[Inhibition of the slow-response action potential of mammalian ventricular myocardium by etafenone (author's transl)].

In isolated papillary muscles of guinea pigs the effect of etafenone on both Na-dependent action potential and slow-response action potential and on isometric contractile force was analyzed. Etafenone (3 mg/l) reduced contractile force to 46.1% of control accompanied by a loss of plateau of the Na-dependent action potential whilst its rate of rise and overshoot remained unchanged. In contrast to this, etafenone significantly diminished upstroke velocity and overshoot of the slow-response action potential and, in some cases, even induced a complete blockade. This inhibition of the slow-response action potential could be overcome by either excess Ca (4 mmol/l) or the beta-adrenergic compound isoproterenol (1 mg/l). Consequently, contractile force increased thereby attaining or even surpassing the control values obtained prior to the etafenone treatment. The results strongly suggest an inhibition of the Ca carried slow inward current that seems to be the reason for the negative inotropic action of etafenone.