Endothelium-dependent contractions to oxygen-derived free radicals in the canine basilar artery

Experiments were designed to determine the effect of oxygen-derived free radicals in isolated canine basilar arteries. Rings with and without endothelium were suspended for isometric tension recording in modified Krebs- Ringer bicarbonate solution bubbled with 95% O₂-5% CO₂ (temperature = 37°C: pH = 7.4). A radioimmunoassay technique was used to measure production of prostaglandins and thromboxane B₂. Xanthine oxidase (1-9 mU/ml, in the presence of 10^-4 M xanthine) and hydrogen peroxide (10^-6 to 10^-4 M) caused concentration-dependent contractions. The removal of endothelium reversed these contractions into relaxations. Contractions to xanthine oxidase and hydrogen peroxide were inhibited in the presence of superoxide dismutase (150 U/ml), catalase (1,200 U/ml), indomethacin (10^-5 M), and SQ 29548 (10^-6 M) but not in the presence of deferoxamine (10^-4 to 10^-3 M) and dimethyl sulfoxide (10^-4 M). N(G)-monomethyl-L-arginine (3 x 10^-5 M) augmented the contractions to hydrogen peroxide. Xanthine oxidase stimulated production of 6-keto-prostaglandin F(1α), prostaglandin F(2α), prostaglandin E₂, and thromboxane B₂. The stimulatory effect was prevented by the removal of endothelial cells. These studies suggest that xanthine oxidase causes endothelium-dependent contractions mediated by 1) hydrogen peroxide-induced stimulation of the endothelial metabolism of arachidonic acid via the cyclooxygenase pathway, leading to activation of prostaglandin H₂-thromboxane A₂ receptors, and 2) inactivation of basal production of nitric oxide by superoxide anions.