Cerebrovascular response to infused noradrenalin and its modification by a catecholamine metabolism blocker

Despite many studies, the cerebrovascular action of infused noradrenalin is still unclear. The study reported here tested the hypothesis that infused noradrenalin is prevented from reaching the adrenoreceptors by being avidly taken up and metabolized in the vascular smooth muscle. Cerebral blood flow was measured by the xenon133 clearance technique in nine baboons before and during blockade of the enzyme catechol-O-methyl transferase, which is known to be important in inactivation of circulating noradrenalin. The cerebral blood flow responses to noradrenalin infused at 8, 16, and 32 mug per minute were also determined. Responses indicated vasodilatation before but vasoconstriction after blockade, with a significant noradrenalin. It is suggested that the cerebral circulation is normally protected against sympathetic vasoconstriction by the avid uptake and metabolism of noradrenalin in extraneuronal and possibly neuronal sites.