Abstract
1. In dog ventricle the spike potential and the maximum depolarization rate of the action potential depend directly on the external Na concentration.
2. The slow change of membrance potential immediately after the rapid depolarization is changed by altering the external Ca concentration in the similar manner observed in frog ventricle, i. e. the slope of the slow potential change increases with elevated Ca concentration. This Ca-effect is augmented by reducing the external Na concentration.
3. The maximum plateau potential is not affected by changing the external Ca concentration, though distinctly altered by varying Na concentration.
4. It is concluded that, in the ventricular muscle of dog, a fraction of the depolarizing current during the action potential is carried by Ca ions.
