The presence of oxytocin receptors in ovine oviduct has been investigated. High-affinity binding sites for [3H]oxytocin were detected in crude membrane fractions prepared from the oviducts of ewes killed during the oestrous period. The dissociation constant calculated for these sites in competition studies was 1·7 nmol/l. Similar dissociation constants were calculated for [Arg8]-vasopressin and the oxytocin-specific agonists [Gly7]-oxytocin and [Thr4, Gly7]-oxytocin, indicating that these sites represent oxytocin receptors. At least one additional site of lower affinity and undetermined identity was present.
The relative concentration of oxytocin-binding sites in preparations of oviduct membranes were estimated in ewes killed at different stages of the oestrous cycle using a single concentration of [3H]oxytocin. Binding was low during the luteal phase of the cycle but increased to a maximum at oestrus (77·7 fmol/mg protein). Binding fell after ovulation, reaching what appeared to be basal concentrations by the early luteal stage of the cycle. Binding to oviductal membranes from prepubertal, anoestrous and pregnant ewes was also low, but in anoestrous animals which had been treated with progesterone and oestrogen it was similar to values measured in ewes at oestrus. These results are consistent with the existence of oviductal oxytocin receptors which are regulated by ovarian steroids.
We conclude that oxytocin receptors are present in the oviduct of the ewe around the time of ovulation. The significance of oxytocin to events taking place in the oviduct at this time remains to be determined.