Structural requirements for VIP interaction with specific receptors in human and rat intestinal membranes: effect of nine partial sequences.

Nine VIP sequences have been tested for their ability to inhibit the specific binding of 125I-VIP and to stimulate adenylate cyclase activity in intestinal epithelial membranes from rat and man. They are VIP 2-28; VIP 1-14; VIP 2-14; VIP 14-28; VIP 15-28; VIP 20-28; VIP 21-28 and two sequences where the N-terminal VIP 1-6 or VIP 1-9 have been joined covalently with the C-terminal VIP 20-28 or VIP 21-28. It appears that only VIP 2-28, VIP 14-28 and VIP 15-18 are able to inhibit competitively the binding of 125I-VIP to human and rat membranes. These analogues are respectively 88, 8,300 and 25,000 times less potent than VIP 1-28 in rat; they are respectively 70, 7,900 and 13,000 times less potent than VIP 1-28 in man. With respect to adenylate cyclase activation, VIP 14-28 and VIP 15-28 are very weak stimulators in the membranes from both species. VIP 2-28 behaves as a full VIP agonist in man whereas it is a partial VIP agonist in rat. These results indicate the structural importance of the whole VIP sequence for interacting with human and rat VIP receptors and further argue for a different structural requirement of rat and human receptors.