Study of oxytocin receptor : II. Gestational changes in oxytocin activity in the human myometrium.

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Abstract
The oxytocin(OT)-specific binding receptors in 20,000 .times. g pellets of nonpregnant, 1st-trimester and tern human myometria were investigated. The receptor analysis was done using the lower uterine segment at term and the lower portion of the anterior uterine body in nonpregnant and 1st-trimester subjects, and no difference was found in the myometrial receptor concentrations in the various uteri. The mean .+-. SD values of the receptor Kd were 3.33 .+-. 0.50, 2.71 .+-. 1.03 and 1.87 .+-. 0.30 nM and the number of binding sites was 0.30 .+-. 0.10, 0.50 .+-. 0.10 and 1.50 .+-. 0.50 pmol/mg protein at each stage studied, indicating that the gestational increase of uterine sensitivity to OT is due to the increase in myometrial OT binding sites as well as its binding affinity. Myometrial OT binding before and after the onset of labor was studied and a marked decrease in total myometrial OT binding was noticed; 35.6 .+-. 13.0% before and 20.2 .+-. 5.0% afterwards. This decrease may be due to the decrease in the number of binding sites from 1.50 .+-. 0.50 to 0.74 .+-. 0.21 pmol/mg protein after the onset of labor (P < 0.01). No changes were found in the dissociation constants. It seems that OT and its receptor coupling trigger labor or are involved in the early steps of labor.