CHARACTERIZATION OF VASOACTIVE-INTESTINAL-PEPTIDE RECEPTORS IN CANINELIVER MEMBRANES

The binding characteristics of vasoactive intestinal peptide (VIP) in the liver membranes of the dog were examined using radioligand binding assay with I-125-VIP and unlabelled peptides and results were compare d with those from the rat. The binding of VIP to canine liver membrane s occurred in a reversible, saturable, specific and temperature-depend ent manner. Guanine nucleotides dose-dependently inhibited VIP binding . The order of potency in competition experiments with unlabelled pept ide was: VIP > pituitary adenylate cyclase activating peptide (PACAP)- 27 > PACAP-38 much greater than peptide histidine isoleucine (PHI) = s ecretin in the dog, and PACAP-27 > PACAP-38 > VIP > PHI > secretin in the rat. PHI and secretin were about 5000 times less potent than VIP i n the dog, but secretin was about 100 times less potent than VIP in th e rat. The VIP binding sites in canine liver membranes have recognitio n sites for VIP which differ from those in rat liver membranes. As mos t of VIP in the portal vein was removed during its passage through the canine liver, the binding sites of canine liver may play a role in de gradation of VIP.