EFFECT OF OVARIECTOMY AND STEROID-HORMONE REPLACEMENT ON THE RECOVERYOF ARTERIAL BLOOD-PRESSURE FOLLOWING HEMORRHAGE IN ANESTHETIZED BRATTLEBORO RATS

There is increasing evidence that ovarian steroids inhibit vascular re sponsiveness to the neurohypophysial hormone vasopressin. The present study examined the recovery of the arterial blood pressure following a single (2 ml/100 g body weight) haemorrhage in ovariectomized (OVX) B rattleboro rats with hereditary hypothalamic diabetes insipidus (BDI) and rats of the parent Long Evans (LE) strain. Some groups of OVX rats received subcutaneous implants of either (17)beta-oestradiol (E(2)) o r progesterone 7 days prior to haemorrhage. The arterial blood pressur e recovery immediately following haemorrhage was significantly impaire d in both groups of steroid-treated OVX LE rats compared with the OVX controls (both comparisons P <0.05). The impairment in blood pressure recovery seen in the steroid-replaced OVX LE rats was similar to that seen in pro-oestrous rats (when ovarian steroid levels are raised) com pared with male rats of this strain (P <0.05). In contrast, ovariectom y with or without steroid replacement in BDI rats had no further effec t on the already attenuated recovery of arterial blood pressure after haemorrhage in this strain. Heart rate responses to haemorrhage also s howed strain differences, which were dependent on steroid treatment, P ro-oestrous female Le rats showed a small decrease in heart rate after haemorrhage, followed by a recovery process, and this initial bradyca rdia was markedly enhanced in the OVX steroid-treated animals. In cont rast, untreated OVX LE rats showed an initial and sustained increase i n heart rate which was significantly higher than in the steroid-treate d OVX animals (P <0.05). ALL BDI rats, irrespective of treatment, cons istently showed an increased heart rate after haemorrhage. In conclusi on, ovarian steroid replacement in OVX LE, but not vasopressin-deficie nt BDI, rats was associated with an attenuated presser recovery after haemorrhage. This provides further evidence for the existence of an im portant inhibitory interaction between ovarian steroids and vasopressi n. The initial decrease in heart rate observed in pro-oestrous and ste roid-treated OVX LE rats after haemorrhage also appears to be related to an ovarian steroid-vasopressin interaction.