Heterogeneity of oxytocin receptors in the pregnant rat myometrium near parturition.

The contractile response of longitudinal and circular strips of rat myometrium to oxytocin (OT), [Arg8]vasotocin (AVT) and [Arg8]vasopressin (AVP) was examined in vitro in late pregnancy (controls at day 20, day 21, day 22) and 24 h after treatment with the antiprogestin, RU 486 (or mifepristone, given on day 20 or day 21 of gestation). In controls, the maximal effect (Emax) of OT in longitudinal strips increased with gestation, but there was no change in OT sensitivity in either myometrial layer. The pA2 values for the OT/AVP V1 antagonist, d(CH2)5[Tyr(Me)2]OVT, against OT, AVT and AVP on longitudinal strips and against AVT and AVP on circular strips were not different. However, the antagonist did not inhibit the effect of OT in circular strips, indicating that the receptor activated by OT is different from that in longitudinal strips. RU 486 treatment did not modify the characteristics of responses to AVT and AVP. RU 486 improved the response to OT in circular strips from rats treated at day 20, however the sensitivity to OT in longitudinal strips was decreased compared to that in the controls.