Glycogen metabolism in muscle: its circadian and seasonal variations

The glycogen content of mice diaphragm and 14-C-glucose incorporation into glycogen was studied around the clock at 4-hr intervals. It was found that glycogen content presents a circadian rhythm with a peak value around 4 hr; this rhythm was reproduced in four different months, but with changes both in the absolute values and in the range of oscillation around the 24-hr mean value. Although labeled glucose incorporation exhibited a circadian variation as well, it peak value was 180 degrees out of phase compared with the glycogen rhythm. When the animals were submitted to a 29-hr fasting, the glycogen-24 hr-variation was still present, with similar oscillation around the mean. However, the absolute glycogen values were one-third as high as the ones obtained in the control animals. Our results suggest that the rhythm of glycogen content is due, at least partly, to the circadian variation of its synthesis. Furthermore, food intake is not the main factor responsible for the above mentioned rhythm.