1. The effects of naloxone on blood pressure recovery after either rapid arterial haemorrhage or prolonged venous haemorrhage were studied in rats lacking vasopressin (Brattleboro strain) and in control (Long Evans) rats.

2. To produce similar reductions in blood pressure, less blood had to be taken from the Brattleboro rats than from the Long Evans rats.

3. After rapid arterial haemorrhage in the absence of naloxone, blood pressure recovery was slower in Brattleboro rats than in Long Evans rats. Naloxone did not affect the response to rapid arterial haemorrhage in Long Evans rats, but improved blood pressure recovery in Brattleboro rats; despite this improvement, the Brattleboro rats remained hypotensive at a time when the Long Evans rats were normotensive. These findings suggest that both the absence of vasopressin and a depressor action of β-endorphins may contribute to the poor ability of Brattleboro rats to cope with rapid haemorrhage.

4. After prolonged venous haemorrhage in the absence of naloxone, there was no difference between the recovery of blood pressure in Brattleboro rats and Long Evans rats. Naloxone improved blood pressure recovery to a similar extent in both strains of rat. These findings suggest that the absence of vasopressin does not impair blood pressure recovery after prolonged haemorrhage.

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