EVIDENCE FOR A LOCAL FETAL INFLUENCE ON MYOMETRIAL OXYTOCIN RECEPTORSDURING PREGNANCY IN THE TAMMAR WALLABY (MACROPUS-EUGENII)

Mesotocin (MT), the oxytocin-like peptide of the tammar wallaby (Macro pus eugenii) is important for delivery of live young. The tammar mesot ocin receptor (MTR) was first characterized using the iodinated oxytoc in receptor antagonist [I-125]d(CH2)(5) [Tyr(Me)(2), Tyr(4), Orn(8), T yr-NH29]-vasotocin. MTR concentrations were then measured in matched s amples of gravid and nongravid myometrium and median vagina at differe nt stages of the 26-day pregnancy. MTR concentrations in both the grav id and nongravid myometrium changed significantly (ANOVA, p < 0.01) du ring pregnancy. There was no difference in MTR concentrations between uteri on Days 8-22. From Day 23 of pregnancy, MTR concentrations in th e gravid myometrium increased (615.8 +/- 144.0 fmol/mg protein), where as in the nongravid myometrium, they remained unchanged (248.6 +/- 65. 5 fmol/mg protein). Receptor concentrations were high in the gravid my ometrium during the last 3 days of pregnancy but decreased significant ly in the nongravid myometrium. In the median vagina, MTR concentratio ns were row compared with myometrial tissues and did not increase at t erm, Changes in MTR concentrations paralleled changes in uterine respo nsiveness to exogenous MT in vitro. Our data show that MTR concentrati ons and the responsiveness to MT differ between the gravid and nongrav id myometrium during pregnancy. The increase in MTRs in the gravid myo metrium and the decrease in the nongravid suggest that different facto rs influence these receptors in the separate uteri, independent of sys temic influence.