EFFECT OF REACTIVE OXYGEN SPECIES ON K+ CONTRACTURES IN THE RAT DIAPHRAGM

Reactive oxygen species (ROS) are postulated to alter low-frequency co ntractility of the unfatigued and fatigued diaphragm. It has been prop osed that ROS affect contractility through changes in membrane excitab ility and excitation-contraction coupling. If this hypothesis is true, then ROS should alter depolarization-dependent K-contractures. Xanthi ne oxidase (0.01 U/ml) + hypoxanthine (1 mM) were used as a source of superoxide anion eliciting oxidative stress on diaphragm fiber bundles in vitro. Diaphragm fiber bundles from 4-mo-old Fischer 344 rats were extracted and immediately placed in Krebs solution bubbled with 95% O -2-5% CO2. After 10 min of equilibration. a K+ contracture (Pre; 135 m M KCl) was induced. Fiber bundles were assigned to the following treat ment groups: normal Krebs-Ringer (KR; Con) and the xanthine oxidase sy stem (XO) in KR solution. After 15 min of treatment exposure, a second (Post) K+ contracture was elicited. Mean time-to-peak tension for con tractures was significantly decreased in Post vs. Pre (16.0 +/- 0.7 vs . 19.8 +/- 1.0 s) with XO; no change was noted with Con. Furthermore, peak contracture tension was significantly higher (31.5%) in the XO gr oup Post compared with Pre; again, no significant change was found wit h KR. The relaxation phase was also altered with XO but not with KR. A dditional experiments were conducted with application of 1 mM hypoxant hine, with results similar to the Con group. We conclude that the appl ication of ROS altered the dynamics of K+ contractures in the rat diap hragm, indicating changes in voltage-dependent excitation-contraction coupling.