[Study of oxytocin receptor activities in human myometrial preparation (author's transl)]

This study was designed to investigate the oxytocin (OT) receptor activities in the human myometrial specimen throughout the course of pregnancy and in labor. The 3H-labeled OT with adequate specific activity (24 Ci/mM) and biologic activity (350 IU/mg) was employed for the OT radioreceptor assay and the apparent dissociation constant (Kds) and the number of binding sites (NBS) were analyzed by Scatchard plot. These activities were predominantly found in 20,000 xg pellets of the myometrial specimens and its binding activities were 2.2 +/- 0.1 nmol/mg protein that were assessed to be composed of two binding fractions: High-affinity--low capacity receptor fraction and low affinity--high capacity fraction. The gestational increase of the binding affinity was significant and so was the increase of the capacity, and the Kd was 1.87 +/- 0.30 nM and the NBS was 1.50 +/- 0.50 pmol/mg protein in the full term specimens and both were significantly higher than those of the nonpregnant specimens (p less than 0.01). When active labor was initiated, a significant decrease of the binding capacity (0.74 +/- 0.21 pmol/mg protein) was found, regardless of the mode of onset of labor; i.e. spontaneous or induced labor, while the binding affinity remained unchanged. These findings suggest that the gestational increase of the uterine sensitivity to OT is due to the increase of both binding affinity and capacity of the OT receptors. And there are OT and OT receptor interaction actively involved during the active stage of labor and this interaction seems to be coupled with the contractile process but is apparently modified when active labor had been already initiated.