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-medi ated Vascular dysfunction and this is paralleled by a loss of endothelial n itric oxide synthase (eNOS) activity, The authors tested whether a direct e xposure to OFR may affect vascular relaxation by altering nitric oxide (NO release, Effects of electrolysis(EL)-generated OFR on basal and agonist-evo ked NO release were monitored in isolated rat hearts by oxyhemoglobin assay . Electrolysis-induced changes were compared with those obtained after 30mi n perfusion with NOS and cyclooxygenase (COX) inhibitors N-G-nitro-L-argini ne methyl ester (L-NAME, 100 muM) and indomethacin (INDO, 1 mM). Electrolys is-generated hydroxyl radical ((OH)-O-.) formed by O-.(2)- and H2O2 via the Fenton reaction as revealed by Electron Paramagnetic Resonance (EPR), Afte r EL, basal NO release declined by 60% and coronary perfusion pressure (CPP ) increased by congruent to 70%, L-NAME/INDO perfusion similarly lowered NO release (- 63%) but increased CPP less than EL (56 +/- 3%; P<0.03 v post-E L), In presence of excess substrates and cofactors eNOS activity was not af fected by EL, Both acetylcholine (ACh; 1 <mu>M) and bradykinin (BK; 10 nM) had minimal effect in reversing EL-induced vasocontriction, whereas both pa rtially reversed L-NAME/INDO-mediated constriction. Sodium nitroprusside (S NP, 1 muM) completely reversed L-NAME/INDO constriction and partly countere d 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 re lease after eNOS/ cyclooxygenase inhibition (P<0.001) but not after EL. In conclusion, OFR severely impair NO-mediated coronary vasorelaxation affecti ng both basal and agonist-evolted NO release but not eNOS activity. However , EL also significantly blunts NOS/COX-independent vasodilation suggesting alteration of other vasodilatative pathways. (C) 2001 Academic Press.