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It is concluded that type 1 diabetes mellitus, although only moderately, lengthens ventricular repolarization. Diabetes attenuates the repolarization reserve by decreasing the density of I(Ks) current, and thereby may enhance the risk of sudden cardiac death.

BACKGROUND AND PURPOSEThe 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. EXPERIMENTAL APPROACHIon currents were recorded using conventional whole-cell voltage clamp and action potential voltage clamp techniques in canine isolated ventricular cells. Action potentials were recorded from canine ventricular preparations using microelectrodes. The contribution of Ito to repolarization was studied using 100 mM chromanol 293B in the presence of 0.5 mM HMR 1556, which fully blocks IKs. KEY RESULTSThe high concentration of chromanol 293B used effectively suppressed Ito without affecting other repolarizing K + currents (IK1, IKr, Ip). Action potential clamp experiments revealed a slowly inactivating and a 'late' chromanol-sensitive current component occurring during the action potential plateau. Action potentials were significantly lengthened by chromanol 293B in the presence of HMR 1556. This lengthening effect induced by Ito inhibition was found to be reverse rate-dependent. It was significantly augmented after additional attenuation of repolarization reserve by 0.1 mM dofetilide and this caused the occurrence of early afterdepolarizations. The results were confirmed by computer simulation. CONCLUSIONS AND IMPLICATIONSThe results indicate that Ito is involved in regulating repolarization in canine ventricular myocardium and that it contributes significantly to the repolarization reserve. Therefore, blockade of Ito may enhance pro-arrhythmic risk. AbbreviationsAPD, action potential duration; APD50 and APD90, action potential durations at 50% and 90% of repolarization; DPP, dipeptidyl-aminopeptidase-like protein; EAD, early afterdepolarization; ICa, L-type calcium current; IK1, inward rectifier potassium current; IKr, rapid component of the delayed rectifier potassium current; IKs, slow component of the delayed rectifier potassium

BackgroundThe 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.MethodsIon 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.ResultsAction 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.ConclusionsDiclofenac 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.

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.