Preferential sensitivity of the left canine purkinje system to cardiac glycosides

Previous studies have shown that the toxic effects of cardiac glycosides are not manifested uniformly throughout the myocardium. The purpose of our study was to determine whether cardiac glycosides exert different effects on the right vs. left peripheral Purkinje systems and to ascertain mechanisms involved. Control in vitro measurements of paired right and left canine Purkinje fibers showed higher spontaneous rates in left (24.2 +/- 1.75 beats/min) than in right (11.6 +/- 1.55 beats/min, P less than 0.01, n = 81) Purkinje fiber bundles. Following overdrive stimulation, left Purkinje fiber bundles also showed earlier escape beats. After ouabain exposure (2 X 10-7 M), left Purkinje fiber bundles showed earlier signs of toxicity in 20 of 28 experiments, as determined by changes in the maximum diastolic potential, the degree of diastolic depolarization, spontaneous escape intervals, and the magnitude of delayed after-depolarizations. The enhanced sensitivity of left Purkinje fiber bundles was independent of the extracellular potassium concentration and glycoside polarity, and was also observed in situ. We conclude that distal Purkinje fibers are functionally dissimilar and that the left Purkinje system shows greater sensitivity to cardiac glycosides than the right Purkinje system. These data also support the observation that digitalis-induced dysrhythmias arise in the left ventricle.