Endothelin-1 (ET-1) is a peptide that plays a crucial role in regulating various physiological processes in the human body. It is primarily produced and secreted by endothelial cells, which are the cells that line the inner walls of blood vessels. ET-1 is a potent vasoconstrictor, meaning it causes blood vessels to constrict or narrow. ET-1 has been shown to be one of the most potent vasoconstrictors.