This is the frog form of urotensin-II, a thirteen-residue peptide whose supplied sequence carries the conserved disulfide-bridged cyclic core shared by all species variants, preceded by an N-terminal extension distinct from the human and rat forms. Urotensin-II was originally described in fish, and the peptide is unusually well conserved across vertebrate evolution, with the cyclic hexapeptide essentially invariant while the flanking residues differ between species. That pattern is itself informative: it identifies the ring as the functional unit under selective constraint and the extension as tolerant of change. Non-mammalian variants are therefore used to probe how much of the observed potency at a given receptor derives from the conserved core versus the species-specific extension. For amphibian physiology the frog peptide is the homologous ligand and the appropriate reagent. In mammalian receptor assays it functions as a comparative tool rather than a substitute for the human peptide, since potency across species variants at a single receptor is exactly the variable under study. As with all members of this family, the disulfide must be intact for activity, and its integrity should be verified after storage. | 