In 1988, a 21-amino acid endothelium-derived bioactive peptide was cloned and named endothelin (ET)1. Later, two other isoforms differing from the original ET-1 in two or six residues, respectively, were cloned and named ET-2 and ET-32. All endothelins bear the common structure with two disulfide links. ETs are among the most powerful vasoconstrictive substances known today. They also act as growth factors involved in fetal development and vascular regulation5. Two types of endothelin receptors have been cloned and named ET-A3 and ET-B4. The two classes differ in their affinity to different ETs. ET-A has a stronger affinity to ET-1 (ET-1>ET-2>> ET-3), while ET-B has an equal affinity towards all three ETs. ET-A and ET-B are both G-protein coupled receptors with extracellular N-termini, 7 transmembrane domains and intracellular C-termini. They share ~60% homology.