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Background and purpose:Assessing the proarrhythmic potential of compounds during drug development is essential. However, reliable prediction of drug-induced torsades de pointes arrhythmia (TdP) remains elusive. Along with QT interval prolongation, assessment of the short-term variability of the QT interval (STV(QT)) may be a good predictor of TdP. We investigated the relative importance of IKs and IKr block in development of TdP together with correlations between QTc interval, QT interval variability and incidence of TdP.Experimental approach:ECGs were recorded from conscious dogs and from anaesthetized rabbits given the IKr blocker dofetilide (DOF), the IKs blocker HMR-1556 (HMR) and their combination, intravenously. PQ, RR and QT intervals were measured and QTc and short-term variability of RR and QT intervals calculated.Key results:DOF increased QTc interval by 20% in dogs and 8% in rabbits. HMR increased QTc in dogs by 12 and 1.9% in rabbits. Combination of DOF+HMR prolonged QTc by 33% in dogs, by 16% in rabbits. DOF or HMR given alone in dogs or HMR given alone in rabbits induced no TdP. Incidence of TdP increased after DOF+HMR combinations in dogs (63%) and following HMR+DOF (82%) and DOF+HMR combinations (71%) in rabbits. STV(QT) markedly increased only after administration of DOF+HMR combinations in both dogs and rabbits.Conclusion and implications:STV(QT) was markedly increased by combined pharmacological block of IKr and IKs and may be a better predictor of subsequent TdP development than the measurement of QTc interval prolongation.British Journal of Pharmacology (2007) 151, 941–951; doi:10.1038/sj.bjp.0707297; published online 29 May 2007 [ABSTRACT FROM AUTHOR]

The antiarrhythmic and cardiac electrophysiological effects of SZV-2649 that contains a 2,6-diiodophenoxy moiety but lacks the benzofuran ring system present in amiodarone, were studied in mammalian cell line, rat and dog cardiac preparations. SZV-2649 exerted antiarrhythmic effects against coronary artery occlusion/reperfusion induced ventricular arrhythmias in rats and in acetylcholine- and burst stimulation induced atrial fibrillation in dogs. SZV-2649 inhibited hERG and GIRK currents in HEK cells (IC 50 : 342 and 529 nM, respectively). In canine ventricular myocytes, SZV-2649 (10 µM) decreased the densities of I Kr , and I to outward and I NaL and I CaL inward currents. The compound (2.5-10 µM) elicited Class IB type V max reducing and Class III type action potential duration prolonging effects in dog right ventricular muscle preparations. In canine atrial muscle, SZV-2629 (2.5-10 µM) moderately prolonged action potential duration and this effect was greatly augmented in preparations pretreated with 1 µM carbachol. In conclusion, SZV-2649, has antiarrhythmic effects based on its multiple ion channel blocking properties. Since its chemical structure substantially differs from that of amiodarone, it is expected that SZV-2649 would exhibit fewer adverse effects than the currently used most effective multichannel inhibitor drug amiodarone and may be a promising molecule for further development., (© 2024. The Author(s).)

Aims: Atrial fibrillation (AF) is the most common chronic/recurrent arrhythmia, which significantly impairs quality of life and increases cardiovascular morbidity and mortality. Therefore, the aim of the present study was to investigate the properties of three repolarizing potassium currents which were shown to contribute to AF-induced electrical remodeling, i.e., the transient outward (I to ), inward rectifier (I K1 ) and acetylcholine-sensitive (I K,ACh ) potassium currents in isolated atrial myocytes obtained from dogs either with sinus rhythm (SR) or following chronic atrial tachypacing (400/min)-induced AF., Methods: Atrial remodeling and AF were induced by chronic (4-6 weeks of) right atrial tachypacing (400/min) in dogs. Transmembrane ionic currents were measured by applying the whole-cell patch-clamp technique at 37 °C., Results: The I to current was slightly downregulated in AF cells when compared with that recorded in SR cells. This downregulation was also associated with slowed inactivation kinetics. The I K1 current was found to be larger in AF cells; however, this upregulation was not statistically significant in the voltage range corresponding with atrial action potential (-80 mV to 0 mV). I K,ACh was activated by the cholinergic agonist carbachol (CCh; 2 µM). In SR, CCh activated a large current either in inward or outward directions. The selective I K,ACh inhibitor tertiapin (10 nM) blocked the outward CCh-induced current by 61%. In atrial cardiomyocytes isolated from dogs with AF, the presence of a constitutively active I K,ACh was observed, blocked by 59% with 10 nM tertiapin. However, in "AF atrial myocytes", CCh activated an additional, significant ligand-dependent and tertiapin-sensitive I K,ACh current., Conclusions: In our dog AF model, I to unlike in humans was downregulated only in a slight manner. Due to its slow inactivation kinetics, it seems that I to may play a more significant role in atrial repolarization than in ventricular working muscle myocytes. The presence of the constitutively active I K,ACh in atrial myocytes from AF dogs shows that electrical remodeling truly developed in this model. The I K,ACh current (both ligand-dependent and constitutively active) seems to play a significant role in canine atrial electrical remodeling and may be a promising atrial selective drug target for suppressing AF.

Introduction: A higher incidence of neural dysfunction in people with obesity has been described. We determined the prevalence of neuropathic lesions in obese women and evaluated their potential association with anthropometric and laboratory parameters., Patients and Methods: In our cross-sectional study, we enrolled female patients with obesity and without diabetes before obesity treatment. Voluntary female subjects were controls with a normal body mass index (BMI). Autonomic function was assessed by Ewing's cardiovascular reflex tests, while comprehensive peripheral neuropathic assessments were conducted utilizing the Neurometer®, Tiptherm®, Monofilament®, and Rydel-Seiffer tuning fork tests. Sudomotor function was assessed by the Neuropad®-test. Body composition was examined using the InBody 770., Results: 71 patients (mean ± SD; age: 36.1 ± 8.3 years; BMI: 40.2 ± 8.5 kg/m 2 ) and 36 controls (age: 36.4 ± 13.3 years; BMI: 21.6 ± 2.1 kg/m 2 ) were enrolled. Patients had significantly higher systolic (patients vs. controls; 137.5 ± 16.9 vs. 114.6 ± 14.8 mmHg, p<0.001) and diastolic (83.0 ± 11.7 vs.69.8 ± 11.2 mmHg, p<0.001) blood pressure compared to controls. Among autonomic tests, only the heart rate response to Valsalva maneuver (Valsalva-ratio) revealed significant impairment in patients (1.4 ± 0.2 vs. 1.7 ± 0.4, p<0.001). Neurometer® at the median nerve revealed increased current perception threshold (CPT) values at all stimulating frequencies in patients (CPT at 2000 Hz: 204.6 ± 70.9 vs. 168.1 ± 66.9, p=0.013; 250 Hz: 84.4 ± 38.9 vs. 56.5 ± 34.8, p<0.001; CPT at 5 Hz: 58.5 ± 31.2 vs 36.9 ± 29.1, p<0.001). The Rydel-Seiffer tuning fork test has revealed a significant impairment of vibrational sensing on the lower limb in patients (right hallux: 6.8 ± 0.9 vs. 7.4 ± 0.8, p=0.030; left hallux: 6.9 ± 0.8 vs. 7.3 ± 0.9, p=0.029). The Neuropad® testing showed a significant impairment of sudomotor function in women with obesity. A negative correlation was found in patients between BMI and the 25-hydroxy-D3/D2-vitamin levels (r=-0.41, p=0.00126) and a positive correlation between the BMI and resting systolic blood pressure (r=0.26, p=0.0325)., Conclusion: Peripheral sensory neuronal and sudomotor function impairments were detected in female patients with obesity compared to the controls with normal BMI. Cardiovascular autonomic dysfunction was also revealed by the Valsalva-ratio in these patients, suggesting the presence of parasympathetic dysfunction. The negative correlation between BMI and the 25-hydroxy-D3/D2-vitamin highlights the potential deficiency of vitamin D in the population affected by obesity., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The author(s) declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process and the final decision, (Copyright © 2024 Keller, Zádori, Lippai, Szöllősi, Márton, Wellinger, Lada, Szűcs, Menyhárt, Kempler, Baczkó, Várkonyi, Lengyel and Vágvölgyi.)

