Effects of decreasing arterial blood pressure on cerebral blood flow in the baboon. Influence of the sympathetic nervous system.

Abstract

The influence of the sympathetic nervous system on the cerebral circulatory response to graded reductions in mean arterial blood pressure was studied in anesthetized baboons. Cerebral blood flow was measured by the 133 Xe clearance method, and arterial blood pressure was decreased by controlled hemorrhage. In normal baboons, the constancy of cerebral blood flow was maintained until mean arterial blood pressure was approximately 65% of the base-line value; thereafter, cerebral blood flow decreased when arterial blood pressure was reduced. Superior cervical sympathectomy of 2-3 weeks duration did not affect the normal response. In contrast, both acute surgical sympathectomy (cervical trunk division) and a-receptor blockade (1.5 mg/kg of phenoxybenzamine) enhanced the maintenance of cerebral blood flow in the face of hemorrhagic hypotension in that cerebral blood flow did not decrease until mean arterial blood pressure was approximately 35% of the base-line value. The results indicate that the sympathetic nervous system is not involved in the maintenance of cerebral blood flow in the face of a fall in arterial blood pressure. Indeed, the implication is that the sympathicoadrenal discharge accompanying hemorrhagic hypotension is detrimental to, rather than responsible for, cerebral autoregulation.• Constancy of cerebral blood flow is maintained in the face of moderate variations in systemic arterial blood pressure (1-4), but the nature of this phenomenon is a matter of some debate. One view is that this mechanism is intrinsic. According to this view, cerebral blood flow is regulated by tissue metabolites (5, 6) or by a myogenic, Bayliss reflex (7-10). If either of these hypotheses is correct, then the blood pressure-flow relationship is a result of autoregulation. The other major view is that the characteristic blood pressure-flow relationship of the cerebral circulation is controlled or modified by the extrinsic innervation of the cerebral vasculature-the neurogenic hypothesis (4,11,12).Some support for the extrinsic mechanism has been taken from morphological studies. Both fluorescent and electron microscopy have confirmed a dual adrenergic and cholinergic innervation of the cerebral circulation (13-15). Sympathetic nerves are the more abundant. The large arteries at the base of the brain receive the densest innervation and the pial vessels are moderately innervated, whereas autonomic nerves have rarely been noted in proximity to the intracerebral arteri- 550oles. However, under conditions of normocapnia and normotension, the largest fraction of the total vascular resistance in the brain is located in the parenchymal arteriolar vessels (16,17).The present study was undertaken to investigate the effects of surgical (acute and chronic) cervical sympathectomy and of a-receptor blockade (phenoxybenzamine) on the cerebral pressure-flow relationship existing during hypotension induced by hemorrhage. MethodsThis study was carried out on 31 young baboons (Papio cynocephalus or Papio anubis) weighing 8-12 kg Th...

Effect of acute exposure to hypergravity (GX vs. GZ) on dynamic cerebral autoregulation

“…For example, in primates, both unilateral superior cervical ganglionectomy and alpha adrenergic blockade with intravenous r------l 12 phenoxybenzamine enhance the maintenance of CBF in the face of hypotension, shifting the autoregulation curve acutely to the left (7,14) and impairing the autoregulatory response to acute increases in BP (14,44). However, these effects are mainly reversed after chronic sympathectomy (7,11). Moreover, stimulation of the cervical sympathetic nerve in primates acutely decreases CBF (13,20) while shifting the autoregulation curve to the right (13 , 17).…”

Spectrum of Altered Reactivity of Isolated Cerebral Arteries following Subarachnoid Haemorrhage—Response to Potassium, pH, Noradrenaline, 5-Hydroxytryptamine, and Sodium Loading

“…Subarachnoid haemorrhage impairs autoregulation and reduces the response to hypercapnia Pickard et al, 1979;Jakubowski et al, 1982;Svendgaard et al, 1983). In contrast, sympathetic denervation acutely shifts the lower limit of autoregulation to the left but chronically has no effect on either the autoreg ulatory curve or the response to hypercapnia (Fitch et al, 1975;Jennett et aI., 1976).…”

Do vasomotor nerves significantly regulate cerebral blood flow?

“…The cerebral vascular response to hypotension is affected by section of the sympathetic nerves 74 and by section of the 7th cranial nerves. 66 The response to hypertension is affected by sympathetic section 59 '^7 5 and stimulation.…”