Effect of oxidant challenge on contractile function of the aging rat diaphragm. Lawler, John M., Camala C. Cline, Zhe Hu, and J. Richard Coast. Respiratory Muscle Laboratory, Eloise Beard Smith Human Performance Laboratories, Dept. of Health and Kinesiology, Texas A&M University, College Station, TX 77843-4243
APStracts 3:0206E, 1996.
Although controversial, there is growing evidence that reactive oxygen species (ROS) alter contractions of skeletal muscle, including the diaphragm. However, the impact of ROS on contractility of the aging diaphragm is unknown. The xanthine oxidase (0.01 U x ml-1) system was used as a ROS generator, imposing an oxidant challenge. Contractile function (twitch tension; twitch time-to-peak tension; twitch one -half relaxation time; tension at 10 and 20 Hz; maximal tetanic tension at 100 Hz) of costal diaphragm fiber bundles from young (4 mo) and old (25 mo) Fischer-344 rats was examined in vitro before (PRE) and after treatment with control Krebs solution (YC & OC), or with xanthine oxidase (0.01 U x ml-1) + hypoxanthine (0.29 mg x ml-1) substrate (YXO & OXO). Contractile function of fiber bundles were re-assessed following oxidant challenge in an unfatigued state (POST-u), or 10 minutes following a fatiguing stimulation protocol (POST-f). Oxidant challenge in the unfatigued fiber bundles increased twitch tension and tension at 10 and 20 Hz in YXO, but not OXO, without increasing maximal tetanic tension (Po). Conversely, xanthine oxidase significantly depressed fatigued diaphragm twitch and low frequency tension in both OXO and YXO, when compared to controls. Po was depressed POST-f in OXO but not YXO. Oxidant challenge also increased twitch one-half relaxation time of the fatigued diaphragm in both age groups Further, fiber bundles from old rats suffered greater fatigue during the stimulation protocol. We conclude that the response to oxidant challenge and increased contractile demand is impaired in the aging diaphragm. 

Received 9 April 1996; accepted in final form 12 September 1996.
APS Manuscript Number E179-6.
Article publication pending Am. J. Physiol. (Endocrinol. Metab.).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 5 November 1996