Disopyramide phosphate effects on slow and depressed fast responses

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Abstract
We studied the effects of disopyramide phosphate on explanted neonatal rat ventricle cells exhibiting depressed fat responses or naturally occurring slow response action potentials together with automatic activity. Disopyramide suppressed the spontaneous activity at a concentration of 2.5 .mu.g/mL with a half-maximal value of 10 .mu.g/mL. Before spontaneous activity was lost, there was an increase in beating rate possibly related to membrane depolarization. In depressed fast and slow response action potentials there was an increase in action potential duration (APD) which was consistently found both at the level of the plateau and at 90% repolarization. Comparison of the APD increase observed after disopyramide treatment and that after exposure to 20 mM tetraethylammonium suggested a block of a potassium conductance as a possible cause underlying the change in APD. The .ovrhdot.Vmax values of the depressed fast response decreased at constant membrane potential and this was attributed to the local anesthetic effect of the drug. In addition, we report two novel findings: (i) a decrease of .ovrhdot.Vmax of the slow response action potentials which may be secondary to membrane depolarization, and (ii) an increase in the duration of slow action potentials, possibly caused by inhibition of a potassium conductance.