Questions: Hepatic Glucose Production: Glycogenolysis and Gluconeogenesis

Question 1

A marathon runner's liver glycogen is nearly depleted after 2 hours of racing, yet their muscles still contain significant glycogen stores. Why can't muscle glycogen rescue blood glucose levels as hepatic stores run out?

• A. Muscle glycogen is a different polymer than liver glycogen and cannot be converted to glucose
• B. Muscle lacks glucose-6-phosphatase, so it cannot release free glucose into the bloodstream — muscle glycogen fuels only muscle contraction
• C. Muscle glycogen is too far from the bloodstream to be transported to the liver for glucose synthesis
• D. Epinephrine during exercise suppresses muscle glycogen breakdown to preserve it for later

Answer: (B)

The key enzyme is glucose-6-phosphatase, which converts glucose-6-phosphate to free glucose that can exit the cell and enter the bloodstream. The liver and kidneys express this enzyme; skeletal muscle does not. When muscle glycogen is broken down, it yields glucose-6-phosphate, which muscle cells use immediately for local glycolysis. Without glucose-6-phosphatase, muscle cannot export glucose to the blood. This is why a well-glycogen-loaded marathon runner still develops exercise-induced hypoglycemia when hepatic stores deplete: the enormous muscle glycogen reservoir is metabolically unavailable for blood glucose maintenance.

Question 2

A patient fasts for 30 hours. Which statement best describes the relative contributions of glycogenolysis and gluconeogenesis to hepatic glucose output at this point?

• A. Glycogenolysis is dominant because hepatic glycogen stores are large enough to last several days
• B. Both contribute equally throughout fasting, with the ratio depending primarily on blood cortisol levels
• C. Gluconeogenesis accounts for essentially all hepatic glucose output because glycogen stores are substantially depleted after 12–18 hours
• D. Neither pathway is active at 30 hours — the brain has switched entirely to ketone bodies by this point

Answer: (C)

Hepatic glycogen stores (roughly 80–100 grams) are progressively depleted during fasting, with glycogenolysis dominating glucose production in the first 12–18 hours. By 30 hours, glycogen is substantially depleted and gluconeogenesis — using lactate, glucogenic amino acids, and glycerol as substrates — accounts for essentially all hepatic glucose output. The brain has not entirely switched to ketones at 30 hours; that transition takes several days of fasting. Cortisol influences the rate of gluconeogenesis but does not independently determine the glycogenolysis/gluconeogenesis ratio.

Question 3

Glucagon activates hepatic glucose production by stimulating both glycogenolysis and gluconeogenesis through cAMP signaling in hepatocytes.

• T. True
• F. False

Answer: True

Glucagon, released by pancreatic alpha cells when blood glucose falls, binds G-protein coupled receptors on hepatocytes, activating adenylyl cyclase and raising intracellular cAMP. This activates protein kinase A, which phosphorylates and activates glycogen phosphorylase (promoting glycogen breakdown) and upregulates key gluconeogenic regulators. The net effect is simultaneous activation of both glucose-releasing pathways, making glucagon highly effective at rapidly restoring blood glucose during hypoglycemia.

Question 4

Because muscle contains large glycogen stores, muscle glycogenolysis is a major direct source of blood glucose during prolonged fasting.

• T. True
• F. False

Answer: False

Muscle glycogen cannot directly contribute to blood glucose because muscle lacks glucose-6-phosphatase. Glycogen breakdown in muscle yields glucose-6-phosphate, which is trapped in the muscle cell and used for local glycolysis, producing lactate. That lactate enters the bloodstream and travels to the liver, where it serves as a gluconeogenic substrate — an indirect contribution called the Cori cycle. So muscle glycogen indirectly supports blood glucose via the Cori cycle, but cannot do so directly through glycogenolysis — which is the common misconception.

Question 5

Why does hepatic gluconeogenesis require dedicated enzymes rather than simply running glycolysis in reverse? What does this tell you about metabolic regulation?

Model answer: Three glycolytic steps are thermodynamically irreversible under physiological conditions: hexokinase (glucose → G6P), phosphofructokinase (F6P → F1,6BP), and pyruvate kinase (PEP → pyruvate). Gluconeogenesis bypasses each with a dedicated enzyme: glucose-6-phosphatase, fructose-1,6-bisphosphatase, and the two-step sequence of pyruvate carboxylase plus PEP carboxykinase. This arrangement allows independent regulation: glucagon/cortisol can activate gluconeogenesis while insulin suppresses it, without the two pathways short-circuiting each other. If gluconeogenesis were simply reverse glycolysis sharing the same enzymes, the cell couldn't run both pathways at different rates or selectively inhibit one.

This principle — irreversible steps require bypass enzymes — is a recurring theme in metabolism and explains why 'just running it backwards' is thermodynamically forbidden at key steps. The existence of dedicated enzymes is not redundancy but regulatory architecture: it allows precise, hormone-responsive control of metabolic direction, which is essential for maintaining blood glucose homeostasis across different nutritional states.