Endometrial and myometrial tissues, obtained from Merino ewes on 5 different days of the estrous cycle, were incubated at 37 C in 30 ml of gassed (95% O2:5% CO2) Krebs-bicarbonate buffer containing, 0, 10, 100 or 1,000 muU/ml oxytocin. Aliquots of the medium were removed at 10 min intervals and examined for prostaglandin F2alpha (PGF2alpha) content by radioimmunoassay. Fresh-frozen (-70 C) samples of endometrial and myometrial tissue were homogenized in Tyrode's solution. Particulate fractions from each tissue, sedimenting between 1,000 X g for 10 min and 165,000 X g for 30 min, were prepared and assayed for [3H]oxytocin-binding activity. Endometrium incubated in vitro released PGF2alpha spontaneously and oxytocin enhanced this release in a dose-dependent manner. The degree of enhancement with low doses of oxytocin appeared to increase as estrus approached, reaching a maximum on the day of estrus. High-affinity binding sites (Kd = 5 to 7 X 10(-10) M) were found in both myometrium and endometrium. The number of high-affinity sites rose to a peak at estrus in both tissues but the binding capacity of endometrium was twice that of the myometrium at this time. Although both tissues released PGF2alpha during incubation, oxytocin enhanced release from endometrial tissue only. The results suggest that (i) the endometrium is a target for oxytocin, (ii) synthesis of PGF2alpha by the uterus may involve interaction between oxytocin and its endometrial receptors and (iii) ovarian steroids may influence uterine PG synthesis by regulating the availability of these receptors.

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