Cardiac Repolarization Reserve Research Articles

E3 ubiquitin ligase rififylin has yin and yang effects on rabbit cardiac transient outward potassium currents (Ito) and corresponding channel proteins

Genome-wide association studies (GWAS) have reported a correlation between a single nucleotide polymorphism of the RING finger E3 ubiquitin protein ligase rififylin (RFFL) and QT interval variability in humans (Newton-Cheh et al., 2009). Previously, we have shown that RFFL down-regulates expression and function of the human-like ether-a-go-go-related gene (hERG) potassium channel and corresponding rapidly activating delayed rectifier potassium current (IKr) in adult rabbit ventricular cardiomyocytes (ARbCMs). Here we report that RFFL also affects the transient outward current (Ito), but in a peculiar way. RFFL overexpression in ARbCMs significantly decreases the contribution of its fast component (Ito,f) from 35% to 21%, and increases the contribution of its slow component (Ito,s) from 65% to 79%. Since Ito,f in rabbits is mainly conducted by Kv4.3, we investigated the effect of RFFL on Kv4.3 expressed in HEK293A cells. We found that RFFL overexpression reduced Kv4.3 expression and corresponding Ito,f in a RING domain-dependent manner in the presence or absence of its accessory subunit KChIP2. On the other hand, RFFL overexpression in Kv1.4-expressing HEK cells leads to an increase in both Kv1.4 expression level and Ito,s, similarly in a RING domain-dependent manner. Our physiologically detailed rabbit ventricular myocyte computational model shows that these yin and yang effects of RFFL overexpression on Ito,f, and Ito,s affect phase 1 of the action potential waveform and slightly decrease its duration in addition to suppressing IKr. Thus, RFFL modifies cardiac repolarization reserve via ubiquitination of multiple proteins that differently affect various potassium channels and cardiac action potential duration (APD).

Abstract 16037: Vaping Associated Sudden Cardiac Arrest in a Young Healthy Patient

Introduction and Importance: While the use of vaping has increased significantly among young individuals, the effects of the various vape aerosol chemical contents on cardiac electrophysiological dynamics remain unknown. Case Presentation: A 22-year-old female with a history of energy vaping but no significant past medical history presented with cardiac arrest. Found to have no pulse, CPR was started and initial rhythm of ventricular tachycardia was obtained. Shock was administered with follow-up rhythm of ventricular fibrillation. Patient was shocked again and entered atrial fibrillation with rapid ventricular response. She later cardioverted into sinus tachycardia and was intubated. Transthoracic echocardiogram showed normal ejection fraction but was otherwise unremarkable. Patient was extubated on day four of hospitalization. Left heart catheterization and cardiac MRI were completely unremarkable. Toxicology, urine analysis, and troponins were all negative. Patient was discharged with metoprolol tartrate, and a life-vest with outpatient follow-ups for possible implantable cardioverter-defibrillator placement in the setting of negative work up for sudden cardiac arrest. Clinical Discussion: Vaping-induced sudden cardiac arrest may be attributed to a reduction in cardiac repolarization reserve. Exposure to vegetable glycerin and propylene glycol, substances present in nearly all vape products, have been found to incite arrhythmias and disrupt cardiac conduction in animals. Acrolein,a byproduct of glycerin, has also been found to induce arrhythmias due to autonomic dysfunction. Increased prevalence of ventricular arrhythmias have also been noted with exposure to aerosols containing nicotine-free propylene glycol (PG).This likely stems from the impact of PG aerosols on the transient outward potassium current, which plays a significant role in early cardiac repolarization. Conclusion: The effects of inhaling vape aerosols remain not fully understood. While there is a perceived notion that nicotine-free aerosols may not be harmful, that remains unclear.Further studies should be conducted to evaluate for pro-arrhythmogenic effects and autonomic dysfunction from the various chemical substances present in vape aerosols.

