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.