Recent experimental data shows that hesperetin, a citrus flavonoid, affects potassium channels and can prolong the QT c interval in humans. Therefore, in the present study we investigated the effects of hesperetin on various transmembrane ionic currents and on ventricular action potentials. Transmembrane current measurements and action potential recordings were performed by patch-clamp and the conventional microelectrode techniques in dog and rabbit ventricular preparations. At 10 µM concentration hesperetin did not, however, at 30 µM significantly decreased the amplitude of the I K1 , I to , I Kr potassium currents. Hesperetin at 3-30 µM significantly and in a concentration-dependent manner reduced the amplitude of the I Ks current. The drug significantly decreased the amplitudes of the I NaL and I CaL currents at 30 µM. Hesperetin (10 and 30 µM) did not change the action potential duration in normal preparations, however, in preparations where the repolarization reserve had been previously attenuated by 100 nM dofetilide and 1 µg/ml veratrine, caused a moderate but significant prolongation of repolarization. These results suggest that hesperetin at close to relevant concentrations inhibits the I Ks outward potassium current and thereby reduces repolarization reserve. This effect in certain specific situations may prolong the QT interval and consequently may enhance proarrhythmic risk., (© 2024. The Author(s).)

The present study was designed to test the hypothesis that the selectivity of blocking the late Na + current (I NaL ) over the peak Na + current (I NaP ) is related to the fast offset kinetics of the Na + channel inhibitor. Therefore, the effects of 1 µM GS967 (I NaL inhibitor), 20 µM mexiletine (I/B antiarrhythmic) and 10 µM quinidine (I/A antiarrhythmic) on I NaL and I NaP were compared in canine ventricular myocardium. I NaP was estimated as the maximum velocity of action potential upstroke (V + ). Equal amounts of I max were dissected by the applied drug concentrations under APVC conditions. The inhibition of I NaL were dissected by the applied drug concentrations under APVC conditions. The inhibition of I NaL block at the physiological cycle length of 700 ms was 2.3% for GS967, 11.4% for mexiletine and 26.2% for quinidine. The respective offset time constants were 110 ± 6 ms, 456 ± 284 ms and 7.2 ± 0.9 s. These results reveal an inverse relationship between the offset time constant and the selectivity of I + over I max inhibition without any influence of the onset rate constant. It is concluded that the selective inhibition of I NaL over I NaP inhibition without any influence of the onset rate constant. It is concluded that the selective inhibition of I NaL over I NaP is related to the fast offset kinetics of the Na + channel inhibitor.

Introduction: Vascular complications and neuropathy may develop in the presence of metabolic syndrome. The aim of our study was to measure the cardiovascular autonomic function following physical training in patients with metabolic syndrome with and without diabetes., Subjects and Methods: 56 patients with metabolic syndrome (32 men/24 women, 40 non-diabetic patients (NDMetS)/16 diabetic patients (DMetS) [mean ± SD]: age: 50.35 ± 8.03 vs. 56.8 ± 9.30 years, p=0.023; baseline BMI: 32.2 ± 7.03 vs. 32.8 ± 5.94 kg/m 2 , p=0.739) were involved in our study. All tests and measurements were carried out before and following a 3-month physical training period. Autonomic function was assessed by means of five standard cardiovascular reflex tests. ECG repolarization parameters, including short-term QT variability and stress-ECG were also measured., Results: In the whole population, Valsalva-ratio (VR) and the autonomic score (AS) improved following training (VR: 1.49 ± 0.24 vs. 1.64 ± 0.34, p=0.001; AS: 2.05 ± 1.73 vs. 1.41 ± 1.36, p=0.015) accompanied by the significant decrease of the systolic (150.3 ± 16.12 vs. 134.1 ± 16.67 mmHg, p<0.001) and diastolic (90.64 ± 12.8 vs. 82.79 ± 11.1 mmHg, p<0.001) blood pressure. An improvement in VR was detected in NDMetS patients following training (1.51 ± 0.24 vs. 1.67 ± 0.31, p= 0.002). No significant changes could be detected in autonomic tests' results in the DMetS patient group following training. The applied exercise training program did not lead to significant changes in ECG repolarization. The stress-ECG test in the whole study population yielded a significant increase in the test duration (12.9 ± 3.76 vs. 15.1 ± 2.96 min, p<0.001) and in the test load (10.5 ± 2.78 vs. 11.6 ± 2.39 MET, p<0.001). The load capability improved significantly in both subgroups: 11.1 ± 2.04 vs. 12.1 ± 1.82, (p<0.001) and 9.0 ± 3.64 vs. 10.4 ± 3.05, (p=0.033) in subpopulations of NDMetS and DMetS, respectively. The DMetS patients achieved a significantly lower MET score at baseline (p=0.039) and following training (p=0.044) in comparison to the NDMetS patients., Conclusion: The three-month exercise program improved the Valsalva-ratio and the AN score in the MetS patients, that is potentially protective against cardiovascular events. The training had some beneficial effect on blood pressure and the results of the stress-ECG tests in both groups. The absence of significant change in the reflex tests in DMetS group reflects an impaired adaptation compared to the NDMestS group., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Vágvölgyi, Ábrahám, Máthéné Köteles, Korom, Barnai, Szűcs, Orosz, Kempler, Menyhárt, Nemes, Várkonyi, Baczkó, Kósa and Lengyel.)

Aims: Atrial fibrillation (AF) is associated with altered cAMP/PKA signaling and an AF-promoting reduction of L-type Ca2+-current (ICa,L), the mechanisms of which are poorly understood. Cyclic-nucleotide phosphodiesterases (PDEs) degrade cAMP and regulate PKA-dependent phosphorylation of key calcium-handling proteins, including the ICa,L-carrying Cav1.2α1C subunit. The aim was to assess whether altered function of PDE type-8 (PDE8) isoforms contributes to the reduction of ICa,L in persistent (chronic) AF (cAF) patients., Methods and Results: mRNA, protein levels, and localization of PDE8A and PDE8B isoforms were measured by RT-qPCR, western blot, co-immunoprecipitation and immunofluorescence. PDE8 function was assessed by FRET, patch-clamp and sharp-electrode recordings. PDE8A gene and protein levels were higher in paroxysmal AF (pAF) vs. sinus rhythm (SR) patients, whereas PDE8B was upregulated in cAF only. Cytosolic abundance of PDE8A was higher in atrial pAF myocytes, whereas PDE8B tended to be more abundant at the plasmalemma in cAF myocytes. In co-immunoprecipitation, only PDE8B2 showed binding to Cav1.2α1C subunit which was strongly increased in cAF. Accordingly, Cav1.2α1C showed a lower phosphorylation at Ser1928 in association with decreased ICa,L in cAF. Selective PDE8 inhibition increased Ser1928 phosphorylation of Cav1.2α1C, enhanced cAMP at the subsarcolemma and rescued the lower ICa,L in cAF, which was accompanied by a prolongation of action potential duration at 50% of repolarization., Conclusion: Both PDE8A and PDE8B are expressed in human heart. Upregulation of PDE8B isoforms in cAF reduces ICa,L via direct interaction of PDE8B2 with the Cav1.2α1C subunit. Thus, upregulated PDE8B2 might serve as a novel molecular mechanism of the proarrhythmic reduction of ICa,L in cAF., Competing Interests: Conflict of interest The authors have no competing interests. R.B.S. reports grants from European Union Horizon 2020, the German Ministry of Research and Education, ERACOSysMed3 and personal fees from BMS/Pfizer, outside this work., (© The Author(s) 2023. Published by Oxford University Press on behalf of the European Society of Cardiology.)

