125I-labelled d(CH2)5[Tyr(Me)2,Thr4,Tyr-NH29]OVT: a selective oxytocin receptor ligand

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Abstract

An oxytocic antagonist, [1-(β-mercapto-β,β-cyclopentamethylenepropionic acid,2-O-methyltyrosine,4-threonine,8-ornithine, 9-tyrosylamide]vasotocin (d(CH2)5[Tyr(Me)2,Thr4,Tyr-NH29]OVT), was monoiodinated at the phenyl moiety of the tyrosylamide residue at position 9. 125I-labelling was performed with 1,3,4,6-tetrachloro-3α,6α-diphenyl-glycoluril. Iodination resulted in an increased affinity for rat uterine oxytocin receptors. A considerably lower affinity for rat vascular V1- and renal V2-receptors was found, resulting in a highly specific oxytocin receptor ligand. 125I-labelled d(CH2)5[Tyr(Me)2,Thr4,Tyr-NH29]OVT was demonstrated to bind selectively to one population of binding sites in rat uterus and ventral hippocampal membrane preparations. Dissociation constants ranged between 0.03 and 0.06 aM. After 3 days of exposure autoradiography revealed binding in regions known to contain oxytocin receptors as well as labelling in some new regions, while no binding was found in the lateral septum, a structure containing mainly [8-arginine]vasopressin receptors. The high specific radioactivity of 125I-labelliag allowed important reductions in membrane protein amount, gain in precision of binding analysis as well as considerably lower exposure times for autoradiography.

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    Present address: Rudolf Magnus Institute for Pharmacology, University of Utrecht, Vondellaan 6, 3521 GD Utrecht, The Netherlands.

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