Electrophysiological effects of imipramine in nontreated and in imipramine-pretreated rat atrial fibres.

1 The effect of imipramine (Imip) in concentrations between 10(-7)M and 5 X 10(-5)M has been studied on rat atrial transmembrane potentials. In another group of experiments the effect of Imip was studied in atrial fibres from rats pretreated for 24 days with twice daily intraperitoneal injections of Imip 7.5 mg/kg or saline. 2 In non-treated atria Imip depressed action potential amplitude and Vmax, reduced th e resting membrane potential and shifted the membrane responsiveness and recovery time curves downward and to the right. 3 Imip also prolonged the action potential duration and the effective refractory period, lengthening the effective refractory period relative to action potential duration. 4 Pretreatment with Imip decreased the resting membrane potential, amplitude and Vmax of the action potential and prolonged the effective refractory period. Further addition of Imip produced similar but more marked changes than in non-treated animals. 5 Imip suppressed the spontaneous atrial automaticity as well as the abnormal automaticity induced by BaCl2, aconitine, ouabain or isoprenaline. 6 The drug produced a negative inotropic effect and depressed the amplitude of the slow contractions elicited by isoprenaline in K-depolarized atria. 7 It is concluded that even when the effects of Imip are similar to those of quinidine (group I of antiarrythmics), it also produces a reduction in Ca and K conductances.