• Blood glucose is the main controller of insulin secretion from pancreatic beta cells (Fig. 79.7).
• Beta cells contain many glucose transporters, so glucose entry into the cells rises as blood glucose rises.
• Inside the beta cell: Glucose → glucose-6-phosphate by glucokinase.
• Glucokinase is the main rate-limiting step in beta-cell glucose metabolism.
• Therefore, glucokinase acts as an important glucose sensor, helping match insulin secretion to blood glucose level.
• Glucose-6-phosphate is metabolized to produce ATP.
• Increased ATP closes ATP-sensitive K⁺ channels.
• Closing K⁺ channels causes beta-cell membrane depolarization.
• Depolarization opens voltage-gated Ca²⁺ channels.
• Ca²⁺ then enters the beta cell.
• Increased intracellular Ca²⁺ causes insulin-containing vesicles to fuse with the cell membrane.
• Insulin is then released by exocytosis.
• The basic sequence is: ↑ Blood glucose → ↑ glucose entry → glucokinase → ↑ ATP → K⁺ channels close → depolarization → Ca²⁺ channels open → ↑ Ca²⁺ entry → insulin exocytosis
• Certain amino acids can also increase ATP inside beta cells and stimulate insulin secretion.
• Glucagon, GLP-1, GIP, and acetylcholine increase intracellular Ca²⁺ through other pathways and enhance glucose-stimulated insulin secretion.
• These substances have little effect on insulin secretion when glucose is absent.
• Somatostatin and norepinephrine acting through α-adrenergic receptors inhibit insulin exocytosis.
• Sulfonylurea drugs stimulate insulin secretion by blocking ATP-sensitive K⁺ channels.
• This causes membrane depolarization and promotes insulin release.
• Therefore, sulfonylureas can stimulate insulin secretion in patients with type 2 diabetes.
• Factors that increase or decrease insulin secretion are summarized in Table 79.1.
• Excess cortisol can cause insulin resistance.
• Insulin resistance then produces a compensatory increase in insulin secretion.
• If cortisol excess is severe and prolonged, type 2 diabetes mellitus may develop.
• Excess growth hormone can produce a similar effect.
• Although normal growth hormone and insulin work together for growth, excessive growth hormone in acromegaly may cause:
  • Insulin resistance
  • Compensatory increased insulin secretion
  • Eventually type 2 diabetes mellitus

KEY CONCEPT

• Glucokinase acts as the beta-cell glucose sensor.
• ↑ Glucose → ↑ ATP → K⁺ channel closure → depolarization → Ca²⁺ entry → insulin release.
• GLP-1, GIP, glucagon, and acetylcholine enhance glucose-stimulated insulin secretion.
• Somatostatin and norepinephrine inhibit insulin secretion.
• Sulfonylureas close ATP-sensitive K⁺ channels → ↑ insulin secretion.
• Excess cortisol or growth hormone → insulin resistance → compensatory ↑ insulin → possible type 2 diabetes.

Conceptual Examples

• After blood glucose rises:
  ↑ Glucose → ↑ ATP → K⁺ channels close → Ca²⁺ enters → insulin is released.
• Sulfonylurea:
  Blocks ATP-sensitive K⁺ channel → depolarization → Ca²⁺ entry → ↑ insulin secretion.
• GLP-1:
  Glucose is present + GLP-1 → stronger intracellular Ca²⁺ effect → more insulin secretion.
• Excess cortisol:
  ↑ Cortisol → insulin resistance → body compensates with ↑ insulin secretion → prolonged severe excess may lead to type 2 diabetes.

Figure 79.7 — Glucose-Stimulated Insulin Secretion

Easiest Concept

Think of the pancreatic β-cell as a glucose sensor:

↑ Blood glucose → β-cell detects it → releases insulin.

Follow the diagram from left → right

• ↑ Glucose outside the cell → more glucose enters the β-cell through GLUT-2.
• Inside the cell, glucokinase converts glucose → glucose-6-phosphate.
• Glucose-6-phosphate is oxidized/burned for energy.
• This produces ↑ ATP.
• Increased ATP closes the ATP-sensitive K⁺ channel.
• Therefore, K⁺ cannot easily leave the cell.
• The cell membrane becomes more positive → depolarization.
• Depolarization opens voltage-gated Ca²⁺ channels.
• Ca²⁺ rushes into the β-cell.
• ↑ intracellular Ca²⁺ makes the insulin-containing vesicles move to and fuse with the membrane.
• Insulin is released outside the cell → ↑ blood insulin.

What do the symbols/arrows mean?

