MUSCLE AND LIVER-GLYCOGEN CONTENT - DIURNAL-VARIATION AND ENDURANCE

Male albino rats were swum to exhaustion at 0700 or 1900 h, after which samples of soleus, white vastus lateralis and red vastus lateralis muscles, as well as liver, were excised and subsequently analyzed for glycogen content. Glycogen content of all tissues from nonexercising control animals was higher in the morning than in the evening. Consequently, animals at 0700 h swam 60% longer than those at 1900 h (209 .+-. 20 min vs. 130 .+-. 23 min, respectively). Because the skeletal tissues of the exhausted animals were not totally depleted of glycogen, it was concluded that fatigue under the swim protocol was the result of hypoglycemia secondary to hepatic glycogen depletion. The physiological consequences of diurnal glycogen fluctuation were demonstrated and experimental support was established for the importance of controlling this variable in rodent exercise investigations.