Endocrine Pancreas & Insulin Secretion
Pancreas Overview
Pancreatic endocrine cells play a key role in regulating blood glucose concentrations; we'll pay special attention to the beta cells and insulin secretion.
The pancreas lies within the superior left quadrant of the abdominal cavity, deep to the stomach.
View
imaging.
Endocrine cells form the "Islets of Langerhans" within the pancreas; histologically, these are visible as distinct cell clusters that appear lighter than the exocrine cells under microscope:
The islets themselves comprise 4 different endocrine cell types; their products are secreted into the
hepatic portal blood and delivered to the liver.
A key function of pancreatic endocrine cells is to maintain basal blood glucose concentration by regulating glucose storage and release. Recall that glucose is a major source of energy.
The islet cells engage in communication via neural, hormonal, and cell-to-cell signaling.
Review
cellular communication.
4 pancreatic endocrine cell types
Alpha cells make up approximately 20% of the islet cells and comprise the rim of the islet.
- Alpha cells secrete glucagon, which increases blood glucose concentrations between meals.
Beta cells comprise approximately 65% of the islet cells; they comprise most of the center of the islet.
- Beta cells secrete insulin, which lowers blood glucose concentrations after a meal.
Delta cells comprise approximately 10% of the islet cells.
- Delta cells secrete somatostatin, which inhibits insulin and glucagon secretion to moderate their effects on blood glucose concentrations.
F cells are rare.
- F cells, aka PP cells, secrete pancreatic polypeptides, whose specific functions are uncertain; they may play a role in satiety.
Insulin & Glucagon
Tutorial
Insulin & Glucagon
Flashcard
Elevated glucose levels, which occurs following a meal, triggers pancreatic beta-cell release of insulin.
Incretins GIP and GLP-1, which are secreted from the gastrointestinal tract after food intake, increase the rate of insulin secretion. This is known as the "incretin effect."
Insulin increases glucose uptake in both striated muscle (skeletal and cardiac muscle) and in adipose tissue; this lowers blood glucose levels. Be aware that insulin has metabolic actions in the liver, too.
As we'll learn elsewhere, beta cells release insulin in response to multiple stimuli, but glucose is the primary stimulus.
We show an enlarged beta cell with a GLUT – 2 receptor.
First, glucose diffuses into the beta cell via GLUT – 2 receptors.
Within the cell, the enzyme glucokinase phosphorylates glucose.
Then, the phosphorylated glucose is oxidized; this produces ATP.
ATP closes ATP-sensitive potassium channels, trapping potassium inside the cell.
The increase in intracellular potassium depolarizes the cell membrane.
In response to depolarization, calcium channels open, and the intracellular calcium concentration increases.
Increased intracellular calcium triggers exocytosis of insulin-containing vesicles; again, this insulin enters the hepatic portal system and is then delivered to the systemic circulation for use by the body tissues.
Be aware that beta cells respond similarly to fatty acids and proteins.
Diabetes - Insulin Deficiency And/Or Resistance
Diabetes is characterized by hyperglycemia (high blood glucose levels) due to insulin deficiency and/or insulin response in the peripheral tissues.
As we learn in more detail elsewhere, Type 1 diabetes (formerly referred to as "childhood diabetes") is caused by destruction of insulin-secreting beta cells; Type 2 diabetes (formerly referred to as "adult-onset diabetes" is caused by peripheral insulin resistance, often with beta-cell dysfunction.
Diabetes Pathophysiology
Tutorial
Diabetes Pathophysiology
Flashcard
Diabetes Mellitus Biochemistry
Tutorial
The islets themselves comprise 4 different endocrine cell types; their products are secreted into the hepatic portal blood and delivered to the liver.
A key function of pancreatic endocrine cells is to maintain basal blood glucose concentration by regulating glucose storage and release. Recall that glucose is a major source of energy.
The islet cells engage in communication via neural, hormonal, and cell-to-cell signaling.
Review cellular communication.