Károly Acsai

Selective inhibition of sodium-calcium exchanger by SEA-0400 decreases early and delayed afterdepolarization in canine heart

The sodium–calcium exchanger (NCX) was considered to play an important role in arrhythmogenesis under certain conditions such as heart failure or calcium overload. In the present study, the effect of SEA‐0400, a selective inhibitor of the NCX, was investigated on early and delayed afterdepolarizations in canine ventricular papillary muscles and Purkinje fibres by applying conventional microelectrode techniques at 37°C. The amplitude of both early and delayed afterdepolarizations was markedly decreased by 1 μM SEA‐0400 from 26.6±2.5 to 14.8±1.8 mV (n=9, P<0.05) and from 12.5±1.7 to 5.9±1.4 mV (n=3, P<0.05), respectively. In enzymatically isolated canine ventricular myocytes, SEA‐0400 did not change significantly the L‐type calcium current and the intracellular calcium transient, studied using the whole‐cell configuration of the patch‐clamp technique and Fura‐2 ratiometric fluorometry. It is concluded that, through the reduction of calcium overload, specific inhibition of the NCX current by SEA‐0400 may abolish triggered arrhythmias.

Combined Na+/Ca2+ Exchanger and L-Type Calcium Channel Block as a Potential Strategy to Suppress Arrhythmias and Maintain Ventricular Function

Background—L-type calcium channel (LTCC) and Na+/Ca2+ exchanger (NCX) have been implicated in repolarization-dependent arrhythmias, but also modulate calcium and contractility. Although LTCC inhibition is negative inotropic, NCX inhibition has the opposite effect. Combined block may, therefore, offer an advantage for hemodynamics and antiarrhythmic efficiency, particularly in diseased hearts. In a model of proarrhythmia, the dog with chronic atrioventricular block, we investigated whether combined inhibition of NCX and LTCC with SEA-0400 is effective against dofetilide-induced torsade de pointes arrhythmias (TdP), while maintaining calcium homeostasis and hemodynamics. Methods and Results—Left ventricular pressure (LVP) and ECG were monitored during infusion of SEA-0400 and verapamil in anesthetized dogs. Different doses were tested against dofetilide-induced TdP in chronic atrioventricular block dogs. In ventricular myocytes, effects of SEA-0400 were tested on action potentials, calcium transients, and early afterdepolarizations. In cardiomyocytes, SEA-0400 (1 &mgr;mol/L) blocked 66±3% of outward NCX, 50±2% of inward NCX, and 33±9% of LTCC current. SEA-0400 had no effect on systolic calcium, but slowed relaxation, despite action potential shortening, and increased diastolic calcium. SEA-0400 stabilized dofetilide-induced lability of repolarization and suppressed early afterdepolarizations. In vivo, SEA-0400 (0.4 and 0.8 mg/kg) had no effect on left ventricular pressure and suppressed dofetilide-induced TdPs dose dependently. Verapamil (0.3 mg/kg) also inhibited TdP, but caused a 15±8% drop of left ventricular pressure. A lower dose of verapamil without effects on left ventricular pressure (0.06 mg/kg) was not antiarrhythmic. Conclusions—In chronic atrioventricular block dogs, SEA-0400 treatment is effective against TdP. Unlike specific inhibition of LTCC, combined NCX and LTCC inhibition has no negative effects on cardiac hemodynamics.

ORM‐10103, a novel specific inhibitor of the Na+/Ca2+ exchanger, decreases early and delayed afterdepolarizations in the canine heart

At present there are no small molecule inhibitors that show strong selectivity for the Na+/Ca2+ exchanger (NCX). Hence, we studied the electrophysiological effects of acute administration of ORM‐10103, a new NCX inhibitor, on the NCX and L‐type Ca2+ currents and on the formation of early and delayed afterdepolarizations.

Na+/Ca2+ exchanger inhibition exerts a positive inotropic effect in the rat heart, but fails to influence the contractility of the rabbit heart

Background and purpose: The Na+/Ca2+ exchanger (NCX) may play a key role in myocardial contractility. The operation of the NCX is affected by the action potential (AP) configuration and the intracellular Na+ concentration. This study examined the effect of selective NCX inhibition by 0.1, 0.3 and 1.0 μM SEA0400 on the myocardial contractility in the setting of different AP configurations and different intracellular Na+ concentrations in rabbit and rat hearts.

