Resistance of endothelium-dependent relaxation to elevation of O 2 − levels in rabbit carotid artery
Endogenous superoxide anion[Formula: see text] interferes with the bioactivity of nitric oxide (NO) in endothelium-dependent arterial relaxation (EDR). Using the lucigenin chemiluminescence assay, we measured[Formula: see text] in the thoracic and abdominal aortas and the carotid artery of rabbits to determine whether ambient[Formula: see text] varies among the three arteries and differentially diminishes the effect of NO. Basal levels of[Formula: see text] were significantly higher in carotid arteries than in the thoracic aorta [23 ± 6.1 vs. 3.9 ± 1.4 chemiluminescence units (CU); P < 0.05], whereas EDR in response to ACh (10−8–10−5M) was not significantly different on ANOVA. After treatment with the superoxide dismutase (SOD) inhibitor diethyldithiocarbamate (DDC; 10 mM), [Formula: see text] levels were significantly elevated, becoming greater in the carotid artery and abdominal aorta than in the thoracic aorta (185 ± 31.2 and 202 ± 40.3 vs. 89 ± 18 CU; P < 0.05). DDC significantly reversed EDR in the thoracic aorta but not in the carotid artery; at 10−6 M ACh, the decrease seen with DDC was 48 ± 6.2 vs. 6.8 ± 8.0% of maximal relaxation in the thoracic aorta and carotid artery, respectively. In the thoracic aorta, exogenous SOD reversed the inhibition of EDR caused by DDC. Moreover, DDC/[Formula: see text]-resistant EDR in the carotid artery was ablated by the addition of nitro-l-arginine methyl ester (300 μM; P < 0.05), an NO synthase inhibitor, consistent with peroxynitrite or an[Formula: see text]-resistant NO donor being involved in carotid relaxation. Indeed, exogenous peroxynitrite caused similar relaxation of the carotid artery and thoracic aorta, which was unaffected by DDC. Our studies show a greater production of nitrite and[Formula: see text] per unit area by the carotid artery, suggesting a greater amount of their product peroxynitrite. These findings support the hypothesis that peroxynitrite is the relaxing agent that resists high [Formula: see text] in the carotid artery.