Adrenergic potentiation on spontaneous activity of canine paranodal fibers

The present study, using in vitro preparations, was designed to determine the anatomic, histological, and automatic properties of canine paranodal fibers. This tissue, together with the atrioventricular (AV) node and His bundle, constituted the three major tissues in the AV junction. The fascicles of the paranodal fibers ran parallel and adjacent to the base of the septal cusp of the tricuspid valve. The distal end of the paranodal fibers joined the lower half of the compact AV node on its convex side. Paranodal fibers when isolated were able to initiate spontaneous activity. Action potentials of many of these fibers showed primary pacemaker characteristics, i.e., a prominent phase 4 depolarization and smooth transition from phases 4 to 0. In 14 preparations, epinephrine (2.0 micrograms injected into the tissue bath) potentiated spontaneous rates to 144 +/- 6.0 beats/min from 61 +/- 5.0, an increase of 136%. Also, under the influence of epinephrine, paranodal fibers consistently generated a spontaneous rate higher than that of the AV node or His bundle, whether they were functionally connected or separated. These findings provide a basis for explaining the junctional tachycardia that occurs under adrenergic influence and demonstrate the presence of three major automatic tissues: the paranodal fibers, AV node, and His bundle in the canine AV junction.