The release of vasopressin in response to haemorrhage and its role in the mechanism of blood pressure regulation.

1. The release of vasopressin in response to haemorrhage and the effects of vasopressin infusions on blood pressure and heart rate have been investigated in anaesthetized dogs. Haemorrhage was produced by the method of Lamson & de Türk (1945), which allows for a precise control of the changes in arterial blood pressure.2. Blood samples were collected from an external jugular vein, from a femoral vein or from a femoral artery and extracted with alcohol; blood extracts were assayed for antidiuretic activity.3. Haemorrhage experiments showed that vasopressin secretion is increased when the fall in diastolic blood pressure (diastolic DeltaP) is 25 mm Hg or more. Mild hypotensions (diastolic DeltaP ranging from 21 to 30 mm Hg) produce an average fourfold increase in the concentration of vasopressin in blood. Such increase is maintained throughout the oligaemic period. Severe hypotensions produce, in most cases, a biphasic secretory response, with an initial high peak followed by a lower, constant, secretory plateau. In all experiments, the retransfusion of blood restored vasopressin to control levels.4. Vasopressin infusion experiments showed that the amounts of hormone secreted in response to haemorrhage are sufficient to cause vasopressor response, provided that the buffering action of blood pressure regulation mechanisms is suppressed. It was also found that the amounts of vasopressin secreted in response to haemorrhage are apparently adequate, if the function of such secretion is to combat the hypotension which follows haemorrhage.5. The effect of hypophysectomy on the blood pressure of animals previously submitted to bilateral division of the vagi and sinus nerves (deafferented animals) was also investigated. It was found that hypophysectomy is followed by a fall in arterial blood pressure which is positively correlated to the previous existing amounts of vasopressin. The time course of this hypotension is similar to that following the stopping of an infusion in a deafferented hypophysectomized animal. In some experiments it was shown that, following hypophysectomy, blood pressure can be restored to its pre-hypophysectomy level by an adequate infusion of vasopressin.6. It is proposed that the release of vasopressin in response to stimuli arising from cardiovascular sensory receptors plays a part in the mechanism of blood pressure regulation.