Vascular effects of a heme oxygenase inhibitor are enhanced in the absence of nitric oxide.
BACKGROUND
Vascular endothelium and smooth muscle express heme oxygenase (HO) that metabolizes heme to biliverdin, iron and carbon monoxide (CO). Carbon monoxide promotes endothelium-independent vasodilation, but also inhibits nitric oxide formation. This study examines the hypothesis that an inhibitor of HO promotes endothelium-independent vasoconstriction, which is attenuated in the presence of unabated nitric oxide formation.


METHODS
In vivo studies were conducted in anesthetized male Sprague-Dawley (SD) rats instrumented with flow probes and arterial catheters. In vitro experiments were performed on pressurized first-order gracilis muscle arterioles isolated from male SD rats superfused with oxygenated modified Krebs buffer.


RESULTS
Vascular smooth muscle and endothelium showed positive HO-1 and HO-2 immunostaining. In anesthetized rats the HO inhibitor chromium mesoporphyrin (CrMP; 45 micromol/kg intraperitoneally) had minimal effect on hindlimb resistance. However, in animals pretreated with N(omega)-nitro-L-arginine methyl ester (L-NAME; 300 mg/kg intraperitoneally), CrMP substantially increased hindlimb resistance. In contrast, in rats infused with phenylephrine to increase blood pressure and vascular tone, CrMP had no effect on hindlimb resistance. In isolated arterioles denuded of endothelium, CrMP (15 micromol/L) caused a powerful vasoconstriction, which was abolished in the presence of a functional endothelium. In arterioles with intact endothelium pretreated with L-NAME (1 mmol/L), or with L-NAME and sodium nitroprusside (10 to 30 nmol/L), CrMP promoted a similarly powerful vasoconstriction as in vessels denuded of endothelium.


CONCLUSIONS
These results suggest that smooth muscle-derived CO may contribute to endothelium-independent regulation of vascular tone by providing a vasodilatory influence. Furthermore, the dilatory effects of endogenous CO are offset by a unique interaction between the CO and nitric oxide systems.
Johnson
Fruzsina K
FK
Department of Physiology, Tulane University Health Sciences Center, New Orleans, Louisiana 70112, USA. fruzsi23@aol.com
Teran
Federico J
FJ
Prieto-Carrasquero
Minolfa
M
Johnson
Robert A
RA
D000818:Animals; D001160:Arterioles / Q000378:metabolism / Q000473:pathology; D002248:Carbon Monoxide / Q000378:metabolism*; D004730:Endothelium, Vascular / Q000187:drug effects / Q000378:metabolism*; D006419:Heme Oxygenase (Decyclizing) / Q000037:antagonists & inhibitors* / Q000378:metabolism; D007150:Immunohistochemistry; D008297:Male; D009131:Muscle, Smooth, Vascular / Q000187:drug effects / Q000378:metabolism; D019331:NG-Nitroarginine Methyl Ester / Q000494:pharmacology*; D009569:Nitric Oxide / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D014664:Vasodilation / Q000187:drug effects
D009569:Nitric Oxide; D002248:Carbon Monoxide; D006419:Heme Oxygenase (Decyclizing); D019331:NG-Nitroarginine Methyl Ester