Enhancement of the toxic effects of veratrine on guinea-pig atrium by threshold inotropic doses of ouabain.

Isolated atria of guinea-pigs were treated with veratrine until the initial signs of toxicity were seen. Ouabain was then added cumulatively, starting with a threshold inotropic concentration, 50 nM, until the tissue became dysrhythmic. It was found that a concentration of ouabain which by itself gave a positive inotropic effect of only 3%, significantly enhanced the toxicity of veratrine. Veratrine had no effect on the (Na+ + K+)-adenosine triphosphatase ((Na+ + K+)-ATPase) enzyme isolated from guinea-pig ventricle. The conclusion drawn is that at threshold inotropic concentrations of ouabain it is likely that the (Na+ + K+)-ATPase is inhibited rather than stimulated.