The health benefits of regular physical exercise are well known. Even so, there is increasing evidence that the exercise regimes of elite athletes can evoke cardiac arrhythmias including ventricular fibrillation and even sudden cardiac death (SCD). The mechanism of exercise-induced arrhythmia and SCD is poorly understood. Here, we show that chronic training in a canine model (12 sedentary and 12 trained dogs) that mimics the regime of elite athletes induces electrophysiological remodeling (measured by ECG, patch-clamp, and immunocytochemical techniques) resulting in increases of both the trigger and the substrate for ventricular arrhythmias. Thus, 4 months sustained training lengthened ventricular repolarization (QTc: 237.1±3.4 ms vs. 213.6±2.8 ms, n=12; APD90: 472.8±29.6 ms vs. 370.1±32.7 ms, n=29 vs. 25), decreased transient outward potassium current (6.4±0.5 pA/pF vs. 8.8±0.9 pA/pF at 50 mV, n=54 vs. 42), and increased the short-term variability of repolarization (29.5±3.8 ms vs. 17.5±4.0 ms, n=27 vs. 18). Left ventricular fibrosis and HCN4 protein expression were also enhanced. These changes were associated with enhanced ectopic activity (number of escape beats from 0/hr to 29.7±20.3/hr) in vivo and arrhythmia susceptibility (elicited ventricular fibrillation: 3 of 10 sedentary dogs vs. 6 of 10 trained dogs). Our findings provide in vivo, cellular electrophysiological and molecular biological evidence for the enhanced susceptibility to ventricular arrhythmia in an experimental large animal model of endurance training., Competing Interests: AP, LT, NZ, NT, JP, ZK, SD, VD, PH, VV, GÁ, ZH, PG, JS, TÁ, MN, AS, NJ, LV, NN, IB, AF, AV No competing interests declared, (© 2023, Polyák, Topal et al.)

Long QT syndrome (LQTS) is an inherited cardiac rhythm disorder associated with increased incidence of cardiac arrhythmias and sudden death. LQTS type 5 (LQT5) is caused by dominant mutant variants of KCNE1, a regulatory subunit of the voltage-gated ion channels generating the cardiac potassium current I Ks . While mutant LQT5 KCNE1 variants are known to inhibit I Ks amplitudes in heterologous expression systems, cardiomyocytes from a transgenic rabbit LQT5 model displayed unchanged I Ks amplitudes, pointing towards the critical role of additional factors in the development of the LQT5 phenotype in vivo. In this study, we demonstrate that KCNE3, a candidate regulatory subunit of I Ks channels minimizes the inhibitory effects of LQT5 KCNE1 variants on I Ks amplitudes, while current deactivation is accelerated. Such changes recapitulate I Ks properties observed in LQT5 transgenic rabbits. We show that KCNE3 accomplishes this by displacing the KCNE1 subunit within the I Ks ion channel complex, as evidenced by a dedicated biophysical assay. These findings depict KCNE3 as an integral part of the I Ks channel complex that regulates I Ks function in cardiomyocytes and modifies the development of the LQT5 phenotype.

Even though rodents are accessible model animals, their electrophysiological properties are deeply different from those of humans, making the translation of rat studies to humans rather difficult. We compared the mechanisms of ventricular repolarization in various animal models to those of humans by measuring cardiac ventricular action potentials from ventricular papillary muscle preparations using conventional microelectrodes and applying selective inhibitors of various potassium transmembrane ion currents. Inhibition of the I K1 current (10 µmol/L barium chloride) significantly prolonged rat ventricular repolarization, but only slightly prolonged it in dogs, and did not affect it in humans. On the contrary, I Kr inhibition (50 nmol/L dofetilide) significantly prolonged repolarization in humans, rabbits, and dogs, but not in rats. Inhibition of the I Kur current (1 µmol/L XEN-D0101) only prolonged rat ventricular repolarization and had no effect in humans or dogs. Inhibition of the I Ks (500 nmol/L HMR-1556) and I to currents (100 µmol/L chromanol-293B) elicited similar effects in all investigated species. We conclude that dog ventricular preparations have the strongest translational value and rat ventricular preparations have the weakest translational value in cardiac electrophysiological experiments.

Aims: Volume overload (VO) induced hypertrophy is one of the hallmarks to the development of heart diseases. Understanding the compensatory mechanisms involved in this process might help preventing the disease progression., Methods and Results: Therefore, the present study used 2 months old Wistar rats, which underwent an aortocaval fistula to develop VO-induced hypertrophy. The animals were subdivided into four different groups, two sham operated animals served as age-matched controls and two groups with aortocaval fistula. Echocardiography was performed prior termination after 4- and 8-month. Functional and molecular changes of several sarcomeric proteins and their signalling pathways involved in the regulation and modulation of cardiomyocyte function were investigated., Results: The model was characterized with preserved ejection fraction in all groups and with elevated heart/body weight ratio, left/right ventricular and atrial weight at 4- and 8-month, which indicates VO-induced hypertrophy. In addition, 8-months groups showed increased left ventricular internal diameter during diastole, RV internal diameter, stroke volume and velocity-time index compared with their age-matched controls. These changes were accompanied by increased Ca 2+ sensitivity and titin-based cardiomyocyte stiffness in 8-month VO rats compared with other groups. The altered cardiomyocyte mechanics was associated with phosphorylation deficit of sarcomeric proteins cardiac troponin I, myosin binding protein C and titin, also accompanied with impaired signalling pathways involved in phosphorylation of these sarcomeric proteins in 8-month VO rats compared with age-matched control group. Impaired protein phosphorylation status and dysregulated signalling pathways were associated with significant alterations in the oxidative status of both kinases CaMKII and PKG explaining by this the elevated Ca 2+ sensitivity and titin-based cardiomyocyte stiffness and perhaps the development of hypertrophy., Conclusions: Our findings showed VO-induced cardiomyocyte dysfunction via deranged phosphorylation of myofilament proteins and signalling pathways due to increased oxidative state of CaMKII and PKG and this might contribute to the development of hypertrophy., (© 2022 The Authors. ESC Heart Failure published by John Wiley & Sons Ltd on behalf of European Society of Cardiology.)

