INFLUENCE OF ESTROUS-CYCLE ON THE PRESSOR RECOVERY FOLLOWING HEMORRHAGE IN ANESTHETIZED BRATTLEBORO RATS

A sexual dimorphism in the presser responsiveness to the neurohypophys ial hormone vasopressin may be associated with a peripheral interactio n between ovarian steroids and the neurohypophysial hormone, indeed, t he ovarian steroids may inhibit the vasopressin-dependent component of the presser response to haemorrhage. The present study examined the r ecovery of the arterial blood pressure following a 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 exa mined in Brattleboro rats with diabetes insipidus (BDI) lacking circul ating vasopressin, All BDI rats had an impaired blood pressure recover y following haemorrhage compared with male rats of the parent LE strai n, and this was irrespective of sex or stage of the oestrous cycle. Wh ile the blood pressure recovery was more impaired in both groups of BD I 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 re covery 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 L E rats having initial decreased heart rates followed by a recovery pro cess, while the heart rate responses of BDI rats increased immediately . The novel use of the female Brattleboro rat in this study provides e vidence for the existence of an important inhibitory interaction betwe en ovarian steroids and vasopressin during the blood pressure recovery phase following haemorrhage, and indicates a possible direct influenc e of gonadal steroids on the recovery process.