The effect of combined oestrogen and progesterone replacement on the renal responses to oxytocin and vasopressin in ovariectomized rats.

Objective: Renal responsiveness to the neurohypophyseal hormones, oxytocin and vasopressin, has been shown in the rat to vary during pregnancy and lactation. A study was performed to determine whether ovarian steroids could contribute to the observed changes.

Design: Using a previously validated method, fluid excretion during infusion of oxytocin or vasopressin was monitored in ovariectomized animals with and without chronic administration of oestrogen and progesterone.

Methods: After 14 days treatment with vehicle or 12.5 mg hydroxyprogesterone caproate and 0.25 mg oestradiol valerate injected every 3 days, rats were infused with 0.077 mol/l NaCl for an equilibration period of approximately 2.5h. Timed urine collections for the determination of volume and electrolytes were then made during a control period of at least 45 min and for 60 min while the infusate was supplemented with vasopressin (40 fmol/min) or oxytocin (50 fmol/min). Further observations were made for a final 90 min of hypotonic saline infusion. In control infusions saline alone was given.

Results: Treatment with ovarian steroids did not affect the volume of urine excreted during hormone infusion. Electrolyte excretion, however, was affected with lower concentrations of sodium and chloride on oxytocin infusion being seen in the steroid-treated animals. During vasopressin infusion, peak electrolyte concentrations were also achieved later in this group of animals.

Conclusion: The increased circulating concentrations of oestrogen and progesterone seen during pregnancy could contribute to variations in the natriuretic response to neurohypophyseal hormones observed in the rat.