Evidence for the Presence of Electrotonic Depression of Pacemakers in the Rabbit Atrioventricular Node. The Effects of Uncoupling from the Surrounding Myocardium

In the isolated AV junctional preparation of the rabbit heart, the presence of electrotonic influences on impulse formation was investigated. After disconnection of the sinus node, impulse formation started in the junctional area with a mean frequency of 72 beats/min (n = 17), which is about 40% of the sinus rate. Intracellular recordings were obtained to determine pacemaker location and activation sequence in the junctional area. The pacemaker was always located in the area of the lower nodal fibers of the AV node (thus distally from the site of maximal conduction delay) and these fibers had the highest rate of diastolic depolarization. Since it is known that pacemaker fibers are electronically influenced by their neighbouring cells, we investigated whether AV nodal automaticity was influenced by its surrounding tissue. Therefore the AV node was isolated from the atrial tissue and His bundle. This caused an enormous increase in diastolic depolarization rate, especially in the lower nodal fibers, accompanied by a rhythm acceleration to a mean of 137 beats/min. From the findings of the present study it was concluded that under normal conditions impulse formation in the lower nodal fibers of the rabbit AV node is electronically depressed by the connecting myocardium.