Abstract 13286: Ryanodine Receptor Inhibition Prevents Sudden Cardiac Death by Preserving Cardiac Repolarization Reserve in Stressed Arrhythmogenic Hearts

Introduction: Current therapies for preventing sudden cardiac death (SCD) are limited. Prior studies have shown that cardiac stress-induced ryanodine receptor (RyR2) hyperactivity increases sarcoplasmic calcium leak, leading to ventricular tachyarrythmias (VT) and SCD. We hypothesized that inhibition of RyR2 with dantrolene sodium (DS) reduces Ca 2+ leak and mitigates VT and SCD risk in pressure-overloaded arrhythmogenic hearts. Methods: We used an established guinea pig model that uniquely recapitulates key features of human SCD, including a high incidence of spontaneous VT in freely ambulating animals, randomized to placebo vs. DS treatment. We performed in vivo (e.g., VT/SCD incidence, ECG heart rate and QT variability, echocardiographic measures of cardiac function) and ex vivo (e.g., pressure-volume, electrophysiology) studies. We performed intention to treat analysis for 4-week VT-free survival. Results: About half the animals receiving placebo had SCD by 4 weeks. By contrast, dantrolene treatment reduced SCD incidence by 50%. Compared to placebo, dantrolene treatment preserved chronotropic competency during transient β-adrenergic challenge, and improved heart rate variability (SDNN: 11±1.1 vs 17±2.8 ms, p<0.05), reduced QT variability (0.52±0.13 vs 0.19±0.08 (log, ms, p<0.05), and improved cardiac function (fractional shortening: 26±1.9 vs 45±4.5 %, p<0.05). In perfused excised hearts, dantrolene treatment mitigated VT (induced by isoproterenol or burst pacing). Conclusion: Inhibition of RyR2 hyperactivity by dantrolene treatment prevents VT and SCD by increasing cardiac repolarization reserve. Dantrolene also improved overall cardiac electromechanical function in stressed arrhythmogenic hearts. Targeted inhibition of RyR2 hyperactivity may be an effective therapeutic strategy for preventing VT and SCD.

B-PO04-011 INHIBITION OF KV11.1 CHANNELS IN ISOLATED HYPOKALEMIC RABBIT HEARTS WAS PROARRHYTHMIC, WHEREAS KCA2 CHANNEL INHIBITION WAS SAFE

Hypokalemia increases the risk of ventricular arrhythmia by reducing the cardiac repolarization reserve and prolonging the QT-interval. The proarrhythmic risk of hypokalemia is further exacerbated by classical Class 3 antiarrhythmic drugs that inhibit Kv11.1-channels such as dofetilide especially at slow heart rates. To examine the proarrhythmic potential of selective Kv11.1 inhibitor (dofetilide), a selective KCa2 inhibitor (AP14145), and a mixed Kv11.1/ KCa2 inhibitor (AP30663) in a setting of normo- and hypokalemia. Isolated rabbit hearts were AV-ablated and perfused with normokalemic (4 mM K+) solution at baseline. This was followed by a 20-minute period perfusion with drug or vehicle control. Finally, a 20-minute period of perfusion followed where drug was still present, but the solution was hypokalemic (2.5 mM K+). Animals were randomly assigned to 4 groups: Time matched controls, dofetilide (10nM), AP14145 (3μM), or AP30663 (1.5 μM). Ventricular action potential duration (APD90) and triangulation (APD30-90) were measured during ventricular pacing at heart rates of 92, 120, and 200 BPM. During unpaced periods, proarrhythmia was quantified by short term variability of repolarisation and a proarrhythmia score. Dofetilide increased APD90, APD30-90, and proarrhythmia markers, compared to time matched controls especially at slow heart rates and during hypokalemia. Neither AP14145 nor AP30663 affected any of these parameters regardless of heart rate or K+ concentration. These results support that KCa2 inhibition may be associated with a better cardiac safety profile than Kv11.1 inhibition.

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.

