Shifts in Pacemaker Dominance within the Sinoatrial Region of Cat and Rabbit Hearts Resulting from Increase of Extracellular Potassium

Simultaneous recording of action potentials from cells in different parts of sinoatrial preparations from cat and rabbit hearts confirms the suggestion by Meek and Eyster that the dominant pacemaker site shifts during treatment with high K + . The direction of the shift can be either upward or downward depending upon the location of the original dominant pacemaker site. In the majority of cases, the original dominant site is located in the upper part of the preparation, consequently the shift is from above downward. In environments with high K + , both true and latent pacemaker cell action potentials show a decrease in both maximal diastolic potential and the slope of the prepotential but these changes seem to be more marked in the true pacemaker cell. It is suggested that the shift of dominant pacemaker site in response to high K + is due to the difference in sensitivity to K + between these two types of pacemaker cells.