Abstract
The binding characteristics of vasoactive intestinal peptide (VIP) in the liver membranes of the dog were examined using radioligand binding assay with 125I-VIP and unlabelled peptides and results were compared with those from the rat. The binding of VIP to canine liver membranes occurred in a reversible, saturable, specific and temperature-dependent manner. Guanine nucleotides dosedependently inhibited VIP binding. The order of potency in competition experiments with unlabelled peptide was: VIP > pituitary adenylate cyclase activating peptide (PACAP)-27 > PACAP-38 > peptide histidine isoleucine (PHI) = secretin 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 in the dog, but secretin was about 100 times less potent than VIP in the rat. The VIP binding sites in canine liver membranes have recognition sites for VIP which differ from those in rat liver membranes. As most 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 degradation of VIP.
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