Attila Kristóf

Role of slow delayed rectifier K+-current in QT prolongation in the alloxan-induced diabetic rabbit heart

Aim:  In diabetes mellitus, several cardiac electrophysiological parameters are known to be affected. In rodent experimental diabetes models, changes in these parameters were reported, but only limited relevant information is available in other species, having cardiac electrophysiological properties more resembling the human, including the rabbit. The present study was designed to analyse the effects of experimental type 1 diabetes on ventricular repolarization and the underlying transmembrane potassium currents in rabbit hearts.

Electrophysiological effects of ivabradine in dog and human cardiac preparations: Potential antiarrhythmic actions

Ivabradine is a novel antianginal agent which inhibits the pacemaker current. The effects of ivabradine on maximum rate of depolarization (V(max)), repolarization and spontaneous depolarization have not yet been reported in human isolated cardiac preparations. The same applies to large animals close to human in heart size and spontaneous frequency. Using microelectrode technique action potential characteristics and by applying patch-clamp technique ionic currents were studied. Ivabradine exerted concentration-dependent (0.1-10 μM) decrease in the amplitude of spontaneous diastolic depolarization and reduction in spontaneous rate of firing of action potentials and produced a concentration- and frequency-dependent V(max) block in dog Purkinje fibers while action potential duration measured at 50% of repolarization was shortened. In the presence of ivabradine, at 400 ms cycle length, V(max) block developed with an onset kinetic rate constant of 13.9 ± 3.2 beat(-1) in dog ventricular muscle. In addition to a fast recovery of V(max) from inactivation (τ=41-46 ms) observed in control, a second slow component for recovery of V(max) was expressed (offset kinetics of V(max) block) having a time constant of 8.76 ± 1.34 s. In dog after attenuation of the repolarization reserve ivabradine moderately but significantly lengthened the repolarization. In human, significant prolongation of repolarization was only observed at 10 μM ivabradine. Ivabradine in addition to the Class V antiarrhythmic effect also has Class I/C and Class III antiarrhythmic properties, which can be advantageous in the treatment of patients with ischemic heart disease liable to disturbances of cardiac rhythm.

Analysis of the contribution of Ito to repolarization in canine ventricular myocardium

BACKGROUND AND PURPOSE The contribution of the transient outward potassium current (Ito) to ventricular repolarization is controversial as it depends on the experimental conditions, the region of myocardium and the species studied. The aim of the present study was therefore to characterize Ito and estimate its contribution to repolarization reserve in canine ventricular myocardium.

Herpesvirus-mediated delivery of a genetically encoded fluorescent ca 2+ sensor to canine cardiomyocytes

We report the development and application of a pseudorabies virus-based system for delivery of troponeon, a fluorescent Ca2+ sensor to adult canine cardiomyocytes. The efficacy of transduction was assessed by calculating the ratio of fluorescently labelled and nonlabelled cells in cell culture. Interaction of the virus vector with electrophysiological properties of cardiomyocytes was evaluated by the analysis of transient outward current (Ito), kinetics of the intracellular Ca2+ transients, and cell shortening. Functionality of transferred troponeon was verified by FRET analysis. We demonstrated that the transfer efficiency of troponeon to cultured adult cardiac myocytes was virtually 100%. We showed that even after four days neither the amplitude nor the kinetics of the Ito current was significantly changed and no major shifts occurred in parameters of [Ca2+]i transients. Furthermore, we demonstrated that infection of cardiomyocytes with the virus did not affect the morphology, viability, and physiological attributes of cells.

Diclofenac Prolongs Repolarization in Ventricular Muscle with Impaired Repolarization Reserve