Background and Purpose: The aim of the present study was to study the antiarrhythmic effects and cellular mechanisms of desethylamiodarone (DEA), the main metabolite of amiodarone (AMIO), following acute and chronic 4-week oral treatments (25-50 mg·kg -1 ·day -1 )., Experimental Approach: The antiarrhythmic effects of acute iv. (10 mg·kg -1 ) and chronic oral (4 weeks, 25 mg·kg -1 ·day -1 ) administration of DEA were assessed in carbachol and tachypacing-induced dog atrial fibrillation models. Action potentials were recorded from atrial and right ventricular tissue following acute (10 μM) and chronic (p.o. 4 weeks, 50 mg·kg -1 ·day -1 ) DEA application using the conventional microelectrode technique. Ionic currents were measured by the whole cell configuration of the patch clamp technique in isolated left ventricular myocytes. Pharmacokinetic studies were performed following a single intravenous dose (25 mg·kg -1 ) of AMIO and DEA intravenously and orally. In chronic (91-day) toxicological investigations, DEA and AMIO were administered in the oral dose of 25 mg·kg -1 ·day -1 )., Key Results: DEA exerted marked antiarrhythmic effects in both canine atrial fibrillation models. Both acute and chronic DEA administration prolonged action potential duration in atrial and ventricular muscle without any changes detected in Purkinje fibres. DEA decreased the amplitude of several outward potassium currents such as I Kr , I Ks , I K1 , I to , and I KACh , while the I CaL and late I Na inward currents were also significantly depressed. Better drug bioavailability and higher volume of distribution for DEA were observed compared to AMIO. No neutropenia and less severe pulmonary fibrosis was found following DEA compared to that of AMIO administration., Conclusion and Implications: Chronic DEA treatment in animal experiments has marked antiarrhythmic and electrophysiological effects with better pharmacokinetics and lower toxicity than its parent compound. These results suggest that the active metabolite, DEA, should be considered for clinical trials as a possible new, more favourable option for the treatment of cardiac arrhythmias including atrial fibrillation., (© 2022 The British Pharmacological Society.)

Aim: Long QT syndrome (LQTS) is a cardiac channelopathy predisposing to ventricular arrhythmias and sudden cardiac death. Since current therapies often fail to prevent arrhythmic events in certain LQTS subtypes, new therapeutic strategies are needed. Docosahexaenoic acid (DHA) is a polyunsaturated fatty acid, which enhances the repolarizing IKs current., Methods and Results: We investigated the effects of DHA in wild type (WT) and transgenic long QT Type 1 (LQT1; loss of IKs), LQT2 (loss of IKr), LQT5 (reduction of IKs), and LQT2-5 (loss of IKr and reduction of IKs) rabbits. In vivo ECGs were recorded at baseline and after 10 µM/kg DHA to assess changes in heart-rate corrected QT (QTc) and short-term variability of QT (STVQT). Ex vivo monophasic action potentials were recorded in Langendorff-perfused rabbit hearts, and action potential duration (APD75) and triangulation were assessed. Docosahexaenoic acid significantly shortened QTc in vivo only in WT and LQT2 rabbits, in which both α- and β-subunits of IKs-conducting channels are functionally intact. In LQT2, this led to a normalization of QTc and of its short-term variability. Docosahexaenoic acid had no effect on QTc in LQT1, LQT5, and LQT2-5. Similarly, ex vivo, DHA shortened APD75 in WT and normalized it in LQT2, and additionally decreased AP triangulation in LQT2., Conclusions: Docosahexaenoic acid exerts a genotype-specific beneficial shortening/normalizing effect on QTc and APD75 and reduces pro-arrhythmia markers STVQT and AP triangulation through activation of IKs in LQT2 rabbits but has no effects if either α- or β-subunits to IKs are functionally impaired. Docosahexaenoic acid could represent a new genotype-specific therapy in LQT2., (Published on behalf of the European Society of Cardiology. All rights reserved. © The Author(s) 2021. For permissions, please email: journals.permissions@oup.com.)

In the present study, the effect of long-term exercise training was investigated on myocardial morphological and functional remodeling and on proarrhythmic sensitivity in a rabbit athlete's heart model. New-Zealand white rabbits were trained during a 12-week long treadmill running protocol and compared with their sedentary controls. At the end of the training protocol, echocardiography, in vivo and in vitro ECG recordings, proarrhythmic sensitivity with dofetilide (nM) were performed in isolated hearts, and action potential duration (APD) measurements at different potassium concentrations (4.5 and 2 mM) were made in the isolated papillary muscles. Expression levels of the slow component of delayed rectifier potassium current and fibrosis synthesis and degradation biomarkers were quantified. Echocardiography showed a significantly dilated left ventricle in the running rabbits. ECG PQ and RR intervals were significantly longer in the exercised group (79 ± 2 vs. 69 ± 2 ms and 325 ± 11 vs. 265 ± 6 ms, p < 0.05, respectively). The in vivo heart rate variability (HRV) (SD of root mean square: 5.2 ± 1.4 ms vs. 1.4 ± 0.2 ms, p < 0.05) and Tpeak-Tend variability were higher in the running rabbits. Bradycardia disappeared in the exercised group in vitro . Dofetilide tended to increase the QTc interval in a greater extent, and significantly increased the number of arrhythmic beats in the trained animals in vitro . APD was longer in the exercised group at a low potassium level. Real-time quantitative PCR (RT-qPCR) showed significantly greater messenger RNA expression of fibrotic biomarkers in the exercised group. Increased repolarization variability and higher arrhythmia incidences, lengthened APD at a low potassium level, increased fibrotic biomarker gene expressions may indicate higher sensitivity of the rabbit "athlete's heart" to life-threatening arrhythmias., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Kui, Polyák, Morvay, Tiszlavicz, Nagy, Ördög, Takács, Leprán, Farkas, Papp, Jost, Varró, Baczkó and Farkas.)

Chronic heart failure is a clinical syndrome with multiple etiologies, associated with significant morbidity and mortality. Cardiac arrhythmias, including ventricular tachyarrhythmias and atrial fibrillation, are common in heart failure. A number of cardiac diseases including heart failure alter the expression and regulation of ion channels and transporters leading to arrhythmogenic electrical remodeling. Myocardial hypertrophy, fibrosis and scar formation are key elements of arrhythmogenic structural remodeling in heart failure. In this article, the mechanisms responsible for increased arrhythmia susceptibility as well as the underlying changes in ion channel, transporter expression and function as well as alterations in calcium handling in heart failure are discussed. Understanding the mechanisms of arrhythmogenic remodeling is key to improving arrhythmia management and the prevention of sudden cardiac death in patients with heart failure.

Introduction: The prevalence of neuropathic lesions in young patients with type 1 diabetes mellitus (T1DM) at the time of transition from pediatric care to adult-oriented diabetes care is poorly studied. A comparative study with healthy volunteers to assess the possible neuropathic condition of this special population and to identify the potential early screening needs has not been performed yet. The results may provide important feedback to pediatric diabetes care and a remarkable baseline reference point for further follow up in adult diabetes care., Patients and Methods: Twenty-nine young patients with T1DM [age: 22.4 ± 2.9 years; HbA1c: 8.5 ± 2.1%, diabetes duration: 12.2 ± 5.8 years; (mean ± SD)] and 30 healthy volunteers (age: 21.5 ± 1.6 years; HbA1c: 5.3 ± 0.3%) were involved in the study. Autonomic function was assessed by standard cardiovascular reflex tests. Complex peripheral neuropathic testing was performed by Neurometer ® , Neuropad ® -test, Tiptherm ® , Monofilament ® , and Rydel-Seiffer tuning fork tests., Results: T1DM patients had significantly higher diastolic blood pressure than controls (80 ± 9 vs. 74 ± 8 mmHg, p < 0.01), but there was no significant difference in systolic blood pressure (127 ± 26 vs. 121 ± 13 mmHg). Cardiovascular reflex tests had not revealed any significant differences between the T1DM patients and controls. No significant differences with Neurometer ® , Neuropad ® -test, and Monofilament ® were detected between the two groups. The vibrational sensing on the radius on both sides was significantly impaired in the T1DM group compared to the controls with Rydel-Seiffer tuning fork test (right: 7.5 ± 1.0 vs. 7.9 ± 0.3; left: 7.5 ± 0.9 vs. 7.9 ± 0.3, p < 0.05). The Tiptherm ® -test also identified a significant impairment in T1DM patients (11 sensing failures vs. 1, p < 0.001). In addition, the neuropathic complaints were significantly more frequently present in the T1DM patient group than in the controls (9 vs. 0, p < 0.01)., Conclusion: In this young T1DM population, cardiovascular autonomic neuropathy and cardiac morphological alterations could not be found. However, Rydel-Seiffer tuning fork and Tiptherm ® -tests revealed peripheral sensory neurological impairments in young T1DM patients at the time of their transition to adult diabetes care., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Vágvölgyi, Maróti, Szűcs, Póczik, Urbán-Pap, Baczkó, Nemes, Csajbók, Sepp, Kempler, Orosz, Várkonyi and Lengyel.)