Modelling sudden cardiac death risks factors in patients with coronavirus disease of 2019: the hydroxychloroquine and azithromycin case.

AimsCoronavirus disease of 2019 (COVID-19) has rapidly become a worldwide pandemic. Many clinical trials have been initiated to fight the disease. Among those, hydroxychloroquine and azithromycin had initially been suggested to improve clinical outcomes. Despite any demonstrated beneficial effects, they are still in use in some countries but have been reported to prolong the QT interval and induce life-threatening arrhythmia. Since a significant proportion of the world population may be treated with such COVID-19 therapies, evaluation of the arrhythmogenic risk of any candidate drug is needed.Methods and resultsUsing the O'Hara-Rudy computer model of human ventricular wedge, we evaluate the arrhythmogenic potential of clinical factors that can further alter repolarization in COVID-19 patients in addition to hydroxychloroquine (HCQ) and azithromycin (AZM) such as tachycardia, hypokalaemia, and subclinical to mild long QT syndrome. Hydroxychloroquine and AZM drugs have little impact on QT duration and do not induce any substrate prone to arrhythmia in COVID-19 patients with normal cardiac repolarization reserve. Nevertheless, in every tested condition in which this reserve is reduced, the model predicts larger electrocardiogram impairments, as with dofetilide. In subclinical conditions, the model suggests that mexiletine limits the deleterious effects of AZM and HCQ.ConclusionBy studying the HCQ and AZM co-administration case, we show that the easy-to-use O'Hara-Rudy model can be applied to assess the QT-prolongation potential of off-label drugs, beyond HCQ and AZM, in different conditions representative of COVID-19 patients and to evaluate the potential impact of additional drug used to limit the arrhythmogenic risk.

Inhibition of KCa2 Channels Decreased the Risk of Ventricular Arrhythmia in the Guinea Pig Heart During Induced Hypokalemia.

BackgroundHypokalemia reduces the cardiac repolarization reserve. This prolongs the QT-interval and increases the risk of ventricular arrhythmia; a risk that is exacerbated by administration of classical class 3 anti-arrhythmic agents.Small conductance Ca2+-activated K+-channels (KCa2) are a promising new atrial selective target for treatment of atrial fibrillation. Under physiological conditions KCa2 plays a minor role in ventricular repolarization. However, this might change under hypokalemia because of concomitant increases in ventriculay -60r intracellur Ca2+.PurposeTo study the effects of pharmacological KCa2 channel inhibition by the compounds AP14145, ICA, or AP30663 under hypokalemic conditions as compared to dofetilide and hypokalemia alone time-matched controls (TMC).MethodsThe current at +10 mV was compared in HEK293 cells stably expressing KCa2.3 perfused first with normo- and then hypokalemic solutions (4 mM K+ and 2.5 mM K+, respectively). Guinea pig hearts were isolated and perfused with normokalemic (4 mM K+) Krebs-Henseleit solution, followed by perfusion with drug or vehicle control. The perfusion was then changed to hypokalemic solution (2.5 mM K+) in presence of drug. 30 animals were randomly assigned to 5 groups: ICA, AP14145, AP30663, dofetilide, or TMC. QT-interval, the interval from the peak to the end of the T wave (Tp–Te), ventricular effective refractory period (VERP), arrhythmia score, and ventricular fibrillation (VF) incidence were recorded.ResultsHypokalemia slightly increased KCa2.3 current compared to normokalemia. Application of KCa2 channel inhibitors and dofetilide prolonged the QT interval corrected for heart rate. Dofetilide, but none of the KCa2 channel inhibitors increased Tp–Te during hypokalemia. During hypokalemia 4/6 hearts in the TMC group developed VF (two spontaneously, two by S1S2 stimulation) whereas 5/6 hearts developed VF in the dofetilide group (two spontaneously, three by S1S2 stimulation). In comparison, 0/6, 1/6, and 1/6 hearts developed VF when treated with the KCa2 channel inhibitors AP30663, ICA, or AP14145, respectively.ConclusionHypokalemia was associated with an increased incidence of VF, an effect that also seen in the presence of dofetilide. In comparison, the structurally and functionally different KCa2 channel inhibitors, ICA, AP14145, and AP30663 protected the heart from hypokalemia induced VF. These results support that KCa2 inhibition may be associated with a better safety and tolerability profile than dofetilide.

