Unilateral and bilateral sympathetic effects on cerebral blood flow during normocapnia

The purposes of this study were to 1) determine the effects of unilateral and bilateral sympathetic stimulation on cerebral blood flow (CBF) during normocapnia and 2) explore the mechanism by which bilateral effects are greater than unilateral effects. In anesthesized rabbits, CBF (15-microns microspheres) and internal carotid artery blood flow (ICBF) were determined during a prestimulation period and electrical stimulation of one or both superior cervical ganglia (8 or 16 Hz, 10 V, 2 ms). Resting CBF ranged from 26 to 34 ml X min-1 X 100 g-1. Unilateral stimulation did not reduce CBF at 8 Hz, but stimulation at 16 Hz reduced ipsilateral blood flow to the cerebrum by 21 +/- 5% (n = 6 from each group). In contrast, bilateral stimulation reduced blood flow to cerebrum by 22 +/- 5 and 40 +/- 4% at 8 and 16 Hz, respectively (P less than 0.05, unilateral vs. bilateral; n = 6 for each group). Similarly, unilateral stimulation at 8 Hz did not reduce ipsilateral ICBF or affect contralateral ICBF. However, bilateral stimulation at 8 Hz reduced ICBF by 51 +/- 6% on both sides. We conclude that 1) bilateral effects of sympathetic nerves on CBF are greater than unilateral effects, and 2) this difference in response may be due to activation of two overlapping sets of sympathetic nerves and release of more neurotransmitter during bilateral sympathetic stimulation.