EFFECT OF OXIDATIVE STRESS AND ACIDOSIS ON DIAPHRAGM CONTRACTILE FUNCTION

Acidosis during exercise has long been associated with skeletal muscle fatigue. Recent evidence also has linked reactive oxygen species (ROS ) with fatigue in skeletal muscle, including the diaphragm. We hypothe sized that acidosis (designed to mimic blood pH during maximal exercis e) would worsen ROS-induced depression of diaphragm contractility. The xanthine oxidase (XO) reaction in solution (0.01 U/ml) allows direct assessment of the effects of oxidant stress by ROS. Costal diaphragm f iber bundles from 24 Sprague-Dawley rats (200-250 g) were divided into four treatment groups: 1) pH 7.4, no XO (H); 2) pH 7.4 + XO (HXO); 3) pH 7.0, no XO (L); and 4) pH 7.0 + XO (LXO). Baseline twitch mechanic s and force-frequency relationships (Pre) were determined in control K rebs solution (pH 7.4, no XO) before treatment. Treatment solutions we re introduced, and the diaphragm underwent 2 min of contractions at 25 Hz (250 ms) at a rate of 1/s. After 10 min of recovery, the control s olution was reintroduced into the bath and postcontractile function (P ost) was measured. Significant reductions in twitch tension and low-fr equency tetanic tension were greater in HXO and LXO compared with H, w ithout an effect on maximal tetanic tension. One-half relaxation time was prolonged only by the combination of acidosis and oxidative stress . Addition of superoxide dismutase (50 U/ml) worsened and catalase (1, 800 U/ml) attenuated XO-induced depression of diaphragm contractility. We concluded that XO induced a reduction of low-frequency tension in the fatigued diaphragm, which was mediated directly or indirectly thro ugh hydrogen peroxide and was exacerbated to a modest extent with acid osis.