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J. E. Hixon and A. P. F. Flint

Summary. Administration of oestradiol-17β benzoate on Days 9 and 10 of the oestrous cycle resulted in episodic secretion of PGF-2α (as indicated by elevated circulating concentrations of 13,14-dihydro-15-ketoprostaglandin F-2α) and a decline in circulating progesterone. Release of PGF-2α began 35 ± 3 h after first injection of oestrogen and progesterone concentrations declined from 42 ± 3 h. Secretion of oxytocin, which was first observed 26 ± 3 h after oestrogen treatment, preceded secretion of PGF-2α; 69% of pulses of oxytocin coincided with episodes of PGF-2α secretion.

Uterine oxytocin receptor concentrations were raised in ewes treated with oestrogen, increases occurring in caruncular endometrium and myometrium by 12 h after treatment and in intercaruncular endometrium by 24 h. Raised receptor concentrations were followed at 24 h by increases in the incorporation of [3H]inositol into phosphatidylinositol and in the hydrolysis of labelled tissue phosphoinositides in response to oxytocin in slices of caruncular endometrium incubated in vitro. The following sequence of events is therefore suggested to occur at oestrogen-induced luteolysis: (a) induction of the oxytocin receptor; (b) increased turnover of phosphoinositides; (c) onset of episodic secretion of PGF-2α; and (d) functional luteolysis.

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J. Zhang, P. G. Weston and J. E. Hixon

Summary. The interaction between oestrogen and progesterone in the regulation of the uterine oxytocin receptor in sheep was evaluated by measuring the binding of oxytocin to membrane preparations of caruncular and intercaruncular endometrium and myometrium. Ovariectomized ewes were assigned in groups of five to each cell of a 4 × 2 factorial design. The four treatments were (a) vehicle (maize oil) for 12 days, (b) progesterone (10 mg day−1) for 9 days, (c) progesterone for 9 days followed by maize oil until day 12 and (d) progesterone for 12 days. The two oestradiol treatments consisted of the administration of implants in the presence or absence of oestradiol. The ewes were killed on day 10 (group b) or day 13 (groups a, c and d) for collection of uterine tissues. The response of the caruncular and intercaruncular endometrium to the treatments was similar. In the absence of oestradiol, treatment with progesterone continuously for either 9 or 12 days reduced the concentration of the oxytocin receptor in comparison with both the control and the progesterone withdrawal group (in which values were similar). The presence of oestradiol reduced the receptor concentrations in control and both 9- and 12-day continuous progesterone treatment groups, but enhanced the concentration in the progesterone withdrawal group. The myometrial oxytocin receptors responded in a similar way to those in the endometrium to progesterone treatment alone, but the addition of oestradiol produced no further effect. In conclusion, progesterone and oestradiol caused downregulation of the endometrial oxytocin receptor. On the other hand, progesterone withdrawal, similar to that which occurs during luteolysis, increased receptor density in the presence of oestradiol. Progesterone may influence the response of the myometrium to oxytocin by causing a reduction in receptor density.

Keywords: endometrium; myometrium; oestradiol; oxytocin receptor; progesterone; sheep