Postrepolarization Refractoriness Research Articles

Targeted activation of human ether-à-go-go-related gene channels rescues electrical instability induced by the R56Q+/- long QT syndrome variant.

Long QT syndrome type 2 (LQTS2) is associated with inherited variants in the cardiac human ether-à-go-go-related gene (hERG) K+ channel. However, the pathogenicity of hERG channel gene variants is often uncertain. Using CRISPR-Cas9 gene-edited hiPSC-derived cardiomyocytes (hiPSC-CMs), we investigated the pathogenic mechanism underlying the LQTS-associated hERG R56Q variant and its phenotypic rescue by using the Type 1 hERG activator, RPR260243. The above approaches enable characterization of the unclear causative mechanism of arrhythmia in the R56Q variant (an N-terminal PAS domain mutation that primarily accelerates channel deactivation) and translational investigation of the potential for targeted pharmacologic manipulation of hERG deactivation. Using perforated patch clamp electrophysiology of single hiPSC-CMs, programmed electrical stimulation showed that the hERG R56Q variant does not significantly alter the mean action potential duration (APD90). However, the R56Q variant increases the beat-to-beat variability in APD90 during pacing at constant cycle lengths, enhances the variance of APD90 during rate transitions, and increases the incidence of 2:1 block. During paired S1-S2 stimulations measuring electrical restitution properties, the R56Q variant was also found to increase the variability in rise time and duration of the response to premature stimulations. Application of the hERG channel activator, RPR260243, reduces the APD variance in hERG R56Q hiPSC-CMs, reduces the variability in responses to premature stimulations, and increases the post-repolarization refractoriness. Based on our findings, we propose that the hERG R56Q variant leads to heterogeneous APD dynamics, which could result in spatial dispersion of repolarization and increased risk for re-entry without significantly affecting the average APD90. Furthermore, our data highlight the antiarrhythmic potential of targeted slowing of hERG deactivation gating, which we demonstrate increases protection against premature action potentials and reduces electrical heterogeneity in hiPSC-CMs.

Amiodarone prevents wave front-tail interactions in patients with heart failure: an in silico study.

Amiodarone (AM) is an antiarrhythmic drug whose chronic use has proved effective in preventing ventricular arrhythmias in a variety of patient populations, including those with heart failure (HF). AM has both class III [i.e., it prolongs the action potential duration (APD) via blocking potassium channels) and class I (i.e., it affects the rapid sodium channel) properties; however, the specific mechanism(s) by which it prevents reentry formation in patients with HF remains unknown. We tested the hypothesis that AM prevents reentry induction in HF during programmed electrical stimulation (PES) via its ability to induce postrepolarization refractoriness (PRR) via its class I effects on sodium channels. Here we extend our previous human action potential model to represent the effects of both HF and AM separately by calibrating to human tissue and clinical PES data, respectively. We then combine these models (HF + AM) to test our hypothesis. Results from simulations in cells and cables suggest that AM acts to increase PRR and decrease the elevation of takeoff potential. The ability of AM to prevent reentry was studied in silico in two-dimensional sheets in which a variety of APD gradients (ΔAPD) were imposed. Reentrant activity was induced in all HF simulations but was prevented in 23 of 24 HF + AM models. Eliminating the AM-induced slowing of the recovery of inactivation of the sodium channel restored the ability to induce reentry. In conclusion, in silico testing suggests that chronic AM treatment prevents reentry induction in patients with HF during PES via its class I effect to induce PRR.NEW & NOTEWORTHY This work presents a new model of the action potential of the human, which reproduces the complex dynamics during premature stimulation in heart failure patients with and without amiodarone. A specific mechanism of the ability of amiodarone to prevent reentrant arrhythmias is presented.

Electropharmacological Characterization of Licorice Using the Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes Sheets and the Chronic Atrioventricular Block Dogs.

Licorice has been traditionally prescribed for palpitation, whereas its overdose has caused lethal arrhythmias including torsade de pointes. Licorice contains glycyrrhizic acid of ≥ 2% (w/w), which is hydrolyzed to glycyrrhetinic acid (GRA) in the intestine. Since their cardiac electropharmacological properties are not fully understood, we assessed them to ask mechanism of licorice-induced torsade de pointes. GRA at 0.1, 1 and 10μg/mL was cumulatively applied to the human induced pluripotent stem cell-derived cardiomyocytes sheets (n = 6). GRA shortened spontaneous activation interval and repolarization period, and decreased maximum contraction velocity, indicating Ca2+ channel blockade. It prolonged effective refractory period and post-repolarization refractoriness with a steep frequency-dependency, whereas it delayed conduction with a modest use-dependency, resembling lidocaine in the mode of Na+ channel-blocking action. Meanwhile, Kanzoto containing a decoction of licorice alone in a dose of 2 or 6g/body/day was orally administered to the conscious chronic atrioventricular block dogs for 3days (n = 4). Kanzoto prolonged QT interval with increasing its temporal dispersion, suggesting K+ channel suppression, and slightly decreased the plasma K+ concentration without inducing torsade de pointes. Moreover, it significantly suppressed atrial and idioventricular rates, leading to sinus arrest along with the onset of ventricular fibrillation in one animal, possibly due to Na+ channel blockade. These results indicate that electropharmacological profile of licorice can be explained by Na+, Ca2+ and K+ channels blockade, which may be associated with low torsadogenic risk, but might contribute to the onset of other types of lethal ventricular arrhythmias.

