Prolonged coronary artery occlusion-reperfusion sequences reduce myocardial free radical production: an electron paramagnetic resonance study

The nitric oxide donor S-nitroso-N-acetylpenicillamine (SNAP) increases free radical generation and degrades left ventricular function after myocardial ischemia–reperfusion

BACKGROUND

During reperfusion of ischemic myocardium nitric oxide (NO) reacts with superoxide radicals to form cardiotoxic peroxynitrite, which causes lipid peroxidation. Our hypothesis was that infusion of a NO donor S-nitroso-N-acetylpenicillamine (SNAP) during ischemia-reperfusion would exacerbate the oxidative damage to the myocardium by increased formation of nitrogen radicals.

METHODS AND RESULTS

In 19 open-chest dogs, left anterior descending (LAD) coronary occlusion (15 min)-reperfusion (15 min) sequences were created. Using electron paramagnetic resonance (EPR), we monitored the coronary sinus concentration of ascorbate free radical (Ascz*-), a measure of free radical generation (total oxidative flux). Seven control dogs (Group 1) received intravenous saline infusion during occlusion-reperfusion, while 12 dogs received SNAP infusion (Group 2: 2.5 microg/min per kg SNAP, and Group 3: 5 microg/min per kg SNAP). Left ventricular fractional area shortening was determined by echocardiography. Dogs in Group 3 receiving a high dose of SNAP (5 microg/min per kg) demonstrated a higher Ascz*- concentration increase than the control group. Percent fractional area shortening in Group 1 declined from 77+/-4.0 (baseline) to 54+/-9.0% during ischemia (P<0.05), and then fully recovered to 74+/-3.7% with reperfusion. In the SNAP-treated dogs, the percent fractional area shortening during reperfusion was significantly lower than baseline in Group 2 (55+/-3.9 vs. baseline 74+/-4.4%, P<0.05) and in Group 3 (49+/-5.0 vs. baseline 71+/-4.5%, P<0.01). In five additional dogs, nitrotyrosine immunohistochemistry showed heavy staining of the ischemic-reperfused myocardium.

CONCLUSIONS

The NO donor SNAP increased free radical concentration and exacerbated myocardial oxidative damage after ischemia-reperfusion.

Nitric oxide synthase inhibitors decrease coronary sinus-free radical concentration and ameliorate myocardial stunning in an ischemia-reperfusion model

OBJECTIVES

Our objective was to determine the effect of a nitric oxide synthase inhibitor, NG-nitro-L-arginine (L-NNA) on free radical generation and myocardial contractility after ischemia-reperfusion.

BACKGROUND

Cardiotoxic free radicals are generated by ischemia-reperfusion sequences. Nitric oxide reacts with superoxide radical to form peroxynitrite, which generates additional free radicals. Our hypothesis was that by inhibiting NO production, free radical formation will be diminished, which should be cardioprotective.

METHODS

We studied 32 dogs. Coronary occlusion-reperfusion (20 min each) sequences were created by intracoronary balloon angioplasty inflation-deflation. Using electron paramagnetic resonance, we monitored the coronary sinus concentration of ascorbate free radical (Asc*-), a measure of total oxidative flux. The L-NNA (4.8 mg/kg total) was infused intravenously during occlusion-reperfusion; control dogs received saline. Immunohistochemical staining demonstrated the peroxynitration product nitrotyrosine.

RESULTS

In the control dogs Asc*- rose from 3.2 +/- SD 0.5 nmol/l to 4.8 +/- 1.1 nmol/l with reperfusion, a 50% rise. With L-NNA the Asc*- rose from 3.2 +/- 0.9 nmol/l to 4.0 +/- 1.2 nmol/l, a 25% rise (p < 0.01, L-NNA vs. control). Echocardiographic left ventricular fractional area shortening (FAS) in the control dogs declined from 38 +/- 19% (baseline) to 26 +/- 14% (ischemia), and to 22 +/- 11% with reperfusion (p < 0.01 vs. baseline). With L-NNA, FAS declined from 36 +/- 13% (baseline) to 27 +/- 12% (ischemia) but then rose to 33 +/- 14 with reperfusion (p = NS vs. baseline). Nitrotyrosine was present in the myocardium subjected to ischemia-reperfusion, but almost absent in dogs receiving L-NNA. Myocardial perfusion was not altered by L-NNA.

CONCLUSIONS

The NO synthase inhibitors decrease coronary sinus free radical concentration and ameliorate myocardial stunning after ischemia-reperfusion.

Transition metal chelators reduce directly measured myocardial free radical production during reperfusion

Transition metals such as iron and copper are present in the myocardium and can act as catalysts for the formation of oxygen free radicals during reperfusion after myocardial ischemia. Previous studies suggested that transition metal chelators such as desferrioxamine reduce the production of such radicals and may thereby attenuate postischemic myocardial dysfunction. These studies used spin trapping agents, commonly nitrone compounds, which may themselves influence the severity of the ischemia and reperfusion events being studied. We evaluated two transition metal chelators, desferrioxamine, an iron chelator, and bathocuproine, a copper chelator, by using a new electron paramagnetic resonance technique that does not require the administration of spin traps. We measured ascorbate free radical, an index of free radical production, in the great cardiac vein effluent. Twenty-eight open-chest dogs underwent 20 min of coronary artery occlusion and 30 min of reperfusion. Ten dogs received no drug, 10 dogs received 750 mg bathocuproine, i.v., and eight dogs received 700 mg desferrioxamine, i.v. Both bathocuproine and desferrioxamine blunted the postreperfusion increase in ascorbate free radical generation: no drug, 36+/-8% increase; desferrioxamine, 13+/-5% increase; bathocuproine, 21+/-6% increase (p < 0.05 vs. baseline). Thus direct free radical measurements indicate that chelation of the transition metals iron and copper reduces free radical generation during reperfusion.