Direct evidence for absence of beta-adrenergic receptors in rat cerebral vessels histochemical study with a fluorescent beta-blocker

The effect of propranolol on cerebral oxygen consumption and blood flow in the rat: measurements during normocapnia and hypercapnia

The cerebral blood flow (CBF) and cerebral oxygen consumption (CMRO2) in the rat during normocapnia and hypercapnia were investigated by means of the intraarterial 133Xenon injection technique; measurements were performed during normocapnia and hypercapnia and the effect of propranolol upon CBF and CMRO2 was studied. The CBF technique applied to rat yield reliable results even in high flow situations. A steady state period of only 10--15 s is all that is necessary to obtain the initial slope of the 133Xenon clearance curve from which CBF is calculated and measurements may be repeated within minutes. Hypercapnia caused an increase in CMRO2 of 35% which confirms the findings of other investigators. The beta-adrenergic receptor blocker propranolol (2 mg per kg i.v.) prevented this increase and could eliminate an increase in CMRO2 already induced; this indicates that CO2 affects adrenergic mechanisms. Although propranolol eliminated the CMRO2 response to hypercapnia, it only reduced the CBF response; this dissociation of CBF and CMRO2 response occurred probably because the beta-receptor blockage only eliminated a CBF increase mediated through an increased CMRO2 (cellular response) whereas a direct CO2 effect upon the arterioles (vascular response) persisted.