Carbon monoxide as an attenuator of vasoconstriction in piglet cerebral arterioles.

Carbon monoxide (CO) is an endogenous dilator in the newborn cerebral circulation. The present study addressed the hypothesis that endogenous CO attenuates pial arteriolar vasoconstriction caused by hypocapnia, platelet activating factor, and elevated blood pressure. Experiments used anesthetized piglets with implanted, closed cranial windows. Topical application of a metal porphyrin inhibitor of heme oxygenase was used to inhibit production of CO. Chromium mesopophyrin increased vasoconstriction in response to hypocapnia. The constrictor response to a topical stimulus, platelet activating factor, was also increased by application of chromium mesoporphyrin. Inhibition of heme oxygenase did not constrict pial arterioles in normotensive newborn pigs (mean arterial pressure of about 70 mmHg), but did constrict pial arterioles of piglets with experimentally induced increases in arterial pressure (mean arterial pressure greater than 90 mmHg). In fact, pial arterioles of normotensive piglets transiently dilated to chromium mesoporphyrin, whereas those of hypertensive piglets progressively constricted during 10 min of chromium mesoporphyrin treatment. Therefore, inhibition of heme oxygenase augments cerebral vasoconstriction in response to several very different constrictor stimuli. These data suggest endogenous CO attenuates vasoconstrictor responses in the newborn cerebral circulation.