SYNTHETIC RAT V-1A VASOPRESSIN RECEPTOR FRAGMENTS INTERFERE WITH VASOPRESSIN BINDING VIA SPECIFIC INTERACTION WITH THE RECEPTOR

To study the vasopressin receptor domains involved in the hormonal bin ding, we synthesized natural and modified fragments of V-1a vasopressi n receptor and tested their abilities to affect hormone-receptor inter actions. Natural fragments mimicking the external loops one, two, and three were able to inhibit specific vasopressin binding to V-1a recept or, In contrast, the natural N-terminal part of the V-1a vasopressin r eceptor was found inactive, One fragment, derived from the external se cond loop and containing an additional C-terminal cysteine amide, was able to fully inhibit the specific binding of both labeled vasopressin agonist and antagonist to rat liver V-1a vasopressin receptor and the vasopressin-sensitive phospholipase C of WRK1 cells, The peptide-medi ated inhibition involved specific interactions between the V-1a recept or and synthetic V-1a vasopressin receptor fragment since 1) it was de pendent upon the vasopressin receptor subtype tested (K-i(app) for the peptide: 3.7, 14.6, and 64.5 mu M for displacing [H-3]vasopressin fro m rat V-1a, V-1b, and V-2 receptors, respectively; 2) it was specific and did not affect sarcosin 1-angiotensin II binding to rat liver memb ranes; 3) it was not mimicked by vasopressin receptor unrelated peptid es exhibiting putative detergent properties; and 4) no direct interact ion between [H-3]vasopressin and synthetic peptide linked to an affini ty chromatography column could be observed, Such an inhibition affecte d both the maximal binding capacity of the V-1a vasopressin receptor a nd its affinity for the labeled hormone, depending upon the dose of sy nthetic peptide used and was partially irreversible, Structure-activit y studies using a serie of synthetic fragments revealed the importance of their size and cysteinyl composition, These data indicate that som e peptides mimicking extracellular loops of the V-1a vasopressin recep tor may interact with the vasopressin receptor itself and modify its c oupling with phospholipase C.