This is the goby form of urotensin-II, and it is the variant of greatest historical interest: urotensin-II was first isolated from the caudal neurosecretory system of teleost fish, and the fish peptide is where the family was characterised before mammalian counterparts were identified. The supplied twelve-residue sequence carries the conserved disulfide-bridged core with a short N-terminal extension. In fish the caudal neurosecretory system is a discrete neuroendocrine organ with no direct mammalian equivalent, and urotensin-II from it was studied for osmoregulatory and smooth muscle effects long before the mammalian receptor was deorphanised. The fish peptide remains the reference point for comparative work, and its potency at mammalian receptors despite considerable divergence in the N-terminal extension is one of the clearest demonstrations that the cyclic core alone carries the pharmacology. Practically this variant is chosen for fish physiology and osmoregulation studies, and in mammalian systems as the evolutionarily most distant comparator in a species panel. Like every member of the family it requires an intact disulfide bridge, and the constrained ring conformation rather than the linear sequence is what the receptor recognises. | 