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M Laburthe, A Couvineau and C Rouyer-Fessard
In order to determine species specificity in growth hormone-releasing factor (GRF) interaction with vasoactive intestinal polypeptide (VIP) receptors, we have tested rat (r) GRF (with a His1 such as in VIP), human (h) GRF and position 1 substituted analogs of hGRF (Ala1, Ac- Tyr1, His1, Phe1, and Trp1 in the place of Tyr1) for their ability to inhibit 125I-VIP binding and to stimulate adenylate cyclase activity in human and rat intestinal epithelial membranes. We show that rGRF has a much higher affinity than hGRF for both human and rat VIP receptors. In humans, the Ki values for inhibiting 125I-VIP binding are 0.5 (VIP), 26 (rGRF), and 830 nM (hGRF). In rats the values are 0.6 (VIP), 46 (rGRF), and 1100 nM (hGRF). This is due in part to the presence of His1 in rGRF since the analog His1 hGRF has a higher affinity than hGRF in man and rat, i.e., Ki = 320 nM and 460 nM, respectively. Studies of adenylate cyclase stimulation reveal that rGRF and His1 hGRF are full VIP agonists in man and rat, whereas hGRF and its other analogs behave as partial agonists in both species. One of the hGRF analogs tested (Ac- Tyr1hGRF) is of great interest since it inhibits 125I-VIP binding to rat intestinal membranes with a Ki = 430 nM but has a negligible intrinsic activity in stimulating adenylate cyclase activity (about 6% of the efficacy of VIP). This analog does inhibit the VIP-stimulated adenylate cyclase activity in a dose-dependent manner, complete inhibition of the VIP (0.01-1 nM) effect being obtained with 30 microM analog. The Schild plot of the inhibitory effect further indicates competitive antagonism. In contrast, Ac-Tyr1hGRF is a partial VIP agonist in humans (about 20% of the efficacy of VIP). These results evidence the important role of His1 for peptide interaction with VIP receptors and provide the first example of a competitive VIP antagonist.
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