Sodium lack prevents strophanthidin toxicity in Purkinje fibers.

The effect of strophanthidin on electrical and mechanical activity of canine cardiac Purkinje fibers perfused in vitro was tested in the presence and in the absence of sodium. The following results were obtained: (1) in Tyrode's solution, strophanthidin causes spontaneous fast rhythms; (2) perfusion of a tetraethylammonium (TEA) Na-free solution results in an initial slowing and quiescence followed by the development of spontaneous small potentials; (3) exposure to strophanthidin during the perfusion of the Na-free solution fails to induce an acceleration of spontaneous discharge; (4) on returning from Na-free to Tyrode's solution there is a temporary acceleration of the small potentials and this acceleration is larger and lasts longer when the Na-free solution contains strophanthidin, and (5) in fibers intoxicated with strophanthidin, exposure to Na-free solution stops the fast rhythms. It is concluded that sodium plays an important role in strophanthidin toxicity and no toxicity occurs when sodium is replaced by TEA.