Cerebral and ocular blood flow during alpha 2-blockade: evidence for a modulated sympathetic response

The effect upon regional blood flows of alpha 2-blockade by yohimbine was investigated in albino rabbits before and during unilateral cervical sympathetic stimulation. The combined effects of i.v. angiotensin, alpha 2-blockade and sympathetic stimulation were also investigated. Regional blood flows were determined using the labelled microsphere method. Yohimbine produced high flow rates in the eyes, the dura, the choroid plexus and in the peripheral organs. Sympathetic stimulation during alpha 2-blockade caused significant blood flow reductions in the brain, the eye and in all other tissues investigated. In the brain the flow reduction was 20-25% in several structures. Angiotensin caused marked reductions in blood flow in the dura and in most peripheral tissues. The effects of sympathetic stimulation were not appreciably changed. The results suggest that vascular alpha 2-receptors affect the vascular tone in some organs and that presynaptic alpha 2-receptors modulate the response to sympathetic stimulation in the brain, the optic nerve and in the choroid plexus.