Effect of varying load magnitude on diaphragmatic glutathione metabolism during loaded breathing

Some studies have suggested that protective mechanisms downregulate diaphragm activity during loaded breathing so as to prevent respiratory- muscle fatigue. Other work has indicated, however, that loading can sometimes elicit significant diaphragmatic fatigue, and that the development of fatigue may be related to alterations in diaphragmatic glutathione concentrations. One potential explanation for these discrepant observations is that the mechanism of respiratory failure may vary as a function of load magnitude, and that some loads evoke little fatigue whereas others produce substantial fatigue and glutathione alterations. The purpose of this study was to examine this issue by determining the diaphragmatic fatigue and alterations in glutathione concentrations produced by a range of inspiratory resistive loads. Experiments were performed on decerebrate rats divided into a control, unloaded group and a group loaded with small, medium, and large inspiratory resistive loads that were applied until respiratory failure occurred. After respiratory arrest, the animals' diaphragms were excised, an in vitro determination was done of diaphragm contractility characteristics, and samples of muscle were assayed for GSH (reduced glutathione) and GSSG (oxidized glutathione). We found that in vitro diaphragm force generation was severely reduced for loaded breathing, and surprisingly, that the magnitude of the low-frequency fatigue present was similar in the three loaded groups. Reductions in diaphragmatic GSH levels and increases in GSSG levels were found in all three loaded groups. Reductions in diaphragmatic GSH levels and increases in GSSG levels were found in all three loaded groups, but again, the magnitude of these changes were similar. These findings indicate that protective mechanisms were unable to prevent severe decrements in diaphragm contractile function or glutathione oxidation at any level of load tested. These data are consistent with the finding that respiratory muscle fatigue plays a role in the development of respiratory failure, and that this is invariably associated with alterations in diaphragmatic glutathione metabolism.