Effect on vasopressin release of microinjection of angiotensin II into the paraventricular nucleus of conscious rats.

We have studied in conscious unrestrained rats the role of angiotensin II receptors in the paraventricular nucleus (PVN) in the control of vasopressin secretion under basal conditions and after 24 h of water deprivation. In euhydrated rats, the microinjection rostral to the PVN of 1 and 5 ng of angiotensin II produced a transient, dose-dependent increase in the plasma vasopressin concentration. There was also a transient, but not dose-dependent, rise in mean arterial blood pressure (MABP), without a change in heart rate. However, the microinjection of angiotensin II into the PVN and sites caudal to it was without effect on the measured variables. In dehydrated rats, on the other hand, microinjection of 5 ng angiotensin II, both into the PVN and rostral to it, resulted in a prolonged increase in the plasma vasopressin concentration. The response from the rostral site was more rapid and was associated with an increased MABP. These findings suggest that angiotensin II receptors rostral to the PVN may participate in the control of vasopressin release in both euhydrated and dehydrated states, whereas angiotensin II receptors in the PVN are effective only following dehydration.