Potassium ions and electrical activity of specialized cardiac fibers

Records of the transmembrane potentials of single fibers in isolated preparations of rabbit atrium show that the specialized fibers of the sinoatrial and atrioventricular node as well as latent pacemaker fibers in the crista terminalis are less sensitive to excess potassium than are atrial muscle fibers. Excitability of these fibers is maintained at potassium concentrations which render the remainder of the atrium inexcitable. Measurements of the potassium content and resting potential of these fibers in solutions containing varying concentrations of K + suggest that the membrane permeability of the specialized fibers may be quantitatively different from that of atrial muscle.