Insulin

Endocrine system - ETP

Endocrine system - ETP

Pharyngeal arches, pouches, and clefts
Endocrine system anatomy and physiology
Anatomy of the thyroid and parathyroid glands
Anatomy of the abdominal viscera: Pancreas and spleen
Pituitary gland histology
Thyroid and parathyroid gland histology
Pancreas histology
Adrenal gland histology
Synthesis of adrenocortical hormones
Adrenocorticotropic hormone
Growth hormone and somatostatin
Hunger and satiety
Antidiuretic hormone
Thyroid hormones
Insulin
Insulins
Glucagon
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Testosterone
Estrogen and progesterone
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Parathyroid hormone
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Multiple endocrine neoplasia: Pathology review
Multiple endocrine neoplasia
Carcinoid syndrome
Neuroblastoma
Opsoclonus myoclonus syndrome (NORD)
Hyperthyroidism medications
Hypothyroidism medications
Hypoglycemics: Insulin secretagogues
Miscellaneous hypoglycemics
Mineralocorticoids and mineralocorticoid antagonists
Adrenal hormone synthesis inhibitors

Flashcards

Insulin

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Questions

USMLE® Step 1 style questions USMLE

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A group of researchers is studying the synthesis and secretion of insulin and C-peptide. Which of the following statements is true regarding insulin synthesis?  

Transcript

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Insulin is a hormone that’s involved in lowering the blood sugar levels or glycemia after a meal. Insulin comes from the latin insula, which means island, because this hormone is produced by some small islands of cells scattered throughout the pancreas that are called pancreatic islets or islets of Langerhans.

The pancreas lies in the upper left part of the abdomen, right behind the stomach. The vast majority of the pancreas is made up by exocrine glands in charge of secreting digestive enzymes into the small intestine to help digestion. But about 1 to 2% of the mass of the pancreas is made up by the islets of Langerhans, which are endocrine glands made up by five different cell types, and each cell type secretes a specific hormone. The most abundant are the beta cells, which produce insulin. But you can also find alpha cells that secrete glucagon, delta cells that secrete somatostatin, gamma cells that secrete pancreatic polypeptide, and finally epsilon cells that secrete ghrelin.

Let’s focus on beta cells. Beta cells are in charge of producing insulin, which is a peptide hormone encoded by the INS gene on chromosome 11. Insulin is first synthesized as a single polypeptide called preproinsulin. A short tail called leader or signal peptide is cleaved from preproinsulin to form proinsulin. Proinsulin consists of three peptide chains in the order B, C for connecting peptide, and A. Proinsulin is then further cleaved at two positions, releasing a fragment called the C-peptide, and leaving the B- and A- chains, and two disulfide bonds which link the B- and A- chains together to form insulin. This mature insulin is stored inside granules within the beta cells where it waits until it’s released into the blood.

The most important trigger for insulin secretion is glucose. Beta cells are sensitive to glucose concentrations in blood, and when blood glucose levels rise, beta cells secrete insulin into the blood to help lower those levels and store glucose. Other stimulatory factors for insulin secretion include hormones like glucagon and cortisol, which stimulate insulin indirectly by increasing blood glucose levels, increased fatty acid or amino acid concentrations in blood, since insulin is also involved in their storage, and acetylcholine from the parasympathetic nervous system, which helps with digestion, and stimulates insulin secretion to decrease the recently absorbed glucose in blood.

On the other hand, insulin secretion is inhibited by norepinephrine from the sympathetic nervous system, which is activated during stressful situations that demand high energy consumption, and need glucose to be readily available in blood. Other molecules that inhibit insulin production include somatostatin, which is classified as an inhibitory hormone that avoids the excessive release of pancreatic hormones and other hormones like growth hormone.

Summary

Insulin is a hormone that helps the body to metabolize sugar (glucose). It is made by the pancreas, specifically the islets of Langerhans.

Insulin controls how much glucose is in the blood. When you eat, your body breaks down food into glucose and other nutrients. These enter the bloodstream and signal the pancreas to make insulin. The insulin then allows the glucose to enter cells in the body, where it is used for energy or stored for later use.

Sources

  1. "Medical Physiology" Elsevier (2016)
  2. "Physiology" Elsevier (2017)
  3. "Human Anatomy & Physiology" Pearson (2018)
  4. "Principles of Anatomy and Physiology" Wiley (2014)
  5. "Insulin: understanding its action in health and disease" British Journal of Anaesthesia (2000)
  6. "Are islet cells the gatekeepers of the pancreas?" Pancreatology (2002)
  7. "A Reappraisal of the Blood Glucose Homeostat which Comprehensively Explains the Type 2 Diabetes Mellitus–Syndrome X Complex" The Journal of Physiology (2003)