12.7 The Endocrine Pancreas
The pancreas is a long, slender organ, most of which is located posterior to the bottom half of the stomach (Figure 12.16). Although it is primarily an exocrine gland, secreting a variety of digestive enzymes, the pancreas has an endocrine function. Its pancreatic islets—clusters of cells formerly known as the islets of Langerhans—secrete the hormones glucagon, insulin, somatostatin and pancreatic polypeptide (PP) among others.
Cells and Secretions of the Pancreatic Islets
The pancreatic islets each contain five main varieties of cells:
- The alpha cell produces the hormone glucagon and makes up approximately 20 percent of each islet. Glucagon plays an important role in blood glucose regulation; low blood glucose levels stimulate its release.
- The beta cell produces the hormones insulin and amylin and makes up approximately 75 percent of each islet. Elevated blood glucose levels stimulate the release of insulin.
- The delta cell accounts for about four percent of the islet cells and secretes the peptide hormone somatostatin. Recall that somatostatin is also released by the hypothalamus (as GHIH), and the stomach and intestines also secrete it. An inhibiting hormone, pancreatic somatostatin inhibits the release of both glucagon and insulin.
- The gamma cell (PP cell) accounts for about one percent of islet cells and secretes the pancreatic polypeptide hormone. It is thought to play a role in appetite, as well as in the regulation of pancreatic exocrine and endocrine secretions. Pancreatic polypeptide released following a meal may reduce further food consumption; however, it is also released in response to fasting.
- The epsilon cell produces the hormone ghrelin and accounts for less than 1 percent of islet cells.
Regulation of Blood Glucose Levels by Insulin and Glucagon
Glucose is required for cellular respiration and is the preferred fuel for all body cells. The body derives glucose from the breakdown of the carbohydrate-containing foods and drinks we consume. Glucose not immediately taken up by cells for fuel can be stored by the liver and muscles as glycogen or converted to triglycerides and stored in the adipose tissue. Hormones regulate both the storage and the utilisation of glucose as required. Receptors located in the pancreas sense blood glucose levels, and subsequently the pancreatic cells secrete glucagon or insulin to maintain normal levels.
Glucagon
Receptors in the pancreas can sense the decline in blood glucose levels, such as during periods of fasting or during prolonged labour or exercise (Figure 12.17). In response, the alpha cells of the pancreas secrete the hormone glucagon, which has several effects:
- It stimulates the liver to convert its stores of glycogen back into glucose. This response is known as glycogenolysis. The glucose is then released into the circulation for use by body cells.
- It stimulates the liver to take up amino acids from the blood and convert them into glucose. This response is known as gluconeogenesis.
- It stimulates lipolysis, the breakdown of stored triglycerides into free fatty acids and glycerol. Some of the free glycerol released into the bloodstream travels to the liver, which converts it into glucose. This is also a form of gluconeogenesis.
Taken together, these actions increase blood glucose levels. The activity of glucagon is regulated through a negative feedback mechanism; rising blood glucose levels inhibit further glucagon production and secretion.
Insulin
The primary function of insulin is to facilitate the uptake of glucose into body cells. Red blood cells, as well as cells of the brain, liver, kidneys, and the lining of the small intestine, do not have insulin receptors on their cell membranes and do not require insulin for glucose uptake. Although all other body cells do require insulin if they are to take glucose from the bloodstream, skeletal muscle cells and adipose cells are the primary targets of insulin.
The presence of food in the intestine triggers the release of gastrointestinal tract hormones such as glucose-dependent insulinotropic peptide (previously known as gastric inhibitory peptide). This is in turn the initial trigger for insulin production and secretion by the beta cells of the pancreas. Once nutrient absorption occurs, the resulting surge in blood glucose levels further stimulates insulin secretion.
Precisely how insulin facilitates glucose uptake is not entirely clear. However, insulin appears to activate a tyrosine kinase receptor, triggering the phosphorylation of many substrates within the cell. These multiple biochemical reactions converge to support the movement of intracellular vesicles containing facilitative glucose transporters to the cell membrane. In the absence of insulin, these transport proteins are normally recycled slowly between the cell membrane and cell interior. Insulin triggers the rapid movement of a pool of glucose transporter vesicles to the cell membrane, where they fuse and expose the glucose transporters to the extracellular fluid. The transporters then move glucose by facilitated diffusion into the cell interior.
Insulin also reduces blood glucose levels by stimulating glycolysis, the metabolism of glucose for generation of ATP. Moreover, it stimulates the liver to convert excess glucose into glycogen for storage, and it inhibits enzymes involved in glycogenolysis and gluconeogenesis. Finally, insulin promotes triglyceride and protein synthesis. The secretion of insulin is regulated through a negative feedback mechanism. As blood glucose levels decrease, further insulin release is inhibited. The pancreatic hormones are summarised in Table 12.6
Table 12.6 Hormones of the pancreas
| Associated hormones | Chemical class | Effect |
| Glucagon (alpha cells) | Protein | Increases blood glucose levels |
| Insulin (beta cells) | Protein | Reduces blood glucose levels |
| Somatostatin (delta cells) | Protein | Inhibits insulin and glucagon release |
| Pancreatic polypeptide (gamma or PP cells) | Protein | Role in appetite |
| Ghrelin (epsilon cells)
|
Protein | Stimulates hunger |
Case study
Max, a 7-year-old male Miniature Schnauzer, was showing signs of polyuria, polydipsia (PU/PD; excessive thirst, frequent urination), weight loss, and increased appetite. Blood tests revealed persistent hyperglycaemia and glycosuria, confirming a diagnosis of diabetes mellitus (DM)—a common endocrine disorder caused by an absolute deficiency of insulin. This condition impairs the body’s ability to regulate glucose, amino acids, and fatty acids. Max was diagnosed with Type I DM, the most common form in dogs, requiring lifelong exogenous insulin therapy. His breed and age placed him at higher risk. Management included twice-daily insulin injections, dietary changes, and regular monitoring of blood glucose.
Miniature Schnauzer by Canarian via Wikimedia Commons, CC BY SA 3.0
Section Review
The pancreas has both exocrine and endocrine functions. The pancreatic islet cell types include alpha cells, which produce glucagon; beta cells, which produce insulin; delta cells, which produce somatostatin; gamma cell which produce pancreatic polypeptide and epsilon cells which produce ghrelin. Insulin and glucagon are involved in the regulation of glucose metabolism. Insulin is produced by the beta cells in response to high blood glucose levels. It enhances glucose uptake and utilisation by target cells, as well as the storage of excess glucose for later use. Dysfunction of the production of insulin or target cell resistance to the effects of insulin causes diabetes mellitus, a disorder characterised by high blood glucose levels. The hormone glucagon is produced and secreted by the alpha cells of the pancreas in response to low blood glucose levels. Glucagon stimulates mechanisms that increase blood glucose levels, such as the catabolism of glycogen into glucose.
Review Questions
Critical Thinking Questions
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