PO-01-148 COMBINATION OF FLECAINIDE AND IBUTILIDE FOR EFFECTIVE AND SAFE MANAGEMENT OF ATRIAL FIBRILLATION

There is a need for improved approaches to rhythm control therapy of atrial fibrillation (AF). To investigate a novel pharmacological approach for rhythm control management of AF. We studied the effectiveness of clinically relevant concentrations of flecainide (1.5 μM) and ibutilide (20 nM), alone and in combination, to prevent the induction of AF and to cardiovert persistent AF, defined here as AF lasting > 1 hr, using canine isolated coronary-perfused right atrial preparations. In addition, we examined the safety of the flecainide and ibutilide (50 nM) combination using canine left ventricular (LV) wedge preparations. Persistent AF could be induced in 100% of preparations following exposure to acetylcholine (0.5 μM; n=5). Persistent AF remained inducible in 60% of atria in the presence of ibutilide alone and in 20% of preparations in the presence of flecainide alone, but in 0% of preparations in the presence of the combination of ibutilide and flecainide (n=5 per group). When used alone, flecainide and ibutilide cardioverted persistent AF in 40% and 20% of atria, respectively, but in 100% of atria when used in combination (n=5 per group). Ibutilide prolonged both atrial and ventricular effective refractory period (ERP; by 15 and 8%, respectively; p<0.05 for both; Cycle length [CL] = 500 ms; n=5 for each). Flecainide significantly prolonged ERP in atria (by 27%; p<0.001) but not in ventricles (2% increase) due to atrial predominant induction of post-repolarization refractoriness. The combination of the two drugs prolonged ERP by 42% in atria (p<0.001 vs. control and 0<0.05 vs. flecainide alone) but by only 7% in ventricles (p<0.05; n=5, CL=500 ms). In separate experiments involving LV wedge preparations, risk factors for Torsade de Pointes were accentuated by ibutilide (50 nM) and slowed pacing (CL= 2000 ms; n=5). Under these conditions, ibutilide induced significant prolongation of QT and Tpeak /Tend intervals (by 25 and 55%, respectively; p<0.01 for both; n=5). Further addition of flecainide (1.5 μM) partially reversed these effects leading to an increase of these parameters by only 15 and 19%, respectively; p<0.05, combination vs. ibutilide; n=5). Torsade de Pointes was never observed with the drug combination. Our experimental data indicate that a combination of flecainide and ibutilide significantly improves pharmacological cardioversion of AF as well as prevention of AF with very little or no risk of long QT-mediated ventricular proarrhythmia.

Impact of late sodium current inhibition on cardiac electrophysiology parameters and ventricular arrhythmias in isolated Langendorff perfused rabbit hearts with short QT interval

