Biochemical Foundations of Human Vital Activity - Volkov N.I., Nesen E.N. 2000

Biochemical Foundations of Human Vital Activity
Carbohydrate Biochemistry
Carbohydrate Metabolism During Muscular Activity

Muscle Glycogen and Blood glucose serve as essential substrates for ATP production in contracting Muscles during prolonged Physical Exercise of submaximal and high power, such as 400m, 800m, 1000m, and 10,000m running. The duration of work depends on the Skeletal Muscle glycogen stores (Fig. 68, a).

Physical exertion enhances glycogen breakdown (mobilization) and glucose oxidation (Glycogenolysis) within muscle fibers. The rate of its breakdown depends on the intensity of physical activity (Fig. 68, b). During low-intensity cycle ergometer exercise (30% $\text{VO}_2\text{max}$), glycogen stores in the gastrocnemius (lateral) muscle decrease by only 20–30% over two hours of work, whereas during high-intensity work (60% $\text{VO}_2\text{max}$), they drop by 80%.

Muscle glycogen breaks down most rapidly During the first few minutes of muscular work. During prolonged exercise, the rate of muscle glycogen breakdown decreases due to the depletion of its stores.

The rate of glycogen breakdown or glucose mobilization varies in fast-twitch (FT) and slow-twitch (ST) types of muscle fibers under METABOLISM/18.html">The Influence of physical loads of varying power (Fig. 68, c). Thus, at moderate work intensity (within 60–75% $\text{VO}_2\text{max}$), enhanced glycogen breakdown occurs in slow-twitch muscle fibers, whereas with an increase in physical load intensity, it shifts to fast-twitch fibers, which exhibit higher glycogenolysis enzyme activity than slow-twitch ones.

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Fig. 66 Scheme of Gluconeogenesis in the Liver

Fig. 67 Lactic acid cycle (Cori cycle) and glucose-Alanine cycle

Fig. 68 Dependence of muscle work duration on their glycogen stores (a), as well as the dependence of glycogen depletion on cycle ergometer exercise intensity in the gastrocnemius muscle (b) and in various types of muscle fibers (c)

The enhancement of carbohydrate mobilization is driven by an increase in The activity of Enzymes that catalyze glycogen breakdown and synthesis. During specific types of muscular work, the activity of Glycogen phosphorylase in the lower limb muscles increases by 2.4-fold, and glycogen synthase by almost 2-fold. The degree of enzyme activity alteration depends on the duration, intensity, and type of load. The activity of these enzymes is regulated by multiple mechanisms, including Hormones (adrenaline), cyclic AMP, and $\text{Ca}^{2+}$ ions, whose metabolism changes during muscular activity (see Chapter 13). Systematic muscular activity leads to an increased glycogen concentration and higher activity of its metabolic enzymes in muscles, which improves their Energy Metabolism during physical exertion.

Muscles also utilize blood glucose for energy production processes. At rest, they consume about 20% of the total glucose entering the blood, whereas at a workload of 60% $\text{VO}_2\text{max}$, they take up more than 80% of blood glucose. This is due to enhanced glucose delivery via blood flow, an increased rate of transport across muscle membranes, and higher muscle utilization.

The degree of blood glucose uptake by muscles is influenced by the type of exercise performed, fitness level, diet, gender characteristics, and the metabolic state of the body. A high level of muscle glycogen—typical of highly trained endurance athletes—as well as an elevated concentration of free Fatty acids in the blood, reduce muscle glucose uptake. Hypoxia stimulates glucose uptake into skeletal muscles. In women, glycogen breakdown and glucose oxidation during muscular work are less pronounced than in men.

During muscular activity, the mobilization of glucose from the liver, where it is stored as glycogen, increases. Glycogen breaks down into glucose, which is released into the blood, preventing The Development of hypoglycemia. The release of glucose from the liver into the bloodstream increases 2- to 3-fold during moderate-intensity muscular activity and 7- to 10-fold during intense work. Due to the homeostatic function of the liver, a high Blood Glucose Level is maintained during muscular activity until liver glycogen stores are exhausted. At the expense of liver glycogen reserves, muscles can perform high-power work for 20–40 minutes.

During strenuous muscular work, liver glycogen stores decrease significantly within 1–2 hours, leading to a drop in blood glucose levels. As exercise duration increases, The process of gluconeogenesis makes a certain contribution to maintaining blood glucose levels.

In the process of gluconeogenesis, which is active in The Liver and Kidneys, glucose is synthesized from Amino Acids, glycerol, lactic acid, and pyruvic acid, thereby preventing tissue glycogen depletion. At the same time, THE CONTRIBUTION OF gluconeogenesis to maintaining blood glucose during short-term exercise is insignificant (10–20%), whereas during prolonged work (lasting several hours), it rises to 50% relative to the total glucose produced in the liver.

Review Questions    

1. Characterize CARBOHYDRATES and their biological role in the body.

2. What classes of carbohydrates and their representatives do you know? What are the Structural Features of them?

3. How is the cyclic form of Monosaccharides formed? What is the advantage of such carbohydrates over linear ones?

4. Write the structural formulas for glucose, fructose, and ribose, as well as their phosphate esters.

5. How are the main Disaccharides structured? Name the enzymes responsible for their Hydrolysis.

6. What is the structural difference between starch and glycogen?

7. Name the Main Pathways of Carbohydrate Metabolism in The Human Body.

8. What are the features of carbohydrate hydrolysis during Digestion and their absorption?

9. What are the mechanisms for maintaining a constant blood glucose concentration?

10. What is meant by carbohydrate storage and mobilization?

11. What are The Essence and energy yield of Glycolysis? Under what types of physical exertion does this process occur in muscles?

12. Name the MAIN STAGES OF aerobic carbohydrate oxidation. What is The Role of The Citric Acid Cycle in their oxidation?

13. What is the role of the Pentose Phosphate Pathway in the body?

14. What are the Specific features of carbohydrate metabolism during muscular activity?

15. How does lactic acid affect physical performance? Why?