Endothelin-1 (ET-1) is produced by many tissues including lung, kidney, brain, endocrine glands, and placenta. It is also expressed by several cell types including, mast cells, endothelial cells, epithelial and smooth muscle cells. The most prominent source is vascular endothelium. ET-1 is best known as a potent vasoconstrictor. Elevated blood ET-1 levels are associated with a variety of diseases, possibly as a function of a stress response.