Objective: To determine the electrophysiological effects and related mechanisms of late sodium current inhibitors on hearts with short QT intervals. Methods: The electrophysiological study was performed on isolated Langendorff perfused rabbit hearts. A total of 80 New Zealand White rabbits were used and 34 hearts without drug treatment were defined as control group A, these hearts were then treated with IKATP opener pinacidil, defined as pinacidil group A. Then, 27 hearts from pinacidil group A were selected to receive combined perfusion with sodium channel inhibitors or quinidine, a traditional drug used to treat short QT syndrome, including ranolazine combined group (n=9), mexiletine combined group (n=9), and quinidine combined group (n=9). Nineteen out of the remaining 46 New Zealand rabbits were selected as control group B (no drug treatments, n=19), and then treated with pinacidil, defined as pinacidil group B (n=19). The remaining 27 rabbits were treated with sodium inhibitors or quinidine alone, including ranolazine alone group (n=9), mexiletine alone group (n=9), and quinidine alone group (n=9). Electrocardiogram (ECG) physiological parameters of control group A and pinacidil group A were collected. In control group B and pinacidil group B, programmed electrical stimulation was used to induce ventricular arrhythmias and ECG was collected. ECG physiological parameters and ventricular arrhythmia status of various groups were analyzed. The concentrations of pinacidil, ranolazine, mexiletine and quinidine used in this study were 30, 10, 30 and 1 μmol/L, respectively. Results: Compared with control group A, the QT interval, 90% of the repolarization in epicardial and endocardial monophasic action potential duration (MAPD90-Epi, MAPD90-Endo) was shortened, the transmural dispersion of repolarization (TDR) was increased, and the effective refractor period (ERP) and post-repolarization refractoriness (PRR) were reduced in pinacidil group A (all P<0.05). Compared with the pinacidil group A, MAPD90-Epi, MAPD90-Endo, QT interval changes were reversed in quinidine combined group and mexiletine combined group (all P<0.05), but not in ranolazine combined group. All these three drugs reversed the pinacidil-induced increases of TDR and the decreases of ERP and PRR. The induced ventricular arrhythmia rate was 0 in control group B, and increased to 10/19 (χ2=13.6, P<0.05) in pinacidil group B during programmed electrical stimulation. Compared with the pinacidil group B, incidences of ventricular arrhythmia decreased to 11% (1/9), 11% (1/9) and 0 (0/9) (χ2=4.5, 4.5, 7.4, P<0.05) respectively in ranolazine group, mexiletine group and quinidine group. Conclusions: Inhibition of late sodium current does not increase but even decreases the risk of malignant arrhythmia in hearts with a shortened QT interval. The antiarrhythmic mechanism might be associated with the reversal of the increase of TDR and the decrease of refractoriness (including both ERP and PRR) of hearts with shortened QT interval.

Dapagliflozin but not empagliflozin prevents atrial fibrillation in an experimental whole-heart model

Abstract Background The SGLT2 inhibitors dapagliflozin and empagliflozin have an evolving role in heart failure therapy. Observational data suggest a lower risk of atrial fibrillation (AF) for patients on SGLT2 inhibitors. So far, it is unclear whether this is related to a direct antiarrhythmic potential of SGLT2 inhibitors or just a marker of improved heart function. Experimental data on the impact of SGTL2 inhibitors on atrial electrophysiology is scarce. Purpose This study aimed at evaluating the effects of the two SGLT2 inhibitors dapagliflozin and empagliflozin on atrial electrophysiology. Methods 25 rabbit hearts were explanted and retrogradely perfused employing the Langendorff-setup. Two catheters were placed on both atria to record monophasic action potentials. Hearts were paced at three different cycle lengths (350ms, 250ms, 150ms), thereby obtaining cycle-length dependent atrial action potential durations (aAPD90) and effective refractory periods (aERP). Atrial post-repolarization (aPRR) refractoriness was determined by the difference between aERP and aAPD90. AF was induced in the presence of acetylcholine (ACh, 1μM) and isoproterenol (Iso, 1μM). Thereafter, hearts were treated with dapagliflozin (n=13, 3 μM) or empagliflozin (n=12, 3 μM). Results Ach/Iso abbreviated aAPD90, aERP and prolonged aPRR in both groups (dapagliflozin group: aAPD90: −49 ms, p&amp;lt;0.01; aERP: −33 ms, p&amp;lt;0.01; aPRR: +18 ms, p=0.06; empagliflozin group: aAPD90: −40 ms, p&amp;lt;0.01; aERP: −25 ms, p&amp;lt;0.05; aPRR: +18 ms, p=0.05). Further treatment with dapagliflozin increased aAPD90 (+13 ms, p&amp;lt;0.01), aERP (+34 ms, p&amp;lt;0.05) and aPRR (+6 ms, p=ns). Additional administration of empagliflozin had no significant effect on aAPD90 (−8 ms, p=ns), aERP (−1 ms, p=ns) or aPRR (+9 ms, p=ns). Vulnerability to AF was assessed by a standardized protocol consisting of several trains of burst pacing. Ach/Iso increased the vulnerability to AF episodes in both groups (dapagliflozin group: 71 episodes vs. 1 under baseline conditions, p&amp;lt;0.01; empagliflozin group: 27 episodes vs. 2 under baseline conditions, p=0.08). Additional perfusion of dapagliflozin significantly reduced the occurrence of AF (to 30 episodes, p&amp;lt;0.05) while additional empagliflozin treatment had no significant antiarrhythmic effect (32 episodes, p=ns). Conclusion Dapagliflozin and empagliflozin had opposing effects on atrial electrophysiology. Dapagliflozin prolonged aAPD90 and aERP, thereby preventing AF. In contrast, empagliflozin did not change aAPD90 or aERP, resulting in no antiarrhythmic effect. Our findings hint at a potential antiarrhythmic benefit of dapagliflozin. Funding Acknowledgement Type of funding sources: Private grant(s) and/or Sponsorship. Main funding source(s): Hans and Gertie Fischer-Foundation (to C.E.)

