Substrate repletion in rat myocardium, liver, and skeletal muscles after exercise

Carbohydrate and lipid substrates were measured in rats during recovery following exercise or a 24-h fast and compared with values from time-matched control (rested, fed) rats. After exercise muscle glycogen recovered at the expense of liver glycogen repletion. Myocardial glycogen supercompensated whereas soleus, red vastus lateralis (RVL) and white vastus lateralis glycogen merely returned to control levels. A similar recovery pattern occurred after fasting with refeeding promoting glycogen synthesis in the liver, skeletal muscles, and even in the myocardium, where glycogen had already been elevated by the fast. Both soleus and RVL muscles, along with the myocardium, exhibited glycogen supercompensation. Both exercise and fasting increased plasma free fatty acid (FFA) levels which favor myocardial glycogen synthesis. Unchanged tissue triglycerides and relatively stable blood glucose levels suggest that these are unlikely influences on glycogen recovery. It is concluded that exercise per se is unlikely to induce glycogen supercompensation in skeletal muscles though myocardial glycogen supercompensation readily occurs, that food restriction prior to exercise quantitatively affects substrate recovery though its impact could go unnoticed because of the qualitative similarities between substrate recovery following exercise or fasting, and that FFA is the only major energy substrate concurrently changing with glycogen after exercise or fasting which could facilitate glycogen synthesis.