Ouabain is the familiar name of g-strophanthin, a poisonous cardiac glycoside. Ouabain is found in the ripe seeds of African plants Strophanthus gratus and the bark of Acokanthera ouabaio. k-strophanthin is found in the ripe seeds of Strophanthus kombe.]
It is well understood that the classical mechanism of action of ouabain involves its binding to and inhibition of the plasma membrane Na+/K+-ATPase (sodium pump) especially at the higher concentrations attainable in vitro or with intravenous dosage. Inhibition of the sodium pump and the secondary effect on the handling of calcium ions by sodium calcium exchanger (NCX) is widely believed to underlie the original beneficial effect as an inotropic agent following intravenous use of ouabain; digoxin is a structurally related and more lipophilic cardiac glycoside that largely replaced ouabain for therapy because of its superior bioavialability per os. Digoxin continues to to be used therapeutically for many of the same indications in which ouabain was used (including atrial fibrillation and congested heart failure).