• Yellow channel at top left = GLUT-2 glucose transporter.
• Blue channel at bottom left = ATP-sensitive K⁺ channel, now closed.
• Green channel at bottom right = Ca²⁺ channel, now open.
• Purple dots = insulin molecules.
• Pink circles containing purple dots = insulin storage vesicles/granules.
• Black arrows = direction of the sequence.
• Dashed K⁺ arrow = reduced K⁺ movement causes depolarization.

🔑 KEY CONCEPT

↑ Glucose → GLUT-2 → glucokinase → ↑ ATP → K⁺ channel closes → depolarization → Ca²⁺ channel opens → ↑ Ca²⁺ → insulin released

One-line memory

“Glucose raises ATP → closes K⁺ → opens Ca²⁺ → releases insulin.”

CONTROL OF INSULIN SECRETION

• Insulin secretion is controlled mainly by blood glucose, but amino acids and other factors also influence it (Table 79.1).

Increased Blood Glucose Stimulates Insulin Secretion

• If blood glucose suddenly rises to 2–3 times normal and stays high, insulin secretion increases in two phases (Fig. 79.8).
• First phase:
  • Plasma insulin rises almost 10-fold within 3–5 minutes.
  • This rapid rise occurs because beta cells release preformed insulin already stored in the islets of Langerhans.
  • This high secretion is brief.
  • Within another 5–10 minutes, insulin falls about halfway back toward normal.
• Second phase:
  • Begins at about 15 minutes.
  • Insulin secretion rises again.
  • It reaches a new plateau within about 2–3 hours.
  • This second phase may be even greater than the first.
  • It results from:
    • Further release of preformed insulin
    • Synthesis and release of new insulin

KEY CONCEPT

• ↑ Blood glucose → biphasic insulin secretion.
• First phase = rapid release of stored insulin.
• Second phase = more stored insulin + newly synthesized insulin.
• Timing:
  • 3–5 min → first peak
  • ~15 min → second rise begins
  • 2–3 h → new plateau

Conceptual Examples

• Immediately after sudden hyperglycemia:
  Stored insulin is rapidly released → first insulin peak.
• If glucose remains high:
  Beta cells continue releasing insulin and make new insulin → second, sustained phase.
• Easy sequence:
  High glucose → stored insulin first → new insulin later.

Figure 79.8 — Biphasic Insulin Response After Sudden Rise in Blood Glucose

Main idea

When blood glucose suddenly rises 2–3 times normal, pancreatic β-cells release insulin in 2 phases:

1st phase = fast, short burst
2nd phase = slower, bigger, prolonged rise

📊 Understand the graph

• X-axis = Minutes → time after glucose suddenly increases.
• Y-axis = Plasma insulin (µU/mL) → amount of insulin in the blood.
• Red curve = insulin concentration over time.

🔴 Before 0 minutes

Insulin stays at a low basal level.

➡️ Blood glucose is still at its previous level, so only normal resting insulin secretion is occurring.

🚀 1st phase: Rapid insulin surge

Immediately after glucose rises:

Insulin shoots upward within about 3–5 minutes.

Why?

➡️ Pancreatic β-cells quickly release preformed insulin that was already stored in secretory granules.

Think:

“Emergency insulin already packed and ready → released immediately.”

📉 Why does insulin then fall?

After the first peak, insulin drops toward about half of its peak.

Why?

➡️ The readily available stored insulin becomes partly depleted.

So:

Stored insulin released quickly → temporary shortage → insulin level falls.

It does not return to normal because blood glucose is still high.

📈 2nd phase: Delayed, prolonged rise

At about 15–20 minutes, insulin begins rising again.

This second rise is:

• slower,
• larger,
• and continues for a long time.

Why?

β-cells now:

mobilize additional stored insulin + synthesize new insulin → continuous secretion.

So insulin continues rising as long as glucose remains high.

🔴 Why does the red curve eventually flatten?

At later times, the curve approaches a plateau.

➡️ Insulin secretion is very high but begins reaching a relatively steady maximal response.

⭐ KEY CONCEPT

Biphasic insulin secretion

High glucose →

Phase 1:
⚡ Immediate release of preformed insulin
→ rapid peak
→ then decline

Phase 2:
⏳ More insulin mobilization + new insulin synthesis
→ begins ~15–20 min later
→ larger, sustained rise

Easy memory

First phase = “Release what is READY.”
Second phase = “Bring more + MAKE more.”

