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HA Singer and MJ Peach
Methacholine, A23187 and arachidonic acid (AA) relax segments of rabbit thoracic aorta via mechanisms which are dependent on an intact endothelium. Relaxation stimulated by AA was previously found by us to be antagonized by eicosatetraynoic acid and nordihydroguaiaretic acid, but not indomethacin. The purpose of the present investigation was to determine the effects of several inhibitors of AA metabolism on endothelium-dependent relaxation stimulated by methacholine and A23187 in rings of rabbit thoracic aorta. Relaxation in response to methacholine and A23187 was: 1) not affected by indomethacin pretreatment (20 microM, 30 min); 2) inhibited by 50 microM eicosatetraynoic acid pretreatment, 53 and 33%, respectively; and 3) inhibited in a dose-dependent manner by nordihydroguaiaretic acid (10- 50 microM) with complete inhibition occurring after 25 and 50 microM pretreatment, respectively. Methacholine-induced relaxation was completely blocked by 10 microM quinacrine (QUIN), whereas A23187- stimulated responses were not significantly affected. QUIN (10 microM) antagonized methacholine-stimulated contractile responses in a noncompetitive manner in rings devoid of endothelium. QUIN may interfere with activation via muscarinic receptors, complicating interpretation of its inhibitory effects. Eicosatetraynoic acid (50 microM), nordihydroguairetic acid (50 microM) and QUIN (10 microM) do not exert significant inhibitory effects at the level of smooth muscle relaxation mechanisms as isoproterenol-induced relaxation was resistant to inhibition by these agents. We conclude that methacholine and A23187 relax rabbit aorta by mechanisms which require intact endothelium and involve noncyclooxygenase metabolites of AA.
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