Glycogen metabolism and the function of fast and slow muscles of the rat.

Changes in glycogen metabolism with function have been explored in a fast (the extensor digitorum longus, EDL) and in a slow (the soleus, S) muscle of rat. The rate of glucose incorporation is not related to the glycogen levels. In the S glycogen levels are lower and glucose incorporation is higher than in EDL and differences almost disappear after denervation and increase with exercise. In the S, synthetic activities may be higher and glycogenolytic activities are lower than in the EDL. The levels are given of many substrates and cofactors which can affect glycogen enzymes "in vivo". On the whole, the data indicate that the rate of glycogen turnover can change dramatically with muscle function even at very light work load and may be higher in the so called oxidative than in the glycolytic muscles.