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1 Anesthesiology, Baylor College of Medicine, Houston, TX, USA
2 Section of Pediatric Critical Care, Baylor College of Medicine, Houston, TX, USA
3 Pharmacology and Phyisology, University of Missouri-Columbia, Columbia, MO, USA
* To whom correspondence should be addressed. E-mail: andresen{at}bcm.tmc.edu.
Carbon monoxide (CO) has been postulated to be a signaling molecule in many tissues including the vasculature. We examined vasomotor responses of adult rat and mouse cerebral arteries to both exogenously applied, and endogenously produced CO. The diameter of isolated, pressurized, and perfused, rat middle cerebral arteries (MCAs) was not altered by authentic CO (10-6 to 10-4 M). Mouse MCAs, however, dilated by 21 ± 10% at 10-4 M CO. Authentic nitric oxide (NO, 10-10 to 10-7 M) dilated both rat and mouse MCAs. At 10-8 M NO, rat vessels dilated by 84 ± 4%, and at 10-7 M NO, mouse vessels dilated by 59 ± 9%. Stimulation of endogenous CO production through heme oxygenase (HO) with the heme precursor, 
-aminolevulinic acid (10-10 to 10-4 M), did not dilate the MCAs of either species. The metalloporphyrin HO inhibitor, chromium mesoporphyrin IX (CrMP), caused profound constriction of the rat MCA (44 ± 2% at 3x10-5 M). Importantly, this constriction was unaltered by exogenous CO (10-4 M), or CO plus 10-5 M biliverdin (both HO products). In contrast, exogenous CO (10-4 M) reversed CrMP-induced constriction in rat gracilis arterioles. Control mouse MCAs constricted by only 3 ± 1% to 10-5 M CrMP. Magnesium protoporphyrin IX (10-5 M), a weak HO inhibitor used to control for non-specific effects of metalloporphyrins, also constricted the rat MCA to a similar extent as CrMP. We conclude that at physiological concentrations, CO is not a dilator of adult rodent cerebral arteries, and that metalloporphyrin HO inhibitors have non-specific constrictor effects in rat cerebral arteries.
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