Pro- and macroglycogenolysis in contracting rat skeletal muscle

Glycogen is present in skeletal muscle in smaller acid-insoluble proglycoge n particles and larger acid-soluble macroglycogen particles. The present st udy was designed to investigate the relative contribution of pro- and macro glycogen 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 conten ts. The following day, isolated hindlimbs were perfused and electrically st imulated to contract for 10 min. Pre- and postcontraction muscle samples of soleus, white and red gastrocnemius were analysed for pro- and macroglycog en. Contractions caused significant reductions in both pro- and macroglycog en in all glycogen groups and muscle types. in glycogen-supercompensated ga strocnemius muscles, the relative utilization of macroglycogen was signific antly 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 abs olute numbers. In conclusion, both proglycogen and macroglycogen are suitab le 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.