Hormones of the exocrine component
The main digestive enzymes found in pancreas can be divided into three major groups. Proteolytic enzymes (protein-splitting enzymes), some of which are secreted as zymogens, carbohydrate-splitting enzymes and lipolytic enzymes (fat-splitting enzymes). The most important representatives of proteases include trypsin(ogen), chymotrypsin, (pro)elastases and carboxypeptidases.
These enzymes cleave proteins at different peptide bonds into smaller peptides. α-Amylase is one of the carbohydrate-cleaving enzymes and hydrolyses glycosidic bonds. In order to break down and finally digest fats contained in food in the duodenum, various lipases (fat-splitting enzymes) are required in addition to bile from the liver. Pancreas contains carboxyl ester lipase, pancreatic lipase and (pro)phospholipase A2, which attack and break ester bonds in fats.
Tasks in blood sugar regulation
The endocrine parts of the pancreas (islets of Langerhans) are located in small cell groups between the closely packed exocrine glands. About one million of these islets of Langerhans occur in humans and are particularly common in the tail part of the pancreas. Microscopically, islets of Langerhans can be recognized as bright areas surrounded by numerous blood vessels (insulo-azinar portal vascular system).
Four cell types can be found in the endocrine tissue: the centrally located β cells, which make up 80% of the islets and produce insulin, the glucagon-producing α cells (20%), somatostatin-producing δ cells (8%) and PP cells, which form pancreatic polypeptide (2%). Insulin and glucagon play a central role in the regulation of blood sugar levels. Insulin is the only hormone that can lower blood sugar levels.
In addition, insulin stimulates the build-up of fat. An acute increase in blood glucose concentration after eating carbohydrate-rich food leads to the release of insulin into the blood. The free insulin docks to insulin receptors on cells and thus leads to the absorption of glucose into the cell.
The target tissues are mainly the liver, skeletal muscle and fatty tissue. This lowers the blood sugar level and the cells have energy in the form of glucose at their disposal. Glucagon acts as an opponent of insulin.
The main function of glucagon is to increase the blood sugar level by stimulating the production of new glucose (gluconeogenesis) in the liver and the splitting of glycogen into glucose. A carbohydrate-rich meal leads to the release of insulin and simultaneous inhibition of glucagon, whereas protein-rich food promotes the secretion of both insulin and glucagon. The precise interaction of both hormones is determined by their antagonistic (opposite) action and by their concentration ratio to each other. In this way blood sugar can be kept constant and large fluctuations (hyper- or hypoglycaemia) can be avoided.
Insulin is a peptide hormone that is synthesized as a prohormone in the β cells of the endocrine pancreas. Due to its short half-life, insulin is secreted pulsatile every 10 – 20 minutes. An acute increase in the glucose concentration in the blood is the strongest stimulus for the secretion of insulin and leads to the rapid removal of the glucose from the blood by introducing the glucose into the target cells.
Other important effects of insulin, besides the increased uptake of glucose into the cells, are the absorption of free fatty acids and amino acids. Insulin also prevents the breakdown of fatty tissue (lipolysis) and inhibits the secretion of glucagon. The antagonist of insulin, glucagon, is also produced as a prohormone in the α cells and is secreted when required.
Besides protein-rich food, the strongest secretion stimulus is low blood sugar (hypoglycaemia). In addition to increasing the blood glucose concentration, glucagon promotes lipolysis. δ cells produce somatostatin (SIH, GHIRH), a short peptide hormone that is also secreted by the hypothalamus.
Rising blood sugar levels stimulate the release of SIH, which, among other things, inhibits insulin and glucagon secretion. Somatostatin also inhibits numerous other hormones and acts as a universal inhibitor. Pancreatic polypeptide is produced in PP cells, secreted after protein-rich meals and has an appetite-suppressing and inhibitory effect on exocrine pancreatic secretion.