P703The small conductance Ca2+-activated K+-channel inhibition tool compound AP14145 protected the guinea pig heart from ventricular arrhythmia during hypokalemia whereas dofetilide was pro-arrhythmic

Abstract Background Hypokalemia is commonly encountered in the clinic. Hypokalemia reduces the cardiac repolarization reserve and causes increases in intracellular calcium. This prolongs the QT-interval and increases the risk of ventricular arrhythmia; a risk that can be further exacerbated by concomitant administration of classical class 3 anti-arrhythmic agents. Small conductance Ca2+-activated K+-channels (SK-channels) are a promising new atrial selective target for treatment of atrial fibrillation (AF). Under physiological conditions SK channels play an insignificant role in ventricular repolarization. However, this might change under hypokalemia because of concomitant increases in intracellur calcium. Purpose To study the effects of SK channel inhibition with the tool compound AP14145 or ICA under hypokalemic conditions as compared to the class 3 anti-arrhythmic agent dofetilide and time matched controls (TMC). Methods Guinea pig hearts were isolated and retrogradely perfused with normokalemic (4.5 mM K+) Krebs-Henseleit solution, followed by perfusion with drug or vehicle control. The perfusion was then changed to hypokalemic solution (2.5 mM K+) in presence of drug for 20 min, followed by 20 min perfusion with normokalemic solution in presence of drug. A total of 24 animals were included in the study and randomly assigned to 4 groups: ICA, AP14145, dofetilide or TMC. QT-interval, ventricular effective refractory period, extra systoles and incidence of ventricular tachycardia (VT) or fibrillation (VF) were recorded for each perfusion period. Results Hypokalemia caused a small increase in QT-interval. Application of SK channel inhibitors did not cause further changes, whereas dofetilide prolonged QT compared to hypokalemia alone. During hypokalemia 3 out of 6 hearts in the TMC group developed VF (one spontaneously, two following S1S2 stimulation) whereas 4 out of 6 hearts developed VF in the dofetilide group (one spontaneously, three following S1S2 stimulation). In comparison only 1 heart out of 6 developed VF when treated with the SK channel inhibitor ICA (spontaneously) or AP14145 (following S1S2 stimulation). Conclusion Hypokalemia was associated with an increased risk of VF, an effect that was exaggerated by co-administration of dofetilide. In comparison, the structurally and functionally different SK channel inhibitors, ICA and AP14145, protected the heart from hypokalemia induced VF. These results with the tool compound AP14145 support that SK inhibition may be associated with a better safety and tolerability profile than dofetilide. Acknowledgement/Funding Innovation Fund Denmark

Unraveling the role of cardiac IKs-related repolarization reserve in human induced pluripotent stem cell-derived cardiomyocytes

Unraveling the role of cardiac IKs-related repolarization reserve in human induced pluripotent stem cell-derived cardiomyocytes

Investigating the state dependence of drug binding in hERG channels using a trapped-open channel phenotype

The hERG channel is a key player in repolarization of the cardiac action potential. Pharmacological blockade of hERG channels depletes the cardiac repolarization reserve, increasing the risk of cardiac arrhythmias. The promiscuous nature of drug interactions with hERG presents a therapeutic challenge for drug design and development. Despite considerable effort, the mechanisms of drug binding remain incompletely understood. One proposed mechanism is that high-affinity drug binding preferentially occurs when channels are in the inactivated state. However, this has been difficult to test, since inactivation is rapid in hERG and access to the drug binding site is limited by slower opening of the activation gate. Here, we have directly assessed the role of inactivation in cisparide and terfenadine drug binding in mutant (I663P) hERG channels where the activation gate is trapped-open. We firstly demonstrate the utility of this approach by showing that inactivation, ion selectivity and high affinity drug binding are preserved in I663P mutant channels. We then assess the role of inactivation by applying cisapride and terfenadine at different membrane voltages, which induce varying degrees of inactivation. We show that the extent of block does not correlate with the extent of inactivation. These data suggest that inactivation is not a major determinant of cisapride or terfenadine binding in hERG channels.

