Key points• Cardiac repolarization, through which heart-cells return to their resting state after having fired, is a delicate process, susceptible to disruption by common drugs and clinical conditions. • Animal models, particularly the dog, are often used to study repolarization properties and responses to drugs, with the assumption that such findings are relevant to humans. However, little is known about the applicability of findings in animals to man.• Here, we studied the contribution of various ion-currents to cardiac repolarization in canine and human ventricle.• Humans showed much greater repolarization-impairing effects of drugs blocking the rapid delayed-rectifier current I Kr than dogs, because of lower repolarization-reserve contributions from two other important repolarizing currents (the inward-rectifier I K1 and slow delayed-rectifier I Ks ).• Our findings clarify differences in cardiac repolarization-processes among species, highlighting the importance of caution when extrapolating results from animal models to man.Abstract 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 I Kr block (50-100 nmol l −1 dofetilide) lengthened AP duration at 90% of repolarization (APD 90 ) >3-fold more in human than dog, suggesting smaller repolarization reserve in humans. Selective I K1 block (10 μmol l −1 BaCl 2 ) and I Ks block (1 μmol l −1 HMR-1556) increased APD 90 more in canine than human right ventricular papillary muscle. Ion current measurements in isolated cardiomyocytes showed that I K1 and I Ks densities were 3-and 4.5-fold larger in dogs than humans, respectively. I Kr density and kinetics were similar in human versus dog. I Ca and I to were respectively ∼30% larger and ∼29% smaller in human, and Na + -Ca 2+ exchange current was comparable. Cardiac mRNA levels for the main I K1 ion channel subunit Kir2.1 and the I Ks accessory subunit minK were significantly lower, but mRNA expression of ERG and KvLQT1 (I Kr and I Ks 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. I K1 and I Ks inhibition increased the APD-prolonging effect of I Kr 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 I K1 and I Ks in repolarization reserve differences. Thus, humans show greater repolarization-delaying effects of I Kr block than dogs, because of lower repolarization reserve contributions from I K1 and I Ks , emphasizing species-specific determinants of repolarization and the limitations of animal models fo...

BACKGROUND AND PURPOSEAt present there are no small molecule inhibitors that show strong selectivity for the Na + /Ca 2+ exchanger (NCX). Hence, we studied the electrophysiological effects of acute administration of ORM-10103, a new NCX inhibitor, on the NCX and L-type Ca 2+ currents and on the formation of early and delayed afterdepolarizations. EXPERIMENTAL APPROACHIon currents were recorded by using a voltage clamp technique in canine single ventricular cells, and action potentials were obtained from canine and guinea pig ventricular preparations with the use of microelectrodes. KEY RESULTSORM-10103 significantly reduced both the inward and outward NCX currents. Even at a high concentration (10 μM), ORM-10103 did not significantly change the L-type Ca 2+ current or the maximum rate of depolarization (dV/dtmax), indicative of the fast inward Na + current. At 10 μM ORM-10103 did not affect the amplitude or the dV/dtmax of the slow response action potentials recorded from guinea pig papillary muscles, which suggests it had no effect on the L-type Ca 2+ current. ORM-10103 did not influence the Na + /K + pump or the main K + currents of canine ventricular myocytes, except the rapid delayed rectifier K + current, which was slightly diminished by the drug at 3 μM. The amplitudes of pharmacologically-induced early and delayed afterdepolarizations were significantly decreased by ORM-10103 (3 and 10 μM) in a concentration-dependent manner. CONCLUSIONS AND IMPLICATIONSORM-10103 is a selective inhibitor of the NCX current and can abolish triggered arrhythmias. Hence, it has the potential to be used to prevent arrhythmogenic events. LINKED ARTICLE

BackgroundIn this study the effects of a new, highly selective sodium-calcium exchanger (NCX) inhibitor, ORM-10962 were investigated on cardiac NCX current, Ca2+ transients, cell shortening and in experimental arrhythmias. The level of selectivity of the novel inhibitor on several major transmembrane ion currents (L-type Ca2+ current, major repolarizing K+ currents, late Na+ current, Na+/K+ pump current) was also determined.MethodsIon currents in single dog ventricular cells (cardiac myocytes; CM), and action potentials in dog cardiac multicellular preparations were recorded utilizing the whole-cell patch clamp and standard microelectrode techniques, respectively. Ca2+ transients and cell shortening were measured in fluorescent dye loaded isolated dog myocytes. Antiarrhythmic effects of ORM-10962 were studied in anesthetized ouabain (10 μg/kg/min i.v.) pretreated guinea pigs and in ischemia-reperfusion models (I/R) of anesthetized coronary artery occluded rats and Langendorff perfused guinea pigs hearts.ResultsORM-10962 significantly reduced the inward/outward NCX currents with estimated EC50 values of 55/67 nM, respectively. The compound, even at a high concentration of 1 μM, did not modify significantly the magnitude of ICaL in CMs, neither had any apparent influence on the inward rectifier, transient outward, the rapid and slow components of the delayed rectifier potassium currents, the late and peak sodium and Na+/K+ pump currents. NCX inhibition exerted moderate positive inotropic effect under normal condition, negative inotropy when reverse, and further positive inotropic effect when forward mode was facilitated. In dog Purkinje fibres 1 μM ORM-10962 decreased the amplitude of digoxin induced delayed afterdepolarizations (DADs). Pre-treatment with 0.3 mg/kg ORM-10962 (i.v.) 10 min before starting ouabain infusion significantly delayed the development and recurrence of ventricular extrasystoles (by about 50%) or ventricular tachycardia (by about 30%) in anesthetized guinea pigs. On the contrary, ORM-10962 pre-treatment had no apparent influence on the time of onset or the severity of I/R induced arrhythmias in anesthetized rats and in Langendorff perfused guinea-pig hearts.ConclusionsThe present study provides strong evidence for a high efficacy and selectivity of the NCX-inhibitory effect of ORM-10962. Selective NCX inhibition can exert positive as well as negative inotropic effect depending on the actual operation mode of NCX. Selective NCX blockade may contribute to the prevention of DAD based arrhythmogenesis, in vivo, however, its effect on I/R induced arrhythmias is still uncertain.

