Insulin

Definitions & Key takeaways

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.

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Introduction0:00–0:22

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.

Pancreas0:22–1:09

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.

Synthesis1:09–2:02

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.

Control2:02–3:14

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.So insulin binds to the insulin receptor located in the membrane of its target cells - mainly in the liver, but also fat or adipose tissue, and skeletal muscles.

Functions3:14–4:57

The insulin receptor is a made up of two alpha subunits that lie outside the cell and bind to insulin, and two beta subunits that span the cell membrane and carry the signal to the inside of the cell.
Insulin is an anabolic hormone, which means that when it binds to the insulin receptor, it promotes the conversion of small energy molecules in the blood - mainly glucose, but also fatty acids and amino acids - into large storage molecules inside insulin’s target cells.
Glucose mainly becomes glycogen in the liver, fatty acids are stored as fat or adipose tissue, and amino acids build up protein in skeletal muscles.
In the liver, insulin promotes glucose conversion into glycogen. Insulin also inhibits glucose production from lactic acid and from noncarbohydrate molecules through gluconeogenesis.
If glycogen storage capacity is reached and excess glucose is still available in blood, insulin prompts the liver to convert glucose into fatty acids through glycolysis, which is the metabolic pathway that converts glucose into pyruvate, and then pyruvate is converted to fatty acids through a reaction with acetyl-CoA.
Fatty acids are then transported and stored in adipose tissue, while breakdown of fatty acids through lipolysis is inhibited.
Insulin can also act on skeletal muscles, where it stimulates amino acid and glucose uptake into the muscle cells, and this helps with protein production and muscle growth.Now, glucose levels are tightly regulated through the interplay of insulin with another hormone secreted by the pancreas, which is glucagon.

Glucose Regulation4:57–5:27

Glucagon has the opposite function, which is to increase glucose levels in blood. Secretion of glucagon by alpha cells is sustained over time during fasting, but after a meal, when glycemia rises, insulin inhibits glucagon secretion.
When glycemia drops, insulin production drops and glucagon is once again secreted.##SummaryAlright, as a quick recap, insulin is a peptide hormone produced by the beta cells of the Langerhans islets in the pancreas.

Review5:27–5:48

After a meal, insulin is secreted into the bloodstream to lower blood glucose levels by triggering glucose absorption and conversion into glycogen or fats to be stored in the liver,