Electrophysiological Properties of the Rabbit Sinoatrial Perinodal Fibers

In the rabbit, the sinus node is surrounded by a group of cells which we have termed perinodal fibers. Since sinus node impulses preferentially proceed to the crista terminalis and then spread to the rest of the atrium, we selected the zone of perinodal fibers lying between the sinus node and the crista terminalis for study. Values of transmembrane potentials and the maximum rate of rise (Vmax) of phase 0 in these perinodal fibers were intermediate between those in sinus node and crista terminalis fibers. Values of phase-0 amplitude and Vmax obtained from perinodal fibers near the crista terminalis were greater than those values from perinodal fibers near the sinus node. Membrane responsiveness was determined using premature stimulation or potassiuminduced depolarization. In perinodal fibers, recovery of phase-0 Vmax of premature responses to peak value showed a strong time dependence outlasting full repolarization and extending throughout diastole. The increasing values of Vmax during diastole occurred despite spontaneous diastolic depolarization. When time was minimized as a variable, using potassium-induced depolarization, Vmax was also shown to be dependent on membrane activation voltage. In addition, recovery of phase-0 amplitude of premature responses to peak value was shown to be somewhat time dependent. The resistance of phase-0 amplitude and Vmax to the depolarizing effects of increased external potassium concentration was greater in perinodal fibers than it was in ordinary a trial fibers. We were able to show that premature responses capable of exciting the crista terminalis blocked in the zone of perinodal fibers.