ENDOCRINE FUNCTION OF THE PANCREAS
The pancreas has both exocrine and endocrine functions. While the exocrine function produces enzymes secreted into the gut to help with digestion, the endocrine function secretes hormones into the circulation. Specialized cells, known as the islets of Langerhans or pancreatic islets, are scat-tered throughout the pancreas and secrete at least 4 peptides with hormonal activity. There are four types of cells in the islets, each secreting a different hor-mone. Two of the hormones, insulin (secreted by the beta or B cells) and glucagon (secreted by the alpha or A cells), are involved with the regulation of carbo-hydrates, proteins, and fat metabolism. The third, so-matostatin (secreted by delta or D cells), plays a rolein regulating the secretions by the islets. The function of the fourth, pancreatic polypeptide (secreted by F cells), is still not settled.
Insulin is anabolic in that it increases the storage of glucose, fatty acids, and amino acids. It is known, therefore, as the “hormone of abundance.” The target cells for insulin have specific receptors on the cell membrane that bind to insulin. The number of re-ceptors and their affinity for insulin are affected by such factors as exercise, food, other hormones, and plasma insulin levels. Exposure to increased insulin decreases the receptor concentration and the affinity of the receptors for insulin and exposure to less in-sulin has the reverse effect. The number of receptors increases in starvation and decreases in obesity; thus, the cells adapt to the plasma levels.
The actions of insulin are complex. Insulin increases the storage of carbohydrates, proteins, and fats. Its actions can be conveniently divided into rapid ac-tions (within seconds), intermediate actions (within minutes), and delayed actions (within hours). Insulin rapidly increases glucose uptake by almost all tissue. The glucose that enters the cells is rapidly converted into storage forms. Insulin speeds the uptake by in-creasing the number of glucose transporters (pro-teins that transport glucose) in the cell membrane.
The intermediate actions of insulin include stimu-lation of protein synthesis and inhibition or degrada-tion of proteins. Within hours, insulin causes an in-crease in the manufacture of enzymes required for the various metabolic processes. Insulin also in-creases the uptake of amino acids by cells and speeds protein synthesis. Formation of fat from fatty acids (lipogenesis) is also accelerated.
The primary stimulus for insulin secretion is an in-crease in blood glucose levels. In addition, insulin se-cretion is stimulated by acetylcholine (a neurotrans-mitter secreted by parasympathetic nerves), glucagon, hormones from the gastrointestinal tract that are re-leased when there is digestion and absorption of pro-teins and carbohydrates, among others. The control of secretion of insulin and glucagon are closely related.
Glucagon is secreted by the A cells of the pancreas. This hormone increases glucose output from the liver and increases breakdown of lipids and formation of glucose from amino acids, increasing the blood glu-cose level. Glucagon secretion is increased by many factors, including sympathetic stimulation, exercise, infection, and stress. Decreased secretion occurs when the blood glucose levels increase or when in-sulin secretion is stimulated.
This hormone is secreted by D cells of pancreatic islets. Somatostatin serves to regulate the secretion of hormones from other pancreatic cells. Somatostatin inhibits the secretion of both insulin and glucagon and reduces the absorption of nutrients from the gut.
The exact function of this hormone is unknown. It has been shown to slow down the secretion of pancreatic digestive enzymes and absorption of food in humans.