Autacoids Mediate Coronary Vasoconstriction Induced by Nitric Oxide Synthesis Inhibition

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Abstract
Inhibition of nitric oxide (NO) synthesis results in coronary vasoconstriction. Using a Langendorff rat heart preparation, we tested the hypothesis that this vasoconstriction is caused by the unopposed effect of the autacoids prostaglandin H2 (PGH2) or thromboxane A2 (TxA2) or both through a mechanism that involves oxygen free radicals. The vasoconstriction induced by NO synthesis inhibition was studied with two different NO synthase inhibitors, N ω-nitro-L-arginine methyl ester (L-NAME) and N ω-monomethyl-L-arginine (L-NMMA). We found that the decrease in coronary flow (CF) induced by L-NAME (from 19.3 ± 0.9 to 13.2 ± 0.9 ml/min; p < 0.001) and L-NMMA (from 20.1 ± 0.4 to 15.0 ± 0.3 ml/min; p < 0.001) was completely blocked by the cyclooxygenase inhibitor indomethacin. A different cyclooxygenase inhibitor (ibuprofen), a PGH2/TxA2-receptor antagonist (SQ29548), and a TxA2 synthase inhibitor (CGS 13080) also completely abolished the vasoconstrictor effect of L-NAME, suggesting that this vasoconstriction is mediated by TxA2. Two different scavengers of superoxide radical anions (O2 −), the enzyme superoxide dismutase (SOD) and a cell-permeable SOD mimic, 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (Tempol), also blocked the vasoconstriction induced by NO synthesis inhibition. In contrast, catalase, which inactivates hydrogen peroxide (H2O2), failed to do so, indicating that O2 − is needed for the vasoconstrictor effect of L-NAME, whereas H2O2 is not. To determine whether O2 − acts on the conversion of PGH2 to TxA2 or at the receptor or postreceptor level, we studied whether the vasoconstriction induced by exogenous PGH2 or the TxA2 receptor agonist U46619 is blocked by scavengers of O2 −. CF decreased by 50% with PGH2 (from 21 ± 2.1 to 10.6 ± 5.8 ml/min; p < 0.01), and this decrease was abolished by SOD and Tempol but not catalase. However, SOD had no effect on the vasoconstriction induced by U46619, which decreased CF by 45% (from 17.3 ± 2.5 to 9.5 ± 1.8 ml/min; p < 0.01). In addition, PGH2 increased the release of TxB2 (the stable metabolite of TxA2) in the coronary effluent (from 5.1 ± 1.2 to 136.1 ± 11.8 pg/ml/min). The release of TxB2 was significantly lower in hearts treated with SOD (76.8 ± 14.2 pg/ml/min) and CGS (65.7 ± 13.9 pg/ml/min). We conclude that the coronary vasoconstriction induced by inhibition of NO synthesis is the result of the unopposed effect of the autacoid TxA2 through activation of its receptor, and that O2 − is necessary for conversion of PGH2 to TxA2.