The role of glycogen in muscles
Glycogen, “a fuel for the body”, has for almost a century been an area of research, however glycogen function and location is still not fully understood. Glycogen is produced by the chemical reaction of many molecules of glucose found in carbohydrates. The tissue in the skeletal muscles has an exceptional plasticity and according to inner and outer demands, the function and structure change to fulfill the metabolic demands from the skeletal muscles.
Glycogen is stored in the skeletal muscle, as an energy reserve, however glycogen is also present in other tissues such as the heart and liver. During exercise, the muscle needs energy to create contractions. A way to create energy is to metabolise glycogen via glycogenolysis^ or glycolysis*. However, other substrates such as lipids and protein can also be metabolised to create energy for contractions. The skeletal muscle has a limited amount of glycogen, which can be affected by diet and training. In the end, this can prolong or shorten the time of exhaustion during prolonged exercise. It is then accepted that the initial size of the glycogen store is a limiting factor for the working capacity.
Despite the above mechanism, research has also shown that other mechanisms were responsible for the time to fatigue during exercise. Glycogen independent mechanisms may play a role in the onset of fatigue. These mechanisms could be exercise condition, by change in substrate availability, dehydration, changes in hydrogen (H+ ), Magnesium (Mg2+ ), calcium (Ca), and potassium(K+ ) balance.
^ the breakdown of glycogen into glucose,
* Series of chemical reactions in which a molecule of glucose is split into two molecules to produce ATPs.
During exercise, glycogen and lipids are the main energy sources. The higher the intensity the more the energy will rely on glycogen as a substrate and during less intense exercise the more lipids will be utilised. If a diet of high lipids is consumed before exercise, then the lipids utilized during exercise will increase. The same is observed with a diet of high carbohydrates. Trained individuals utilises less carbohydrates during exercise at same relative load compared to untrained. Carbohydrates intake can increase glycogen content in the muscle by 11 %, after 3 hours of ingestion. However, if carbohydrates are consumed before and during exercise, a reduced utilisation of glycogen by 30 % is observed, with a reduced utility in type 1 fibres, however type 2 fibres were unaffected This is in line with previous investigation which showed differences in glycogen particle size, glycogen content and distribution in skeletal muscle fibre types. Also, glycogen is metabolised from different fibre and compartments, depending on the type of exercise. Prolonged endurance exercise almost depleted type 1 fibre compared to repetitive sprint exercise, which metabolised from both type 1 and 2, however to a smaller extend. Furthermore it is observed that the size of the glycogen particles varies within the compartments and after exhausting exercise, the skeletal muscle is selective in replenishing the glycogen stores, first by particle number, hereafter particle size.
The subcellular compartments are divided into three subcellular locations when analyzed by electron microscopy. 1) The IMF glycogen, which is the intermyofibrillar glycogen, located close to the sarcoplasmic reticulum (SR) and mitochondria between the myofibrils. The IMF glycogen is responsible for approximately 75 % of the total amount of glycogen (tgly) in the skeletal muscle. 2) The Intra glycogen, which most often is located in the I-bad of the sarcomere, interspersed between the myofibrils and represent approximately 5-15 % of the tgly in the skeletal muscle. 3) The SS glycogen, which is located close to the mitochondria, lipids, Golgi apparatus and nuclei under the surface membrane represent approximately 5-15 % of tgly in the skeletal muscle.
A study showed no difference between the glycogen distribution in fibre types in sedentary obese subjects or recreationally active young or elderly men. Weather endurance trained athletes have 30 % more SS and 80 % more Intra glycogen in type I fibre compared to type II fibre. However, type II fibre have 10 % more IMF glycogen compared to type I fibre. The absolute and relative distribution changes with training, and it is shown that endurance trained athletes have 60% more Intra glycogen in type I fibre and 23 % and 63 % more glycogen in IMF and SS in both type I and II fibres compared to untrained individuals. In recovery after exercise without carbohydrates availability Intra glycogen resynthesis was inhibited compared to SS and IMF glycogen. Similar, it was found after two weeks of disuse the Intra glycogen decrease with 50 %, while SS and IMF remained unchanged. It is thought that SS and Intra glycogen are related to Na+-K+ activity in the sarcolemma and t-system and trained individuals have a higher requirement of activity here compared to untrained, which is the reason for larger compartments.
It is well known that glycogen content in the skeletal muscle increases with training. However, more research is needed to clarify and better understand glycogen role in the skeletal muscle during various conditions. Is it possible to optimized the storing of glycogen, is the storing space limited, how are the compartments affected by training in healthy individuals, why are the glycogen distributed as it is. This is some of the questions which still remain unanswered. This might in the end help athletes, recreational or elite, to perform better, during competition or training.