group of 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.