Blood Glucose Regulation

Maintaining a stable blood glucose concentration is essential for providing a constant energy supply to cells, particularly the brain, which relies almost exclusively on glucose for respiration. Blood glucose regulation involves the pancreas, the hormones insulin and glucagon, and several metabolic pathways in the liver. Understanding this topic also provides the foundation for studying diabetes mellitus.

Key Definition: Blood glucose regulation is the homeostatic maintenance of blood glucose concentration within narrow limits (approximately 4–6 mmol dm⁻³ in a fasting individual), primarily through the antagonistic actions of the hormones insulin and glucagon.

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The Role of the Pancreas

The pancreas is both an exocrine gland (producing digestive enzymes) and an endocrine gland (producing hormones). The endocrine function is carried out by clusters of cells called the islets of Langerhans.

Islets of Langerhans

The islets contain two key cell types:

Cell Type	Hormone Produced	Stimulus for Secretion	Effect on Blood Glucose
Beta (β) cells	Insulin	High blood glucose (e.g., after a meal)	Lowers blood glucose
Alpha (α) cells	Glucagon	Low blood glucose (e.g., during fasting or exercise)	Raises blood glucose

Beta cells are more numerous, making up approximately 60–80% of the islet cells. Alpha cells account for approximately 15–20%.

How Beta Cells Detect High Blood Glucose

1. Glucose enters the beta cell via GLUT2 transporters (facilitated diffusion — not insulin-dependent).
2. Glucose is metabolised by glycolysis and aerobic respiration, producing ATP.
3. The increased ATP:ADP ratio causes ATP-sensitive potassium channels to close.
4. K⁺ accumulates inside the cell, causing depolarisation of the cell membrane.
5. Voltage-gated calcium channels open, and Ca²⁺ flows into the cell.
6. Ca²⁺ causes insulin-containing vesicles to fuse with the cell membrane and release insulin by exocytosis.

Exam Tip: This mechanism linking glucose concentration to insulin secretion is a common 6-mark question. Learn the sequence: glucose → GLUT2 → glycolysis → ATP → K⁺ channels close → depolarisation → Ca²⁺ channels open → exocytosis of insulin.

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The Actions of Insulin (When Blood Glucose is Too High)

Insulin is a protein hormone that binds to tyrosine kinase receptors on target cell membranes. Its effects include:

In the Liver (Hepatocytes)

• Glycogenesis: Insulin activates the enzyme glycogen synthase, which catalyses the conversion of glucose to glycogen for storage. Glycogen is a large, insoluble, branched polymer of glucose that does not affect the water potential of the cell.
• Increased glycolysis: Insulin activates enzymes of glycolysis (e.g., glucokinase), increasing the rate of glucose oxidation.
• Increased lipogenesis: Excess glucose is converted to fatty acids and then to fat for long-term storage.
• Decreased gluconeogenesis: Insulin inhibits the conversion of non-carbohydrate sources (amino acids, glycerol) to glucose.

In Muscle and Adipose Tissue

• Insulin causes GLUT4 transporter proteins to be inserted into the cell membrane by exocytosis of vesicles.
• This increases the rate of glucose uptake by facilitated diffusion.
• In muscle cells, glucose is used for respiration or stored as glycogen.
• In adipose cells, glucose is converted to fat for storage.

Net Effect

Blood glucose concentration falls back towards the set point (approximately 5 mmol dm⁻³).

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The Actions of Glucagon (When Blood Glucose is Too Low)

Glucagon is also a protein hormone. It acts primarily on the liver (hepatocytes) via a second messenger system involving cyclic AMP (cAMP).

Mechanism of Glucagon Action

1. Glucagon binds to a G-protein-coupled receptor on the hepatocyte membrane.
2. This activates a G protein, which activates the enzyme adenylyl cyclase.
3. Adenylyl cyclase converts ATP to cyclic AMP (cAMP) — the second messenger.
4. cAMP activates protein kinase A, which phosphorylates and activates target enzymes.

Effects on the Liver