Sepsis increases contraction-related generation of reactive oxygen species in the diaphragm

Recent work indicates that free radicals mediate sepsis-induced diaphragmatic dysfunction. These previous experiments have not, however, established the source of the responsible free radical species. In theory, this phenomenon could be explained if one postulates that sepsis elicits an upregulation of contraction-related free radical formation in muscle. The purpose of the present study was to test this hypothesis by examination of the effect of sepsis on contraction-related free radical generation [i.e., formation of reactive oxygen species (ROS)] by the diaphragm. Rats were killed 18 h after injection with either saline or endotoxin. In vitro hemidiaphragms were then prepared, and ROS generation during electrically induced contractions (20-Hz trains delivered for 10 min) was assessed by measurement of the conversion of hydroethidine to ethidium. ROS generation was negligible in noncontracting diaphragms from both saline- and endotoxin- treated groups (2.0 ± 0.6 and 2.8 ± 1.0 ng ethidium/mg tissue, respectively), but it was marked in contracting diaphragms from saline- treated animals (19.0 ± 2.8 ng/mg tissue) and even more pronounced (30.0 ± 2.8 ng/mg tissue) in diaphragms from septic animals (P < 0.01). This enhanced free radical generation occurred despite the fact that the force-time integral (i.e., the area under the curve of force vs. time) for control diaphragms was higher than that for the septic group. In additional studies, in which we altered the stimulation paradigm in control muscles to achieve a force-time integral similar to that achieved in septic muscles, an even greater difference between control and septic muscle ROS formation was observed. These data indicate that ROS formation during contraction is markedly enhanced in diaphragms from endotoxin-treated septic animals. We speculate that ROS generated in this fashion plays a central role in producing sepsis-related skeletal muscle dysfunction.