Ouabain binds to and inhibits the transport activity of the Na⁺/K⁺/ATPase (i.e., sodium pump).

Ouabain is plant derived and belongs to the class of drugs referred to as cardiac glycosides. Similar to other cardiac glycosides, ouabain increase heart muscle contractility. However, ouabain is used only experimentally and not in humans (as for example digoxin is for the treatment of congestive heart failure).

There is some evidence that ouabain may be produced endogenously in humans.

Vanadate

Ouabain (Wikipedia)