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 Ca 2+ current (I Ca ), late Na + current (I Na-late ), rapid and slow components of the delayed rectifier K + current (I Kr and I Ks , respectively), inward rectifier K + current (I K1 ), transient outward K + current (I to1 ), and Na + /Ca 2+ exchange current (I NCX ) 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 I K1 and the recovery kinetics of I to . I K1 displayed a largely four-fold larger density in canine than human myocytes, and I to 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 I K1 , the repolarization reserve is more pronounced in canine cells, and moderate differences in the frequency-dependent repolarization patterns can also be anticipated.

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

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 (I Kr ) and slow (I Ks ) delayed rectifier, the transient outward (I to ) and inward rectifier (I K1 ) 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 I Kr , I Ks and I to (only in dog) significantly with corresponding estimated EC 50 values of 4.9, 3.1 and 5 µM, respectively, without changing I K1 . Although the EC 50 value of CBD was found to be higher than literary C max 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.

Aim: To obtain a quantitative expression profile of the main genes involved in the cAMP-signaling cascade in human control atria and in different cardiac pathologies., Methods and Results: Expression of 48 target genes playing a relevant role in the cAMP-signaling cascade was assessed by RT-qPCR. 113 samples were obtained from right atrial appendages (RAA) of patients in sinus rhythm (SR) with or without atrium dilation, paroxysmal atrial fibrillation (AF), persistent AF or heart failure (HF); and left atrial appendages (LAA) from patients in SR or with AF. Our results show that right and left atrial appendages in donor hearts or from SR patients have similar expression values except for AC7 and PDE2A. Despite the enormous chamber-dependent variability in the gene-expression changes between pathologies, several distinguishable patterns could be identified. PDE8A, PI3Kγ and EPAC2 were upregulated in AF. Different phosphodiesterase (PDE) families showed specific pathology-dependent changes., Conclusion: By comparing mRNA-expression patterns of the cAMP-signaling cascade related genes in right and left atrial appendages of human hearts and across different pathologies, we show that 1) gene expression is not significantly affected by cardioplegic solution content, 2) it is appropriate to use SR atrial samples as controls, and 3) many genes in the cAMP-signaling cascade are affected in AF and HF but only few of them appear to be chamber (right or left) specific., Topic: Genetic changes in human diseased atria., Translational Perspective: The cyclic AMP signaling pathway is important for atrial function. However, expression patterns of the genes involved in the atria of healthy and diseased hearts are still unclear. We give here a general overview of how different pathologies affect the expression of key genes in the cAMP signaling pathway in human right and left atria appendages. Our study may help identifying new genes of interest as potential therapeutic targets or clinical biomarkers for these pathologies and could serve as a guide in future gene therapy studies., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Enhancement of the late Na + current (I NaL ) increases arrhythmia propensity in the heart, while suppression of the current is antiarrhythmic. GS967 is an agent considered as a selective blocker of I NaL . In the present study, effects of GS967 on I NaL and action potential (AP) morphology were studied in canine ventricular myocytes by using conventional voltage clamp, action potential voltage clamp and sharp microelectrode techniques. The effects of GS967 (1 µM) were compared to those of the class I/B antiarrhythmic compound mexiletine (40 µM). Under conventional voltage clamp conditions, I NaL was significantly suppressed by GS967 and mexiletine, causing 80.4 ± 2.2% and 59.1 ± 1.8% reduction of the densities of I NaL measured at 50 ms of depolarization, and 79.0 ± 3.1% and 63.3 ± 2.7% reduction of the corresponding current integrals, respectively. Both drugs shifted the voltage dependence of the steady-state inactivation curve of I NaL towards negative potentials. GS967 and mexiletine dissected inward I NaL profiles under AP voltage clamp conditions having densities, measured at 50% of AP duration (APD), of -0.37 ± 0.07 and -0.28 ± 0.03 A/F, and current integrals of -56.7 ± 9.1 and -46.6 ± 5.5 mC/F, respectively. Drug effects on peak Na + current (I NaP ) were assessed by recording the maximum velocity of AP upstroke (V + max ) in multicellular preparations. The offset time constant was threefold faster for GS967 than mexiletine (110 ms versus 289 ms), while the onset of the rate-dependent block was slower in the case of GS967. Effects on beat-to-beat variability of APD was studied in isolated myocytes. Beat-to-beat variability was significantly decreased by both GS967 and mexiletine (reduction of 42.1 ± 6.5% and 24.6 ± 12.8%, respectively) while their shortening effect on APD was comparable. It is concluded that the electrophysiological effects of GS967 are similar to those of mexiletine, but with somewhat faster offset kinetics of V + and I max at the same concentration, the current view that GS967 represents a new class of drugs that selectively block I + has to be questioned and it is suggested that GS967 should be classified as a class I/B antiarrhythmic agent.max and I NaL at the same concentration, the current view that GS967 represents a new class of drugs that selectively block I NaL has to be questioned and it is suggested that GS967 should be classified as a class I/B antiarrhythmic agent.

In search of more efficacious and safe pharmacological treatments for atrial fibrillation (AF), atria-selective antiarrhythmic agents have been promoted that target ion channels principally expressed in the atria. This concept allows one to engage antiarrhythmic effects in atria, but spares the ventricles from potentially proarrhythmic side effects. It has been suggested that cardiac small conductance Ca 2+ -activated K + (SK) channels may represent an atria-selective target in mammals including humans. However, there are conflicting data concerning the expression of SK channels in different stages of AF, and recent findings suggest that SK channels are upregulated in ventricular myocardium when patients develop heart failure. To address this issue, RNA-sequencing was performed to compare expression levels of three SK channels ( KCNN1 , KCNN2 , and KCNN3 ) in human atrial and ventricular tissue samples from transplant donor hearts (no cardiac disease), and patients with cardiac disease in sinus rhythm or with AF. In addition, for control purposes expression levels of several genes known to be either chamber-selective or differentially expressed in AF and heart failure were determined. In atria, as compared to ventricle from transplant donor hearts, we confirmed higher expression of KCNN1 and KCNA5 , and lower expression of KCNJ2 , whereas KCNN2 and KCNN3 were statistically not differentially expressed. Overall expression of KCNN1 was low compared to KCNN2 and KCNN3 . Comparing atrial tissue from patients with AF to sinus rhythm samples we saw downregulation of KCNN2 in AF, as previously reported. When comparing ventricular tissue from heart failure patients to non-diseased samples, we found significantly increased ventricular expression of KCNN3 in heart failure, as previously published. The other channels showed no significant difference in expression in either disease. Our results add weight to the view that SK channels are not likely to be an atria-selective target, especially in failing human hearts, and modulators of these channels may prove to have less utility in treating AF than hoped. Whether targeting SK1 holds potential remains to be elucidated., Competing Interests: TN, MS, and SR were employed by Amgen Inc., at the time of submission of the manuscript. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Darkow, Nguyen, Stolina, Kari, Schmidt, Wiedmann, Baczkó, Kohl, Rajamani, Ravens and Peyronnet.)

