VASOPRESSIN-BINDING SITES IN THE PIG PITUITARY-GLAND - COMPETITION BY NOVEL VASOPRESSIN ANTAGONISTS SUGGESTS THE EXISTENCE OF AN UNUSUAL RECEPTOR SUBTYPE IN THE ANTERIOR LOBE

Arginine vasopressin (AVP) acts in the pituitary gland, in synergy with corticotrophin-releasing factor, to induce ACTH release in response to stressful stimuli. Pituitary AVP receptors in the rat are coupled to phospholipase C, as are the so-called V-1-type AVP receptors. The present study examined [H-3]AVP binding in membranes prepared from the anterior lobe of the pituitary gland of the pig. [H-3]AVP, alone or in competition with analogues, bound to sites in the pig anterior lobe which are pharmacologically similar to those described previously by others in the rat pituitary gland. For comparison, the same competition studies were performed on membrane preparations from the rat liver which contain the classic V-1-type AVP receptor. Pituitary and liver AVP-binding sites were dissimilar; both cyclic and linear V-1 antagonists had, in general, a much lower affinity for pituitary AVP-binding sites than for those in the liver. Thus, Phaa-D-Tyr(Et)-Phe-GLn-Asn-Lys-Pro-Arg-NH2, (Phaa = phenylacetyl) has a 2500-fold greater affinity for the latter (negative logarithm of inhibition constant (pK(i))= 9.64) than for the former (pK(i) = 6.22). One linear antagonist, Pa-D-Tyr-Phe-Val-Asn-Arg-Pro-Arg-Arg-NH2, (Pa = propionyl) had about equal affinities for Liver and pituitary membranes (pK(i) = 6.39 and 6.53 respectively). Another compound, Phaa-D-Tyr-Phe-Val-Asn-Arg-Pro-Arg-Arg-NH2 had the highest affinity found to date for binding to AVP sites in the pituitary (pK(i) = 7.43). These findings suggest some ideas for the design of more potent and/or selective AVP analogues acting in the pituitary gland.