Evidence for presence of a new type of neurohypophysial hormone receptor in fish gill epithelium.

Arginine vasotocin (AVT) and isotocin (IT) induced direct inhibition of adenylate cyclase activity in gill plasma membranes of the rainbow trout adapted to freshwater (FW) and seawater (SW). The maximal inhibition was obtained with 10(-11)-10(-10) M (50% inhibitory concentration approximately 10(-13) M), values in agreement with the circulating levels of AVT in trout blood. Specific V1 or V2 agonists or antagonists (with reference to vasopressin) were used to define the specificity of the neurohypophysial peptide receptors involved in this inhibition. The V1 agonist [Phe2,Orn8]vasotocin ([Phe2]OVT) inhibited the basal and glucagon-stimulated adenylate cyclase activity, and this effect in SW (20%) was twice more than in FW (10%). The V2 agonist 1-deamino[Val4,Arg8]-vasopressin (dVDAVP), however, produced a stimulation that was of the same amplitude (10%) in both media. The V1 antagonist [(1-beta-mercapto-beta-beta-cyclopentamethylenepropionic acid), 1-(O-ethyl)Tyr2,Orn8]vasotocin (d(CH2)5[Tyr(Et)2]OVT) totally reversed the AVT- or IT-induced inhibition of basal or glucagon-stimulated cyclase activity, whereas the V1/V2 antagonist [(1-beta-mercapto-beta-beta-cyclopentamethylene propionic acid), 1-(O-ethyl)D-Tyr2,Val4,Arg8]vasopressin (d(CH2)5[D-Tyr(Et)2]-VAVP) (previously used as V2 antagonist in amphibians) had no such effect. When active, all analogues had their maximal activity at 10(-11)-10(-10) M (50% maximal activity approximately 10(-13) M), as observed with the natural peptides. These results, together with our previous observations, point to the gill epithelium as a direct target organ for neurohypophysial peptides and suggest that the hormone receptors involved in fish belong predominantly, if not exclusively, to a new type that we propose to designate as fish neurohypophysial hormone (NHF) receptors while awaiting further characterization.