Demonstration of Alpha and Beta Adrenergic Receptors in Canine Cerebral Vasculature

The cerebral vascular effects of various adrenomimetic agents were examined in 12 isolated canine brains perfused with blood at constant flow. Changes in cerebral vascular resistance (CVR) and the magnitude of pressor or depressor responses observed following drug injection were examined. The presence of alpha adrenergic receptors in this vascular bed is indicated by the increased CVR observed when phenylephrine, norepinephrine, and epinephrine were administered and by the reduced or reversed pressor actions of these compounds following treatment with the alpha adrenergic blocking drug phenoxybenzamine HCl. The presence of beta adrenergic receptors in this vascular bed is indicated by the decreased CVR noted when isoproterenol was given and by the reduced depressor actions of this compound following treatment with the beta adrenergic blocking drug propranolol. Further evidence for the presence of beta adrenergic receptors is demonstrated by the vasomotor reversal to epinephrine which was observed following alpha suppression with phenoxybenzamine. As catecholamine blood levels in intact dogs are low in comparison to those achieved in these studies, it appears doubtful that circulating catecholamines play an important physiological role in the regulation of CVR. Possible explanations are considered for the lower response of the cerebral vasculature to catecholamines when this response is compared to that observed in other vascular beds.