When muscle biopsies first began to be used routinely in research on exercise physiology five decades ago, it soon become clear that the muscle content of glycogen is an important determinant of exercise performance. r Glycogen particles are stored in distinct pools within the muscles, but the role of each pool during exercise and how this is affected by diet is unknown. r Here, the effects of diet and exercise on these pools, as well as their relation to endurance during prolonged cycling were examined. r We demonstrate here that an improved endurance capacity with high carbohydrate loading is associated with a temporal shift in the utilisation of the distinct stores of glycogen pools and is closely linked to the content of the glycogen pool closest to actin and myosin (intramyofibrillar glycogen). r These findings highlight the functional importance of distinguishing between different subcellular microcompartments of glycogen in individual muscle fibres.

Glycogen supercompensation after glycogen-depleting exercise can be achieved by consuming a carbohydrate-enriched diet, but the associated effects on the size, number and localization of intramuscular glycogen particles are unknown. We investigated how a glycogen-loading protocol affects fibre type-specific glycogen volume density, particle diameter and numerical density in three subcellular pools: between (intermyofibrillar) or within (intramyofibrillar) the myofibrils or beneath the sarcolemma (subsarcolemmal). Resting muscle biopsies from 11 physically active men were analysed using transmission electron microscopy after mixed (MIX), LOW or HIGH carbohydrate consumption separated by glycogen-lowering cycling at 75% of maximal oxygen consumption until exhaustion. After HIGH, the total volumetric glycogen content was 40% [95% confidence interval 16, 68] higher than after MIX in type I fibres (P < 0.001), with little to no difference in type II fibres (9% [95% confidence interval −9, 27]). Median particle diameter was 22.5 (interquartile range 20.8-24.7) nm across glycogen pools and fibre types, and the numerical density was 61% [25, 107] and 40% [9, 80] higher in the subsarcolemmal (P < 0.001) and intermyofibrillar (P < 0.01) pools of type I fibres, respectively, with little to no difference in the intramyofibrillar pool (3% [−20, 32]). In LOW, total glycogen was in the range of 21-23% lower, relative to MIX, in both fibre types, reflected in a 21-46% lower numerical density across pools. In comparison to MIX, particle diameter was unaffected by other diets ([−1.4, 1.3] nm). In conclusion, glycogen supercompensation after prolonged cycling is exclusive to type I fibres, predominantly in the subsarcolemmal pool, and involves an increase in the numerical density rather than the size of existing glycogen particles.

Glycogen is a key energy substrate in excitable tissue, including in skeletal muscle fibers where it also contributes to local energy production. Transmission electron microscopy imaging has revealed the existence of a heterogenic subcellular distribution of three distinct glycogen pools in skeletal muscle, which are thought to reflect the requirements for local energy stores at the subcellular level. Here, we show that the three main energy-consuming ATPases in skeletal muscles (Ca2+, Na+,K+, and myosin ATPases) utilize different local pools of glycogen. These results clearly demonstrate compartmentalized glycogen metabolism and emphasize that spatially distinct pools of glycogen particles act as energy substrate for separated energy requiring processes, suggesting a new model for understanding glycogen metabolism in working muscles, muscle fatigue, and metabolic disorders. These observations suggest that the distinct glycogen pools can regulate the functional state of mammalian muscle cells and have important implications for the understanding of how the balance between ATP utilization and ATP production is regulated at the cellular level in general and in skeletal muscle fibers in particular.

Glycogen is a key energy substrate in excitable tissue and especially in skeletal muscle fibers it contributes with a substantial, but also local energy production. A heterogenic subcellular distribution of three distinct glycogen pools in skeletal muscle is proved by transmission electron microscopy (TEM), which is thought to represent the requirements for local energy stores at the subcellular level. Here, we show that the three main energy-consuming ATPases in skeletal muscles (Ca2+-, Na+,K+-, and myosin ATPases) utilize different local pools of glycogen. These results clearly demonstrate compartmentalized glycogen metabolism and emphasize that spatially distinct pools of glycogen particles act as energy substrate for separated energy requiring processes, suggesting a new paradigm for understanding glycogen metabolism in working muscles, muscle fatigue and metabolic disorders.