Pro- and macroglycogenolysis in contracting rat skeletal muscle.

Glycogen is present in skeletal muscle in smaller acid-insoluble proglycogen particles and larger acid-soluble macroglycogen particles. The present study was designed to investigate the relative contribution of pro- and macroglycogen to glycogenolysis during muscle contractions. Rats were subjected to a glycogen-depleting exercise bout and refed with either a carbohydrate-rich or fat-rich diet, resulting in widely different muscle glycogen contents. The following day, isolated hindlimbs were perfused and electrically stimulated to contract for 10 min. Pre- and postcontraction muscle samples of soleus, white and red gastrocnemius were analysed for pro- and macroglycogen. Contractions caused significant reductions in both pro- and macroglycogen in all glycogen groups and muscle types. In glycogen-supercompensated gastrocnemius muscles, the relative utilization of macroglycogen was significantly higher than the relative utilization of proglycogen. In muscles with normal to low initial glycogen contents, proglycogen was much more abundant than macroglycogen and therefore contributed more to glycogenolysis in absolute numbers. In conclusion, both proglycogen and macroglycogen are suitable substrates during skeletal muscle contractions, although macroglycogen, when amply available, seems to be more easily broken down. This may provide an explanation for the dependence of the glycogenolytic rate on the total muscle glycogen content.