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OP Rorstad, I Wanke, DH Coy, A Fournier and M Huang
Department of Medicine, University of Calgary, Alberta, Canada.
The structure-activity relationships for vasoactive intestinal peptide (VIP) receptor binding were studied using N-terminally modified VIP analogs. VIP fragments, and VIP receptor antagonists. Tissue sources included bovine coronary artery, rat mesenteric artery, rat pituitary, rat brain synaptosomes, and rat liver. Experimental conditions for receptor binding were maintained as near to identical as possible. The competitive binding curves for VIP analogs were similar in the bovine and rat vascular preparations. However, appreciable differences were observed between the vascular and other preparations. The vascular receptors discriminated between [D-His1]VIP and [Phe1]VIP, whereas the receptors in other tissues did not. The greatest selectivity was found for [D-Ala4]VIP, which was among the lowest affinity analogs tested on the vasculature but among the highest affinity analogs in the other preparations. The rank orders of analog potencies were comparable for the rat brain and pituitary receptors. The rat liver VIP receptor differed from its counterpart in brain and pituitary predominantly by discriminating between [D-Phe2]VIP and [D-Arg2]VIP. The two VIP receptor antagonists bound weakly and nonselectively to all receptor preparations. Integrity of the full VIP molecule was necessary for full potency of binding to the vascular receptor. We conclude that the vascular VIP receptor possesses recognition properties that are distinct from those for VIP receptors in liver, pituitary, or brain.
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