Safe electrophysiologic profile of dexmedetomidine in different experimental arrhythmia models

Previous studies suggest an impact of dexmedetomidine on cardiac electrophysiology. However, experimental data is sparse. Therefore, purpose of this study was to investigate the influence of dexmedetomidine on different experimental models of proarrhythmia. 50 rabbit hearts were explanted and retrogradely perfused. The first group (n = 12) was treated with dexmedetomidine in ascending concentrations (3, 5 and 10 µM). Dexmedetomidine did not substantially alter action potential duration (APD) but reduced spatial dispersion of repolarization (SDR) and rendered the action potentials rectangular, resulting in no proarrhythmia. In further 12 hearts, erythromycin (300 µM) was administered to simulate long-QT-syndrome-2 (LQT2). Additional treatment with dexmedetomidine reduced SDR, thereby suppressing torsade de pointes. In the third group (n = 14), 0.5 µM veratridine was added to reduce the repolarization reserve. Further administration of dexmedetomidine did not influence APD, SDR or the occurrence of arrhythmias. In the last group (n = 12), a combination of acetylcholine (1 µM) and isoproterenol (1 µM) was used to facilitate atrial fibrillation. Additional treatment with dexmedetomidine prolonged the atrial APD but did not reduce AF episodes. In this study, dexmedetomidine did not significantly alter cardiac repolarization duration and was not proarrhythmic in different models of ventricular and atrial arrhythmias. Of note, dexmedetomidine might be antiarrhythmic in acquired LQT2 by reducing SDR.

Ion Channel Impairment and Myofilament Ca2+ Sensitization: Two Parallel Mechanisms Underlying Arrhythmogenesis in Hypertrophic Cardiomyopathy

Life-threatening ventricular arrhythmias are the main clinical burden in patients with hypertrophic cardiomyopathy (HCM), and frequently occur in young patients with mild structural disease. While massive hypertrophy, fibrosis and microvascular ischemia are the main mechanisms underlying sustained reentry-based ventricular arrhythmias in advanced HCM, cardiomyocyte-based functional arrhythmogenic mechanisms are likely prevalent at earlier stages of the disease. In this review, we will describe studies conducted in human surgical samples from HCM patients, transgenic animal models and human cultured cell lines derived from induced pluripotent stem cells. Current pieces of evidence concur to attribute the increased risk of ventricular arrhythmias in early HCM to different cellular mechanisms. The increase of late sodium current and L-type calcium current is an early observation in HCM, which follows post-translation channel modifications and increases the occurrence of early and delayed afterdepolarizations. Increased myofilament Ca2+ sensitivity, commonly observed in HCM, may promote afterdepolarizations and reentry arrhythmias with direct mechanisms. Decrease of K+-currents due to transcriptional regulation occurs in the advanced disease and contributes to reducing the repolarization-reserve and increasing the early afterdepolarizations (EADs). The presented evidence supports the idea that patients with early-stage HCM should be considered and managed as subjects with an acquired channelopathy rather than with a structural cardiac disease.

Canine Myocytes Represent a Good Model for Human Ventricular Cells Regarding Their Electrophysiological Properties

Due to the limited availability of healthy human ventricular tissues, the most suitable animal model has to be applied for electrophysiological and pharmacological studies. This can be best identified by studying the properties of ion currents shaping the action potential in the frequently used laboratory animals, such as dogs, rabbits, guinea pigs, or rats, and comparing them to those of human cardiomyocytes. The authors of this article with the experience of three decades of electrophysiological studies, performed in mammalian and human ventricular tissues and isolated cardiomyocytes, summarize their results obtained regarding the major canine and human cardiac ion currents. Accordingly, L-type Ca2+ current (ICa), late Na+ current (INa-late), rapid and slow components of the delayed rectifier K+ current (IKr and IKs, respectively), inward rectifier K+ current (IK1), transient outward K+ current (Ito1), and Na+/Ca2+ exchange current (INCX) were characterized and compared. Importantly, many of these measurements were performed using the action potential voltage clamp technique allowing for visualization of the actual current profiles flowing during the ventricular action potential. Densities and shapes of these ion currents, as well as the action potential configuration, were similar in human and canine ventricular cells, except for the density of IK1 and the recovery kinetics of Ito. IK1 displayed a largely four-fold larger density in canine than human myocytes, and Ito recovery from inactivation displayed a somewhat different time course in the two species. On the basis of these results, it is concluded that canine ventricular cells represent a reasonably good model for human myocytes for electrophysiological studies, however, it must be borne in mind that due to their stronger IK1, the repolarization reserve is more pronounced in canine cells, and moderate differences in the frequency-dependent repolarization patterns can also be anticipated.

