A CHIMERIC VIP-PACAP ANALOG BUT NOT VIP PSEUDOPEPTIDES FUNCTION AS VIP RECEPTOR ANTAGONISTS

The ability to assess the importance of VIP in different physiological processes is limited by the lack of specific potent antagonists. In t he present study, we have adopted two different approaches used succes sfully with other peptides in an attempt to identify new VIP receptor antagonists. One involves the formation of pseudopeptides by insertion of reduced peptide bonds in the NH2-terminus from position 2 to 8 of VIP. The other methodology involves the formation of a COOH-terminal c himeric analogue by combining VIP(6-28) and PACAP(28-38). The ability of each of these peptides to function as an antagonist was compared wi th reported VIP antagonists. All of the peptides inhibited [I-125]VIP binding to VIP receptors on guinea pig pancreatic acini. For the pseud opeptides the affinities were: [psi 3-4]VIP (0.2 mu M) = 4 X [psi 4-5] VIP = 8 X [psi 8-9]VIP = 14 X [psi 6-7]VIP, [psi 2-3]VIP = 25 X [psi 5 -6]VIP. Each nonpseudopeptide analogue also inhibited VIP binding with relative potencies of VIP(6-28)-PACAP(28-38) (1 mu M)= 2.5 X [4-Cl-D- Phe(6),Leu(17)]VIP, VIP(10-28), neurotensin(6-11)-VIP(7-28) = 6 X [Ac- Tyr(1),D-Phe(2)]GRF. All pseudopeptides were agonists with relative po tencies: [psi 3-4]VIP > [psi 6-7], [psi 4-5]VIP > [psi 5-6] > [psi 8-9 ]VIP > [psi 2-3]VIP. The reported VIP receptor antagonist, neurotensin (6-1l)-VIP(7-28), was also an agonist. Each of the remaining peptides had no agonist activity; however, each inhibited VIP-stimulated amylas e release with potencies of: VIP(6-28)-PACAP(28-38) = VIP(6-28) (K-i = 0.3 mu M) = 2 X [4-Cl-D-Phe(6),Leu(17)]VIP = 3 x VIP(10-28) = 9 X [Ac -Tyr(1),D-Phe(2)]GRF. We conclude that the strategy of making reduced peptide bond analogues at the NH2-terminus of VIP does not result in r eceptor antagonists as it did with secretin or GRF. A chimeric analogu e, VIP(6-28)-PACAP(28-38), is two times more potent than any existing VIP antagonist; however, its increase in affinity is due to the presen ce of the VIP(6-28) moiety entirely. This raises the possibility that additional, more potent, antagonists may be developed by modifying VIP (6-28).