Cardiovascular diseases are the leading causes of mortality. Sudden cardiac death is most commonly caused by ventricular fibrillation (VF). Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia and a major cause of stroke and heart failure. Pharmacological management of VF and AF remains suboptimal due to limited efficacy of antiarrhythmic drugs and their ventricular proarrhythmic adverse effects. In this study, the antiarrhythmic and cardiac cellular electrophysiological effects of SZV-270, a novel compound, were investigated in rabbit and canine models. SZV-270 significantly reduced the incidence of VF in rabbits subjected to coronary artery occlusion/reperfusion and reduced the incidence of burst-induced AF in a tachypaced conscious canine model of AF. SZV-270 prolonged the frequency-corrected QT interval, lengthened action potential duration and effective refractory period in ventricular and atrial preparations, blocked I Kr ) at cycle lengths smaller than 1000 ms in ventricular preparations (Class I/B effect). Importantly, SZV-270 did not provoke Torsades de Pointes arrhythmia in an anesthetized rabbit proarrhythmia model characterized by impaired repolarization reserve. In conclusion, SZV-270 with its combined Class I/B and III effects can prevent reentry arrhythmias with reduced risk of provoking drug-induced Torsades de Pointes.V max ) at cycle lengths smaller than 1000 ms in ventricular preparations (Class I/B effect). Importantly, SZV-270 did not provoke Torsades de Pointes arrhythmia in an anesthetized rabbit proarrhythmia model characterized by impaired repolarization reserve. In conclusion, SZV-270 with its combined Class I/B and III effects can prevent reentry arrhythmias with reduced risk of provoking drug-induced Torsades de Pointes.

Increased transmural dispersion of repolarization is an established contributing factor to ventricular tachyarrhythmias. In this study, we evaluated the effect of chronic amiodarone treatment and acute administration of dofetilide in canine cardiac preparations containing electrotonically coupled Purkinje fibers (PFs) and ventricular muscle (VM) and compared the effects to those in uncoupled PF and VM preparations using the conventional microelectrode technique. Dispersion between PFs and VM was inferred from the difference in the respective action potential durations (APDs). In coupled preparations, amiodarone decreased the difference in APDs between PFs and VM, thus decreasing dispersion. In the same preparations, dofetilide increased the dispersion by causing a more pronounced prolongation in PFs. This prolongation was even more emphasized in uncoupled PF preparations, while the effect in VM was the same. In uncoupled preparations, amiodarone elicited no change on the difference in APDs. In conclusion, amiodarone decreased the dispersion between PFs and VM, while dofetilide increased it. The measured difference in APD between cardiac regions may be the affected by electrotonic coupling; thus, studying PFs and VM separately may lead to an over- or underestimation of dispersion.

Sudden cardiac death among top athletes is very rare, however, it is 2-4 times more frequent than in the age-matched control population. In the present study, the electrophysiological consequences of long-term exercise training were investigated on Ca 2+ homeostasis and ventricular repolarization, together with the underlying alterations of ion channel expression, in a rat athlete's heart model. 12-week swimming exercise-trained and control Wistar rats were used. Electrophysiological data were obtained by using ECG, patch clamp and fluorescent optical measurements. Protein and mRNA levels were determined by the Western immunoblot and qRT-PCR techniques. Animals in the trained group exhibited significantly lower resting heart rate, higher incidence of extrasystoles and spontaneous Ca 2+ release events. The Ca 2+ content of the sarcoplasmic reticulum (SR) and the Ca 2+ transient amplitude were significantly larger in the trained group. Intensive physical training is associated with elevated SR Ca 2+ content, which could be an important part of physiological cardiac adaptation mechanism to training. However, it may also sensitize the heart for the development of spontaneous Ca 2+ release and extrasystoles. Training-associated remodeling may promote elevated incidence of life threatening arrhythmias in top athletes.

Background and Purpose: Reliable prediction of pro-arrhythmic side effects of novel drug candidates is still a major challenge. Although drug-induced pro-arrhythmia occurs primarily in patients with pre-existing repolarisation disturbances, healthy animals are employed for pro-arrhythmia testing. To improve current safety screening, transgenic long QT (LQTS) rabbit models with impaired repolarisation reserve were generated by overexpressing loss-of-function mutations of human HERG (HERG-G628S, loss of I Kr ; LQT2), KCNE1 (KCNE1-G52R, decreased I Ks ; LQT5), or both transgenes (LQT2-5) in the heart., Experimental Approach: Effects of K + channel blockers on cardiac repolarisation and arrhythmia susceptibility were assessed in healthy wild-type (WT) and LQTS rabbits using in vivo ECG and ex vivo monophasic action potential and ECG recordings in Langendorff-perfused hearts., Key Results: LQTS models reflect patients with clinically "silent" (LQT5) or "manifest" (LQT2 and LQT2-5) impairment in cardiac repolarisation reserve: they were more sensitive in detecting I Kr -blocking (LQT5) or I K1 /I Ks -blocking (LQT2 and LQT2-5) properties of drugs compared to healthy WT animals. Impaired QT-shortening capacity at fast heart rates was observed due to disturbed I Ks function in LQT5 and LQT2-5. Importantly, LQTS models exhibited higher incidence, longer duration, and more malignant types of ex vivo arrhythmias than WT., Conclusion and Implications: LQTS models represent patients with reduced repolarisation reserve due to different pathomechanisms. As they demonstrate increased sensitivity to different specific ion channel blockers (I Kr blockade in LQT5 and I K1 and I Ks blockade in LQT2 and LQT2-5), their combined use could provide more reliable and more thorough prediction of (multichannel-based) pro-arrhythmic potential of novel drug candidates., (© 2020 The British Pharmacological Society.)

Cardiovascular diseases, including coronary artery disease, ischemic heart diseases such as acute myocardial infarction and postischemic heart failure, heart failure of other etiologies, and cardiac arrhythmias, belong to the leading causes of death. Activation of capsaicin-sensitive sensory nerves by the transient receptor potential vanilloid 1 (TRPV1) capsaicin receptor and other receptors, as well as neuropeptide mediators released from them upon stimulation, play important physiological regulatory roles. Capsaicin-sensitive sensory nerves also contribute to the development and progression of some cardiac diseases, as well as to mechanisms of endogenous stress adaptation leading to cardioprotection. In this review, we summarize the role of capsaicin-sensitive afferents and the TRPV1 ion channel in physiological and pathophysiological functions of the heart based mainly on experimental results and show their diagnostic or therapeutic potentials. Although the actions of several other channels or receptors expressed on cardiac sensory afferents and the effects of TRPV1 channel activation on different non-neural cell types in the heart are not precisely known, most data suggest that stimulation of the TRPV1-expressing sensory nerves or stimulation/overexpression of TRPV1 channels have beneficial effects in cardiac diseases.

