Oxytocin receptor binding activity in explants of caruncular and intercaruncular endometrium collected from luteal phase ewes increased during culture. An initial rise in binding activity occurred during the first 24 h of culture in both tissues; binding activity in intercaruncular endometrium continued to increase until day 6, remained unchanged on day 8 and had decreased by day 10 of culture. The maximum concentration of receptors in caruncular endometrium was significantly lower than that in intercaruncular endometrium (P < 0.001) and did not change significantly between days 4 and 10 of culture. Apparent dissociation constants and maximal binding of oxytocin receptor in cultured caruncular and intercaruncular endometrium were 3.09 and 2.72 nmol l−1 and 249 and 459 fmol [3H]oxytocin bound mg−1 protein, respectively. Concentrations of oxytocin receptor remained constant in myometrium during 96 h of culture. The rise in endometrial oxytocin receptor concentration did not result from exposure to fetal calf serum, phenol red or insulin in the culture medium. Substituting fetal calf serum with sheep serum or BSA did not block the rise in receptor binding activity. Actinomycin D and cycloheximide inhibited the rise in receptor concentration in both tissues. Co-culture of lung or kidney with endometrium had no effect on binding activity, whereas co-culture with luteal tissue effectively reduced the rise in oxytocin receptor concentration. To establish whether synthesis of functional oxytocin receptors occurred during culture, the effect of oxytocin on prostaglandin F2α (PGF2α) production was assessed in fresh tissue and after 48 h in culture. No PGF2α was secreted in response to oxytocin on the day of tissue collection, but after 48 h oxytocin-induced PGF2α production occurred in intercaruncular endometrium and to a lesser extent in caruncular endometrium. Indomethacin reduced basal and oxytocin-induced PGF2α production in both cultured tissues. These data show that de novo synthesis of functional oxytocin receptors occurs during culture and can be inhibited by a secretory product of the corpus luteum.
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