Dilatation of pial arterial vessels in hypercapnia and in acute hypertension

Pial arterial vessels were inspected in hypercapnia (PaCO₂ 97±9 (S.E.) mmHg) and in acute hypertension induced by 5 μg kg^-1 noradrenaline i.v. (mean arterial pressure 210±5 mmHg) by means of a cranial window. The diameter of arterial vessels with a resting diameter of 10 to 150 μm was measured either from photographs or by aid of an image splitting eyepiece. Arterioles with a resting diameter ?30 μm exhibited the highest degree of dilatation both in hypercapnia and acute hypertension. The mean dilatation was higher in hypertension than in hypercapnia in vessels of all sizes, the difference being statistically significant in arterioles up to 50 μm resting diameter.     

Neurogenic modification of the vulnerability of the blood-brain barrier during acute hypertension in conscious rats

To study the possible influence of sympathetic adrenergic tone on the blood-brain barrier function during acute hypertension in conscious unrestrained rats with indwelling catheters in the aorta and a jugular vein the blood pressure was increased by noradrenaline, 6-hydroxydopamine (6-OHDA) or baclofen. One or 60 min later the rats were sacrificed and the extravasation of ¹²⁵I labelled albumin determined in the brain. After i. v. injection of noradrenaline the baroreceptor reflex will decrease the sympathetic tone whereas the blood pressure increase induced by the other two drugs is accompanied by an increased sympathetic activity. One minute after a corresponding rise in blood pressure the albumin content in the brain was considerably lower in rats given 6-OHDA than in those given noradrenaline. 60 min after the injection of 6-OHDA or baclofen the extravasation in the brain did not differ despite a considerably more rapid increase in pressure after 6-OHDA. Pretreatment with clonidine increased the blood-brain barrier dysfunction in rats given 6-OHDA but not in those given baclofen, probably because the slower rise in pressure facilitates myogenic autoregulation. It is concluded that neurogenic influences on vessel tone can modify the response of the blood-brain barrier during acute hypertension in conscious rats.