The Na+/Ca2+ exchange blocker SEA0400 fails to enhance cytosolic Ca2+ transient and contractility in canine ventricular cardiomyocytes

AIMS This study was designed to evaluate the effects of the Na(+)/Ca(2+) exchange (NCX) inhibitor SEA0400 on Ca(2+) handling in isolated canine ventricular myocytes. METHODS AND RESULTS Intracellular Ca(2+) ([Ca(2+)](i)) transients, induced by either field stimulation or caffeine flush, were monitored using Ca(2+) indicator dyes. [Ca(2+)](i)-dependent modulation of the inhibitory effect of SEA0400 on NCX was characterized by the changes in Ni(2+)-sensitive current in voltage-clamped myocytes. Sarcoplasmic reticulum (SR) Ca(2+) release and uptake were studied in SR membrane vesicles. Gating properties of single-ryanodine receptors were analysed in lipid bilayers. Ca(2+) sensitivity of the contractile machinery was evaluated in chemically skinned myocytes. In myocytes paced at 1 Hz, neither diastolic [Ca(2+)](i) nor the amplitude of [Ca(2+)](i) transients was significantly altered by SEA0400 up to the concentration of 1 microM, which was shown to inhibit the exchange current. The blocking effect of SEA0400 on NCX decreased with increasing [Ca(2+)](i), and it was more pronounced in reverse than in forward mode operation at every [Ca(2+)](i) examined. The rate of decay of the caffeine-induced [Ca(2+)](i) transients was decreased significantly by 1 microM SEA0400; however, this effect was only a fraction of that observed with 10 mM NiCl(2). Neither SR Ca(2+) release and uptake nor cell shortening and Ca(2+) sensitivity of the contractile proteins were influenced by SEA0400. CONCLUSION The lack of any major SEA0400-induced shift in Ca(2+) transients or contractility of myocytes can well be explained by its limited inhibitory effect on NCX (further attenuated by elevated [Ca(2+)](i) levels) and a concomitant reduction in Ca(2+) influx due to the predominantly reverse mode blockade of NCX and suppression of L-type Ca(2+) current.

Self-augmentation of the lengthening of repolarization is related to the shape of the cardiac action potential: Implications for reverse rate dependency

Background and purpose:  The aims of the present work were to study the mechanism of the reverse rate dependency of different interventions prolonging cardiac action potential duration (APD).

Selective Na+/Ca2+ exchanger inhibition prevents Ca2+ overload‐induced triggered arrhythmias

Augmented Na+/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 Ca2+i overload. Hence, we used NCX inhibitors SEA0400 and the more selective ORM10103 to evaluate the efficacy of NCX inhibition against arrhythmogenic Ca2+i rise in conditions when [Ca2+]i was augmented via activation of the late sodium current (INaL) or inhibition of the Na+/K+ pump.

T-Tubule Remodelling and Ryanodine Receptor Organization Modulate Sodium-Calcium Exchange

The Na(+)/Ca(2+) exchanger (NCX) is a key regulator of intracellular Ca(2+) in cardiac myocytes, predominantly contributing to Ca(2+) removal during the diastolic relaxation process but also modulating excitation-contraction coupling. NCX is preferentially located in the T-tubules and can be close to or within the dyad, where L-type Ca(2+) channels face ryanodine receptors (RyRs), the Ca(2+) release channels of the sarcoplasmic reticulum. However, especially in larger animals, not all RyRs are in dyads or adjacent to T-tubules, and a substantial fraction of Ca(2+) release from the sarcoplasmic reticulum thus occurs at distance from NCX. This chapter deals with the functional consequences of NCX location and how NCX can modulate diastolic and systolic Ca(2+) events. The loss of T-tubules and the effects on RyR function and NCX modulation are explored, as well as quantitative measurement of local Ca(2+) gradients at the level of the dyadic space.

The Effect of a Novel Highly Selective Inhibitor of the Sodium/Calcium Exchanger (NCX) on Cardiac Arrhythmias in In Vitro and In Vivo Experiments

Background In 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. Methods Ion 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. Results ORM-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. Conclusions The 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.

Selective Na(+) /Ca(2+) exchanger inhibition prevents Ca(2+) overload-induced triggered arrhythmias.

Augmented Na+/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 Ca2+i overload. Hence, we used NCX inhibitors SEA0400 and the more selective ORM10103 to evaluate the efficacy of NCX inhibition against arrhythmogenic Ca2+i rise in conditions when [Ca2+]i was augmented via activation of the late sodium current (INaL) or inhibition of the Na+/K+ pump.