Pharmacological characterization of microminipig as a model to assess the drug-induced cardiovascular responses for non-clinical toxicity and/or safety pharmacology studies.

We tried to establish the halothane-anesthetized microminipigs as an alternative animal model for non-clinical toxicity and/or safety pharmacology studies. In order to characterize the halothane-anesthetized microminipigs, we firstly clarified the effects of halothane anesthesia on their cardiovascular system (n = 5). Then, we examined the cardiovascular effects of dl-sotalol in doses of 0.1, 0.3 and 1 mg/kg, i.v. on the halothane-anesthetized microminipigs (n = 6). Induction of the halothane anesthesia by itself prolonged the QT interval as well as QTcF, suggesting that the halothane anesthesia can reduce the cardiac repolarization reserve in microminipigs like in dogs. dl-Sotalol showed more potent negative chronotropic, dromotropic and hypotensive effects together with repolarization delay in microminipigs than in dogs, although each cardiovascular response to dl-sotalol was directionally similar between them, suggesting greater basal sympathetic tone and/or smaller volume of distribution of the drug in microminipigs than in dogs. Analyses of proarrhythmic surrogate markers indicate that Tpeak-Tend and short-term variability of QT interval may be more sensitive to detect the dl-sotalol-induced direct electrophysiological changes in microminipigs than in dogs, but its reverse will be true for J-Tpeakc. Thus, these results may help better understand the drug-induced cardiovascular responses in microminipigs.

A Role of Preload Variability in Quantifying Cardiac Repolarization Reserve in Baseline

Ventricular arrhythmias compose a heterogeneous group of cardiac complications that can lead to sudden cardiac death. An increased susceptibility to (prolonged) repolarization associated arrhythmias is suggested to be the result of a reduced 'repolarization reserve', associated with an increase in the beat-to-beat variability in repolarization (BVR). Multiple parameters, related to ventricular remodelling as occurs in the chronic, complete atrioventricular block (CAVB) dog model or in patients in heart failure have already been demonstrated to increase BVR in baseline, which further increases upon infusion with action potential duration (APD) prolonging drugs. We demonstrate that the beat-to-beat variability in preload, as a result of atrioventricular dyssynchrony, can assist to determine the status of repolarization reserve. The increase in BVR, induced by varying the preload, could be diminished by infusion with streptomycin, an inhibitor of the stretch-activated channels (SACs). We therefore identified the SACs as important components of the process that contributes to the beat-to-beat variability in preload to increase the BVR. The preload variability (PLV) induced increase in BVR was however not associated with an increase in the incidence of ventricular arrhythmias. Therefore, the effect of variability in preload is able to unmask a latent decrease of repolarization reserve. The identification of a new method to determine repolarization reserve status opens numerous doors for future research in the field of arrhythmias. For instance, PLV could be used in the optimization of the BVR as a predictor of arrhythmias. Furthermore, clinical applicability and anti-arrhythmic efficacy of diminishing effects of variable preload on SACs in cardiac patients as (part of) an anti-arrhythmic therapy could also be further explored.

Electrocardiographic Early Repolarization: A Scientific Statement From the American Heart Association.