Electrophysiological Safety Profile of Antiestrogenic Therapies in the Isolated Rabbit Heart

Introduction: Hormone-mediated therapies are on the rise and are key therapies in the treatment of some of the most common cancer entities. Proarrhythmic effects have been described in patients treated with SERMs while little to none is known about the electrophysiological effects of the potentially less arrhythmogenic selective estrogen receptor degraders. Methods: Twenty hearts of female New Zealand White rabbits were excised and retrogradely perfused employing a Langendorff setup. An electrophysiology study was performed to obtain CL-dependent action potential duration at 90% of repolarization (APD90), QT interval, effective refractory period (ERP), and post-repolarization refractoriness (PRR = ERP-APD90). After generating baseline data, the hearts were assigned to two groups and perfused with (n = 10) increasing doses of fulvestrant (1 µM and 5 µM; n = 10) or tamoxifen (2.5 µM and 7.5 µM; n = 10), and the protocol was repeated again. Results: Fulvestrant led to a decrease in APD90 and QT interval and an increased PRR. The inducibility of ventricular tachycardia (VT) episodes was unaltered. Tamoxifen showed similar effects, resulting in a shortened APD90 and QT interval and no increased VT incidence in the setting of a prolonged PRR. Conclusion: The present study shows no acute proarrhythmic effect of tamoxifen and fulvestrant in an established whole heart model when employing clinically relevant concentrations.

Pharmacological activation of the hERG K+ channel for the management of the long QT syndrome: A review.

In the human heart, the rapid delayed rectifier K+ current (I Kr) contributes significantly to ventricular action potential (AP) repolarization and to set the duration of the QT interval of the surface electrocardiogram (ECG). The pore‐forming (α) subunit of the I Kr channel is encoded by KCNH2 or human ether‐à‐go‐go‐related gene 1 (hERG1). Impairment of hERG function through either gene mutation (congenital) or pharmacological blockade by diverse drugs in clinical use (acquired) can cause a prolongation of the AP duration (APD) reflected onto the surface ECG as a prolonged QT interval or Long QT Syndrome (LQTS). LQTS can increase the risk of triggered activity of ventricular cardiomyocytes and associated life‐threatening arrhythmia. Current treatments all focus on reducing the incidence of arrhythmia or terminating it after its onset but there is to date no prophylactic treatment for the pharmacological management of LQTS. A new class of hERG modulators (agonists) have been suggested through direct interaction with the hERG channel to shorten the action potential duration (APD) and/or increase the postrepolarisation refractoriness period (PRRP) of ventricular cardiomyocytes protecting thereby against triggered activity and associated arrhythmia. Although promising drug candidates, there remain major obstacles to their clinical development. The aim of this review is to summarize the latest advances as well as the limitations of this proposed pharmacotherapy.

Atrial resting membrane potential confers sodium current sensitivity to propafenone, flecainide and dronedarone

BackgroundAlthough atrial fibrillation ablation is increasingly used for rhythm control therapy, antiarrhythmic drugs (AADs) are commonly used, either alone or in combination with ablation. The effectiveness of AADs is highly variable. Previous work from our group suggests that alterations in atrial resting membrane potential (RMP) induced by low Pitx2 expression could explain the variable effect of flecainide.ObjectiveThe purpose of this study was to assess whether alterations in atrial/cardiac RMP modify the effectiveness of multiple clinically used AADs.MethodsThe sodium channel blocking effects of propafenone (300 nM, 1 μM), flecainide (1 μM), and dronedarone (5 μM, 10 μM) were measured in human stem cell–derived cardiac myocytes, HEK293 expressing human NaV1.5, primary murine atrial cardiac myocytes, and murine hearts with reduced Pitx2c.ResultsA more positive atrial RMP delayed INa recovery, slowed channel inactivation, and decreased peak action potential (AP) upstroke velocity. All 3 AADs displayed enhanced sodium channel block at more positive atrial RMPs. Dronedarone was the most sensitive to changes in atrial RMP. Dronedarone caused greater reductions in AP amplitude and peak AP upstroke velocity at more positive RMPs. Dronedarone evoked greater prolongation of the atrial effective refractory period and postrepolarization refractoriness in murine Langendorff-perfused Pitx2c+/– hearts, which have a more positive RMP compared to wild type.ConclusionAtrial RMP modifies the effectiveness of several clinically used AADs. Dronedarone is more sensitive to changes in atrial RMP than flecainide or propafenone. Identifying and modifying atrial RMP may offer a novel approach to enhancing the effectiveness of AADs or personalizing AAD selection.

Effects of Ranolazine and its Combination with Amiodarone on Rapid Pacing-induced Reentrant Atrial Tachycardia in Rabbits.

