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[Physiology](https://durgical.com/en/physiology/) · [Endocrine](https://durgical.com/en/physiology/#s-endocrine)

# Islets and insulin: secretion and actions

- Regulation
- Exam priority: high

After a meal the pancreatic β cell releases insulin, which makes liver, muscle and fat store glucose and energy.

Updated 2026-11-01

Also called: β cell, Fed-state hormone

**See it in 3D:** [Pancreas](https://durgical.com/en/anatomy/pancreas/) · [Liver](https://durgical.com/en/anatomy/liver/)

## Key points

- Glucose → GLUT2 → glucokinase → ↑ ATP/ADP → K-ATP channel closes → depolarization → Ca²⁺ entry → exocytosis.
- Sulfonylureas close the same K-ATP channel; diazoxide opens it.
- Oral glucose releases more insulin than intravenous glucose: the incretin effect (gut GLP-1 and GIP).
- C-peptide is released equimolar with insulin and is not extracted by the liver: it measures own secretion (low in T1D, high in insulinoma).
- Insulin drives potassium into cells (Na⁺/K⁺-ATPase): plasma potassium falls.
- Brain, liver and red cells take up glucose without needing insulin; muscle and fat need GLUT4.

## What it is

Insulin is the hormone of the fed state. The β cell "measures" glucose through its own metabolism: more ATP closes potassium channels and triggers insulin release. Insulin reaches the liver first through the portal vein and then muscle and fat, where it drives storage of glucose, fat and protein and stops the making of glucose and ketones. The signaling (tyrosine kinase receptor, PI3K, GLUT4) is in Biochemistry.

## How it works, step by step

- Glucose enters the β cell and glucokinase phosphorylates it; this low-affinity enzyme makes the rate track blood glucose.
- Glycolysis and the mitochondria raise the ATP/ADP ratio.
- ATP closes the K-ATP channel (Kir6.2 with SUR1) and the membrane depolarizes.
- L-type calcium channels open; calcium triggers fusion of insulin vesicles.
- Phase 1: fast release of ready granules (minutes). Phase 2: sustained secretion while hyperglycemia lasts.
- In the liver insulin drives glycogen and fat synthesis and stops gluconeogenesis; in muscle and fat it moves GLUT4 to the membrane.

## Guide values

| Parameter | Value | Note |
| --- | --- | --- |
| Normal fasting glucose | ≈70–99 mg/dL (3.9–5.5 mmol/L) | |
| Plasma insulin half-life | ≈5–6 min | the liver extracts ≈50% on first pass |
| Islet make-up | β ≈60–70%, α ≈20–25%, δ ≈5–10% | |

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

## Regulation

- *+* **Glucose**: Main stimulus; threshold close to the fasting glucose level.
- *+* **Amino acids (arginine, leucine)**: Stimulate insulin after a protein meal.
- *+* **GLP-1 and GIP (incretins)**: Amplify secretion only when glucose is high.
- *+* **Vagus (acetylcholine, M3 receptor)**: Cephalic phase: primes the response before the meal is absorbed.
- *−* **Sympathetic nerves and epinephrine (α2 receptor)**: Brake insulin during stress and exercise.
- *−* **Somatostatin**: Paracrine inhibition from δ cells.

## Insulin actions by tissue

| Tissue | Carbohydrate | Fat and protein |
| --- | --- | --- |
| Liver | ↑ glycogen, ↓ glycogenolysis and gluconeogenesis | ↑ fatty acid synthesis, ↓ ketogenesis |
| Muscle | ↑ GLUT4 uptake, ↑ glycogen | ↑ protein synthesis, ↓ proteolysis |
| Adipose tissue | ↑ GLUT4 uptake for glycerol | ↑ lipoprotein lipase, ↓ hormone-sensitive lipase |

## Clinical notes

- Insulinoma: fasting hypoglycemia with high insulin and high C-peptide; high insulin with low C-peptide means the insulin came from outside.
- Type 1 diabetes: autoimmune destruction of β cells; C-peptide falls to very low levels.
- Insulin lowers potassium: in nursing practice (NCLEX) potassium is monitored during ketoacidosis management.

## Memory aid

Insulin is the pantry key: it opens the door (GLUT4) and locks the exit (liver glucose and ketones).

## Test yourself

Think of the answer, then tap the question to see it.

## Why does a sulfonylurea stimulate insulin even when glucose is normal?

It closes the K-ATP channel directly (SUR1 subunit), skipping the step glucose normally performs (raising ATP) and depolarizing the β cell; hence it can cause hypoglycemia.

## Why does oral glucose release more insulin than the same amount given intravenously?

Glucose in the gut releases GLP-1 and GIP, which amplify glucose-dependent insulin secretion (the incretin effect).

## Hypoglycemia with high insulin and suppressed C-peptide: what does it suggest?

Exogenous insulin: injected insulin carries no C-peptide, and hypoglycemia shuts down own secretion, so C-peptide falls.

## Why does insulin lower plasma potassium?

It activates Na⁺/K⁺-ATPase in muscle and liver and moves potassium into cells; this is why it is used in hyperkalemia and can cause hypokalemia.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/islets-and-insulin-secretion-and-actions/)

## See it in 3D

The structures of Durgical's 3D anatomy where it happens.

- [Pancreas](https://durgical.com/en/anatomy/pancreas/)
- [Liver](https://durgical.com/en/anatomy/liver/)

## Related topics

- [Physiology of diabetes and ketoacidosis](https://durgical.com/en/physiology/physiology-of-diabetes-and-ketoacidosis/)
- [Glucagon and the counter-regulatory hormones](https://durgical.com/en/physiology/glucagon-and-the-counter-regulatory-hormones/)
- [Blood glucose, glucose tolerance test and HbA1c](https://durgical.com/en/physiology/blood-glucose-glucose-tolerance-test-and-hba1c/)
- [The physiological response to hypoglycemia](https://durgical.com/en/physiology/the-physiological-response-to-hypoglycemia/)
- [Potassium balance](https://durgical.com/en/physiology/potassium-balance/)
- [Fed and fasting states: the hormonal switch](https://durgical.com/en/physiology/fed-and-fasting-states-the-hormonal-switch/)

## Related conditions

- [Type 1 diabetes mellitus](https://durgical.com/en/conditions/type-1-diabetes-mellitus/)
- [Type 2 diabetes mellitus](https://durgical.com/en/conditions/type-2-diabetes-mellitus/)
- [Hypokalaemia](https://durgical.com/en/conditions/hypokalaemia/)

## In Biochemistry

- [Insulin signalling](https://durgical.com/en/biochemistry/topics/insulin-signalling/)
- [Insulin receptor](https://durgical.com/en/biochemistry/enzymes/insulin-receptor/)
- [Glucose](https://durgical.com/en/biochemistry/molecules/glucose/)

## In Pharmacology

- [Insulins](https://durgical.com/en/pharmacology/classes/insulins/)
- [Regular human insulin](https://durgical.com/en/pharmacology/regular-human-insulin/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [17.9 The Endocrine Pancreas](https://openstax.org/books/anatomy-and-physiology-2e/pages/17-9-the-endocrine-pancreas) (OpenStax (Rice University), 2022); [Insulin C-peptide test: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003701.htm) (MedlinePlus, US National Library of Medicine, 2024); [Type 1 Diabetes](https://www.ncbi.nlm.nih.gov/books/NBK507713/) (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 79). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/islets-and-insulin-secretion-and-actions/