In a paper which we published recently,1 we showed that digitalis exerts a direct action on the systolic force of mammalian cardiac muscle. Our preparation was the papillary muscle of the cat's right ventricle immersed in Ringer's solution. We measured tension with the isometric lever and recorded it photographically. Since the muscle was driven at a fixed rate, since there was no coronary circulation or systemic circulation and since the drug reversed the direction of failing systolic tension while the initial (diastolic) tension was maintained constant, we concluded that the digitalis glucosides increase the force of contraction of failing mammalian cardiac muscle by direct action on the muscle. This effect applies to all the digitalis glucosides which have been used: ouabain, digitoxin (Merck), digitalin (Nativelle) and digilanid C (Sandoz). Figure 1, reproduced from that paper, shows the essentials of the results.
Since then we have explored the matter further.
GOLD H, CATTELL M. MECHANISM OF DIGITALIS ACTION IN ABOLISHING HEART FAILURE. Arch Intern Med (Chic). 1940;65(2):263-278. doi:10.1001/archinte.1940.00190080045003