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L-NAME, but not Phenylephrine Enhances the Effects of Endogenous Carbon Monoxide on Vascular Tone
 in Vivo
 and
 in Vitro


 4
 
 
 Vascular endothelium and smooth muscle express heme oxygenase, which metabolizes heme to form biliverdin, free iron and carbon monoxide. Carbon monoxide promotes endothelium-independent vasodilation, but also inhibits nitric oxide formation. We previously found, that heme-derived carbon monoxide dilates isolated gracilis arterioles pretreated with a nitric oxide synthase inhibitor, N
 ω
 -nitro-L-arginine methyl ester (L-NAME), and lowers blood pressure in hypertensive rat models. This current study examines the hypothesis that nitric oxide modifies the effects of endogenous carbon monoxide on vascular tone both
 in vitro
 and
 in vivo
 . For this purpose
 in vitro
 studies were conducted on rat isolated pressurized first-order gracilis arterioles superfused with oxygenated (14%O
 2
 /5%CO
 2
 balanced with N
 2
 ) modified Krebs’ buffer (CaCl
 2
 1.4mmol/L). A heme oxygenase inhibitor, chromium mesoporphyrin (CrMP, 15μmol/L), constricted gracilis arterioles (Δ-32±6μm, t
 1/2max
 =5min; n=6, P<.05). This effect was markedly amplified by pretreatment with 1mmol/L L-NAME (Δ-65±11μm; t
 1/2max
 =5min; n=7; P<.05), but not by preconstriction with 100nmol/L phenylephrine (n=6; P>.05). In addition,
 in vivo
 studies were performed in anesthetized rats instrumented with flow probes and arterial catheters. In these animals the heme oxygenase inhibitor CrMP (45μmol/Kg,IP) had no significant effect on hindlimb resistance (Δ 2.3±0.1 mmHg/mL/min; n=5; P>.05). But, in animals pretreated with L-NAME (100mg/Kg), CrMP increased hindlimb resistance (Δ10.7 ± 0.7 mmHg/mL/min; n=4; P<.05). In contrast, in animals infused with PE (3μg/min) to increase blood pressure and vascular tone, CrMP had no effect on hindlimb resistance (Δ1.8±0.2 mmHg/mL/min; n=5; P>.05). This study shows the vascular actions of an inhibitor of endogenous carbon monoxide formation are enhanced by an inhibitor of nitric oxide synthase, but not by an alternative vasoconstrictor. These findings suggest the endogenous heme-heme oxygenase-carbon monoxide system exerts vasodilatory influences on basal vascular tone which are uniquely enhanced in the absence of nitric oxide synthesis.
