Ouabain, a glycoside poison, inhibits

It is a poisonous cardiac glycoside. It binds and inhibits sodium potassium pump. Ouabain 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.