Endothelin, a 21–amino acid peptide, has potent and sustained vasoconstrictive effects on vascular smooth muscle both in vivo and in vitro and is found in three different isoforms. ET-1 binds and activates two G-protein coupled receptors: ET-A and ET-B. While the former has been attributed to vasoconstriction, the role of ET-B depends mostly on its location; on one hand it could promote vasoconstriction and on the other stimulate nitric oxide (NO) release -a known vasodilator.