Diaphragmatic lipid peroxidation in chronically loaded rats

Recent work indicates that free radical-mediated lipid peroxidation takes p lace within the diaphragm on strenuous contraction. This phenomenon has onl y been demonstrated using fairly artificial experimental models and has not been studied during the type of sustained respiratory loading typically se en in patients with lung disease. The purpose of the present study was to m easure the levels of several biochemical markers of protein oxidation (prot ein carbonyl levels) and lipid peroxidation (8-isoprostane, reduced glutath ione, and oxidized glutathione levels) in diaphragms of rats subjected to c hronic respiratory loading. Respiratory loading was accomplished by trachea l banding; groups of animals were loaded far 4, 8, or 12 days, and a group of sham-operated unloaded animals was used as controls. After loading, anim als were killed, diaphragm contractility was assessed in vitro by using a p ortion of the excised diaphragm, and the remaining diaphragm and the soleus muscles were used for biochemical analysis. We found diminished force gene ration in diaphragms from all groups of banded animals compared with muscle s from controls. For example, twitch force averaged 7.8 +/- 0.8 (SE) N/cm(2 ) in unloaded animals and 4.0 +/- 0.4, 3.0 +/- 0.4, and 3.4 +/-: 0.4 N/cm(2 ) in animals loaded for 4, 8, and 12 days, respectively (P < 0.0001). Loadi ng also elicited increases in diaphragmatic protein carbonyl concentrations (P < 0.001), and the time course of alterations in carbonyl levels paralle led loading-induced alterations in the diaphragm force-frequency relationsh ip. Although loading was also associated with increases in diaphragmatic 8- isoprostane levels (P < 0.003) and reductions in diaphragm reduced glutathi one levels (P < 0.003), the time course of changes in these latter paramete rs did not correspond to alterations in force. Soleus glutathione and carbo nyl levels were not altered by banding. We speculate that respiratory loadi ng-induced alterations in diaphragmatic force generation may be related to free radical-mediated protein oxidation, but not to free radical-induced li pid peroxidation.