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

Abstract

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.

Propafenone and Its Metabolites Preferentially Inhibit IKrin Rabbit Ventricular Myocytes

“…Propafenone is effective in the medical therapy of a variety of supraventricular as well as ventricular tachyarrhythmias in children (Paul and Janousek, 1994) and adults (Grant, 1996). Although categorized as a sodium channel-blocking agent in the Vaughan Williams antiarrhythmic drug classification scheme, propafenone potently inhibits a number of repolarizing potassium currents in cardiac myocytes isolated from humans (Gross and Castle, 1998;Seki et al, 1999) as well as from animals (Duan et al, 1993;Slawsky and Castle, 1994;Delpón et al, 1995;Christé et al, 1999;Cahill et al, 2001). Most repolarizing current investigations have focused on I to inhibition (Duan et al, 1993;Slawsky and Castle, 1994;Gross and Castle, 1998;Seki et al, 1999;Cahill et al, 2001).…”

“…Although categorized as a sodium channel-blocking agent in the Vaughan Williams antiarrhythmic drug classification scheme, propafenone potently inhibits a number of repolarizing potassium currents in cardiac myocytes isolated from humans (Gross and Castle, 1998;Seki et al, 1999) as well as from animals (Duan et al, 1993;Slawsky and Castle, 1994;Delpón et al, 1995;Christé et al, 1999;Cahill et al, 2001). Most repolarizing current investigations have focused on I to inhibition (Duan et al, 1993;Slawsky and Castle, 1994;Gross and Castle, 1998;Seki et al, 1999;Cahill et al, 2001). However, more recent studies involving heterologously expressed HERG channels, which mediate I Kr , suggest that HERG is an important molecular target for propafenone (Mergenthaler et al, 2001;Paul et al, 2002).…”

“…Transient outward current (I to ) and its inhibition were analyzed as previously described (Gross and Castle, 1998;Cahill et al, 2001). Briefly, I to was measured as the time integral of spontaneously decaying outward current observed in response to depolarizing 800-ms voltage steps from a holding potential of Ϫ80 mV, adjusted for the "steady-state" current remaining at the end of the step.…”

“…Propafenone blocks the transient outward (I to ) and ultrarapidly activating delayed rectifier (I Kur ) currents in human (Gross and Castle, 1998;Seki et al, 1999) and rabbit (Duan et al, 1993) atrial myocytes as well as in adult (Slawsky and Castle, 1994) and neonatal (Cahill et al, 2001) rat ventricular myocytes. In guinea pig ventricular myocytes, propafenone potently and preferentially inhibits the rapidly activating component of delayed rectifier current, I Kr (Delpón et al, 1995).…”

“…Available studies with 5-hydroxypropafenone indicate potent I to inhibition in cultured neonatal rat ventricular myocytes (Cahill et al, 2001) and electropharmacologic effects in intact organisms and in vitro preparations consistent with inhibition of a number of cardiac myocyte ionic currents (von Philipsborn et al, 1984;Delgado et al, 1987;Valenzuela et al, 1987Valenzuela et al, , 1988Malfatto et al, 1988;Rouet et al, 1989;Case et al, 1991;Haefeli et al, 1991;Boucher et al, 1996;Franqueza et al, 1998). The cardiac cellular electropharmacology of N-depropylpropafenone has not been previously studied, although limited data suggesting ionic current blockade are available from intact animals and tissue preparations (Malfatto et al, 1988;Rouet et al, 1989).…”