Effect of reproductive status on plasma oxytocin concentrations and the renal response to oxytocin in the conscious rat.

The magnitude of diuresis and natriuretis produced by oxytocin in the female rat has been shown to be dependent on the stage of the oestrous cycle. A study has been performed to determine the role of ovarian steroids in modulating the renal response to oxytocin infused at a rate of 100 fmol min-1 in hypotonic saline. Observations were performed on ovariectomised rats with and without steroid treatment and rats with suppressed oestrous cycles following treatment with the long-acting gonadotrophin-releasing hormone analogue Zoladex, given as a 100 mg S.C. depot, or the antioestrogen tamoxifen, given as three daily injections of 1 mg. Steroid treatment comprised a single dose of 10 microgram oestradiol benzoate or 2.0 mg progesterone, or a combination of the two hormones given 30 h apart. Ovariectomy had no significant effect on plasma oxytocin concentrations, although progesterone treatment produced an increase. The natriuresis in ovariectomised animals of 27% was smaller (P < 0.05) than that seen on the day of pro-oestrus and closer to that seen in the intact rat at oestrus. The responses in animals with suppressed oestrous cycles following treatment with tamoxifen were not significantly different from those observed following ovariectomy, neither was the diuresis following Zoladex. The renal responses were greater following treatment with oestradiol than in the ovariectomised group. Thus ovarian steroids do influence the renal responsiveness to oxytocin with oestradiol augmenting the response.