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Hormonal Regulation
of Metabolism

Discover how insulin, glucagon, and epinephrine orchestrate your body's metabolic pathways to maintain energy balance.

Biochemistry

What is Metabolic Regulation?

Your body must continuously balance energy storage and energy mobilization. This balance depends on circulating hormones that act as chemical messengers, telling cells when to store glucose as glycogen or fat, and when to break those stores down for fuel.

Meet the Three Key Hormones

Insulin
Released by pancreatic beta cells when blood glucose is high. Promotes glucose uptake and storage.
Glucagon
Released by pancreatic alpha cells when blood glucose is low. Promotes glycogen breakdown and gluconeogenesis.
Epinephrine
Released by the adrenal medulla during stress or exercise. Rapidly mobilizes glucose and fatty acids.

Why Hormonal Control Matters

Without hormonal regulation, your body would have no way to coordinate the activities of distant organs. The liver, skeletal muscle, and adipose tissue must all respond in a unified way to changes in nutrient availability.

1
Brain depends on glucose: The brain consumes about \( 120 \text{ g} \) of glucose per day and cannot easily use fatty acids.
2
Organ specialization: Liver exports glucose; muscle consumes it; adipose stores lipids. Coordination is essential.
3
Disease relevance: Dysregulation causes diabetes mellitus, metabolic syndrome, and hypoglycemia.

The Fed State: Insulin Signaling

High Blood Glucose Pancreas (Beta Cells) releases Insulin Liver Glycogenesis
\( \text{Glucose} \rightarrow \text{Glycogen} \)
 Muscle Glucose Uptake
via \( \text{GLUT4} \) transporters
 Adipose Lipogenesis
\( \text{Glucose} \rightarrow \text{Fatty Acids} \)
 Blood Glucose Decreases

The Fasted State: Glucagon Signaling

Low Blood Glucose Pancreas (Alpha Cells) releases Glucagon Liver Glycogenolysis
\( \text{Glycogen} \rightarrow \text{Glucose} \)
 Liver Gluconeogenesis
\( \text{Amino acids} \rightarrow \text{Glucose} \)
 Adipose Lipolysis
\( \text{TAG} \rightarrow \text{Fatty Acids} \)
 Blood Glucose Increases

Epinephrine: Fight-or-Flight Response

Stress / Exercise Adrenal Medulla Epinephrine Released Liver Glycogenolysis
(via \( \beta \) receptors)
 Muscle Glycogenolysis (local fuel only) Adipose Lipolysis
(releases \( \text{FFAs} \))
 Rapid Energy Mobilization

The \( \text{cAMP} \) Signaling Cascade

Both glucagon and epinephrine share the same intracellular mechanism: the \( \text{cAMP} \)-\( \text{PKA} \) cascade. This amplification system ensures that a tiny amount of hormone produces a massive cellular response.

1
Hormone binds \( \text{GPCR} \) on the cell surface, activating a G-protein.
2
G-protein activates adenylyl cyclase, which converts \( \text{ATP} \) to \( \text{cAMP} \).
3
\( \text{cAMP} \) activates \( \text{PKA} \) (Protein Kinase A), which phosphorylates target enzymes.
4
Phosphorylation cascades activate glycogen phosphorylase and inhibit glycogen synthase.
5
Signal amplification: One hormone molecule can activate millions of enzyme molecules.

Insulin vs. Glucagon: Opposing Forces

Feature Insulin Glucagon
Trigger High blood glucose Low blood glucose
Source Beta cells Alpha cells
\( \text{cAMP} \) effect Decreases \( \text{cAMP} \) Increases \( \text{cAMP} \)
Enzyme state Dephosphorylated (active synthases) Phosphorylated (active phosphorylases)
Net effect Storage (anabolic) Mobilization (catabolic)
Primary target Liver, muscle, adipose Liver (mainly)

Phosphorylation as the Master Switch

Glycogen Synthase Active (Dephospho) Makes glycogen Inactive (Phospho) Glycogen synthesis off
\( \text{PKA} \)
\( \text{PP1} \)
 Glycogen Phosphorylase Inactive (Dephospho) No glycogen breakdown Active (Phospho) Breaks down glycogen Phosphorylase Kinase
\( \text{PP1} \)
 Glucagon/Epinephrine → Phosphorylation (right) Insulin → Dephosphorylation (left)

