Anisotropic Conduction Properties of Canine Ventricular Muscles. Influence of High Extracellular K+ Concentration and Stimulation Frequency

The effects of fiber direction on the conduction of excitation and the characteristics of both extracellular and intracellular action potentials were examined in isolated dog ventricular muscle preparations. Conduction velocity and amplitude of extracellular potentials during propagation in the direction parallel to the long axis of fibers (longitudinal propagation) were significantly greater than those during propagation in the direction perpendicular to it (transverse propagation). However, the maximum rate of rise and amplitude of intracellular action potential were significantly greater during transverse propagation than during longitudinal propagation. Compared with the transverse propagation, the longitudinal propagation was more susceptible to conduction failure under high extracellular K+ concentration, suggesting a lower safety factor of the latter mode of propagation in depolarized cardiac tissue. In contrast, high frequency stimulation caused a greater decrease in the conduction velocity during transverse than longitudinal propagation resulting in augmentation of anisotropic conduction properties. These results suggest that the anisotropic conduction properties of cardiac tissue may play important roles in conduction disturbances leading to arrhythmias under various pathological conditions.