[125I]-[d(CH2)5, Sar7] AVP: A Selective Radioligand for V1Vasopressin Receptors

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Abstract
Arginine8-vasopressin (AVP) acts on vascular and hepatic V1 receptors to influence blood pressure and glycogenolysis respectively. We have radioiodinated the AVP V1 receptor antagonist, [1-(β-mercapto-β,β-cyclopentamethylenepropionic-acid), 7-sarcosine, 8-arginine] vasopressin ([d(CH2)5, Sar7]AVP) and determined its receptor-binding properties in rat liver and kidney plasma membranes. The binding was of high affinity to single classes of receptors (liver: Kd=3.0nM and Bmax=530±10 fmol/mg protein, kidney: Kd=0.5±0.9nM and Bmax=11±8 fmol/mg protein). Competition of [125I]-[d(CH2)5, Sar7]AVP binding by unlabelled AVP analogues gave the following order of potency in both tissues, consistent with that expected for binding to a V1 receptor: [d(CH2)5, Tyr(Me)2]AVP > AVP > [d(CH2)5, D-Ile2, Ile4] AVP > DDAVP. No degradation of [125I]-[d(CH2)5, Sar7]AVP during incubation or storage was detected by HPLC analysis. We have used [125I]-[d(CH2)5, Sar7]AVP and in vitro autoradiography to demonstrate its use in localizing brain AVP receptors. These studies suggest that [125I]-[d(CH2)5, Sar7]AVP is a suitable selective radioligand for labelling V1 receptors and will provide a valuable tool for the study of the localization and regulation of AVP V1 receptors in tissues and in receptor isolation.