Osmotically-induced release of vasopressin and oxytocin in non-pregnant women - influence of estrogen and progesterone

Background. Circulating vasopressin and oxytocin are influenced by ovarian steroid blood levels, but the effect of estrogen and progestogen treatment on induced release of the posterior pituitary hormones is not clear. Methods. Eight postmenopausal women who had not been on hormonal replacemen t therapy for at least two months were included in the study. The women wer e treated for four weeks with transdermal administration of estradiol-17 be ta in a daily dose of 100 mu g with the addition of 5 mg tablets of medoxyp rogesterone twice daily for the last two weeks. A 25 minute intravenous inf usion of hypertonic saline (0.06 mg/kg/min) was given before hormonal treat ment, and after two and four weeks with serial plasma sampling for assay of vasopressin and oxytocin. Results. The mean basal concentration of vasopressin, which was 0.83+/-0.13 (SE) pmol/L before hormonal treatment, increased to a statistically signif icant degree after estradiol alone to 1.18+/-0.11 pmol/L and decreased afte r combined estrogen/progestogen treatment to 0.31+/-0.02 pmol/L. Sodium con centration and osmolality increased in a similar way during all three infus ions, but the resultant increase in vasopressin concentration was significa ntly smaller and slower after treatment with estradiol alone than in the fi rst experiment without pretreatment. The areas under the concentration curv e for the second and third infusion were significantly smaller than when no hormone treatment was given. The induced hyperosmolality also caused a ris e in oxytocin levels, but no influence of ovarian hormone treatment was obs erved. Conclusions. Ovarian hormone administration influences vasopressin secretio n, affecting both the basal levels in plasma and the responses to an increa se in plasma osmolality. The influence of ovarian hormones on oxytocin secr etion is minimal.