Urotensins (UI and UII) were first expected to have osmoregulatory functions in fishes because they are secreted from the urophysis, which is upstream and in direct circulatory contact with the kidney and intestine (McCrohan et al.). U1, the orthologue of which in mammals was named urocortin, is a member of the corticotropin-releasing hormone (CRH) family and thus stimulates ACTH secretion, resulting in cortisol secretion (Lovejoy and Balment). UI (urocortin) and CRH bind to CRH type 1 (CRHR1) and type 2 (CRHR2) receptors. The CRHR1 has similar affinities to both ligands, whereas the CRHR2 exhibits higher affinity to UI. The goby (Gillichthys mirabilis), transferred from SW to FW, increased urophyseal UI content after 24 h, showing osmotic sensitivity of the urophysis (Larson and Madani). Initial physiological studies reported that UI inhibited water and NaCl absorption in isolated anterior intestinal segments of FW-acclimated, but not SW-acclimated, Mozambique tilapia (Mainoya and Bern). In the flounder, UI appears to stimulate cortisol secretion directly and interact synergistically with ACTH (Kelsall and Balment). Given cortisol's role in both FW and SW acclimation , these results implicate a possible role of UI in FW acclimation.
UII is a member of the somatostatin superfamily and thus likely to be involved in the inhibition of GH secretion (Tostivint et al). Expression of the UII gene and its receptor (UT) gene was reduced after acute transfer of euryhaline flounder from SW to FW, and the UT gene expression in the gills and kidney is downregulated in FW-acclimated fish, although plasma UII levels do not differ between SW and FW fishes (Lu et al). However, plasma UII concentration was reduced for some time after transfer of the flounder from SW to FW, showing the rapid-acting nature of UII (Bond et al). Initial physiological studies revealed that UII has a direct action on the intestine to increase water and NaCl absorption in SW-acclimated tilapia (Mainoya and Bern) and 5% SW-acclimated goby (Loretz et a) and on the urinary bladder of SW-acclimated goby (Loretz and Bern). UII inhibited prolactin secretion from the rostral part of Mozambique tilapia pituitary (Grau et al). These results suggest a role of UII in SW acclimation. However, UII inhibited short-circuit current in the goby skin, suggesting an inhibition of active Cl− secretion, probably via ionocytes (Marshall and Bern). As UI reversed the effect of UII in the goby skin, UI and UII may be involved in the acclimation to opposite osmotic environments. More recently, UII was found to potently inhibit drinking in eels through its action on the brain (Nobata et al). UII stimulated cortisol secretion from the interrenal cells of rainbow trout (Arnold-Reed and Balment).