Ranolazine (RAN) has previously been shown to lower the onset of cholinergic atrial fibrillation in intact animals; however, its efficacy in the setting of atrial tachycardia (AT) is unknown. The purpose of this study was to investigate the effects of RAN alone or in combination with amiodarone (AMIO) on rapid pacing-evoked right AT in rabbit hearts. Right atrial monophasic action potentials (MAPs) were recorded in 11 anesthetized rabbits, using combination MAP pacing catheters. Vulnerability to AT was tested by employing consecutive trains of rapid burst pacing prior to and after 2.4 mg/kg of RAN alone delivered intravenously and then in combination with 3 mg/kg of AMIO as a 15-minute infusion. Primary endpoints were postdrug AT reproducibility as well as cycle length (CL) and tachycardia duration. MAP duration at 75% repolarization and the effective refractory period (ERP) were assessed during programmed pacing to calculate the atrial postrepolarization refractoriness (aPRR = ERP – MAPD75%). AT was elicited in eight out of 11 rabbits; only these animals were included for further investigation. RAN did not abolish the inducibility of AT in any experiment; however, it prolonged its CL (baseline vs. RAN: 120 ± 16 ms vs. 138 ± 18 ms; p = 0.053). Supplemental AMIO further increased the AT CL (baseline vs. RAN + AMIO: 120 ± 16 ms vs. 152 ± 23 ms; p = 0.006), without affecting arrhythmia reinducibility. Slowing of the tachycardia after RAN or RAN + AMIO was associated with spontaneous termination of the arrhythmia. RAN prolonged the aPRR significantly, while AMIO in addition to RAN potentiated this effect. Neither RAN alone nor its combination with AMIO abolished the elicitation of AT in this model. However, both agents synergistically prolonged the aPRR, resulting in the slowing of AT and promoting spontaneous termination of the arrhythmia.

Blockade of sodium‑calcium exchanger via ORM-10962 attenuates cardiac alternans

Repolarization alternans, a periodic oscillation of long-short action potential duration, is an important source of arrhythmogenic substrate, although the mechanisms driving it are insufficiently understood. Despite its relevance as an arrhythmia precursor, there are no successful therapies able to target it specifically. We hypothesized that blockade of the sodium‑calcium exchanger (NCX) could inhibit alternans. The effects of the selective NCX blocker ORM-10962 were evaluated on action potentials measured with microelectrodes from canine papillary muscle preparations, and calcium transients measured using Fluo4-AM from isolated ventricular myocytes paced to evoke alternans. Computer simulations were used to obtain insight into the drug's mechanisms of action. ORM-10962 attenuated cardiac alternans, both in action potential duration and calcium transient amplitude. Three morphological types of alternans were observed, with differential response to ORM-10962 with regards to APD alternans attenuation. Analysis of APD restitution indicates that calcium oscillations underlie alternans formation. Furthermore, ORM-10962 did not markedly alter APD restitution, but increased post-repolarization refractoriness, which may be mediated by indirectly reduced L-type calcium current. Computer simulations reproduced alternans attenuation via ORM-10962, suggesting that it is acts by reducing sarcoplasmic reticulum release refractoriness. This results from the ORM-10962-induced sodium‑calcium exchanger block accompanied by an indirect reduction in L-type calcium current. Using a computer model of a heart failure cell, we furthermore demonstrate that the anti-alternans effect holds also for this disease, in which the risk of alternans is elevated. Targeting NCX may therefore be a useful anti-arrhythmic strategy to specifically prevent calcium driven alternans.

Antiarrhythmic effect of ranolazine in combination with selective NCX-inhibition in an experimental model of atrial fibrillation

