ULTRAMICROELECTRODE RECORDING FROM THE CARDIAC PACEMAKER

¹ Department of Pharmacology, University of Washington School of Medicine, Seattle, Washington

The site of impulse origination in surviving rabbit atria has been localized electrically. With the aid of the ultramicroelectrode, membrane potentials during spontaneous activity were recorded from pacemaker and from non-pacemaker regions.

The pacemaker region lies in a flat band of tissue separating the two caval orifices in the right atrium. In contrast to sites elsewhere in the atria, recordings from this area exhibit a diastolic prepotential, relatively slow depolarization and slow repolarization. The magnitude of the resting membrane potential is less in pacemaker cells and overshoot during activity characteristically is diminished or absent. By means of a second microelectrode to establish a time reference, pacemaker cells showing early firing may be located, although all cells associated with a prepotential are not necessarily the site of impulse origination.

Shifts in pacemaker locus are demonstrated and often appear following the application of drugs acting on the sinoatrial node.

The significance of these results to cardiac pharmacology is discussed.