Adrenergic receptors and vascular resistance in cerebral circulation of the cat

Infusion of catecholamines and pharmacologic blockade were used to demonstrate the presence of an adrenergic receptor system with both alpha and beta components inthe feline cerebral vasculature. For this purpose, the anatomically isolated brain preparation was perfused under controlled constant-pressure conditions to eliminate active autoregulatory changes and passive fluctuations in calculated cerebral vascular resistance (CVR) secondary to alterations of perfusion pressure. Alpha adrenergic activity was demonstrated as substantial cerebral vasoconstriction in response to infusions of l-norepinephrine and epinephrine; cerebral blood flow (CBF) was decreased by mean values of 25% and 29%, respectively, with calculated increases in CVR of 82% and 62%, respectively. Marked reductions in the vasoconstriction produced by these two catecholamines followed the use of the alpha receptor blocking drug, phenoxybenzamine. Isoproterenol consistently produced cerebral vasodilation (mean CVR decrease of 22%), and this vasodilation was blocked during infusion of a specific beta adrenergic blocking agent, propranolol. Histamine vasodilation (mean CBF increase 49%) appeared to be independent of the classic adrenergic mechanisma. The observed responses are explained on the basis of a functionally significant cerebrovascular adrenergic system having high specificity and demonstrating considerable potency. The data also indicate a predominance of alpha over beta adrenergic cerebrovascular reception.