The Endocrine Pancreas

The pancreas is located posterior to the stomach and near the curve of the duodenum. It consists of a head, a body, and a tail. The pancreas is both an endocrine and an exocrine organ. Ninety-nine percent of the pancreas is made up of clusters of cells called acini. The cells contained in the acini produce digestive enzymes that are secreted into the small intestine. The exocrine function of the pancreas will be discussed in more detail in Unit 18.

Within the pancreas, among the acini, there are 1-2 million tiny clusters of endocrine tissue called pancreatic islets, or islets of Langerhans.

The pancreas is comprised of two types of tissue. The exocrine pancreas, which secretes fluid and enzymes which aid digestion. This is about 99% of the pancreatic tissue.

There is also an endocrine pancreas, which we will focus on in this objective. The endocrine pancreas is made up of a minority of pancreatic tissue, that is found within the islets.

The pancreatic islets contain four different types of endocrine cells (alpha, beta, delta, and F cells), each secreting separate hormones. Glucagon functions to raise blood sugar. Insulin lowers blood sugar levels. Somatostatin inhibits the release of insulin and glucagon, and pancreatic polypeptide acts as an inhibitor of digestion. The focus of this unit will be on glucagon and insulin.

Glucagon and insulin are antagonists. 

Alpha Cells Secrete Glucagon

Low blood glucose levels stimulate the alpha cells to secrete glucagon.

Glucagon activates multiple mechanisms to increase the blood glucose level:

1. Stimulates the liver to convert its stores of glycogen back into glucose (glycogenolysis). This glucose is then released into the circulation raising blood glucose levels.
2. Stimulates the liver to take up amino acids from the blood and convert them into glucose (gluconeogenesis).
3. Stimulates lipolysis, the breakdown of stored triglycerides into free fatty acids and glycerol. Some of the free glycerol travels to the liver where it is converted to glucose, another form of gluconeogenesis.

Beta Cells Secrete Insulin

Elevated blood glucose levels stimulate the beta cells to release insulin. Insulin acts on cells throughout the body in a number of different ways, all of which will result in a decrease in an individual’s blood sugar:

1. increases facilitated diffusion of glucose out of the blood and into the cells by increasing the number of glucose transporters;
2. increases amino acid uptake into cells and increases protein synthesis;
3. increases fatty acid synthesis;
4. converts glucose to glycogen; and
5. decreases gluconeogenesis.