The early repolarization (ER) pattern (ERP), initially described as elevation of the ST segment of ≥1 leads on the 12-lead ECG, has long been considered a benign phenomenon. However, more recent studies have demonstrated positive, negative, and neutral associations between an ERP and various end points, including all-cause, cardiac, and arrhythmic mortality. These recent studies have used more complex or heterogeneous definitions of ER, including J-wave or J-point elevation and QRS complex notching or slurring, with or without concomitant ST-segment elevation. Other studies have identified an increased prevalence of ERP in survivors of spontaneous ventricular fibrillation (VF) or cardiac arrest. As a result, considerable confusion remains concerning the definition of ERP, its prognostic significance, and whether additional evaluation or treatment is warranted. These issues are especially important because the prevalence of ERP may be as high as 10% in the general population and even higher in some healthy subgroups. The writing group performed a comprehensive literature search and developed recommendations based on the results of the current literature. We searched MEDLINE (via PubMed), EMBASE, and the Cochrane Library to identify relevant primary scientific articles, guideline statements, and review articles in the literature. Search terms included, but were not limited to, early repolarization, J-point elevation, J wave, Haissaguerre syndrome, sudden cardiac death, idiopathic ventricular fibrillation, ventricular fibrillation, cardiac repolarization reserve, sudden unexplained death syndrome, and Osborn waves. We also reviewed the references of highly cited documents and citing documents. The recommendations were voted on, and 80% agreement was required for inclusion. The purposes of this document are as follows:

Quantification of the Ionic Current Contributions to Cardiac Repolarization Reserve Based on a Novel Piecewise-Linear Approximation Approach

Cardiac repolarization reserve is an important protective physiological mechanism. The assessment of single current contributions to action potential (AP) duration (APD) changes allows investigating the complex interplay of ionic mechanisms underlying such mechanism. We present a new method to quantify the contributions of each membrane current to APD changes due to a perturbation from the basal to a different condition.The method is based on a piecewise-linear approximation of the AP repolarization, which is dissected into uniform “discrete decreases” deltaV (e.g. −1 mV), splitting the repolarization timecourses into unevenly long time intervals. For each deltaV, APs are approximated by a linear decreasing trend and each current by its mean value; under this assumption the local contributions of each current to the prolongation/shortening can be computed.We tested our method on the O’Hara-Rudy (ORd) computational model of human ventricular AP in case of 50% block of the IKr current, which prolongs APD by 118 ms.Our analysis revealed that when the contribution of IKr only is considered APD results extremely prolonged (+329 ms), but other currents tend to compensate, mostly ICaL (−183 ms) and IKs to a lesser extent (−35 ms). Time-independent currents, like IK1, do not contribute to APD change. The dynamic adaptation to IKr block from the perturbation until the steady state was also quantified.Our method enables the quantification of the adaptive and compensatory mechanisms implemented by the (in silico model of) cardiac cells in response to a perturbation, such as the block of a repolarizing current.According to the ORd model, and surprisingly enough, the cardiac repolarization reserve in case of IKr block is largely attributable to a decrease in ICaL rather than to an increase in other potassium currents.

Cardiac Small Conductance Calcium-Activated K Channels Maintain Repolarization Reserve in a Pharmacological Model of Type 3 Long QT Syndrome

Drug-induced long QT syndrome (diLQT) is a major public health problem. Whether small conductance calcium-activated K (SK) channels are involved in modulating cardiac repolarization reserve in diLQT is unclear. To examine their role in diLQT, we determined the effects of SK current inhibition on optically measured action potential duration at 80% repolarization (APD80) in isolated perfused, immobilized (10 microM ±-blebbistatin) mouse hearts, in which ventricular repolarization was prolonged by the late Na current (INa-L) activator ATX-II (15 nM). ATX-II at this concentration had no significant effect on the activity of cardiac SK channels expressed in HEK293 cells, but significantly (P 0.05). Vehicles for ATX-II and apamin did not significantly alter APD80 (n = 3; P>0.05). Identical results were obtained in isolated perfused rabbit hearts (n = 2), indicating that apamin effects on APD80 were not species-specific. In conclusion, these findings suggest that SK currents are acutely upregulated in a pharmacological model of type 3 LQT syndrome. Increased SK channel activity may serve as an endogenous rescue mechanism protecting normal and diseased ventricles from arrhythmias associated with reduced repolarization reserve. Apamin caused further prolongation of the APD80. Therefore, SK channel blockade removes this rescue mechanism, unmasking diLQT.

