Transient Outward Current Inhibition by Propafenone and 5-Hydroxypropafenone in Cultured Neonatal Rat Ventricular Myocytes

The antiarrhythmic agent propafenone and its primary electropharmacologically active metabolite, 5-hydroxypropafenone, are known inhibitors of cardiac myocyte repolarizing currents. We recently documented potent propafenone inhibition of the transient outward potassium current (Ito) in human atrial myocytes from patients in the newborn and infant age range. In the current study we characterized ventricular Ito inhibition by propafenone and 5-hydroxypropafenone in neonatal myocytes enzymatically isolated from 2-day-old Sprague-Dawley rat pups. Using the whole-cell patch-clamp technique in ventricular myocytes kept in primary culture for 1-4 days, we observed comparably potent Ito inhibition by both agents, yielding 50% maximal inhibitory concentration (IC50) values of 2.1 +/- 0.5 and 1.5 +/- 0.2 microM for propafenone and 5-hydroxypropafenone, respectively. Ito blockade by both of these agents was time, concentration, and voltage dependent, but use independent. There was no drug effect on steady-state voltage dependence of Ito inactivation, or on the time course of Ito recovery from inactivation. These findings are consistent with an open channel-blocking mechanism as suggested by other models. We conclude that both propafenone and 5-hydroxypropafenone are potent Ito inhibitors in neonatal rat ventricular myocytes, with potencies exceeding those demonstrated for propafenone in adult rat ventricular myocytes or in human atrial myocytes from patients of all ages.