Skeletal muscle glycogen stores

The primary fuel used to support muscle contraction depends on the magnitude and duration of exercise as well as the major fibers involved. Skeletal muscle has stores of both glycogen and some triglycerides. Blood glucose and free fatty acids also may be used. 

Glycogen is stored in the liver and skeletal muscle. Glycogen s)mthesis and degradation in the liver are carefully regulated. As we will see in Section 21.7, these two processes are intimately involved in keeping blood glucose levels constant. 

What would happen to a 70-kg man if the 135,000 kcal stored as triacylglycerols in his 18 kg of adipose tissue were stored instead as skeletal muscle glycogen It would take approximately 34 kg glycogen to store as many calories. Glycogen, because it is a polar molecule with -OH groups, binds approximately 4 times its weight in water, or 136 kg. Thus, his fuel stores would weigh 170 kg. 

Caffeine enhances the contractility of skeletal and cardiac muscle, and helps metabolize fat, thereby sparing muscle glycogen stores. It is also a central nervous system stimulant, which can aid in activities that require concentration. Many small studies using randomized, double-blind design have associated caffeine use with increased endurance times in athletes. - ... 

At rest, skeletal muscle burns free fatty acids almost exclusively (85-90 % of total energy) [25]. During exercise, muscle glycogen stores... 

Skeletal muscle utilizes glucose as a fuel, forming both lactate and CO2. It stores glycogen as a fuel for its use in muscular contraction and synthesizes muscle protein from plasma amino acids. Muscle accounts for approximately 50% of body mass and consequently represents a considerable store of protein that can be drawn upon to supply amino acids for gluconeogenesis in starvation. 

During the recovery period from exercise, ATP (newly produced by way of oxidative phosphorylation) is needed to replace the creatine phosphate reserves — a process that may be completed within a few minutes. Next, the lactic acid produced during glycolysis must be metabolized. In the muscle, lactic acid is converted into pyruvic acid, some of which is then used as a substrate in the oxidative phosphorylation pathway to produce ATP. The remainder of the pyruvic acid is converted into glucose in the liver that is then stored in the form of glycogen in the liver and skeletal muscles. These later metabolic processes require several hours for completion. 

All muscle types require ATP to achieve contraction. Glucose, fatty acids and amino acids may all be used as oxidizable substrates to produce ATP and all three energy sources may be obtained from stored intracellular sources (glycogen, triglyceride and protein) or imported from the blood stream. In quantitative terms, skeletal muscle is... 

The major fuels of skeletal muscle are glucose and fatty acids. Because of the enormous bulk, skeletal muscle is the body s major consumer of fuel. After a meal, under the influence of insulin, skeletal muscle takes up glucose to replenish glycogen stores and amino acids that are used for protein synthesis. Both excess glucose and amino acids can also be oxidized for energy. 

Glutamine is found in all cells in a combined form in peptides or proteins, but also in a free form. The highest free concentration of glutamine is found in muscle, where it acts as a store for use by other tissues. In fact, the total amount in all the skeletal muscle in the body is about 80 g, which is synthesised in the muscle from glucose and branched-chain amino acids (see Chapter 8). As with glycogen in the liver and triacylglycerol in adipose tissue. 

Glutamine is stored in muscle approximately 80 g in the total skeletal muscle in the body. This is an amount similar to that of glucose which is stored as glycogen in the liver (Chapters 2 and 6). 

Glycogen stores in skeletal muscle are mainly held in reserve to satisfy the organ s need for a burst of energy during exercise, and thus are rapidly depleted upon activity during a fast. 

The main stores of glycogen in the body are found in skeletal muscle, where they serve as a fuel reserve for the synthesis of ATP during muscle contraction, and in the liver, where glycogen is used to maintain the blood glucose concentration, particularly during the early stages of a fast. 

Glycogen is stored mainly by the liver and skeletal muscle as an energy reserve. The role of stored glycogen in muscle is to provide a source of energy upon prolonged muscle contraction. In contrast, glycogen stored in the liver is used to maintain blood glucose levels. 

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