Blood pressure maintenance in awake dehydrated rats: renin, vasopressin, and sympathetic activity.

The role of vasopressin, the renin system, and sympathetic activity in sustaining blood pressure in the dehydrated state was investigated in normotensive nonanesthetized male Wistar rats. After 48-h dehydration, plasma arginine vasopressin was 14.0 +/- 1.7 pg/ml and plasma norepinephrine 0.46 +/- 0.05 ng/ml. In another group of rats in which the angiotensin converting enzyme inhibitor (MK 421, 5 mg po twice daily) was administered throughout the dehydration period, blood pressure was reduced by more than 20% (P less than 0.001), and both plasma arginine vasopressin and norepinephrine were higher at 23.4 +/- 3.9 pg/ml (P less than 0.01) and 0.83 +/- 0.07 ng/ml (P less than 0.01), respectively. Taken together, in rats with or without converting enzyme blockade, there was an inverse correlation between mean blood pressure and plasma arginine vasopressin (r = 0.67, P less than 0.01) as well as plasma norepinephrine (r = 0.82, P less than 0.01) levels. The acute administration of a specific vasopressin pressor inhibitor (dPVDAVP) reduced mean blood pressure in the rats with a blocked renin system by 16.9 mmHg (P less than 0.001). In rats without converting enzyme inhibition, the induced fall was only 6.4 mmHg. These results indicate that following 48-h dehydration the renin angiotensin system interacts with the vasopressin secretory mechanism to sustain blood pressure, with renin playing a predominant role. They further suggest that, following blockade of the renin system, activation of the sympathetic nervous system probably also contributes to blood pressure maintenance.