Cerebral and ocular blood flow during α2-blockade: evidence for a modulated sympathetic response

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Abstract
The effect upon regional 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 labeled 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. Vascular .alpha.2-receptor apparently affect the vascular tone in some organs. Presynaptic .alpha.2-receptors modulate the response to sympathetic stimulation in the brain, the optic nerve and in the choroid plexus.