Diaphragmatic free radical generation increases in an animal model of heart failure

Heart failure evokes diaphragm weakness, but the mechanism(s) by which this occurs are not known. We postulated that heart failure increases diaphragm free radical generation and that free radicals trigger diaphragm dysfunction in this condition. The purpose of the present study was to test this hypothesis. Experiments were performed using halothaneanesthetized sham-operated control rats and rats in which myocardial infarction was induced by ligation of the left anterior descending coronary artery. Animals were killed 6 wk after surgery, the diaphragms were removed, and the following were assessed: 1) mitochondrial hydrogen peroxide (H₂O₂) generation, 2) free radical generation in resting and contracting intact diaphragm using a fluorescent-indicator technique, 3) 8-isoprostane and protein carbonyls (indexes of free radical-induced lipid and protein oxidation), and 4) the diaphragm force-frequency relationship. In additional experiments, a group of coronary ligation animals were treated with polyethylene glycol-superoxide dismutase (PEG-SOD, 2,000 units-kg^-1·day^-1) for 4 wk. We found that coronary ligation evoked an increase in free radical formation by the intact diaphragm, increased diaphragm mitochondrial H₂O₂ generation, increased diaphragm protein carbonyl levels, and increased diaphragm 8-isoprostane levels compared with controls (P < 0.001 for the first 3 comparisons, P < 0.05 for 8-isoprostane levels). Force generated in response to 20-Hz stimulation was reduced by coronary ligation (P < 0.05); PEG-SOD administration restored force to control levels (P < 0.03). These findings indicate that cardiac dysfunction due to coronary ligation increases diaphragm free radical generation and that free radicals evoke reductions in diaphragm force generation.