Characterization of a novel, linear radioiodinated vasopressin antagonist: an excellent radioligand for vasopressin V1a receptors.

We report on the pharmacological properties of a potent and selective linear vasopressin (AVP) V1a receptor antagonist HO-Phenylacetyl1-D-Tyr(Me)2-Phe3-Gln4-Asn5-Arg6-Pro7-Arg8-NH2 (HO-LVA). Iodinated on the phenolic substituent at position 1, [125I]-HO-LVA displayed the highest affinity for rat liver V1a receptors (8 pM) ever reported. Furthermore, affinities of HO-LVA and I-HO-LVA for V1b, V2 and oxytocin (OT) receptors was 400- to 1,000-fold lower than for V1a receptors, rendering it a highly selective ligand.