Drug-induced proarrhythmia constitutes a potentially lethal side effect of various drugs. Most often, this proarrhythmia is mechanistically linked to the drug's potential to interact with repolarizing cardiac ion channels causing a prolongation of the QT interval in the ECG. Despite sophisticated screening approaches during drug development, reliable prediction of proarrhythmia remains very challenging. Although drug-induced long-QT-related proarrhythmia is often favored by conditions or diseases that impair the individual's repolarization reserve, most cellular, tissue, and whole animal model systems used for drug safety screening are based on normal, healthy models. In recent years, several transgenic rabbit models for different types of long QT syndromes (LQTS) with differences in the extent of impairment in repolarization reserve have been generated. These might be useful for screening/prediction of a drug's potential for long-QT-related proarrhythmia, particularly as different repolarizing cardiac ion channels are impaired in the different models. In this review, we summarize the electrophysiological characteristics of the available transgenic LQTS rabbit models, and the pharmacological proof-of-principle studies that have been performed with these models-highlighting the advantages and disadvantages of LQTS models for proarrhythmia research. In the end, we give an outlook on potential future directions and novel models., (Copyright © 2020 Baczkó, Hornyik, Brunner, Koren and Odening.)

Introduction: Re-entry is a basic mechanism of ventricular fibrillation, which can be elicited by extrasystolic activity, but the timing of an extrasystole can be critical. The action potential duration (APD) of an extrasystole depends on the proximity of the preceding beat, and the relation between its timing and its APD is called electrical restitution. The aim of the present work was to study and compare the effect of several antiarrhythmic drugs on restitution in preparations from undiseased human ventricular muscle, and other mammalian species., Methods: Action potentials were recorded in preparations obtained from rat, guinea pig, rabbit, and dog hearts; and from undiseased human donor hearts using the conventional microelectrode technique. Preparations were stimulated with different basic cycle lengths (BCLs) ranging from 300 to 5,000 ms. To study restitution, single test pulses were applied at every 20th beat while the preparation was driven at 1,000 ms BCL., Results: Marked differences were found between the animal and human preparations regarding restitution and steady-state frequency dependent curves. In human ventricular muscle, restitution kinetics were slower in preparations with large phase 1 repolarization with shorter APDs at 1000 ms BCL compared to preparations with small phase 1. Preparations having APD longer than 300 ms at 1000 ms BCL had slower restitution kinetics than those having APD shorter than 250 ms. The selective I Kr inhibitors E-4031 and sotalol increased overall APD and slowed the restitution kinetics, while I Ks inhibition did not influence APD and electrical restitution. Mexiletine and nisoldipine shortened APD, but only mexiletine slowed restitution kinetics., Discussion: Frequency dependent APD changes, including electrical restitution, were partly determined by the APD at the BCL. Small phase 1 associated with slower restitution suggests a role of I to in restitution. APD prolonging drugs slowed restitution, while mexiletine, a known inhibitor of I Na , shortened basic APD but also slowed restitution. These results indicate that although basic APD has an important role in restitution, other transmembrane currents, such as I Na or I to , can also affect restitution kinetics. This raises the possibility that ion channel modifier drugs slowing restitution kinetics may have antiarrhythmic properties by altering restitution., (Copyright © 2020 Árpádffy-Lovas, Baczkó, Baláti, Bitay, Jost, Lengyel, Nagy, Takács, Varró and Virág.)

Cardiac adverse effects are among the leading causes of the discontinuation of clinical trials and the withdrawal of drugs from the market. The novel concept of 'hidden cardiotoxicity' is defined as cardiotoxicity of a drug that manifests in the diseased (e.g. ischemic/reperfused), but not in the healthy heart or as a drug-induced deterioration of cardiac stress adaptation (e.g. ischemic conditioning). Here, we aimed to test if the cardiotoxicity of a selective COX-2 inhibitor rofecoxib that was revealed during its clinical use, i.e., increased occurrence of proarrhythmic and thrombotic events, could have been revealed in early phases of drug development by using preclinical models of ischemia/reperfusion (I/R) injury. Rats that were treated with rofecoxib or vehicle for four weeks were subjected to 30 min. coronary artery occlusion and 120 min. reperfusion with or without cardioprotection that is induced by ischemic preconditioning (IPC). Rofecoxib increased overall the arrhythmias including ventricular fibrillation (VF) during I/R. The proarrhythmic effect of rofecoxib during I/R was not observed in the IPC group. Rofecoxib prolonged the action potential duration (APD) in isolated papillary muscles, which was not seen in the simulated IPC group. Interestingly, while showing hidden cardiotoxicity manifested as a proarrhythmic effect during I/R, rofecoxib decreased the infarct size and increased the survival of adult rat cardiac myocytes that were subjected to simulated I/R injury. This is the first demonstration that rofecoxib increased acute mortality due to its proarrhythmic effect via increased APD during I/R. Rofecoxib did not interfere with the cardiprotective effect of IPC; moreover, IPC was able to protect against rofecoxib-induced hidden cardiotoxicity. These results show that cardiac safety testing with simple preclinical models of I/R injury uncovers hidden cardiotoxicity of rofecoxib and might reveal the hidden cardiotoxicity of other drugs., Competing Interests: The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. P.F. is the founder and CEO of Pharmahungary, a group of R&D companies.

Although late sodium current (I Na-late ) has long been known to contribute to plateau formation of mammalian cardiac action potentials, lately it was considered as possible target for antiarrhythmic drugs. However, many aspects of this current are still poorly understood. The present work was designed to study the true profile of I Na-late in canine and guinea pig ventricular cells and compare them to I Na-late recorded in undiseased human hearts. I Na-late was defined as a tetrodotoxin-sensitive current, recorded under action potential voltage clamp conditions using either canonic- or self-action potentials as command signals. Under action potential voltage clamp conditions the amplitude of canine and human I Na-late monotonically decreased during the plateau (decrescendo-profile), in contrast to guinea pig, where its amplitude increased during the plateau (crescendo profile). The decrescendo-profile of canine I Na-late could not be converted to a crescendo-morphology by application of ramp-like command voltages or command action potentials recorded from guinea pig cells. Conventional voltage clamp experiments revealed that the crescendo I Na-late profile in guinea pig was due to the slower decay of I Na-late in this species. When action potentials were recorded from multicellular ventricular preparations with sharp microelectrode, action potentials were shortened by tetrodotoxin, which effect was the largest in human, while smaller in canine, and the smallest in guinea pig preparations. It is concluded that important interspecies differences exist in the behavior of I Na-late . At present canine myocytes seem to represent the best model of human ventricular cells regarding the properties of I Na-late . These results should be taken into account when pharmacological studies with I Na-late are interpreted and extrapolated to human. Accordingly, canine ventricular tissues or myocytes are suggested for pharmacological studies with I Na-late inhibitors or modifiers. Incorporation of present data to human action potential models may yield a better understanding of the role of I Na-late in action potential morphology, arrhythmogenesis, and intracellular calcium dynamics., Competing Interests: Declaration of Competing Interest None declared., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Unexpected cardiac adverse effects are the leading causes of discontinuation of clinical trials and withdrawal of drugs from the market. Since the original observations in the mid-90s, it has been well established that cardiovascular risk factors and comorbidities (such as ageing, hyperlipidaemia, and diabetes) and their medications (e.g. nitrate tolerance, adenosine triphosphate-dependent potassium inhibitor antidiabetic drugs, statins, etc.) may interfere with cardiac ischaemic tolerance and endogenous cardioprotective signalling pathways. Indeed drugs may exert unwanted effects on the diseased and treated heart that is hidden in the healthy myocardium. Hidden cardiotoxic effects may be due to (i) drug-induced enhancement of deleterious signalling due to ischaemia/reperfusion injury and/or the presence of risk factors and/or (ii) inhibition of cardioprotective survival signalling pathways, both of which may lead to ischaemia-related cell death and/or pro-arrhythmic effects. This led to a novel concept of 'hidden cardiotoxicity', defined as cardiotoxity of a drug that manifests only in the diseased heart with e.g. ischaemia/reperfusion injury and/or in the presence of its major comorbidities. Little is known on the mechanism of hidden cardiotoxocity, moreover, hidden cardiotoxicity cannot be revealed by the routinely used non-clinical cardiac safety testing methods on healthy animals or tissues. Therefore, here, we emphasize the need for development of novel cardiac safety testing platform involving combined experimental models of cardiac diseases (especially myocardial ischaemia/reperfusion and ischaemic conditioning) in the presence and absence of major cardiovascular comorbidities and/or cotreatments., (© The Author(s) 2018. Published by Oxford University Press on behalf of the European Society of Cardiology.)