Feedback Regulation of Blood Glucose and Insulin Secretion

• When blood glucose rises above about 100 mg/100 mL, insulin secretion increases rapidly.
• At blood glucose levels of about 400–600 mg/100 mL, insulin secretion may reach 10–25 times the basal level (Fig. 79.9).
• When blood glucose falls back to the fasting level, insulin secretion decreases rapidly within about 3–5 minutes.
• This forms an important negative-feedback mechanism: ↑ Blood glucose → ↑ insulin → ↑ glucose uptake by liver, muscle, and other cells → ↓ blood glucose toward normal

Other Factors That Stimulate Insulin Secretion

Amino Acids

• Some amino acids stimulate insulin secretion, especially arginine and lysine.
• Amino acids alone cause only a small increase in insulin if blood glucose is not elevated.
• When blood glucose is also high, amino acids can increase glucose-stimulated insulin secretion by as much as twofold.
• Therefore, amino acids strongly potentiate the effect of glucose on insulin secretion.
• The increased insulin then promotes:
  • Amino acid uptake into cells
  • Protein formation

Gastrointestinal Hormones

• Several gastrointestinal hormones can increase insulin secretion:
  • Gastrin
  • Secretin
  • Cholecystokinin
  • GLP-1
  • GIP
• GLP-1 and GIP are the most powerful and are called incretins.
• Incretins increase insulin release from beta cells when plasma glucose rises.
• They also inhibit glucagon secretion from alpha cells.
• These hormones are released after a meal and cause an early rise in insulin before absorbed glucose and amino acids fully enter the blood.
• They increase the sensitivity of beta cells to glucose and may almost double insulin secretion as blood glucose rises.
• Drugs that mimic or enhance incretin actions are used in treatment of diabetes mellitus and other components of metabolic syndrome, including obesity.

Other Hormones and the Autonomic Nervous System

• Hormones that increase insulin secretion or strengthen the glucose stimulus include:
  • Glucagon
  • Growth hormone
  • Cortisol
  • To a lesser degree, progesterone and estrogen
• Prolonged excessive secretion of these hormones can cause:
  • Insulin resistance
  • Compensatory ↑ insulin secretion
  • Eventual beta-cell dysfunction or injury
  • Increased risk of type 2 diabetes mellitus
• Diabetes is especially associated with:
  • Excess growth hormone in acromegaly or gigantism
  • Excess glucocorticoids in Cushing syndrome
• Pancreatic islets receive both parasympathetic and sympathetic nerves.
• During hyperglycemia:
  • Parasympathetic stimulation → ↑ insulin secretion
• During hypoglycemia:
  • Sympathetic stimulation → ↑ glucagon + ↓ insulin secretion
• Glucose levels are detected by specialized neurons in the hypothalamus and brain stem and by peripheral glucose-sensing cells such as those in the liver.

KEY CONCEPT

• ↑ Blood glucose → ↑ insulin → glucose enters tissues → blood glucose falls toward normal.
• Arginine and lysine strongly enhance glucose-stimulated insulin secretion.
• GLP-1 and GIP = incretins → ↑ insulin + ↓ glucagon.
• Prolonged excess growth hormone or cortisol → insulin resistance → compensatory hyperinsulinemia → possible type 2 diabetes.
• Parasympathetic activity favors insulin secretion; sympathetic activity during hypoglycemia favors glucagon and suppresses insulin.

Conceptual Examples

• High blood glucose:
  ↑ Glucose → ↑ insulin → ↑ tissue glucose uptake → glucose returns toward normal.
• Protein-rich meal:
  Glucose + amino acids → stronger insulin response → ↑ amino acid uptake and protein formation.
• After a meal:
  GLP-1 + GIP → early increase in insulin + decreased glucagon.
• Hypoglycemia:
  Sympathetic stimulation → ↓ insulin + ↑ glucagon.
• Long-term hormone excess:
  Excess growth hormone or cortisol → insulin resistance → ↑ insulin demand → possible beta-cell dysfunction and type 2 diabetes.

Figure 79.9 — Effect of Plasma Glucose on Insulin Secretion

🧠 Main idea

Blood glucose is the main controller of insulin secretion.

As glucose rises:

Glucose ↑ → pancreatic β-cells stimulated → insulin secretion ↑↑

But the response is not linear. It follows an S-shaped curve.

📊 Understand every part of the graph

X-axis = Plasma glucose concentration (mg/100 mL)
This means the amount of glucose in the blood.

• Moving left → right = blood glucose increasing
• 100 mg/100 mL = 100 mg/dL, approximately the normal fasting range.

Y-axis = Insulin secretion (× normal)
This tells us how many times insulin secretion is above its normal basal level.

For example:

• 1× = normal insulin secretion
• 5× = five times normal
• 10× = ten times normal
• 20× = twenty times normal

🔴 Red curve — Insulin secretion

1️⃣ Low glucose: almost flat line

When glucose is below about 100 mg/dL, insulin secretion is very low.

➡️ There is little need for insulin because blood glucose is already low.

