Nitric oxide induces hydroxyl radical generation in rat hearts via depolarization-induced nitric oxide synthase activation.

Abstract

We examined the effect of NG-nitro-L-arginine methyl ester (L-NAME), a NOS inhibitor, on extracellular potassium ion concentration ([K+]o) and induced hydroxyl free radical (.OH) generation by an in vivo microdialysis technique. A flexibly mounted microdialysis technique was used to detect the generation of .OH in in-vivo rat hearts. The microdialysis probe was implanted in the left ventricular myocardium of anesthetized rats and tissue was perfused with Ringer's solution through the microdialysis probe at a rate of 1.0 microl/min. To measure the level of .OH, sodium salicylate in Ringer's solution (0.5 nmol/microl per min) was infused directly through a microdialysis probe to detect the generation of .OH as reflected by the nonenzymatic formation of 2,3-dihydroxybenzoic acid (2,3-DHBA). Induction of high-concentration [K+]o (20, 70 and 140 mM) significantly increased formation of .OH trapped as 2,3-DHBA in a concentration-dependent manner. However, the application of L-NAME (50 mg/kg, i.v.) and allopurinol, a xanthine oxidase inhibitor, abolished the [K+]o depolarization-induced .OH generation. Tyramine (1.0 mM) increased the level of 2,3-DHBA. However, the application of L-NAME did not change the level of 2,3-DHBA. On the other hand, pretreatment with allopurinol (10 mg/kg, i.v.) abolished the KCl- or tyramine-induced .OH generation. Moreover, when iron (II) was administered to [K+]o (70 mM)-pretreated animals, there was a marked increased in the level of 2,3-DHBA. However, the application of L-NAME was not related to a Fenton-type reaction via [K+]o depolarization-induced .OH generation. To examine the effect of L-NAME on ischemic/reperfused rat myocardium, the heart was subjected to myocardial ischemia for 15 min by occlusion by left anterior descending coronary artery branch (LAD). When the heart was reperfused, a marked elevation of the level of 2,3-DHBA was observed. However, L-NAME attenuated .OH generation by ischemic/reperfused rat heart. These results suggest that NOS inhibition is associated with a cardioprotective effect due to the suppression of [K+]o depolarization-induced .OH generation.