Lesions of A1 noradrenergic cells affect AVP release and heart rate during hemorrhage

The role of A1 noradrenergic cells of the ventrolateral medulla in the changes in mean arterial pressure (MAP), heart rate (HR), and plasma arginine vasopressin (AVP) after slow continuous hemorrhage (2% blood vol/min up to 35%) was examined by comparing responses in conscious rabbits before and 3 wk after a sham operation or A1 lesions. In the control experiments, MAP fell minimally up to the withdrawal of 20% of blood volume after which it fell abruptly to 20-30 mmHg below control by the 35% level. Plasma AVP increased nonlinearly during progressive hemorrhage with significant increases occurring only after 25% of blood volume was removed. In contrast, HR increased linearly after the onset of bleeding. After A1 lesions, which destroyed 84% (range 80-94%) of the noradrenergic cells, the amount of AVP released and the tachycardia during hemorrhage were reduced by 83 and 61%, respectively (P less than 0.005), but the fall in MAP was minimally affected. Basal values of MAP, HR, or plasma AVP were not affected by the lesions. These results suggest that during hemorrhage in conscious rabbits A1 noradrenergic neurons are important for the secretion of AVP and the reflex tachycardia but play little role in the maintenance of blood pressure.