POTASSIUM AND CAFFEINE CONTRACTURES OF MOUSE MUSCLES BEFORE AND AFTERFATIGUING STIMULATION

To assess the impairment of muscle membrane excitation, excitation-con traction (E-C) coupling, and contractility during muscle fatigue, we m onitored the contracture responses of resting and fatigued muscles on exposure to high potassium and caffeine. On exposure to 140 mmol/L pot assium, mouse extensor digitorum longus (EDL) developed a contracture which was 15.7% of tetanic tension before fatigue and 31.7% after fati gue, while soleus developed 59.4% contracture before and 68.8% after f atigue. Potassium causes contractures by depolarizing the muscle fiber membrane. Hence, membrane excitation is reduced in fatigued EDL and s oleus. On exposure to 32 mmol/L caffeine, the contracture was 7.1% in resting EDL, 8.5% in fatigued EDL, 50.1% in resting soleus, and 43.7% in fatigued soleus. On exposure to 1 mmol/L caffeine followed by rapid cooling, the contracture was 3.0% in resting EDL, 3.2% in fatigued ED L, 21.5% in resting soleus, and 10.3% in fatigued soleus. Caffeine cau ses contracture by releasing Ca++ from the sarcoplasmic reticulum. Our results indicate reduced E-C coupling attributable to reduced membran e excitation in fatigued EDL, and reduced contractility in fatigued so leus. (C) 1994 John Wiley & Sons, Inc.