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Substances obtained from species of Digitalis, Strophanthus, and other plants that contain specific steroid glycosides or their semisynthetic derivatives and used in congestive heart failure. They increase the force of cardiac contraction without significantly affecting other parameters, but are very toxic at larger doses. Their mechanism of action usually involves inhibition of the Na (+)-K (+)-exchanging ATPase and they are often used in cell biological studies for that purpose.