The cardiovascular effects of vasopressin after haemorrhage in anaesthetized rats

The cardiovascular effects of an acute haemorrhage (2% of the body weight) were studied over a 60 min period in three groups of rats: (a) Brattleboro rats with hereditary hypothalamic diabetes insipidus (b.d.i.) lacking circulating vasopressin, (b) control rats of the parent Long Evans (l.e.) strain, and (c) l.e. rats treated with an antagonist of the vascular action of vasopressin. Prior to the haemorrhage there were no significant differences between the three groups of rats with respect to mean arterial blood pressure, cardiac output, stroke volume or total peripheral resistance. Following the haemorrhage cardiac output and stroke volume were severely reduced in all three groups of rats. Total peripheral resistance was relatively unaffected in antagonist-treated l.e. rats and b.d.i. rats, but rose substantially in response to the loss of blood in the control l.e. group. Both total peripheral resistance and mean arterial blood pressure were markedly greater in the untreated l.e. control rats than in the other two groups of animals during the first 20 min after haemorrhage. The mean heart rate measured in Brattleboro rats was elevated compared with that of control l.e. rats throughout the experiment and, in addition, significantly greater than that of antagonist-treated l.e. rats during the first 40 min after the haemorrhage. Survival rate for the b.d.i. rats following the 2% haemorrhage was lower than that for l.e. control rats and antagonist-treated l.e. rats. The results indicate that the recovery of the blood pressure following an acute arterial haemorrhage is significantly influenced by vasopressin, particularly during the first 20 min, and that the predominant effect of the hormone is to increase the total peripheral resistance. The higher mortality associated with volume depletion in the b.d.i. rats is unlikely to be directly related to the absence of the vascular action of vasopressin, since administration of the vasopressin antagonist to normal l.e. rats does not reduce their survival rate.