Phase action of ouabain on the electrical and contractile activity of the frog myocardium

: Ouabain effects (5 X 10(-8)-5 X 10(-5) M) on the isometric contraction and intracellular action potentials were studied on frog atrial and ventricular muscles. It was found that ouabain produced a three-phase action on the electrical and contractile activity of the myocardium under different patterns of stimulation both in normal Ringer and D-600-containing solutions. The data suggest that the three-phase ouabain action is a consequence of its influence on at least two component processes of the excitation-contraction coupling in the frog heart muscle: the slow Na-Ca channels of the sarcolemma and Na+-K+ pump. The contribution of each mechanism depends on the glycoside concentration and the duration of exposure to ouabain.