation, which prolongs repolarization, could induce early afterdepolarizations (EADs) in ferret Purkinje fibers. We used standard microelectrodes to study the effects of phenylephrine 10(-6)M in the presence of metoprolol 1.5 x 10(-6)M, on action potentials (AP) recorded from isolated ferret Purkinje fibers superfused with normal Tyrode's solution. Phenylephrine induced a time-dependent prolongation of the AP duration at 60 and 90% of full repolarization (APD60, APD90) from 223 +/- 8 and 269 +/- 9 ms, respectively, during control to 279 +/- 11 and 329 +/- 12 ms after 1-h superfusion (n = 29; p < 0.05 vs. control for both parameters) and 334 +/- 13 and 385 +/- 15 ms after 4 h (n = 29, p < 0.05 vs. control and 1-h superfusion for both parameters) without changing the other parameters. About one third of the fibers studied developed EADs that could be either phase 2 or phase 3 EADs. The alpha1-adrenoceptor antagonist WB 4101 (10(-7)M) limited the AP prolongation and prevented the occurrence of EADs. On the other hand, the alpha1-adrenoceptor alkylating agent chloroethylclonidine (10(-7)M) had no effect. The calcium chelator BAPTA [1,2-bis(2-aminophenoxy) ethane-N,N,N',N'-tetra-acetic acid] (2 x 10(-3)M) did not prevent the induction of EADs by phenylephrine, although it suppressed the twitch tension, showing that it did chelate the intracellular calcium. Our results show that alpha1-adrenergic stimulation prolongs repolarization in ferret Purkinje fibers and can induce EADs. This effect is mainly mediated by the WB 4101-sensitive alpha1-adrenoceptors (alpha1A- and/or alpha1C-adrenoceptors) and does not depend on intracellular calcium. alpha1-Adrenergic stimulation may have arrhythmogenic effects in patients with long QT syndrome (LQTS).

Role of Purkinje fibers in induction of early afterdepolarizations and triggered ventricular tachycardia in isolated rabbit hearts

Role of Purkinje fibers in induction of early afterdepolarizations and triggered ventricular tachycardia in isolated rabbit hearts

Arrhythmogenic effects of isoproterenol-activated Cl − current in guinea-pig ventricular myocytes

Possible arrhythmogenic effects of the isoproterenol-activated Cl current were examined in isolated guinea-pig ventricular myocytes under various intra- and extracellular Cl − concentrations. Experiments were carried out with external K + concentration ([K +] o) decreased to 2 or 3 m m. Under symmetrical concentrations of Cl − in intra- and extra-cellular solutions (E Cl=0 mV), 1 μ m isoproterenol (ISP) depolarized resting membrane potential (RMP) by 6.2±1.1 mV and slowed repolarization with induction of early afterdepolarizations (EADs) in 9 out of 9 cells. EADs appeared at voltages positive to −40 mV, where L-type Ca 2+ current is assumed to be activated. When Cl − concentrations were settled near physiological conditions (E Cl=−40–50 mV), ISP depolarized RMP by 2.8±0.4 mV and elicited abnormal repolarization with occasional EADs in 6 out of 19 cells. When E Cl was set to −80 mV, however, ISP depolarized RMP by only 0.5±0.5 mV without induction of abnormal activities. Thus, depolarizing effects of ISP and incidence of repolarization abnormalities including EADs were increased as E Cl shifted to more positive potential levels. At [K +] o=4 m m, no abnormal activities were observed when E Cl was around −50 mV ( 0 8 ), and 6 out of 6 cells showed abnormal activities when E Cl was set to 0 mV. ISP-elicited abnormal activities were abolished by 1 m m DNDS (4,4′-dinitrostilbene-2,2′-disulphonic acid), a blocker for Cl − channels. We suggest that during conditions of hypokalemia or decreased K + conductance, ISP-activated Cl − current may contribute to the genesis of arrhythmias through facilitation of membrane depolarization and abnormal automaticity in combination with the concomitantly enhanced Ca 2+ current by ISP.

Evidence for direct arrhythmogenic action of endothelin

We studied electrophysiological effects of endothelin on canine cardiac tissues. Endothelin prolonged action potential duration and decreased spontaneous firing rate of the right bundle branch cells. At a concentration of 2 x 10(-7)M the plateau phase of action potentials was flattened, followed by the abrupt occurrence of early afterdepolarizations (EADs). ET, at a concentration as low as 2 x 10(-9)M, was capable of inducing EADs although their incidence was low. The EADs were initiated from the membrane potential less negative than -30mV and were suppressed by nicardipine, suggesting the involvement of dihydropyridine-sensitive Ca2+ channels in the induction of EADs. Because EADs are considered to underlie certain types of arrhythmias endothelin per se may have arrhythmogenic action.

Rate-Dependent Class III Antiarrhythmic Action, Negative Chronotropy, and Positive Inotropy of a Novel Ik Blocking Drug, UK-68,798

The electromechanical effects of UK-68,798 (UK), a novel class III antiarrhythmic drug, were studied in guinea pig and rat papillary muscles (PMs) and atria in vitro using conventional microelectrode technique. UK (10(-8)-10(-6) M) prolonged the action potential duration (APD) by 21-58% and effective refractory period in parallel, without affecting the resting potential or maximum rate of depolarization in guinea pig PM stimulated at 1 Hz. UK increased the contractile force without prolonging the time to peak force or relaxation. In comparison, 5 x 10(-5) M d-sotalol was needed to induce the same electrophysiological effects as 10(-8) M UK. UK prolonged the APD significantly less at 2 Hz than at 1 and 0.5 Hz. Early afterdepolarizations (EADs) developed in 2 of 11 preparations after 10(-6) M at 0.5 Hz. No reversal of drug effect was seen after up to 2 h washout. UK (10(-9)-10(-5) M) reduced the spontaneous heart rate and prolonged the sinus node recovery time of guinea pig right atria. No effects on rat PM or atria, even after 10(-5) M, indicate a selective action of UK on the delayed rectifying outward potassium current, Ik. These results indicate a potent and selective, rate-dependent class III antiarrhythmic action of UK-68,798 linked with positive inotropy. Increased APD, bradycardia, and induction of EADs, however, represent a potential arrhythmogenic combination.