The purpose of the present study was to investigate the presence of high-affinity oxytocin-binding sites (putative oxytocin receptors) in the cervix of the non-pregnant ewe. [3H]Oxytocin binding to the peripheral layers of cervical tissue (comprising the serosal layer and the least dense collagenous and muscular layers) and the remaining dense collagenous cervical tissue were studied separately. [3H] Oxytocin-binding sites were detected in membrane fractions prepared from both of these regions, but binding to the peripheral cervix was variable and binding site concentrations were low, hence these were not characterized further. A high-affinity oxytocin-binding site, having a dissociation constant of 1·8 nmol/l, was characterized in the dense collagenous regions of the cervix of ewes killed during the oestrous period. Similar dissociation constants were determined for [Arg8]-vasopressin and the oxytocin-specific agonist [Thr4, Gly7]-oxytocin in competition studies.
[3H] Oxytocin binding to peripheral cervical tissue and to the dense collagenous cervix was generally low or undetectable during the luteal phase, but increased in both tissues around the time of luteolysis. Although specific binding to both tissues was variable during the oestrous period, it was higher at this time than during the luteal phase. [3H] Oxytocin-binding site concentrations were also found to be higher within the inner dense collagenous cervix of oestrous ewes than of pregnant, ovariectomized or anoestrous animals. During the oestrous cycle, oxytocin-binding site concentrations reached a maximum in the dense collagenous cervical tissue on the day of oestrus (141·8 ±44 (s.e.m.) fmol/mg protein), showing a general decline during the following days back to luteal phase values. This compared with concentrations of 513·3 ±132·1 and 216·1 ± 13·9 fmol/mg protein, measured for comparative purposes in endometrial and myometrial membrane preparations, respectively, on the day of oestrus in the same group of ewes. However, in membrane preparations of peripheral cervical tissue higher concentrations were measured on day 14 than on the following 2 days and maximal concentrations were not reached until the day after oestrus (52·7 ± 4·2 fmol/mg protein). Concentrations were maintained at similar values during the subsequent 2 days and significant specific binding was still measurable in both regions of the cervix on day 5.
The localization of oxytocin-binding sites within dense collagenous cervical tissue was investigated autoradiographically using the 125I-labelled specific oxytocin receptor antagonist [1(β-mercapto-β,β-cyclopentamethylene propionic acid), 2-(ortho-methyl)-Tyr2, Thr4, Orn8, Tyr9 -NH2]-vasotocin. The only clear specific labelling was localized to the luminal epithelium of the uterine section of the cervix of oestrous ewes, with labelling in ewes in the luteal phase clearly reduced or absent.
The results demonstrate the presence of a high-affinity oxytocin-binding site within the cervix of the oestrous ewe which is associated with secretory cells and which undergoes similar changes in concentration during the oestrous cycle to uterine oxytocin receptor sites. The significance of this novel putative site of oxytocin action remains to be established.