Glutathione metabolic responses to loaded breathing: variation among respiratory muscles

Several studies have shown that loaded breathing elicits an oxidation of reduced glutathione (GSH) to oxidized glutathione (GSSG) within the diaphragm, but the effects of loaded breathing on GSH and GSSG levels in other respiratory muscles have not been examined. The present experiment examined this issue by using decerebrate unanesthetized rats in which a large inspiratory resistive load was applied until respiratory arrest. Subsequently, muscle samples from the triangularis sterni, diaphragm (Dia), parasternal intercostal (PI), upper rib cage lateral intercostal, lower rib cage lateral intercostal, and soleus were assayed for GSH and GSSG. Glutathione levels were also measured on samples from unloaded control animals. We found that the Dia from loaded animals had a lower GSH level than did control animals (i.e., 653 +/- 99 and 928 +/- 40 nmol/gm for loaded and control groups, respectively; P < 0.05), higher GSSG level (68 +/- 14 and 32 &/- 7 nmol/gm for loaded and control groups, respectively; P < 0.05), and higher GSSG-to-GSH ratios (GSSG/GSH; 17.0 +/- 6.0 and 3.7 +/- 0.9% for loaded and control groups, respectively; (P <0.05). Of the other muscles examined, only the PI muscles had comparable alterations in glutathione levels in response to loading. Specifically, for the PI muscles of loaded and control groups, GSH was 427 +/- 75 and 618 +/- 40 nmol/g, (P < 0.05), GSSG was 71 +/- 16 and 20 +/- 5 nmol/g (P < 0.01), and GSSG/GSH was 22 +/- 8 and 3.6 +/- 1.2%, respectively (P < 0.05). No other muscle demonstrated a significant increase in GSSG or GSSG/GSH with loading, and only the lower rib cage lateral intercostal had a significant reduction in GSH. These findings indicate variation in the degree of glutathione oxidation elicited by inspiratory loading among the different respiratory muscles. The fact that quantitatively similar glutathione alterations were observed in the Dia and PI muscles suggests that these muscle groups may share a similar propensity to generate free radicals during inspiratory loading.