XANTHINE OXIDASE INDUCED OXIDANT CHALLENGE IN THE UNFATIGUED RAT DIAPHRAGM: TIME AND DOSE DEPENDENCE IN VITRO 160

Abstract

Previously, we have demonstrated that in vitro exposure of the rat diaphragm to xanthine oxidase (0.01-0.02 U·ml-1) generation(XO) of reactive oxygen species (ROS) potentiaties contractility in the unfatigued state and depresses contractility in the fatigued state(Am. J. Physiol., In Press). The purpose of the current study was to determine the effects of increased XO dose and time exposure on the contractility of the unfatigued diaphragm. Two diaphragm fiber bundles were extracted from each of thirteen Fischer-344 rats (4 mo. old) and placed in 36°C Krebs Ringers solution bubbled with 95% O2, 5% CO2. Baseline twitch tension (TW), tension @ 20 Hz (low-frequency tetanic), and maximal tetanic tension (Po) @ 120 Hz were then measured(PRE). One of five treatments were then applied: 0.01 U·ml-1 XO+ 0.5 mM hypoxanthine (1D); 0.02 U·ml-1(2D); 0.03 U·ml-1 (3D); 0.05 U·ml-1 (5D); or Krebs Ringers with 2.5 mM hypoxanthine as a control (CON). Diaphragm contractile function was then re-assessed 10 (POST-1) and 70 (POST-2) minutes after treatment introduction. Repeated measures ANOVA revealed significant effects of treatment across time (p<0.0001) for TW, 20 Hz, and Po. Significant potentiation of TW and 20 Hz for 1D,2D, and 3D were found at POST-1, however, significant depression of TW, 20 Hz, andPo for 3D and 5D were found atPOST-2. These data indicate a time and dose dependence of XO-induced oxidant challenge on diaphragm contractility and suggest a critical threshold of ROS exposure. Figure