Sudden cardiac death in athletes is rare and most often unexpectable. For a better understanding of cardiac remodeling, this study presents the effects of chronic vigorous exercise on cardiac structure and electrophysiology in new rabbit and dog athlete's heart models. Rabbits and dogs were randomized into sedentary ('Sed'), exercised (subjected to 16 weeks chronic treadmill exercise ('Ex') groups, and a testosterone-treated ('Dop') group in dogs. Echocardiography and electrocardiogram were performed. Proarrhythmic sensitivity and autonomic responses were tested in conscious dogs. 'Ex' animals exhibited left ventricular enlargement with bradycardia (mean RR in 'Ex' vs. 'Sed' rabbits: 335 ± 15 vs. 288 ±19 ms, p ≤ 0.05, and in 'Dop' vs. 'Ex' vs. 'Sed' dogs: 718 ± 6 vs. 638 ± 38 vs. 599 ± 49 ms) accompanied by an increase of heart rate variability in both species (e.g. SD RR in 'Ex' vs. 'Sed' rabbits: 3.4 ± 0.9 vs. 1.4 ± 0.1 ms, p ≤ 0.05, and in 'Dop' vs. 'Ex' vs. 'Sed' dogs: 156 ± 59 vs. 163 ± 44 vs. 111 ± 49 ms) indicating an increased vagal tone. A lower response to parasympatholytic agent atropine and more pronounced QTc interval lengthening after dofetilide challenge were found in 'Ex' and 'Dop' dogs compared to the 'Sed' group. No morphological and functional changes were found after chronic steroid treatment in dogs. The structural-functional findings share more similarities with human athlete's heart. Slight repolarization sensitivity in the exercised dogs may indicate an increased risk of arrhythmias in athletes under different circumstances. These animal models might be useful for the further investigations of the cardiovascular effects of competitive training., Competing Interests: The authors declare that there is no conflict of interest., (© 2018 Polyák et al. Published by IMR press. All rights reserved.)

Background: Myocardial metabolic impairment is a major feature in chronic heart failure. As the major coenzyme in fuel oxidation and oxidative phosphorylation and a substrate for enzymes signaling energy stress and oxidative stress response, nicotinamide adenine dinucleotide (NAD + ) is emerging as a metabolic target in a number of diseases including heart failure. Little is known on the mechanisms regulating homeostasis of NAD + in the failing heart., Methods: To explore possible alterations of NAD + homeostasis in the failing heart, we quantified the expression of NAD + biosynthetic enzymes in the human failing heart and in the heart of a mouse model of dilated cardiomyopathy (DCM) triggered by Serum Response Factor transcription factor depletion in the heart (SRF HKO ) or of cardiac hypertrophy triggered by transverse aorta constriction. We studied the impact of NAD + precursor supplementation on cardiac function in both mouse models., Results: We observed a 30% loss in levels of NAD + in the murine failing heart of both DCM and transverse aorta constriction mice that was accompanied by a decrease in expression of the nicotinamide phosphoribosyltransferase enzyme that recycles the nicotinamide precursor, whereas the nicotinamide riboside kinase 2 (NMRK2) that phosphorylates the nicotinamide riboside precursor is increased, to a higher level in the DCM (40-fold) than in transverse aorta constriction (4-fold). This shift was also observed in human failing heart biopsies in comparison with nonfailing controls. We show that the Nmrk2 gene is an AMP-activated protein kinase and peroxisome proliferator-activated receptor α responsive gene that is activated by energy stress and NAD + depletion in isolated rat cardiomyocytes. Nicotinamide riboside efficiently rescues NAD + synthesis in response to FK866-mediated inhibition of nicotinamide phosphoribosyltransferase and stimulates glycolysis in cardiomyocytes. Accordingly, we show that nicotinamide riboside supplementation in food attenuates the development of heart failure in mice, more robustly in DCM, and partially after transverse aorta constriction, by stabilizing myocardial NAD + levels in the failing heart. Nicotinamide riboside treatment also robustly increases the myocardial levels of 3 metabolites, nicotinic acid adenine dinucleotide, methylnicotinamide, and N1-methyl-4-pyridone-5-carboxamide, that can be used as validation biomarkers for the treatment., Conclusions: The data show that nicotinamide riboside, the most energy-efficient among NAD precursors, could be useful for treatment of heart failure, notably in the context of DCM, a disease with few therapeutic options., (© 2017 American Heart Association, Inc.)

Aims: Quantitative real-time RT-PCR (RT-qPCR) has become the method of choice for mRNA quantification, but requires an accurate normalization based on the use of reference genes showing invariant expression across various pathological conditions. Only few data exist on appropriate reference genes for the human heart. The objective of this study was to determine a set of suitable reference genes in human atrial and ventricular tissues, from right and left cavities in control and in cardiac diseases., Methods and Results: We assessed the expression of 16 reference genes (ACTB, B2M, GAPDH, GUSB, HMBS, HPRT1, IPO8, PGK1, POLR2A, PPIA, RPLP0, TBP, TFRC, UBC, YWHAZ, 18S) in tissues from: right and left ventricles from healthy controls and heart failure (HF) patients; right-atrial tissue from patients in sinus rhythm with (SRd) or without (SRnd) atrial dilatation, patients with paroxysmal (pAF) or chronic (cAF) atrial fibrillation or with HF; and left-atrial tissue from patients in SR or cAF. Consensual analysis (by geNorm and Normfinder algorithms, BestKeeper software tool and comparative delta-Ct method) of the variability scores obtained for each reference gene expression shows that the most stably expressed genes are: GAPDH, GUSB, IPO8, POLR2A, and YWHAZ when comparing either right and left ventricle or ventricle from healthy controls and HF patients; GAPDH, IPO8, POLR2A, PPIA, and RPLP0 when comparing either right and left atrium or right atria from all pathological groups. ACTB, TBP, TFRC, and 18S genes were identified as the least stable., Conclusions: The overall most stable reference genes across different heart cavities and disease conditions were GAPDH, IPO8, POLR2A and PPIA. YWHAZ or GUSB could be added to this set for some specific experiments. This study should provide useful guidelines for reference gene selection in RT-qPCR studies in human heart., (Published on behalf of the European Society of Cardiology. All rights reserved. © The Author 2017. For Permissions, please email: journals.permissions@oup.com.)