Physiology
Insulin and glucagon: blood glucose control
This is the textbook example of negative feedback, and one of the clearest antagonistic hormone pairs in human physiology.
The two hormones
| Insulin | Glucagon | |
|---|---|---|
| Secreted by | Beta cells of the pancreas | Alpha cells of the pancreas |
| Trigger | High blood glucose | Low blood glucose |
| Overall effect | Lowers blood glucose | Raises blood glucose |
| Metabolic mode | Anabolic (storage) | Catabolic (mobilization) |
Both are produced in the islets of Langerhans — an easy detail to lose marks on, since alpha and beta cells are frequently swapped in exam answers. Mnemonic: Insulin comes from beta cells and moves glucose in.
What insulin does
- Triggers GLUT4 transporters to move to the cell membrane in muscle and fat tissue, allowing glucose uptake
- Stimulates glycogenesis — storing glucose as glycogen in liver and muscle
- Stimulates lipogenesis and inhibits fat breakdown
- Promotes amino acid uptake and protein synthesis
What glucagon does
- Stimulates glycogenolysis — breaking liver glycogen back down into glucose
- Stimulates gluconeogenesis — making new glucose from amino acids, lactate and glycerol
- Promotes lipolysis, releasing fatty acids for fuel
Liver vs. muscle glycogen
Only the liver can release glucose back into the bloodstream, because muscle lacks the enzyme glucose-6-phosphatase. Muscle glycogen fuels the muscle that stores it and cannot raise blood glucose for the rest of the body.
When the system fails
In type 1 diabetes, autoimmune destruction of beta cells means little or no insulin is produced. In type 2 diabetes, insulin is produced but target tissues respond poorly to it — insulin resistance — and beta-cell function may decline over time. In both, glucose accumulates in the blood because it cannot be moved into cells efficiently.
Frequently asked questions
Which pancreatic cells make insulin and glucagon?
Beta cells in the islets of Langerhans secrete insulin in response to high blood glucose. Alpha cells secrete glucagon in response to low blood glucose.
Why can't muscle glycogen raise blood sugar?
Muscle lacks glucose-6-phosphatase, the enzyme needed to release free glucose into the bloodstream. Only liver glycogen can be broken down and exported to raise blood glucose for the rest of the body.
What is the difference between type 1 and type 2 diabetes?
In type 1, autoimmune destruction of pancreatic beta cells means little or no insulin is produced. In type 2, insulin is produced but tissues respond poorly to it, and beta-cell function may decline over time.
Keep reading: What is glycogen? · Homeostasis explained · The glycemic index