How to Predict Metabolic Outcomes

1
Identify the metabolic state: Is blood glucose high (fed), low (fasted), or is the body under stress?
2
Determine the dominant hormone: Fed → insulin. Fasted → glucagon. Stress → epinephrine.
3
\( \text{cAMP} \) direction: Insulin lowers \( \text{cAMP} \). Glucagon and epinephrine raise \( \text{cAMP} \).
4
Predict the enzyme state: High \( \text{cAMP} \) → phosphorylated enzymes. Low \( \text{cAMP} \) → dephosphorylated enzymes.
5
State the pathway outcome: Phosphorylated → glycogenolysis, lipolysis, gluconeogenesis. Dephosphorylated → glycogenesis, lipogenesis, glycolysis.

Example 1: After a Meal

Scenario: A student eats a large carbohydrate-rich meal. Blood glucose rises to \( 140 \text{ mg/dL} \). What happens in the liver?

1
State: Fed state. Blood glucose is elevated.
2
Dominant hormone: Insulin is released from beta cells.
3
\( \text{cAMP} \) decreases in hepatocytes. \( \text{PKA} \) is inactive.
4
Enzymes: Glycogen synthase is dephosphorylated (active). Glycogen phosphorylase is dephosphorylated (inactive).
5
Outcome: Liver converts excess glucose to glycogen (glycogenesis) and fatty acids (lipogenesis).

Example 2: Overnight Fasting

Scenario: A person has been sleeping for \( 8 \) hours with no food intake. Blood glucose drops to \( 72 \text{ mg/dL} \). What metabolic changes occur?

1
State: Fasted. Blood glucose is low.
2
Dominant hormone: Glucagon rises from alpha cells. Insulin falls.
3
\( \text{cAMP} \) increases in the liver. \( \text{PKA} \) is activated.
4
Liver: Glycogen phosphorylase is phosphorylated (active). Glycogenolysis releases glucose into the blood. Gluconeogenesis begins from amino acids.
5
Adipose: Lipolysis releases fatty acids and glycerol for hepatic beta-oxidation and gluconeogenesis substrate.

Example 3: Sprint Exercise

Scenario: An athlete begins a \( 100 \)-meter sprint. The sympathetic nervous system activates. What happens in skeletal muscle and liver?

Muscle Response
Epinephrine activates glycogen phosphorylase via \( \text{cAMP} \). Glycogen is rapidly broken down to \( \text{glucose-6-phosphate} \), which enters glycolysis locally. Muscle cannot export glucose.
Liver Response
Epinephrine stimulates hepatic glycogenolysis. \( \text{Glucose-6-phosphatase} \) converts \( \text{G6P} \) to free glucose, which is released into the blood to supply the brain and other tissues.

Common Mistakes to Avoid

✕ Confusing cell types
Alpha cells release glucagon. Beta cells release insulin. Not the other way around. Remember: A comes before B, Glucagon before Insulin alphabetically.
✕ Muscle exports glucose
Skeletal muscle lacks \( \text{glucose-6-phosphatase} \) and cannot release free glucose into the blood. Only the liver (and kidney) can do this.
✕ Phosphorylation always activates
Phosphorylation activates phosphorylase but inactivates glycogen synthase. The effect depends on the specific enzyme.

Comprehensive Hormone Summary

Feature Insulin Glucagon Epinephrine
Stimulus High glucose Low glucose Stress/exercise
Source Beta cells Alpha cells Adrenal medulla
Receptor Tyrosine kinase \( \text{GPCR} \) \( \text{GPCR} \) (\( \beta \))
\( \text{cAMP} \) Decreases Increases Increases
Key target organs Liver, muscle, adipose Liver Liver, muscle, adipose
Net metabolic effect Anabolic (storage) Catabolic (release) Catabolic (rapid fuel)

Question 1 of 5

Which pancreatic cell type releases glucagon in response to low blood glucose?

Question 2 of 5

When glucagon binds its receptor on a hepatocyte, what is the immediate intracellular effect?

Question 3 of 5

During intense exercise, epinephrine stimulates glycogenolysis in skeletal muscle. Why can't muscle release glucose into the bloodstream?

Question 4 of 5

In the fed state, insulin promotes the dephosphorylation of glycogen synthase. What is the functional consequence?

Question 5 of 5

A patient with Type 1 diabetes cannot produce insulin. Which metabolic abnormality would you most expect in the liver after a meal?