✳️ Red star around 100 mg/dL

The star highlights approximately the normal glucose level.

At about 100 mg/dL, insulin secretion is around its normal basal level.

This is also where increasing glucose begins to produce a much stronger insulin response.

2️⃣ About 100–200 mg/dL: insulin rises rapidly

Once glucose rises above about 100 mg/dL:

Glucose ↑ → insulin secretion begins increasing steeply ↑↑

Example from the curve:

~200 mg/dL glucose → insulin secretion ≈ 8–10× normal

➡️ Pancreatic β-cells strongly sense the increased glucose.

3️⃣ About 200–400 mg/dL: very strong insulin response

The red curve continues steeply upward.

Approximately:

• ~300 mg/dL → ~15–17× normal
• ~400 mg/dL → ~19× normal

➡️ Higher glucose produces progressively greater insulin secretion.

4️⃣ About 400–600 mg/dL: plateau

At very high glucose concentrations, the curve begins to flatten near:

~20× normal insulin secretion

Why?

➡️ β-cells are approaching their maximum secretory capacity.

So increasing glucose further produces relatively little additional insulin secretion.

⭐ Why is the curve S-shaped?

Think of it as three stages:

Low glucose
→ β-cells barely stimulated
→ little insulin

Moderately/high glucose
→ β-cells strongly stimulated
→ rapid insulin increase

Extremely high glucose
→ β-cells near maximum capacity
→ plateau

⭐ KEY CONCEPT

Glucose below ~100 mg/dL → little insulin secretion

Glucose above ~100 mg/dL → insulin secretion rises dramatically

Glucose ~400–600 mg/dL → insulin secretion reaches about 20× normal and approaches maximum

Easy memory:

More glucose → more insulin, until β-cells reach their maximum.

✳️ Star = approximately normal glucose/normal insulin point.

THE ROLE OF INSULIN (AND OTHER HORMONES) IN “SWITCHING” BETWEEN CARBOHYDRATE AND LIPID METABOLISM

• Insulin acts like a metabolic switch between carbohydrate and fat use.
• When insulin is high, cells mainly use carbohydrates (glucose) for energy and fat use decreases.
• When insulin is low, most tissues shift toward fat utilization, while the brain continues to depend mainly on glucose.
• The main signal controlling this switch is the blood glucose concentration.
• When blood glucose is low:
  • Insulin secretion decreases.
  • Fat becomes the main energy source for most tissues.
• When blood glucose is high:
  • Insulin secretion increases.
  • Cells use carbohydrates instead of fat.
  • Excess glucose is stored as:
    • Liver glycogen
    • Muscle glycogen
    • Liver fat
• Therefore, an important function of insulin is to determine moment by moment whether cells use glucose or fat for energy.
• Four other hormones also help control this metabolic switching:
  • Growth hormone
  • Cortisol
  • Epinephrine
  • Glucagon
• Growth hormone and cortisol are released during hypoglycemia.
• They decrease cellular glucose utilization and increase fat utilization.
• Their effects develop slowly and may require many hours to become maximal.
• Epinephrine is especially important during stress when the sympathetic nervous system is activated.
• It rapidly increases blood glucose by causing glycogenolysis in the liver.
• Therefore: Liver glycogen → glucose → ↑ blood glucose within minutes
• Epinephrine also activates hormone-sensitive lipase in adipose tissue.
• This causes lipolysis and increases free fatty acids in blood.
• The rise in fatty acids is much greater than the rise in blood glucose.
• Therefore, epinephrine strongly promotes fat utilization during stress, especially in:
  • Exercise
  • Circulatory shock
  • Anxiety

KEY CONCEPT

• ↑ Blood glucose → ↑ insulin → carbohydrate use + energy storage.
• ↓ Blood glucose → ↓ insulin → fat use increases.
• Growth hormone + cortisol → ↓ glucose use + ↑ fat use during hypoglycemia.
• Epinephrine → ↑ liver glycogenolysis + ↑ adipose lipolysis → ↑ glucose and especially ↑ fatty acids.
• Insulin is the major hormone that switches metabolism between glucose use and fat use.

Conceptual Examples

• After a carbohydrate-rich meal:
  ↑ Glucose → ↑ insulin → glucose used for energy + excess stored as glycogen or fat.
• Between meals:
  ↓ Glucose → ↓ insulin → fat becomes the main fuel for most tissues.
• During prolonged hypoglycemia:
  ↑ Growth hormone + ↑ cortisol → ↓ glucose use + ↑ fat use.
• During exercise or stress:
  ↑ Epinephrine → liver glycogen breakdown + adipose fat breakdown → ↑ blood glucose + much greater ↑ fatty acids.

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