Lactic acidosis and prolonged in vitro diaphragmatic fatigue

Previous work from our laboratory showed that lactic acidosis did not decrease contractility of the in vitro isolated diaphragm at pH levels attained during exercise in humans. These results suggest that lactate and H ion efflux from exercising tocomotor muscle do not adversely effect the diaphragm. Others have indicated that long-term fatigue (>10 min) may respond more to outside perturbations than initial contractility or short-term (<2 min) fatigue. Therefore, this study evaluated the effects of a physiological level of lactic acidosis on prolonged fatigue in the isolated in vitro rat diaphragm strip. Twelve strips were removed from the lateral costal portion of the diaphragms of six female Sprague-Dawiey rats (X = 345 gm) and placed in a bath with a control Ringer's solution. After a 15 min equilibration period, a force-frequency relationship was established for the strips using frequencies from twitch to 200 Hz. The bath was then replaced with a second control solution or a Ringer's solution with 10mM lactic acid added and buffered to a pH of 7 1. Following another 15 min equilibration, a second force-frequency curve was measured and the strip was then fatigued with 250 msec trains at 25 Hz (1 tr/sec) for 30 minutes. No differences were found in the force-frequency curves across time or condition. Fatigue was not different at 1,2, 5,10,20, or 30 min, although the fatigue index was slightly increased in the lactate strips at all time periods. This study indicates that physiological arterial levels of lactic acid and pH do not adversely effect the isolated diaphragm and are not likely responsible for exercise-induced diaohraam fatiaue.