Summary. The concentrations of oxytocin receptors were measured in intercaruncular and caruncular endometrium, fetal cotyledons, chorioallantois and amnion during pregnancy and parturition in cows. Tissues were obtained on days 20 (endometrium only), 50, 100, 150, 200, 225, 250, 275, at term (days 280–284), during labour and within 24 h after calving. Receptor concentrations in intercaruncular endometrium were low on day 20 of pregnancy, 39 ± 11 fmol mg−1 protein. By day 50, receptor concentrations had increased more than tenfold to 572 ± 52 fmol and rose steadily until day 250 and then levelled off at about 4500 fmol mg−1. Shortly before parturition, on day 282 ± 1, a further rise to 7300 ± 1418 fmol mg−1 was observed, these concentrations were maintained throughout labour. By contrast, caruncular endometrial receptor concentrations remained low until term, mean 145 ± 15 fmol mg−1, and then rose to 720 ± 163 fmol mg−1 during labour (cervix 17 cm – fully dilated). Fetal cotyledons and membranes had very low oxytocin receptor concentrations during most of pregnancy, on average only 20 fmol mg−1 protein. At term and during labour, receptor concentrations were significantly increased in both tissues. Mean concentrations during labour were 163 ± 36 fmol mg−1 for cotyledons, 270 ± 61 fmol mg−1 for chorioallantois and 311 ± 121 fmol mg−1 for amnion. The affinities of the oxytocin binding sites in the cotyledons and caruncular endometrium were similar and significantly higher than in the other tissues (caruncles and cotyledons, mean Kd: 5·70 × 10−10 mol l−1; versus intercaruncular endometrium, Kd 9·15 × 10−10 mol l−1 or fetal membranes, mean Kd 8·50 × 10−10 mol l−1, P < 0·001 and <0·005, respectively). Receptor affinity did not change during gestation in any of the tissues. The steep rise in oxytocin receptor concentrations in fetal membranes, placentomes and endometrium shortly before and during parturition of cows suggests a role for oxytocin in addition to its contractile effect on the myometrium. Oxytocin receptors in these tissues probably mediate prostanoid release which in turn results in enhancement of the contractile effects of the hormone. The low caruncular oxytocin receptor concentrations may prevent the onset of labour before term despite the high concentrations of oxytocin receptors in the intercaruncular endometria during the last trimester of pregnancy.
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