Background The aim of the present work was to characterize the electrophysiological effects of the non-steroidal anti-inflammatory drug diclofenac and to study the possible proarrhythmic potency of the drug in ventricular muscle. Methods Ion currents were recorded using voltage clamp technique in canine single ventricular cells and action potentials were obtained from canine ventricular preparations using microelectrodes. The proarrhythmic potency of the drug was investigated in an anaesthetized rabbit proarrhythmia model. Results Action potentials were slightly lengthened in ventricular muscle but were shortened in Purkinje fibers by diclofenac (20 µM). The maximum upstroke velocity was decreased in both preparations. Larger repolarization prolongation was observed when repolarization reserve was impaired by previous BaCl2 application. Diclofenac (3 mg/kg) did not prolong while dofetilide (25 µg/kg) significantly lengthened the QTc interval in anaesthetized rabbits. The addition of diclofenac following reduction of repolarization reserve by dofetilide further prolonged QTc. Diclofenac alone did not induce Torsades de Pointes ventricular tachycardia (TdP) while TdP incidence following dofetilide was 20%. However, the combination of diclofenac and dofetilide significantly increased TdP incidence (62%). In single ventricular cells diclofenac (30 µM) decreased the amplitude of rapid (IKr) and slow (IKs) delayed rectifier currents thereby attenuating repolarization reserve. L-type calcium current (ICa) was slightly diminished, but the transient outward (Ito) and inward rectifier (IK1) potassium currents were not influenced. Conclusions Diclofenac at therapeutic concentrations and even at high dose does not prolong repolarization markedly and does not increase the risk of arrhythmia in normal heart. However, high dose diclofenac treatment may lengthen repolarization and enhance proarrhythmic risk in hearts with reduced repolarization reserve.

L-364,373 (R-L3) enantiomers have opposite modulating effects on IKs in mammalian ventricular myocytes.

Activators of the slow delayed rectifier K⁺ current (IKs) have been suggested as promising tools for suppressing ventricular arrhythmias due to prolongation of repolarization. Recently, L-364,373 (R-L3) was nominated to activate IKs in myocytes from several species; however, in some studies, it failed to activate IKs. One later study suggested opposite modulating effects from the R-L3 enantiomers as a possible explanation for this discrepancy. Therefore, we analyzed the effect of the RL-3 enantiomers on IKs in ventricular mammalian myocytes, by applying standard microelectrode and whole-cell patch-clamp techniques at 37 °C. We synthesized 2 substances, ZS_1270B (right) and ZS_1271B (left), the 2 enantiomers of R-L3. In rabbit myocytes, ZS_1270B enhanced the IKs tail current by approximately 30%, whereas ZS_1271B reduced IKs tails by 45%. In guinea pig right ventricular preparations, ZS_1270B shortened APD90 (action potential duration measured at 90% repolarization) by 12%, whereas ZS_1271B lengthened it by approximately 15%. We concluded that R-L3 enantiomers in the same concentration range indeed have opposite modulating effects on IKs, which may explain why the racemic drug R-L3 previously failed to activate IKs. ZS_1270B is a potent IKs activator, therefore, this substance is appropriate to test whether IKs activators are ideal tools to suppress ventricular arrhythmias originating from prolongation of action potentials.

Effects of diclofenac on ventricular muscle repolarization: proarrhythmic implications

Results Diclofenac (30 μM) decreased the amplitude of rapid (IKr) and slow (IKs) delayed rectifier and L-type calcium currents (ICa) without influencing transient outward (Ito) and inward rectifier (IK1) potassium currents. The action potential was slightly lengthened in ventricular muscle but shortened in Purkinje fibres by diclofenac (20 μM). The maximum upstroke velocity (Vmax) was decreased in both preparations. Larger repolarization lengthening was observed when repolarization reserve was impaired by previous BaCl2 application. Diclofenac (3 mg/kg) did not prolong the QTc interval, while the potassium channel blocker dofetilide (25 μg/kg) significantly lengthened QTc in anaesthetized rabbits. The combination of diclofenac and dofetilide significantly prolonged QTc. Diclofenac alone did not induce torsades de pointes ventricular tachycardia (TdP) while TdP incidence following dofetilide was 20%. However, the combination of diclofenac and dofetilide led to a significant increase in the incidence of TdP.

Properties of the transient outward, ultra-rapid delayed rectifier and acetylcholine-sensitive potassium currents in isolated atrial myocytes from dogs: sinus rhythm and tachypaced model of permanent atrial fibrillation.

Background Atrial fibrillation (AF) is a common and severe arrhythmia, which largely affects quality of life. State-of-the-art treatment of AF still relies heavily on pharmacological modalities. Therefore, the aim of the present study was to investigate and compare the properties of three repolarizing currents which contribute to AF-induced remodeling, i.e. the transient outward (Ito), ultra-rapid delayed rectifier (IKur) and acetylcholine-sensitive potassium currents (IK, ACh ) in isolated atrial myocytes obtained from normal (SR) and tachypaced model of permanent atrial fibrillation (ATR) dogs. Methods The tachypaced atrial fibrillation model was performed in dogs. Transmembrane ionic currents were investigated by applying the whole-cell patch clamp technique at 37°C, and ECG was recorded in conscious dogs.