Abstract Background Previous studies showed an antiarrhythmic effect of ranolazine in different clinical and experimental studies. The aim of this study was to investigate the effects of a combination of ranolazine with different selective NCX-inhibitors in an established experimental model of atrial fibrillation. Methods and results 18 hearts of New Zealand white rabbits were retrogradely perfused. Left and right atrial catheters were used to record monophasic action potentials. Hearts were paced at three different cycle lengths (350ms, 250ms, 150ms). Thereby, cycle-length dependent atrial action potential durations (aAPD90), atrial effective refractory periods (aERP) and atrial post-repolarization refractoriness (aPRR=aERP-aAPD90) were obtained. Vulnerability to AF was tested by a standardized protocol employing several trains of burst pacing. After generating baseline data, the hearts were perfused with a combination of acetylcholine (ACh, 1μM) and isoproterenol (Iso, 1μM) to increase occurrence of AF. Afterwards, the hearts were assigned to two groups and additionally perfused with a combination of 10 μM ranolazine and 1 μM of the selective NCX-inhibitor ORM-10103 (group 1: Rano-ORM) or 10 μM ranolazine and 1 μM of another NCX-inhibitor, SEA0400 (group 2: Rano-SEA). Infusion of ACh/Iso led to a shortening of aAPD90 (group 1/2: −27.2ms/−24.5ms p&amp;lt;0.05), aERP (−29.2ms/−35.6ms p&amp;lt;0.05), aPRR (−12.2ms/−13.7ms p=ns) and the occurrence of AF-Episodes was significantly increased (group 1: baseline 6 episodes to 34 episodes under ACh/Iso p&amp;lt;0.05, group 2: baseline 7 episodes to 26 episodes under ACh/Iso p&amp;lt;0.05). Additional perfusion with ranolazine and ORM-10103 did not alter aAPD90 (−0,47ms) while effective refractory periods (+21.4ms, p&amp;lt;0.05) and aPRR (+43ms, p&amp;lt;0.05) were significantly prolonged and AF episodes were effectively reduced to 15 episodes (p&amp;lt;0.05). In group 2, Rano-SEA led to a slight decrease in aAPD90 (−9.5ms) while aERP (+13.3ms, p&amp;lt;0.05) and aPRR (+19.8ms, p&amp;lt;0.05) were prolonged. The occurrence of AF episodes was reduced to 15 episodes (p=0.1). Conclusion To our knowledge this is the first study investigating the effect of ranolazine combined with different selective NCX-inhibitors in an isolated whole-heart model of AF. Atrial repolarization was not significantly changed with either of the combinations. However, both combinations prolonged aERP and aPRR and thereby suppressed induction of AF. Combining novel cellular targets may therefore lead to new potentially interesting options for antiarrhythmic AF therapy that have to be tested in clinical studies. Funding Acknowledgement Type of funding source: None

Antiarrhythmic Effect of Ranolazine in Combination with Selective NCX-Inhibition in an Experimental Model of Atrial Fibrillation

The aim of this study was to investigate the effects of a combination of ranolazine with different selective inhibitors of the Na+/Ca2+-exchanger (NCX) in an established experimental model of atrial fibrillation (AF). Eighteen hearts of New Zealand white rabbits were retrogradely perfused. Atrial catheters were used to record monophasic action potentials (aPRR). Hearts were paced at three different cycle lengths. Thereby, atrial action potential durations (aAPD90), atrial effective refractory periods (aERP) and atrial post-repolarization refractoriness were obtained. Isoproterenol and acetylcholine were employed to increase the occurrence of AF. Thereafter, the hearts were assigned to two groups (n = 9 each group) and additionally perfused with a combination of 10 µM ranolazine and 1 µM of the selective NCX-inhibitor ORM-10103 (group A: Rano-ORM) or 10 µM ranolazine and 1 µM of another NCX-inhibitor, SEA0400 (group B: Rano-SEA). The infusion of Iso/ACh led to a shortening of aAPD90, aERP, aPRR and the occurrence of AF episodes was significantly increased. Additional perfusion with ranolazine and ORM-10103 (group A) significantly prolonged the refractory periods and aPRR and AF episodes were effectively reduced. In group B, Rano-SEA led to a slight decrease in aAPD90 while aERP and aPRR were prolonged. The occurrence of AF episodes was consecutively reduced. To our knowledge, this is the first study investigating the effect of ranolazine combined with different selective NCX-inhibitors in an isolated whole-heart model of AF. Both combinations prolonged aERP and aPRR and thereby suppressed the induction of AF.

Ranolazine depresses conduction of rapid atrial depolarizations in a beating rabbit heart model.

Previous clinical studies have shown that ranolazine (RAN) added to amiodarone (AMIO) might accelerate the termination of recent-onset atrial fibrillation. This study was undertaken to delineate possible mechanisms that contribute to the enhancement of the antiarrhythmic efficacy of RAN-AMIO coadministration. Ten rabbits were anesthetized and two monophasic action potential (MAP) catheters were sequentially inserted into the right atrium. One MAP electrode was used to pace and record; the other electrode was used only for recording MAP from an adjacent atrial region. Intraatrial conduction time (IACT), 2:1 intraatrial conduction block (IACB), and atrial post-repolarization refractoriness (aPRR) were consecutively determined by high-rate atrial burst pacing and programmed stimulation, respectively. All parameters were evaluated during baseline and following AMIO (3 mg/kg iv) or AMIO+RAN (2.4 mg/kg iv bolus +0.134 mg/kg/min maintenance infusion). The IACT remained unchanged post AMIO compared with baseline (37.6 ± 3.8 vs 36.4 ± 2.4 ms), whereas the addition of RAN to AMIO significantly prolonged IACT (50.4 ± 3.6 ms, p < .001). The pacing cycle length producing 2:1 IACB was 101.2 ± 21.7 ms at baseline , 117.5 ± 15 ms after AMIO (p = 0.265), and 150 ± 14 ms after AMIO+RAN (p < .001). Baseline aPRR was longer following AMIO treatment (35 ± 5 vs 50 ± 9 ms, p < .01) but remarkably prolonged with RAN supplementation (105 ± 11 ms, p < .001). RAN significantly prolonged the propagation time of rapid atrial depolarizations and potentiated the AMIO-induced moderate increases in aPRR. These mechanisms possibly contribute to the earlier termination of atrial fibrillation when RAN is co-administered with AMIO.

