Oxytocin receptors in nonpregnant human uterus

The concentration and distribution of specific [3H]oxytocin-binding sites in the nonpregnant human uterus were studied. Oxytocin was bound with an apparent Kd of about 1 nM in the crude membrane fractions of the fundus and corpus. A second class of sites with lower affinity and higher capacity for oxytocin also was found. Consistent with its high uterotonic potency in nonpregnant uteri, arginine vasopressin was as effective as oxytocin in inhibiting [3H]oxytocin binding to corpus and fundus membrane preparations. The concentrations of high affinity binding sites in fundus and corpus were similar and were significantly higher than those of membrane fractions from isthmus or Fallopian tubes. The lowest concentration of sites was found in the cervix. Endometrial membrane preparations contained oxytocin-binding sites in about the same concentration as that in the myometrium. The concentrations of oxytocin receptors in all parts of the nonpregnant uterus were somewhat higher in the luteal phase than in the follicular phase. Concentrations were lowest in postmenopausal uteri. The concentrations of oxytocin receptors in nonpregnant uteri were 50-100 times lower than those in uteri near the end of gestation. These differences correspond to the differences in sensitivity to oxytocin between luteal phase and follicular phase and between the nonpregnant and late term uterus. These findings add support to the evidence that the binding sites for oxytocin represent true oxytocin receptors.