endothelin-1 is a peptide (comprising 21 amino acids) that is produced in endothelial cells. it acts on the endothelin receptors eta and etb in various cell types including vascular smooth muscle cells and fibroblasts, leading to vasoconstriction, hypertrophy, proliferation, inflammation, and fibrosis. it also acts on etb receptors in endothelial cells; this leads to the release of both vasoconstrictors and vasodilators from those cells, and clears endothelin-1 from the system.