The effects of oestradiol, progesterone, oxytocin and combinations of these hormones on oxytocin receptor binding in explants of uteri from ovariectomized ewes were determined. Receptor binding remained unchanged after 96 h in culture in control medium. Oestradiol at concentrations of 1 pmol–10 μmol l−1 did not alter receptor binding activity in tissue cultured for 96 h, but at 100 μmol l−1 oestradiol significantly reduced (P < 0.01) receptor binding activity. Progesterone and oxytocin significantly reduced receptor binding activity in explants cultured for 96 h (P < 0.05). Explants cultured in medium containing progesterone and oestradiol or oxytocin and oestradiol showed receptor binding characteristics similar to those found in tissue cultured with progesterone or oxytocin alone. When explants were cultured for 72 h in medium containing oestradiol followed by 24 h in medium containing oestradiol alone, oestradiol with oxytocin, oestradiol with progesterone, oxytocin alone, progesterone alone, or in medium with no added hormones, receptor binding activity was always reduced in the presence of progesterone and oxytocin whether or not oestradiol was present in the medium. Receptor binding activity in explants cultured for the final 24 h in medium containing oestradiol or no added hormones were similar to those in tissue cultured in control medium for a total of 96 h. These data show that progesterone and oxytocin reduce oxytocin receptor binding activity in cultured uterine tissue and, in contrast to its effect on the rat uterus, that oestradiol is not a potent stimulator of oxytocin receptor synthesis in uterine tissue of ovariectomized ewes in vitro. These data also raise the possibility that post-receptor events resulting in increased secretion of prostaglandin F2α, rather than oxytocin receptor synthesis, are controlled by oestrogen in sheep.
E. L. Sheldrick and H. C. Flick-Smith
D. C. Wathes, H. Flick Smith, S. T. Leung, K. R. Stevenson, S. Meier and G. Jenkin
The development of uterine oxytocin receptors is an important regulatory step in the initiation of labour. Paracrine production of oxytocin by uterine and placental tissues may also be involved in some species. Placentome, intercotyledonary endometrium, myometrium and fetal membranes were collected from 3–5 ewes each, at regular intervals throughout pregnancy and from eight ewes during labour. Localization of mRNA encoding oxytocin and its receptor was by in situ hybridization; oxytocin peptide concentrations were measured by radioimmunoassay and oxytocin receptor concentrations were measured by autoradiography and radioreceptor assay. In the intercotyledonary endometrium, mRNA encoding the oxytocin receptor was located in the luminal epithelium only. Both the epithelial and myometrial receptors were detected at low concentrations from the fourth week of gestation onwards, with a major increase associated with the onset of labour. In the placentomes, oxytocin receptors were localized to a stromal capsule surrounding the placental villi. Expression in this region was maximal in mid-gestation, declining in the second half of pregnancy and remaining low during labour. Cervical oxytocin receptors were detected at low concentrations in the epithelium and the muscular/connective tissue layers from day 22 of pregnancy onwards. There was no evidence for the local uterine production of oxytocin in the ewe; mRNA encoding oxytocin was undetectable and oxytocin concentrations were always <23 pg g−1 wet mass of tissue. These results suggest that regulation of the timing of oxytocin receptor development varies between the different tissue types, despite a similar steroidal background. The receptors in the luminal epithelium are probably associated with the ability of exogenous oxytocin to induce the release of PGF2α throughout most of pregnancy. The increase in receptors in both the intercotyledonary endometrium and myometrium at term suggest an involvement in labour, whereas their role in caruncular stroma in mid-pregnancy is unknown.
E. L. Sheldrick, H. C. Flick-Smith and G. J. Dos Santos Cruz
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.