Potent vasoconstrictor.Human Urotensin II is composed of only 11 amino acid residues, while fish and frog Urotensin II possess 12 and 13 amino acid residues, respectively (2). Human Urotensin II is found within both vascular and cardiac tissue, including coronary atheroma, and effectively constricts isolated arteries from nonhuman primates (3). Urotensin II may act as an autocrine and/or paracrine hormone rather than as a circulating hormone, by playing an important role in the development of ventricular hypertrophy induced by chronic hypoxia (4).