endothelin (ET) /-thē″lin/ , any of a group of vasoconstrictive peptides produced by endothelial cells. Three known endothelins, designated ET-1, ET-2, and ET-3, are chemically related to asp venom. ET-1 is the most potent vasoconstrictor yet discovered, being 10 times stronger than the second-most potent vasoconstrictor known, angiotensin II.