/Ca2+ exchanger (NCX) activity may play a crucial role in cardiac arrhythmogenesis; however, data regarding the anti-arrhythmic efficacy of NCX inhibition are debatable. Feasible explanations could be the unsatisfactory selectivity of NCX inhibitors and/or the dependence of the experimental model on the degree of Ca 2+ i overload. Hence, we used NCX inhibitors SEA0400 and the more selective ORM10103 to evaluate the efficacy of NCX inhibition against arrhythmogenic Ca 2+ i rise in conditions when [Ca 2+ ]i was augmented via activation of the late sodium current (INaL) or inhibition of the Na EXPERIMENTAL APPROACHAction potentials (APs) were recorded from canine papillary muscles and Purkinje fibres by microelectrodes. NCX current (INCX) was determined in ventricular cardiomyocytes utilizing the whole-cell patch clamp technique. Ca KEY RESULTSEnhanced INaL increased the Ca 2+ load and AP duration (APD). SEA0400 and ORM10103 suppressed INCX and prevented/reversed the anemone toxin II (ATX-II)-induced [Ca 2+ ]i rise without influencing APD, CaT or cell shortening, or affecting the ATX-II-induced increased APD. ORM10103 significantly decreased the number of strophanthidin-induced spontaneous diastolic Ca 2+ release events; however, SEA0400 failed to restrict the veratridine-induced augmentation in Purkinje-ventricle APD dispersion. CONCLUSIONS AND IMPLICATIONSSelective NCX inhibition -presumably by blocking revINCX (reverse mode NCX current) -is effective against arrhythmogenesis caused by [Na]i elevation, without influencing the AP waveform. Therefore, selective INCX inhibition, by significantly reducing the arrhythmogenic trigger activity caused by the perturbed Ca 2+ i handling, should be considered as a promising anti-arrhythmic therapeutic strategy.

*These two authors contributed equally to this work. †These two senior authors contributed equally to this work. BACKGROUND AND PURPOSEThe reliable assessment of proarrhythmic risk of compounds under development remains an elusive goal. Current safety guidelines focus on the effects of blocking the KCNH2/HERG ion channel-in tissues and animals with intact repolarization. Novel models with better predictive value are needed that more closely reflect the conditions in patients with cardiac remodelling and reduced repolarization reserve. EXPERIMENTAL APPROACHWe have developed a model for the long QT syndrome type-5 in rabbits (LQT5 ) with cardiac-specific overexpression of a mutant (G52R) KCNE1 β-subunit of the channel that carries the slow delayed-rectifier K + -current (I Ks ). ECG parameters, including short-term variability of the QT interval (STV QT ), a biomarker for proarrhythmic risk, and arrhythmia development were recorded. In vivo, arrhythmia susceptibility was evaluated by i.v. administration of the I Kr blocker dofetilide. K + currents were measured with the patch-clamp technique. KEY RESULTSPatch-clamp studies in ventricular myocytes isolated from LQT5 rabbits revealed accelerated I Ks and I Kr deactivation kinetics. At baseline, LQT5 animals exhibited slightly but significantly prolonged heart-rate corrected QT index (QTi) and increased STV QT . Dofetilide provoked Torsade-de-Pointes arrhythmia in a greater proportion of LQT5 rabbits, paralleled by a further increase in STV QT . CONCLUSION AND IMPLICATIONSWe have created a novel transgenic LQT5 rabbit model with increased susceptibility to drug-induced arrhythmias that may represent a useful model for testing proarrhythmic potential and for investigations of the mechanisms underlying arrhythmias and sudden cardiac death due to repolarization disturbances. AbbreviationsHERG, human ether-a-go-go gene; I Ca,L , L-type Ca 2+ current; I K1 , inward rectifier potassium current; I Kr , rapid delayed rectifier potassium current; I Ks , slow delayed rectifier potassium current; I to , transient outward potassium current; KCNE1, potassium voltage-gated channel subfamily E member 1; LQT5, long QT syndrome type 5; minK, minimum sequence required for a potassium current; QTi, heart rate-corrected QT index; STV QT , short-term variability of the QT interval; STV RR , short-term variability of the RR interval; TdP, Torsade-de-Pointes; TG, transgenic; WT, wild type