Influence of oestrous cycle on the pressor recovery following haemorrhage in anaesthetized Brattleboro rats

Department of Physiology, Charing Cross and Westminster School, London, UK.

A sexual dimorphism in the pressor responsiveness to the neurohypophysial hormone vasopressin may be associated with a peripheral interaction between ovarian steroids and the neurohypophysial hormone. Indeed, the ovarian steroids may inhibit the vasopressin-dependent component of the pressor response to haemorrhage. The present study examined the recovery of the arterial blood pressure following it single large (2% v/w) haemorrhage in anaesthetized male Long Evans (LE) rats and females of the same strain during either pro-oestrous or di-oestrous phases of the reproductive cycle. In addition the same recovery process was examined in Brattleboro rats with diabetes insipidus (BDI) lacking circulating vasopressin. All BDI rats had an impaired blood pressure recovery following haemorrhage compared with male rats of the parent LE strain, and this was irrespective of sex or stage of the oestrous cycle. While the blood pressure recovery was more impaired in both groups of BDI female rats than in the males of the same strain during the first 20 min after haemorrhage (both comparisons p < 0.001; ANOVA), there was no difference between the recoveries of the female rats in pro-oestrus or di-oestrus. In contrast a significantly impaired blood pressure recovery was observed in female LE rats at pro-oestrus, when circulating ovarian steroid concentrations are raised, compared with male (p < 0.001: ANOVA) and di-oestrous (p < 0.02: ANOVA) rats of the same strain. Heart rate responses to haemorrhage showed strain differences, with LE rats having initial decreased heart rates followed by a recovery process, while the heart rate responses of BDI rats increased immediately. The novel use of the female Brattleboro rat in this study provides evidence for the existence of an important inhibitory interaction between ovarian steroids and vasopressin during the blood pressure recovery phase following haemorrhage, and indicates a possible direct influence of gonadal steroids on the recovery process.