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

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

Exercise Fuel Powers & Howley, Figure 4-11

Exercise Fuel Powers & Howley, Figure 4-13

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

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

Muscle Glycogenolysis Metabolism, Figure 12-7

Muscle Glycogen Degradation

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

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

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

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

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

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

“The Big Picture” Exercise of Increasing Duration: Exercise of Increasing Duration: Onset: Glycogen depletion is most rapid now accounting for most of CHO oxidation. 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. 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. Glucose uptake increases with time, and more is utilized as exercise time progresses and glycogen depletes.

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

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

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

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