RECEPTOR-MEDIATED UTERINE EFFECTS OF VASOPRESSIN AND OXYTOCIN IN NONPREGNANT WOMEN

Objective To study in nonpregnant women myometrial actions of vasopressin and oxytocin and the involvement in these effects of specific uterine receptors. Subjects Twenty-eight women undergoing hysterectomy for benign gynaecological disorders. Interventions Intrauterine pressure recordings. Intravenous bolus injections of 10 pmol/kg body weight of vasopressin and oxytocin. Repeated blood sampling for measurement of vasopressin and oxytocin concentrations in plasma. Recording of effects of vasopressin and oxytocin on isolated myometrium. Estimation of myometrial concentrations of vasopressin Via and oxytocin receptors. Measurement of plasma oestradiol and progesterone. Main outcome measures Vasopressin- and oxytocin-induced increases of the area under the in vivo recording curve over 10 minutes and EC(50) concentrations of dose-responses in vitro. Concentrations of vasopressin Via and oxytocin receptors. Results Vasopressin was on average four times more potent than oxytocin in vivo. The effect of vasopressin premenstrually was more pronounced than in women under oestrogen influence only (proliferative phase-hyperproliferation; P = 0.02), and tended to be more marked than in those in the luteal phase (P = 0.07). No significant variation in oxytocin response with the hormonal state was observed. EC(50) concentrations of vasopressin were more than 20 times lower than those of oxytocin. The median concentration of the vasopressin Via receptor was 208 (range 139-343) fmol/mg protein and that of the oxytocin receptor 49 (38-87) fmol/mg protein. Vasopressin receptor concentrations and in vivo effects of this peptide did not correlate, whereas for those of oxytocin a significant correlation was observed (P = 0.02). Conclusion The high potency of vasopressin in nonpregnant women, particularly premenstrually, firmly supports an aetiological importance of this peptide in the uterine hyperactivity of primary dysmenorrhoea. Oxytocin seems to be less important in this condition in view of its much smaller potency and the absence of increase in effect premenstrually. Vasopressin appears to influence both the oxytocin and the vasopressin Via receptor sites in the uterus, whereas oxytocin acts specifically on its own receptor.