A toxic glycoside derived from the seeds of certain African plants, such as Strophanthus gratus, that has been used historically as a drug to stimulate contractions of heart muscle in treating heart failure and other conditions. It specifically blocks the action of sodium/potassium ATPase (see sodium pump), a crucial membrane-bound transport protein that maintains differential ion concentrations between a cell’s interior and the external environment. For this property, ouabain is used widely in biomedical research, and it is now known to occur naturally in the body at very low concentrations. The adrenal glands are thought to be the main source of endogenous ouabain in humans, which may play a role in sodium metabolism, e.g. modulating the effects of dietary salt intake on blood pressure.