The electrophysiological effects of cannabidiol on action potentials and transmembrane potassium currents in rabbit and dog cardiac ventricular preparations

Cannabis use is associated with known cardiovascular side effects such as cardiac arrhythmias or even sudden cardiac death. The mechanisms behind these adverse effects are unknown. The aim of the present work was to study the cellular cardiac electrophysiological effects of cannabidiol (CBD) on action potentials and several transmembrane potassium currents, such as the rapid (IKr) and slow (IKs) delayed rectifier, the transient outward (Ito) and inward rectifier (IK1) potassium currents in rabbit and dog cardiac preparations. CBD increased action potential duration (APD) significantly in both rabbit (from 211.7 ± 11.2. to 224.6 ± 11.4 ms, n = 8) and dog (from 215.2 ± 9.0 to 231.7 ± 4.7 ms, n = 6) ventricular papillary muscle at 5 µM concentration. CBD decreased IKr, IKs and Ito (only in dog) significantly with corresponding estimated EC50 values of 4.9, 3.1 and 5 µM, respectively, without changing IK1. Although the EC50 value of CBD was found to be higher than literary Cmax values after CBD smoking and oral intake, our results raise the possibility that potassium channel inhibition by lengthening cardiac repolarization might have a role in the possible proarrhythmic side effects of cannabinoids in situations where CBD metabolism and/or the repolarization reserve is impaired.

When Manual Analysis of 12-Lead ECG Holter Plays a Critical Role in Discovering Unknown Patterns of Increased Arrhythmogenic Risk: A Case Report of a Patient Treated with Tamoxifen and Subsequent Pneumonia in COVID-19

Several medicines, including cancer therapies, are known to alter the electrophysiological function of ventricular myocytes resulting in abnormal prolongation and dispersion of ventricular repolarization (quantified by multi-lead QTc measurement). This effect could be amplified by other concomitant factors (e.g., combination with other drugs affecting the QT, and/or electrolyte abnormalities, such as especially hypokalemia, hypomagnesaemia, and hypocalcemia). Usually, this condition results in higher risk of torsade de point and other life-threatening arrhythmias, related to unrecognized unpaired cardiac ventricular repolarization reserve (VRR). Being VRR a dynamic phenomenon, QT prolongation might often not be identified during the 10-s standard 12-lead ECG recording at rest, leaving the patient at increased risk for life-threatening event. We report the case of a 49-year woman, undergoing tamoxifen therapy for breast cancer, which alteration of ventricular repolarization reserve, persisting also after correction of concomitant recurrent hypokalemia, was evidenced only after manual measurements of the corrected QT (QTc) interval from selected intervals of the 12-lead ECG Holter monitoring. This otherwise missed finding was fundamental to drive the discontinuation of tamoxifen, shifting to another “safer” therapeutic option, and to avoid the use of potentially arrhythmogenic antibiotics when treating a bilateral pneumonia in recent COVID-19.

Drug-induced and transgenic LQTS rabbit models with reduced repolarization reserve to study proarrhythmic drug effects

Proarrhythmia - the triggering of arrhythmias following drug therapy - is a rare, but potentially lethal side-effect of various drugs, and therefore, a major safety concern during drug development. Most often proarrhythmia is caused by the drugs’ potential to interact with various K+-channels in the heart, leading to a prolongation of cardiac repolarization that is usually observed on the ECG as prolonged QT interval (drug-induced acquired long QT syndrome; aLQTS). Although drug-induced long-QT-related proarrhythmia is most frequently found in patients with impaired cardiac repolarization due to disease-induced structural and/or electrophysiological remodelling of the heart; most cellular, tissue and whole animal model systems used for drug safety screening are based on normal, healthy models. This approach has serious limitations; therefore, novel animal models that mimic the pathophysiological conditions under which drugs display the highest proarrhythmic risk - such as models with impaired cardiac repolarization - would be desirable for proarrhythmia safety testing. The aims of the present study: Drug-induced (HMR-1556 to block IKs) acquired LQTS, and various transgenic (congenital) LQTS rabbit models with impaired cardiac repolarization due to cardio-selective overexpression of loss-of-function mutations of human KCNH2 (HERG-G628S, α-subunit of IKr, loss of IKr, LQT2), KCNE1 (KCNE1-G52R, β-subunit of IKs, decreased IKs, LQT5)[1] or both KCNQ1 and KCNE1 transgenes (LQT2-5) were used to investigate: - the proarrhythmic potential of SZV-270, a novel antiarrhythmic drug candidate with combined Class I/B and Class III effects (acquired LQTS model). - the electrophysiological characteristics of a newly generated, double-transgenic LQT2-5 rabbit model - the utility of transgenic LQT2, LQT5 and LQT2-5 rabbit models for more reliable prediction of drug-induced ventricular arrhythmias Main findings: The acquired LQTS rabbit proarrhythmia model with pharmacologically reduced repolarization reserve (by the IKs inhibitor HMR-1556) was able to predict the known torsadogenic potential of the IKr blocker dofetilide, while indicated no SZV-270-induced proarrhythmia risk. This advantageous electrophysiological effect of the SZV-270 - prolongation of ventricular repolarization without increased arrhythmia risk - is assumed to be attributed to its combined IKr (Class III) and INa (Class I/B) blocking characteristics. Transgenic LQTS rabbit models reflected patients with clinically ‘silent’ - normal QT interval (LQT5) - or 'manifest' - prolonged QT interval (LQT2 and LQT2-5) - impairment in cardiac repolarization reserve capacity due to different pathomechanisms. The LQTS animals were more sensitive in detecting IKr - (LQT5) or IK1/IKs - (LQT2 and LQT2-5) blocking properties of drugs compared to healthy wild type (WT) animals. Impaired QT-shortening capacity at fast heart rates was observed due to disturbed IKs function in LQT5 and LQT2-5. Importantly, the transgenic LQTS models did not only show more pronounced changes in different proarrhythmia markers in response to potassium channel blockers but also exhibited higher incidence, longer duration and more malignant type of ex vivo arrhythmias than WT. Conclusions: Drug-induced and transgenic LQTS rabbit models reflect human pathophysiological settings - patients with reduced repolarization reserve - that favour drug-induced arrhythmia formation. As they demonstrate increased sensitivity to different specific ion-channel blockers (IKr-blockade in LQT5 or in HMR-1556 induced acquired LQTS model, IK1 - and IKs - blockade in LQT2 and LQT2-5), their combined use could provide more reliable, and more thorough prediction of (multi-channel-based) pro-arrhythmic potential of novel drug candidates especially in the setting of impaired cardiac repolarization reserve.

