Urotensin II (UII) was originally identified in fish, but isoforms are now known to be present in the human and other mammalian species. Human UII is an 11-amino acid peptide. Major sites of UII expression in humans include the brain, spinal cord, and kidneys. UII is also present in plasma, and potential sources of this circulating peptide include the heart, lungs, liver, and kidneys. The stimulus to UII release has not been identified but increased blood pressure has been implicated in some studies.
In vitro, UII is a potent constrictor of vascular smooth muscle; its activity depends on the type of blood vessel and the species from which it was obtained. Vasoconstriction occurs primarily in arterial vessels, where UII can be more potent than ET-1, making it the most potent known vasoconstrictor. However, under some conditions, UII may cause vasodilation. In vivo, UII has complex hemodynamic effects, the most prominent being regional vaso-constriction and cardiac depression. In some ways, these effects resemble those produced by ET-1. Nevertheless, the role of the peptide in the normal regulation of vascular tone and blood pres-sure in humans appears to be minor.
The actions of UII are mediated by a Gq protein-coupled receptor referred to as the UT receptor. UT receptors are widely distributed in the brain, spinal cord, heart, vascular smooth mus-cle, skeletal muscle, and pancreas. Some effects of the peptide including vasoconstriction are mediated by the phospholipase C, inositol trisphosphate-diacylglycerol signal transduction pathway.
Urantide (“urotensin antagonist peptide”), a penicillamine-substituted derivative of UII, is a UII receptor antagonist. A non-peptide antagonist, palosuran, has also been developed and used in clinical studies.
Although UII appears to play only a minor role in health, evi-dence is accumulating that it is involved in cardiovascular and other diseases. In particular, it has been reported that plasma UII levels are increased in hypertension, heart failure, atherosclerosis, diabetes mellitus, and renal failure, and palosuran may benefit diabetic patients with renal disease. Nevertheless, the role of UII in disease is poorly understood. Indeed it is possible that rather than contribut-ing to these diseases, UII may actually play a protective role.