Indomethacin potentiates acetylcholine-induced vasodilation by increasing free radical production

We studied the effects of indomethacin on endothelium-dependent and -independent vascular relaxation in rat thoracic aortic rings and its role in superoxide anion (O(2)(-)) production. We measured isometric force changes in response to acetylcholine (Ach, 1 nM-0.1 mM), sodium nitroprusside (SNP, 0.1 nM-0.1 microM; a nitric oxide (NO) donor) and cromakalim (1 nM-0.1 mM; a K(ATP)-channel opener) in aorta rings contracted with norepinephrine (NE, 0.1 microM). Indomethacin (10 microM; 20 min) significantly increased Ach-induced vasodilation (EC(50) decreased from 8.99 microM to 16 nM). The free radical scavengers superoxide dismutase and 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl completely reverted these effects. Indomethacin did not affect SNP- or cromakalim-induced vasodilation. Neither acetylsalicylic acid (ASA, 5-100 microM; 15 min) nor ketoprofen (1-100 microM; 15 min) affected Ach, SNP and cromakalim concentration-response curves. Incubation of the aorta with Ach (1 microM) rapidly and markedly increased intracellular NO fluorescence in the aorta endothelium. Indomethacin did not affect Ach-induced NO production. We measured intracellular O(2)(-) in the aorta endothelium with dihydroethidium (DHE) dye. Indomethacin significantly increased O(2)(-) fluorescence versus controls. Neither ASA nor ketoprofen affected O(2)(-) fluorescence. Nitrotyrosine staining was increased in indomethacin-treated aorta sections exposed to Ach, which indicates endogenous formation of peroxynitrite. It was low in aorta sections exposed to Ach alone or with ASA or ketoprofen. We cannot judge if indomethacin-induced endothelium-dependent vasodilation damages or protects the cardiovascular system. Here, we show that indomethacin acts on the cardiovascular system regardless of cyclooxygenase inhibition.