Exogenous norepinephrine constricts cerebral arterioles via alpha 2-adrenoceptors in newborn pigs.

The purpose of this study was to determine whether exogenous norepinephrine mediates cerebrovascular constriction via alpha 1- or alpha 2-adrenoceptors in anesthetized neonatal pigs. Diameters of pial arterioles in anesthetized piglets, 1--6 days old, were investigated using a "closed" cranial window. We examined constrictor effects of norepinephrine on pial arterioles in the absence and presence of relatively selective alpha 1-(prazosin) and alpha 2-(yohimbine) adrenoceptor antagonists (1 mg/kg i.v.). Yohimbine and prazosin inhibited pial arteriolar constriction induced by topical application of clonidine and phenylephrine (10(-6) and 10(-4) M, respectively), and yohimbine did not affect the response to topical phenylephrine. In one group diameter was 188 +/- 13 (mean +/- SEM) micron during control and 146 +/- 12 micron during 10(-5) M norepinephrine (22 +/- 5% constriction). Following yohimbine the same vessels did not constrict significantly. In another group 10(-5) M norepinephrine constricted arterioles by 22 +/- 5%, and this response was unaffected by prazosin (24 +/- 5% constriction). We conclude that pial arterioles are responsive to both alpha 1- and alpha 2-adrenoceptor agonists, that intravenous administration of prazosin and yohimbine results in these drugs crossing the blood-brain barrier and inhibiting constrictor effects of agonists, and that norepinephrine constricts pial arterioles predominantly via alpha 2-adrenoceptors.