The effects of ovariectomy and ovarian steroids on uterine oxytocin receptors in the anoestrous ewe

Abstract

The effects of treatment with progesterone, oestradiol-17β or the presumptive oestrogen agonist Tamoxifen on the concentrations of uterine oxytocin receptors (OTr) were investigated in Merino ewes during the anoestrous season. Three groups of ewes were ovariectomised (Ovx) and subsequently given progesterone ( n = 5), oestradiol-17β ( n = 4) or no treatment (control-Ovx, n = 5) for 6 days. Two groups of ewes were sham-operated and given either Tamoxifen ( n = 5) or no treatment (control-intact, n = 5) for 6 days. All ewes were then given an intravenous (i.v.) injection of oxytocin (10 IU) to determine the prostaglandin (PG)F 2α metabolite, 13, 14-dihydro-15-keto-PGF 2α (PGFM) response. Immediately thereafter, the concentrations of uterine OTr were measured in uterine tissues after hysterectomy. The untreated ewes during the anoestrous season (control-intact) had high uterine OTr concentrations. There was no effect of ovariectomy on uterine OTr concentrations in anoestrous ewes (control-Ovx ewes). In contrast, the concentrations of uterine OTr were reduced ( P < 0.05) both in the progesterone-treated and oestradiol-17β-treated Ovx ewes compared with the control-Ovx ewes. The Tamoxifen-treated intact ewes had lower ( P < 0.05) uterine OTr concentrations than did the control-intact ewes. In all groups, the oxytocin-induced responses of PGFM were negligible. These results indicate that anoestrous ewes have high uterine OTr although the oxytocin-induced PGFM response was low. Ovariectomy of anoestrous ewes had no effect on these variables. In addition, treatment of the ovariectomised anoestrous ewe with progesterone or oestradiol-17β for 6 days reduced uterine OTr but did not affect the oxytocin-induced PGFM response.