P1905Propofol suppresses atrial fibrillation in an experimental whole-heart model

Abstract Background Many clinical studies suggest an impact of propofol on the occurrence of atrial arrhythmias. Purpose Since experimental data is sparse, purpose of this study was to investigate the influence of propofol on atrial electrophysiology and the vulnerability to atrial fibrillation in a sensitive whole-heart model of atrial fibrillation. Methods 11 hearts of New Zealand white rabbits were isolated and Langendorff-perfused. Two catheters were placed on both atria to record atrial monophasic action potentials. Hearts were stimulated at three different cycle lengths (350ms, 250ms, 150ms), thereby obtaining cycle-length dependent atrial action potential durations (aAPD90) and effective refractory periods (aERP). Atrial post-repolarization (aPRR) refractoriness was defined as the difference between aERP and aAPD90. To increase the occurrence of atrial fibrillation, hearts were perfused with a combination of acetylcholine (ACh, 1μM) and isoproterenol (Iso, 1μM) after obtaining baseline data. Vulnerability to atrial fibrillation was tested by a standardized protocol consisting of several trains of burst pacing. Results Acetylcholine/isoproterenol infusion reduced aAPD90 (−3ms), aERP (−16ms) and aPRR (−14ms). Additional treatment with propofol prolonged aAPD90 (+5ms), aERP (+24ms) and aPRR (+20ms). Under baseline conditions, 13 episodes of atrial fibrillation lasting longer than 1s occurred. Additional treatment with acetylcholine/isoproterenol provoked more episodes of atrial fibrillation (33 episodes, p&lt;0.05). Infusion of propofol on top of acetylcholine/isoproterenol significantly suppressed atrial fibrillation (2 episodes, p&lt;0.05). Conclusion In this study, propofol prolonged aAPD90 and significantly reduced the occurrence of atrial fibrillation. Prolongation of effective refractory periods and especially of post-repolarization refractoriness protects the atrial myocardium against premature excitation and thereby prevents arrhythmias.

Hypothermia induces postrepolarization refractoriness in the atrial myocardium of the hibernating and active ground squirrel Citellus undulatus

The heart of the hibernating mammals demonstrates tolerance to the cold-induced arrhythmias and the electrophysiological mechanisms that underlie this phenomenon expecially in the atrial myocardium is still not elucidated. This study is aimed to the investigation of the hypothermia-induced changes of the atrial action potentials (AP) and refractoriness of the hibernating ground squirrel Citellus undulatus. APs were recorded with usage of standard sharp-electrode technique in the isolated, perfused multicellular continuously paced atrial myocardium preparations obtained from hibernating (HS) and summer active (SAS) squirrels. Action potentials duration (APD) and refractoriness duration (RD) were estimated at 37-17 °С and at various pacing cycle length (200-500 ms). It has been demonstrated that hypothermia causes prolongation of both APD and RD similarly in HS and SAS animals. However, the duration of the refractoriness in the atrial myocardium significantly exceeds the duration of the APs during hypothermia (27-17 °C). Revealed phenomenon may be considered as postrepolarization refractoriness (PRR). Proposed hypothermia-induced PRR probably substantially contributes to the insusceptibility of the hibernators to the cold induced arrhythmias by preventing the afterdepolarizations.

Hypothermia-Induced Postrepolarization Refractoriness Is the Reason of the Atrial Myocardium Tolerance to the Bioelectrical Activity Disorders in the Hibernating and Active Ground Squirrel Citellus undulatus.

The electrophysiological mechanism of the atrial myocardium resistance to the cold-induced arrhythmias was studied in the hibernating ground squirrel Citellus undulatus. The atrial action potentials (APs) and refractoriness were recorded with microelectrodes in isolated multicellular preparations of the atrial myocardium taken from the hibernating and summer active ground squirrels (HS and SAS, respectively) at 37, 27, and 17°С to estimate the AP and refractoriness durations. In both HS and SAS, hypothermia increased the duration of the AP and refractoriness period (APD and RD, respectively), and in both animal groups RD was longer than APD under hypothermia but not at 37°С. This last observation can be a result of the postrepolarization refractoriness (PRR), which seems to contribute substantially to the atrial myocardium tolerance of the hibernating animals to the hypothermia-induced arrhythmias because it prevents afterdepolarizations.

