Nifekalant hydrochloride (MS-551), a class III antiarrhythmic agent, is a IKr potassium channel blocker with an IC50 of 10 μM. Nifekalant hydrochloride can be used for refractory ventricular tachyarrhythmias research[2].In Vitro: Nifekalant interacts with the cardiac M2 and the peripheral M3 receptors with a Ki value of 27 and 74 mM, respectively. Nifekalant dose dependently blocks HERG channels with an IC50 value of 7.9 mM, but Nifekalant does not block minK currents in the Xenopus oocyte expression system. Nifekalant blocks HERG channels mainly in their open state in a frequency dependent manner. As a pure K+ channel blocker, Nifekalant does not have negative inotropic effects which amiodarone has via a β-blocking action and does not affect cardiac conduction[2].In Vivo: In rats (a species deficient in functional cardiac IK), before coronary ligation, 3 mg/kg and 10 mg/kg MS-551 decreased the heart rate by 6% and 12%, and increased mean arterial pressure (MAP) by 14% and 33%, respectively. MS-551 prolongs the QT interval and reduced the incidence of sustained ventricular fibrillation (VF) after reperfusion.