Relationship between acetylstrophanthidin-induced aftercontractions and the strength of contraction of canine ventricular myocardium

Contractile activity was recorded from isolated canine ventricular muscle exposed to acetylstrophanthidin (AS), 0.5 to 2 x 10^-7 g/ml. Development of the positive inotropic effect of AS was accompanied by the appearance of aftercontractions (AC) coupled to the driven responses. The amplitudes of AC increased with the number of preceding beats. Test beats occurring during the ascending limb of AC were potentiated and the potentiation increased with the amplitude of the AC. Beats falling during the descending limb decreased in strength as the amplitude of the AC increased. AS eliminated the frequency dependence of restitution. However, staircase phenomena persisted and were then found to be dependent on the phase relationship of each beat to underlying AC. These findings suggest an important role for AC in the inotropic actions of digitalis and in determining strength-interval relationships of ventricular muscle treated with digitalis.