A new vasoactive intestinal peptide antagonist discriminates VIP receptors on guinea pig trachea and human neuroblastoma cells

Abstract VIP is a widely distributed neuropeptide of 28 amino acids, whose central part is proposed to be an amphiphilic α-helix. In order to gain an understanding of the effect of this α helix on receptor binding and stimulation, a human VIP analog has been designed in which the residues 12 to 19 were replaced by a spacer of the same length, (γ-aminobutyryl) 2 . This peptide altered neither the basal guinea pig tracheal smooth muscle tonus nor the VIP-induced relaxation. Conversely, the VIP analog was found to displace VIP from its binding sites on LA-N-2 human neuroblastoma cells (VIP IC 50 : 5.4 nM; VIP analog IC 50 : 52.2 nM) and to inhibit the VIP-induced cyclic AMP production of 58±15% at 1 μM and 95±2% at 10 μM. It seems that the α helix structure might only play the role of a spacer holding the important residues, at the N- and C-ends, respectively, at an appropriate distance. In the VIP analog structure, the (γ-aminobutyryl) 2 chain introduced in place of the α helix plays the role of adequate spacer to bind the LA-N-2 receptors but probably does not induce the active conformation for receptor stimulation. The lack of VIP analog effects on the tracheal receptors related to relaxation argues for a possible heterogeneity of VIP receptors on a pharmacological basis.