Cerebral circulation: effects of sympathetic nerves and protective mechanisms during hypertension.

The first goal of this study was to examine bilateral effects of reflex activation of sympathetic nerves on the cerebral circulation. Seizures, which activate sympathetic nerves, were induced in animals with intact nerves and after bilateral cervical sympathetic denervation. Increases in cerebral blood flow (microspheres) and decreases in cerebral vascular resistance were similar in denervated and innervated animals. Thus, during intense metabolic stimulation, metabolic factors are the primary determinant of cerebral blood flow, and bilateral effects of sympathetic nerves are minimal. The second goal of this study was to examine the role of vascular hypertrophy in protection of the cerebral circulation. Cerebral perfusion pressure was decreased on one side by clipping one carotid artery in 4-week-old stroke-prone spontaneously hypertensive rats. Two to four months later, the clip was removed, and seizures were induced. Disruption of the blood-brain barrier in the cerebrum occurred predominantly on the clipped side. We suggest that reduction in perfusion pressure attenuates development of cerebral vascular hypertrophy and thereby increases susceptibility to disruption of the blood-brain barrier. Thus, hypertrophy of cerebral vessels during chronic hypertension may protect the cerebral circulation.