Effects of veratrine on repolarization in the canine right bundle branch

Superfusion of Purkinje fibers in the canine right bundle branch with Tyrode solution containing veratrine, less than or equal to 1 mug/ml, prolonged repolarization phases 2 and 3 and induced a long-lasting negative after-potential that ended in some experiments with a slightly hyperpolarized resting transmembrane potential. The alterations in repolarization time were reversed by washout with veratrine-free Tyrode solution or by adding tetrodotoxin to the veratrine-containing Tyrode solution to a concentration of 0.1 and 1 mug/ml. The effect of veratrine on the repolarization time was more prominent at slower driving rates. Its effect on repolarization of ventricular muscle cells was smaller than that on the repolarization of Purkinje fibers. In the concentrations used, veratrine did not induce multiple responses.