Cardiac glycogen in long-evans rats: diurnal pattern and response to exercise.

The 24-h pattern of cardiac glycogen was determined in normally active, caged male Long-Evans rats. Relatively small fluctuation was observed during a 24-h cycle with maximal difference between mean values ranging from 3.41 +/- 0.28 (dark room) to 5.15 +/- 0.19 (light room) mg glycogen/g wet wt heart, suggesting that the substantial diurnal variation of cardiac glycogen reported in Wistar rats is not a universally observed phenomenon. Cardiac glycogen during and following a single bout of moderate running was compared to a bout of strenuous running in fed male Long-Evans rats. Moderate continuous running at 20 m/min for 30 min did not decrease cardiac glycogen below the average control level (4.09 +/- 0.10 mg glycogen/g heart) but did cause a short period of supercompensation, which reached a peak of 6.27 +/- 0.19 mg/g heart at 2 h postexercise. Strenuous running in bouts at 30 m/min over a 2-h period for a distance of 1,413 m caused a significant decrease in cardiac glycogen to 2.66 +/- 0.20 mg/g heart followed by an extended period of supercompensation, which reached a peak of 9.01 +/- 1.41 mg/g heart at 4 h postexercise and remained significantly elevated during the next 13 h. Thus, the severity of exercise in normal, fed rats determines not only the extent of cardiac depletion, but also the supercompensation pattern of glycogen repletion following exercise.