Oxygen radical-mediated reduction in basal and agonist-evoked no release in isolated rat heart

Oxygen free radicals (OFR) play a primary role in ischemia - reperfusion-mediated vascular dysfunction and this is paralleled by a loss of endothelial nitric oxide synthase (eNOS) activity. The authors tested whether a direct exposure to OFR may affect vascular relaxation by altering nitric oxide (NO) release. Effects of electrolysis(EL)-generated OFR on basal and agonist-evoked NO release were monitored in isolated rat hearts by oxyhemoglobin assay. Electrolysis-induced changes were compared with those obtained after 30 min perfusion with NOS and cyclooxygenase (COX) inhibitors N^G-nitro-L-arginine methyl ester (L-NAME. 100 μM) and indomethacin (INDO. 1 mM). Electrolysis-generated hydroxyl radical (^·OH) formed by ^·O₂^- and H₂O₂ via the Fenton reaction as revealed by Electron Paramagnetic Resonance (EPR). After EL, basal NO release declined by 60% and coronary perfusion pressure (CPP) increased by ≅70%. L-NAME/INDO perfusion similarly lowered NO release (-63%) but increased CPP less than EL (56 ± 3%; P<0.03 ν post-EL). In presence of excess substrates and cofactors eNOS activity was not affected by EL. Both acetylcholine (ACh; 1 μM) and bradykinin (BK; 10 nM) had minimal effect in reversing EL-induced vasocontriction, whereas both partially reversed L-NAME/INDO-mediated constriction. Sodium nitroprusside (SNP. 1 μM) completely reversed L-NAME/INDO constriction and partly countered that after EL (-38 ± 2.5. P<0.001). Acetylcholine-evoked NO release was nearly abolished by both treatments whereas BK still elicited partial NO release after eNOS/cyclooxygenase inhibition (P<0.001) but not after EL. In conclusion. OFR severely impair NO-mediated coronary vasorelaxation affecting both basal and agonist-evoked NO release but not eNOS activity. However, EL also significantly blunts NOS/COX-independent vasodilation suggesting alteration of other vasodilatative pathways.