Interactions Between Muscle Glycogen and Blood Glucose During Exercise - A Review

1. Interactions Between Muscle Glycogen and Blood Glucose During Exercise A Review by Mark Hargreaves, Presented by Anna Zorn

2. Objectives • Discuss primary CHO forms of fuel during exercise • Review major CHO sources • Explore glycogen as an exercise fuel • Explore glucose as an exercise fuel • “The Big Picture”: CHO interaction • View hypotheses regarding CHO-ingestion

3. Exercise Fuel Powers & Howley, Figure 4-11

4. Exercise Fuel Powers & Howley, Figure 4-13

5. Carbohydrates • FORMS: • Muscle Glycogen • Blood Glucose • SOURCES: • Glycogenesis: glycogen synthesis • Glycogenolysis: glycogen degradation • Gluconeogenesis: glucose synthesis • Dietary intake: Glucose absorption via intestines

6. Muscle Glycogenolysis • Glycogen Degradation: • Glycogen phosphorylase: regulatory enzyme • Activation: • Glucagon / Epinephrine – Response to low BG • Cascade amplification • High Ca2+ concentrations • Inactivation: • Insulin – Response to high BG

7. Muscle Glycogenolysis Metabolism, Figure 12-7

8. Muscle Glycogen Degradation

9. Activators of Glycogenolysis • Allosteric Activators: reflect intracellular energy • ADP, Pi • More bi-products = ↑ energy demand • Intense exercise = ↑ATP turnover, ↑ allosteric activators. ↑ activation of GP • Other Activators: • ↓ FFA in plasma = ↑ glycogen degradation • ↑ Glycogen = ↑ Glycogenolysis

10. Inhibition of Glycogenolysis • Allosteric Inhibitors: • ATP, NADH • ↓ intracellular energy demand = ↓ GP • Other Inhibitors: • ↑ FFA in plasma: ↓ rate of glycogenolysis • ↓ Glycogen = ↓ Glycogenolysis

11. Gluconeogenesis • Figure 9-2: metabolism pg. 131 Metabolism, Figure 9-2

12. Glucose Uptake • Facilitation of Glucose Uptake: • Insulin + Glucose • Insulin binds to insulin receptor • Glucose binds with GLUT-4 • Muscle contractions • Exercise = increased blood flow • GLUT– 4 Translocation = movement of transport protein from intracellular site to membrane

13. Glucose Uptake • Increased muscle glucose uptake and GLUT-4 translocation (due to exercise) increase with or without insulin! • Effects of insulin and exercise on glucose uptake are additive. • Local factors (Intracellular) are more influential in glucose uptake than systemic factors (circulatory)

14. “The Big Picture” • Glycogen / Glucose Utilization Are Affected By • Exercise Duration • Exercise Intensity • Glycogen and Glucose Availability • Availability of other fuels (FFA)

15. “The Big Picture” • Exercise of Increasing Duration: • Onset: Glycogen depletion is most rapid now accounting for most of CHO oxidation. • Glycogenolysis continues to substantially contribute as long as intensity is low. With time, glycogen depletes and less glycogenolysis occurs. • Glucose uptake increases with time, and more is utilized as exercise time progresses and glycogen depletes.

16. “The Big Picture” • Exercise of Increasing Intensity: • Low: Relatively low glucose uptake, but high utilization. Muscle glycogen degradation is low. • Moderate: Moderate rate of glucose uptake and utilization. Relatively greater glycogen degradation. • High: High rate of glucose uptake, but little utilization. Quick rate of glycogenolysis.

17. Carbohydrate Ingestion • CHO – Loading • CHO – Supplementation • CHO AS AN ERGOGENIC AID? • Varying Results • Depends on exercise duration, intensity, and mode

18. Research Conclusions • The interaction between glycogen and glucose fuel is complex, and even more so regarding exercise. • Muscle glycogen influences muscle glucose uptake. • Glucose uptake increases when muscle glycogen decreases. • Prolonged exercise = more glucose uptake as glycogen decreases from glycogenolysis. • These relationships have not been proven causal.

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