Application of human induced pluripotent stem cell-derived cardiomyocytes sheets with microelectrode array system to estimate antiarrhythmic properties of multi-ion channel blockers

We examined electrophysiological indices of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) sheets in order to quantitatively estimate Na+, K+ and Ca2+ channel blocking actions of bepridil and amiodarone using microelectrode array system in comparison with that of E-4031. We analyzed the field potential duration, effective refractory period, current threshold and conduction property using a programmed electrical stimulation protocol to obtain the post repolarization refractoriness and coefficient a of the relationship between the pacing cycle length and field potential duration. Electropharmacological profile of each drug was successfully characterized; namely, 1) the changes in the current threshold and conduction property provided basic information of Na+ channel blocking kinetics, 2) the relationship between pacing cycle length and field potential duration reflected drug-induced inhibition of human ether-à-go-go-related gene (hERG) K+ channel, 3) the post repolarization refractoriness indicated the relative contribution of these drugs to Na+ and K+ channel blockade, and 4) L-type Ca2+ channel blocking action was more obvious in the field potential waveform of the hiPSC-CMs sheets than that expected in the electrocardiogram in humans. Thus, this information may help to better utilize the hiPSC-CMs sheets for grasping the properties and net effects of drug-induced Na+, Ca2+ and K+ channel blockade.

Antiarrhythmic properties of ivabradine in an experimental model of Short-QT- Syndrome.

The If channel inhibitor ivabradine is recommended for treatment of chronic heart failure. However, ivabradine also inhibits human ether-a-go-go (hERG) mediated potassium currents. The aim of the present study was to assess the electrophysiologic effects of ivabradine in an experimental model of short-QT-syndrome. Twelve rabbit hearts were isolated and Langendorff-perfused. After obtaining baseline data, pinacidil, an IK-ATP channel opener, was infused (1μmol/L). Eight endo- and epicardial monophasic action potentials and a 12-lead ECG showed a significant abbreviation of QT interval (-32ms, P<.05) and shortening of action potential duration at 90% of repolarization (APD90; -22ms, P<.05). The shortening of ventricular repolarization was accompanied by a reduction of effective refractory period (ERP; -20ms, P<.05). Thereafter, hearts were additionally treated with ivabradine (5μmol/L) leading to an increase of QT interval (+31ms, P<.05), APD90 (+15ms, P<.05) as well as of ERP (+38ms, P<.05) and post-repolarization refractoriness (PRR, +33ms, P<.05) as compared with sole pinacidil infusion. Under baseline conditions, ventricular fibrillation (VF) was inducible by a standardized pacing protocol including programmed stimulation and burst stimulation in 3 of 12 hearts (6 episodes). After application of 1μmol/L pinacidil, 6 of 12 hearts were inducible (22 episodes). Additional infusion of 5μmol/L ivabradine led to a significant suppression of VF. Only two episodes could be induced in 1 of 12 hearts. In the present study ivabradine reversed the electrophysiologic effects of pharmacologically simulated short-QT syndrome. Ivabradine demonstrated antiarrhythmic properties based on an increase of both ERP and PRR.

The anti-influenza drug oseltamivir reduces atrial fibrillation in an experimental whole-heart model.

Recent experimental studies suggested direct effects of the anti-influenza drug oseltamivir on cardiac electrophysiology. We therefore aimed at analyzing potential antiarrhythmic effects of oseltamivir on atrial fibrillation (AF) in an experimental whole-heart model. Twelve rabbit hearts were isolated and Langendorff perfused. Thereafter, hearts were paced at cycle lengths of 350, 250, and 200ms in the atrium. A standardized protocol employing atrial burst pacing induced AF in 4 of 12 hearts under baseline conditions (33%, 11 episodes). Subsequently, a combination of acetylcholine (1μM) and isoproterenol (1μM) was administered to increase AF occurrence. Two monophasic action potential recordings on the left and two on the right atrial epicardium displayed a decrease of atrial action potential duration (aAPD, -38ms, p<0.01) and atrial effective refractory period (aERP; -20ms, p<0.05). Under the influence of acetylcholine/isoproterenol AF was inducible in 8 of 12 hearts (66%; 69 episodes). Additional infusion of oseltamivir (100μM) resulted in a significant increase of both aAPD (+29ms, p<0.05) and aERP (+40ms, p<0.01) leading to an increase of atrial post-repolarization refractoriness (aPRR). Under the influence of oseltamivir only 3 of 12 hearts (25%, 8 episodes) remained inducible. In six additional hearts oseltamivir (50μM and 100μM) did not significantly alter ventricular APD, QRS duration and QT interval but induced a significant increase of ventricular ERP. In the present experimental study, acute infusion of the anti-influenza drug oseltamivir reduced atrial fibrillation. The antiarrhythmic effect can be explained by a significant increase in aERP and aPRR. These results suggest an antiarrhythmic potential of oseltamivir in atrial arrhythmias.