Changes in rat atrial action potentials induced by promazine and thioridazine.

Abstract

The action potentials of electrically stimulated left rat atria were studied in an organ‐bath continuously supplied with a Ringer solution. Action potentials derived from single myocardial cells were displayed on a Tektronix dual‐beam oscilloscope by means of fine glass capillary electrodes and photographed. Promazine 10‐5 M and thioridazine 5 × 10‐5 M reduced the maximum rate of rise (dv/dt) and the overshoot of the action potential, without significantly altering the resting membrane potential. The drugs also caused a slight reduction in the rate of repolarization. The decrease in the rate of the depolarization due to promazine was minimized when the external potassium concentration was reduced from 5.3 to 1.325 meq./1, a procedure which increased the resting membrane potential. An increase in the potassium concentration from 5.3 to 7.95 meq./1 reduced the resting membrane potential and augmented the effect of the drug on the rate of depolarization. It is suggested that the findings described in this study may explain the phenothiazine induced alterations in electrocardiogram of isolated rat hearts as well as the changes in excitability and refractoriness of isolated rat atria.