Evidence for Specialized Fibers in the Canine Right Atrium

Right atria from excised beating dog hearts were immersed in Tyrode solution with the endocardial surface of the anterior wall exposed. Glass microelectrodes were used to impale fibers in different anatomical areas of this preparation. Fibers with action potentials similar in contour to those of ventricular Purkinje fibers were found along the caval border of the crista terminalis in the area of the posterior internodal tract. The action potential of these fibers (plateau fibers) had a resting potential of -85 to -95 mv, a sharp spiked overshoot, a long plateau phase, and inherent diastolic depolarization. The maximum velocity of the action potential upstroke of plateau fibers was consistently greater than that of simultaneously recorded fibers which lacked a plateau phase (regular fibers). Epinephrine or isoproterenol produced an increase in both rate and magnitude of diastolic depolarization of plateau fibers and on occasion converted them to true pacemakers. Acetylcholine accelerated repolarization of plateau fibers with disappearance of the plateau. Increases in extracellular concentration of potassium ions from 2.7 to 10.8 mM rendered regular atrial fibers inexcitable, but plateau fibers continued to show action potentials. Plateau atrial fibers possess several characteristics exhibited by specialized conducting and impulse generating fibers. The possibility that these fibers constitute the posterior internodal tract and function in preferential conduction of excitation to the A-V node was discussed.