Ouabain is a glycoside poison that binds to and inhibits the action of the Na⁺/K⁺ pump in the cell membrane (see here). The Na⁺/K⁺ pump is essential for maintaining the balance of these ions across cell walls.

Ouabain and digitoxin are used to stimulate the heart muscle. They work by binding to the Na⁺/K⁺ ATPase and inhibiting its action. The result of this is that Na⁺ leaks back into the cell. When this happens, the cell tries to maintain the osmotic balance pumping the sodium out with the Na⁺/Ca²⁺ pump. This pumps Ca²⁺ into the cell, which triggers muscle contraction.