Inward Rectifier K + Currents Contribute to the Proarrhythmic Electrical Phenotype of Atria Overexpressing Cyclic Adenosine Monophosphate Response Element Modulator Isoform CREM‐IbΔC‐X

BACKGROUND Transgenic mice (TG) with heart‐directed overexpresion of the isoform of the transcription factor cyclic adenosine monophosphate response element modulator (CREM), CREM‐IbΔC‐X, display spontaneous atrial fibrillation (AF) and action potential prolongation. The remodeling of the underlying ionic currents remains unknown. Here, we investigated the regulatory role of CREM‐IbΔC‐X on the expression of K + channel subunits and the corresponding K + currents in relation to AF onset in TG atrial myocytes. METHODS AND RESULTS ECG recordings documented the absence or presence of AF in 6‐week‐old (before AF onset) and 12‐week‐old TG (after AF onset) and wild‐type littermate mice before atria removal to perform patch clamp, contractility, and biochemical experiments. In TG atrial myocytes, we found reduced repolarization reserve K + currents attributed to a decrease of transiently outward current and inward rectifier K + current with phenotype progression, and of acetylcholine‐activated K + current, age independent. The molecular determinants of these changes were lower mRNA levels of Kcnd2/3 , Kcnip2 , Kcnj2/4 , and Kcnj3/5 and decreased protein levels of K + channel interacting protein 2 (KChIP2 ), Kir2.1/3, and Kir3.1/4, respectively. After AF onset, inward rectifier K + current contributed less to action potential repolarization, in line with the absence of outward current component, whereas the acetylcholine‐induced action potential shortening before AF onset (6‐week‐old TG mice) was smaller than in wild‐type and 12‐week‐old TG mice. Atrial force of contraction measured under combined vagal‐sympathetic stimulation revealed increased sensitivity to isoprenaline irrespective of AF onset in TG. Moreover, we identified Kcnd2 , Kcnd3 , Kcnj3 , and Kcnh2 as novel CREM‐target genes. CONCLUSIONS Our study links the activation of cyclic adenosine monophosphate response element–mediated transcription to the proarrhythmogenic electrical remodeling of atrial inward rectifier K + currents with a role in action potential duration, resting membrane stability, and vagal control of the electrical activity.

The electrophysiological effect of cannabidiol on hERG current and in guinea-pig and rabbit cardiac preparations

Cannabis use is associated with cardiovascular adverse effects ranging from arrhythmias to sudden cardiac death. The exact mechanism of action behind these activities is unknown. The aim of our work was to study the effect of cannabidiol (CBD), tetrahydrocannabinol and 11-nor-9-carboxy-tetrahydrocannabinol on cellular cardiac electrophysiological properties including ECG parameters, action potentials, hERG and IKr ion channels in HEK cell line and in rabbit and guinea pig cardiac preparations. CBD increased action potential duration in rabbit and guinea pig right ventricular papillary muscle at lower concentrations (1 µM, 2.5 µM and 5 µM) but did not significantly change it at 10 µM. CBD at high concentration (10 µM) decreased inward late sodium and L-type calcium currents as well. CBD inhibited hERG potassium channels with an IC50 value of 2.07 µM at room temperature and delayed rectifier potassium current with 6.5 µM at 37 °C, respectively. The frequency corrected QT interval (QTc) was significantly lengthened in anaesthetized guinea pig without significantly changing other ECG parameters. Although the IC50 value of CBD was higher than literary Cmax values after CBD smoking and oral intake, our results raise the possibility that hERG and potassium channel inhibition might have a role in the possible proarrhythmic adverse effects of cannabinoids in situations where metabolism of CBD impaired and/or the repolarization reserve is weakened.