A RADIOIODINATED LINEAR VASOPRESSIN ANTAGONIST - A LIGAND WITH HIGH-AFFINITY AND SPECIFICITY FOR V1A RECEPTORS

A linear vasopressin antagonist, Phaa-D-Tyr(Me)-Phe-Gln-Asn-Arg-Tyr-NH2 (Linear AVP Antag) (Phaa = Phenylacetyl), was monoiodinated at the phenyl moiety of the tyrosylamide residue at position 9. This antagonist appeared to be a highly potent anti-vasopressor peptide with a pA2 value in vivo of 8.94. It was demonstrated to bind to rat liver membrane preparations with a very high affinity (K(d) = 0.06 nM). The affinity for the rat uterus oxytocin receptor was lower (K(i) = 2.1 nM), and affinities for the rat kidney- and adenohypophysis-vasopressin receptors were much lower (K(i) = 47 nM and 92 nM, respectively), resulting in a highly specific vasopressin V(la) receptor ligand. Autoradiographical studies using rat brain slices showed that this ligand is a good tool for studies on vasopressin receptor localization and characterization.