Faulty cardiac repolarization reserve in alternating hemiplegia of childhood broadens the phenotype.

Alternating hemiplegia of childhood is a rare disorder caused by de novo mutations in the ATP1A3 gene, expressed in neurons and cardiomyocytes. As affected individuals may survive into adulthood, we use the term 'alternating hemiplegia'. The disorder is characterized by early-onset, recurrent, often alternating, hemiplegic episodes; seizures and non-paroxysmal neurological features also occur. Dysautonomia may occur during hemiplegia or in isolation. Premature mortality can occur in this patient group and is not fully explained. Preventable cardiorespiratory arrest from underlying cardiac dysrhythmia may be a cause. We analysed ECG recordings of 52 patients with alternating hemiplegia from nine countries: all had whole-exome, whole-genome, or direct Sanger sequencing of ATP1A3. Data on autonomic dysfunction, cardiac symptoms, medication, and family history of cardiac disease or sudden death were collected. All had 12-lead electrocardiogram recordings available for cardiac axis, cardiac interval, repolarization pattern, and J-point analysis. Where available, historical and prolonged single-lead electrocardiogram recordings during electrocardiogram-videotelemetry were analysed. Half the cohort (26/52) had resting 12-lead electrocardiogram abnormalities: 25/26 had repolarization (T wave) abnormalities. These abnormalities were significantly more common in people with alternating hemiplegia than in an age-matched disease control group of 52 people with epilepsy. The average corrected QT interval was significantly shorter in people with alternating hemiplegia than in the disease control group. J wave or J-point changes were seen in six people with alternating hemiplegia. Over half the affected cohort (28/52) had intraventricular conduction delay, or incomplete right bundle branch block, a much higher proportion than in the normal population or disease control cohort (P = 0.0164). Abnormalities in alternating hemiplegia were more common in those ≥16 years old, compared with those <16 (P = 0.0095), even with a specific mutation (p.D801N; P = 0.045). Dynamic, beat-to-beat or electrocardiogram-to-electrocardiogram, changes were noted, suggesting the prevalence of abnormalities was underestimated. Electrocardiogram changes occurred independently of seizures or plegic episodes. Electrocardiogram abnormalities are common in alternating hemiplegia, have characteristics reflecting those of inherited cardiac channelopathies and most likely amount to impaired repolarization reserve. The dynamic electrocardiogram and neurological features point to periodic systemic decompensation in ATP1A3-expressing organs. Cardiac dysfunction may account for some of the unexplained premature mortality of alternating hemiplegia. Systematic cardiac investigation is warranted in alternating hemiplegia of childhood, as cardiac arrhythmic morbidity and mortality are potentially preventable.

Combined inhibition of key potassium currents has different effects on cardiac repolarization reserve and arrhythmia susceptibility in dogs and rabbits.

A reliable assessment of the pro-arrhythmic potential for drugs in the development phase remains elusive. Rabbits and dogs are commonly used to create models of pro-arrhythmia, but the differences between them with respect to repolarizing potassium currents are poorly understood. We investigated the incidence of drug-induced torsades de pointes (TdP) and measured conventional ECG parameters and the short-term variability of the QT interval (STVQT) following combined pharmacological inhibition of IK1+IKs and IK1+IKr in conscious dogs and anesthetized rabbits. A high incidence of TdP was observed following the combined inhibition of IK1+IKs in dogs (67% vs. 14% in rabbits). Rabbits exhibited higher TdP incidence after inhibition of IK1+IKr (72% vs. 14% in dogs). Increased TdP incidence was associated with significantly larger STVQT in both models. The relatively different roles of IK1 and IKs in dog and rabbit repolarization reserve should be taken into account when extrapolating the results from animal models of pro-arrhythmia to humans. A stronger repolarization reserve in dogs (likely due to stronger IK1 and IKs), and the more human-like susceptibility to arrhythmia of rabbits argues for the preferred use of rabbits in the evaluation of adverse pro-arrhythmic effects.

