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Some of the binding characteristics of a novel oxytocin receptor ligand ¹²⁵I-labelled [1-(β-mercapto-β, β-cyclopentamethylene propionic acid), 2-(ortho-methyl)-Tyr²,Thr⁴,Orn⁸,Tyr⁹-NH₂]-vasotocin ([¹²⁵I]OTA) have been determined in the sheep uterus. The compound was subsequently used for the autoradiographic localization of oxytocin receptors in the uterus and oviduct of the ewe.

Specific binding of [¹²⁵I]OTA to crude membrane fractions of ovine endometrium was time-dependent and was unaffected by the addition of cations to incubation media. Endometrial membranes contained a single population of saturable, high-affinity binding sites for the iodinated ligand (dissociation constant (K_d) 0·23±0·08 nmol/l) and unlabelled oxytocin competed with [¹²⁵I]OTA for binding sites with high affinity (K_d 1·29±0·4 nmol/l) in the presence of Mg²⁺ In contrast, unlabelled OTA was able to compete with high affinity (K_d 1·13±0·16 nmol/l) in the absence of cation. Competition studies with a number of oxytocin analogues and related peptides and the tissue distribution of [¹²⁵I]OTA binding sites also indicated that [¹²⁵I]OTA bound to the ovine oxytocin receptor. This was further validated by autoradiographic studies which showed specific labelling with [¹²⁵I]OTA to be greater to uterus and oviduct obtained from ewes which had been killed within 2 days of oestrus than to similar tissue from ewes killed during the luteal phase. In both the ampullary and isthmic regions of the oviduct and the myometrium, [¹²⁵I]OTA binding sites were confined to smooth muscle. Endometrial binding sites for [¹²⁵I]OTA were consistently located on the luminal epithelium and epithelial cells lining secretory glands. In addition, in one ewe which had been killed 2 days after cloprostenol treatment, stromal cells were labelled in a caruncular region of the endometrium. The consistency of this observation between similar animals remains to be determined.

The autoradiographic technique demonstrated appears sufficiently sensitive to allow further studies into the distribution of the endometrial oxytocin receptor throughout the oestrous cycle, and into its regulation at luteolysis and during the establishment of pregnancy.

Journal of Endocrinology (1991) 128, 187–195

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