The obligatory role of endothelial cells in the relaxation of arterial smooth muscle by acetylcholine.

In 1978 Furchgott discovered a substance in endothelial cells that relaxes blood vessels, calling it endothelium-derived relaxing factor (EDRF). By 1986, he had worked out EDRF's nature and mechanism of action, and determined that EDRF was in fact nitric oxide (NO), an important compound in many aspects of cardiovascular physiology. 

See also: "Studies on relaxation of rabbit aorta by sodium nitrite: the basis for the proposal that the acid-activatable inhibtory factor from retractor penis in inorganic nitrite and the endothelium-derived relaxing factor is nitric oxide" IN: Vasodilatation: Vascular smooth muscle, petides, and endothelium, edited by P. M. Vanhoutte, (1988) 401-414.

In 1998 the Nobel Prize in Physiology or Medicine was awarded jointly to Robert F. Furchgott, Louis J. Ignarro and Ferid Murad "for their discoveries concerning nitric oxide as a signalling molecule in the cardiovascular system."