Potential role of K+ currents in the repolarization reserve: the importance of cardiac repolarization reserve in understanding the proarrhythmic side effects of antiarrhythmic drugs

Potential role of K+ currents in the repolarization reserve: the importance of cardiac repolarization reserve in understanding the proarrhythmic side effects of antiarrhythmic drugs

Potential role of potassium currents in the repolarization reserve: the importance of cardiac repolarization reserve in safety pharmacology

E-mail: jost.norbert@gmail.com Cardiac repolarization is a delicate process and the determinants of repolarization in various species are poorly understood. This study compared the contribution of IK1, IKr and IKs to cardiac repolarization in canine and human ventricular preparations by applying conventional microelectrode, whole cell patch-clamp and molecular biological techniques. Pharmacological inhibition of IKr evokes modest prolongation of repolarization in the dog ventricle, while it largely lengthens it in the human. These results suggest that in human, in addition to its known effect to establish the resting membrane potential, IK1 is contributing only to the repolarization reserve, while in the dog, IK1 also plays a role to secure the normal repolarization. Therefore, the smaller IK1 in human comparing to the dog makes the ventricle more susceptible to repolarization lengthening in the human than in the dog. This should be taken into consideration when pharmacological results from the dog studies are extrapolated to the human, i.e. the effect on repolarization of drugs potentially blocking IKr/HERG channels can be underestimated in the human based on experiments performed in the dog. We conclude that humans show much greater repolarizationdelaying effects of IKr-block than dogs, because of lower repolarization-reserve contributions from IK1 and IKs. These results are relevant to understanding species-specific determinants of repolarization and emphasize the limitations of animal models for the human disease.

Ionic mechanisms limiting cardiac repolarization reserve in humans compared to dogs

The species-specific determinants of repolarization are poorly understood. This study compared the contribution of various currents to cardiac repolarization in canine and human ventricle. Conventional microelectrode, whole-cell patch-clamp, molecular biological and mathematical modelling techniques were used. Selective IKr block (50-100 nmol l(-1) dofetilide) lengthened AP duration at 90% of repolarization (APD90) >3-fold more in human than dog, suggesting smaller repolarization reserve in humans. Selective IK1 block (10 μmol l(-1) BaCl2) and IKs block (1 μmol l(-1) HMR-1556) increased APD90 more in canine than human right ventricular papillary muscle. Ion current measurements in isolated cardiomyocytes showed that IK1 and IKs densities were 3- and 4.5-fold larger in dogs than humans, respectively. IKr density and kinetics were similar in human versus dog. ICa and Ito were respectively ~30% larger and ~29% smaller in human, and Na(+)-Ca(2+) exchange current was comparable. Cardiac mRNA levels for the main IK1 ion channel subunit Kir2.1 and the IKs accessory subunit minK were significantly lower, but mRNA expression of ERG and KvLQT1 (IKr and IKs α-subunits) were not significantly different, in human versus dog. Immunostaining suggested lower Kir2.1 and minK, and higher KvLQT1 protein expression in human versus canine cardiomyocytes. IK1 and IKs inhibition increased the APD-prolonging effect of IKr block more in dog (by 56% and 49%, respectively) than human (34 and 16%), indicating that both currents contribute to increased repolarization reserve in the dog. A mathematical model incorporating observed human-canine ion current differences confirmed the role of IK1 and IKs in repolarization reserve differences. Thus, humans show greater repolarization-delaying effects of IKr block than dogs, because of lower repolarization reserve contributions from IK1 and IKs, emphasizing species-specific determinants of repolarization and the limitations of animal models for human disease.