Pharmacologic inhibition of small-conductance calcium-activated potassium (SK) channels by NS8593 reveals atrial antiarrhythmic potential in horses

BACKGROUND Small-conductance calcium-activated potassium (SK) channels have been found to play an important role in atrial repolarization and atrial fibrillation (AF). OBJECTIVE The purpose of this study was to investigate the existence and functional role of SK channels in the equine heart. METHODS Cardiac biopsies were analyzed to investigate the expression Level of the most prominent cardiac ion channels, with special focus on SK channels, in the equine heart. Subcellular distribution of SK isoform 2 (SK2) was assessed by immunohistochemistry and confocal microscopy. The electrophysiologic and anti-AF effects of the relative selective SK channel. inhibitor NS8593 (5 mg/kg IV) were evaluated in anesthetized horses, focusing on the potential of NS8593 to terminate acute pacing-induced AF, drug-induced changes in atrial effective refractory period, AF duration and vulnerability, and ventricular depolarization and repolarization times. RESULTS Analysis revealed equivalent mRNA transcript Levels of the 3 SK channel isoforms in atria compared to ventricles. Immunohistochemistry and confocal microscopy displayed a widespread distribution of SK2 in both atrial and ventricular cardiomyocytes. NS8593 terminated all induced AF episodes (duration >= 15 minutes), caused pronounced prolongation of atrial effective refractory period, and reduced AF duration and vulnerability. QRS duration and QTc interval were not affected by treatment. CONCLUSION SK channels are widely distributed in atrial and ventricular cardiomyocytes and contribute to atrial repolarization. Inhibition by NS8593 terminates pacing-induced AF of short duration and decreases AF duration and vulnerability without affecting ventricular conduction and repolarization. Thus, inhibition by NS8593 demonstrates clear atrial antiarrhythmic properties in healthy horses.