Sympathetic nerves reduce cerebral blood flow during hypoxia in awake rabbits

Effects of sympathetic nerves on cerebral blood flow (CBF) during normoxia or hypoxia combined with hemorrhagic hypotension were examined in awake rabbits. One superior cervical ganglion was excised and sham surgery was performed on the other side. Five rabbits each were exposed to normoxia [arterial O2 tension (Pao2) greater than or equal to 78 mmHg] or hypoxia (Pao2 less than or equal to 39 mmHg). Cerebral blood flow was measured with 15-micron microspheres three times in each group: normotension [mean arterial pressure (MAP) 80-86 mmHg]; moderate hypotension (MAP 62-65 mmHg); and severe hypotension (MAP 49-50 mmHg). During normoxia, blood flow to cerebrum was 70 +/- 8 (SE) ml X min-1 X 100 g-1 during normotension and did not decrease during hypotension. Sympathetic nerves had no effect on CBF during normoxia. During hypoxia, blood flow to cerebrum was 243 +/- 21 ml X min-1 X 100 g-1 during normotension and fell as arterial pressure was lowered. Blood flow to denervated cerebrum was higher than on the intact side during normotension (13 +/- 4%), moderate hypotension (11 +/- 5%), and severe hypotension (12 +/- 4%). Similarly, blood flow to denervated cortical gray matter was higher during normotension (19 +/- 4%), moderate hypotension (16 +/- 6%), and severe hypotension (17 +/- 6%) when compared with the innervated side. In addition, blood flow to denervated cerebellum was higher than the innervated side during severe hypotension (27 +/- 17%).(ABSTRACT TRUNCATED AT 250 WORDS)