PANCREATIC GLAND
1. Delineate the endocrine and exocrine functions of the pancreas.
2. What are the four pancreatic hormones? Briefly describe the functions of each (providing that they are known).
3. What are the pancreatic islets?
4. Does insulin activity increase or decrease blood glucose concentration?
5. How does glucagon increase blood glucose?
The pancreas has both exocrine and endocrine functions. The exocrine functions are those associated with digestion and include digestive enzyme and bicarbonate secretion.
Hormones of the Pancreas
The hormones of the pancreas are insulin, glucagon, somatostatin, and pancreatic polypeptide. They are secreted by specific cells located in islets scattered throughout the pancreas. Four major types of cells are found in the islets, each responsible for.the secretion of one hormone. The cells are identified as alpha cells (glucagon), beta cells (insulin), delta cells (somatostatin), and F cells (pancreatic polypeptide). The pancreatic hormones are polypeptides.
Insulin
The tissues differ in regard to their sensitivity to insulin. Whereas the liver, muscle, adipose tissue, and leukocytes respond readily to insulin, the brain, kidney, intestines, and erythrocytes show little response. The principal effect of insulin on carbohydrate metabolism in those tissues sensitive to insulin (except the liver) is to allow the transport of glucose across the cell membrane (see Chapter 11). In these tissues, insulin enhances facilitated diffusion. In the liver, insulin enhances glucose uptake by stimulating enzymes in the liver cells that assist in the production of glycogen and lipogenesis and by inhibiting enzymes that catalyze glycogenolysis. Generally, insulin promotes fat deposition and protein synthesis. The result of insulin activity is lowering of the blood glucose concentration.
Glucagon
The result of glucagon activity is elevation of the blood glucose concentration.
This is achieved by the activation of adenylcyclase in liver cells, which in turn stimulates phosphorylase and results in the breakdown of glycogen. In addition, glucagon increases gluconeogenesis, increases metabolic rate, and stimulates lipolysis. Another action of glucagon is stimulation of the secretion of insulin (so that the new glucose can diffuse into cells) and of somatostatin.Somatostatin
Somatostatin usually seems to act as an inhibitory agent to slow the output of nutrients into the circulation and to moderate the metabolic effects of insulin, glucagon, and growth hormone. In this regard, somatostatin inhibits the secretion of insulin and glucagon. Also, as a moderator, it inhibits the secretion of gastrin, secretin, cholecystokinin, pancreatic exocrine secretion, and gastric acid. Somatostatin also moderates gastrointestinal motility and the absorption of glucose.
Pancreatic Polypeptide
Pancreatic polypeptide secretion is stimulated by the ingestion of protein, by exercise, and by fasting. No definite function has been established for pancreatic polypeptide.
Control of Insulin and Glucagon Secretion
The secretion of insulin and glucagon is controlled directly by the blood glucose concentration.
Because of the dual control (insulin decreases, glucagon increases) of glucose concentration, blood levels show little variation.
Important stimulatory effects on insulin secretion are caused by the gastrointestinal hormones, gastrin, secretin, cholecystokinin, and other hormones. The gastrointestinal hormones are secreted in response to food ingestion and actually cause insulin to be secreted before glucose absorption. Insulin secretion is also.stimulated by pancreatic glucagon (see previous text).
Glucagon secretion is stimulated by hypoglycemia, gastrin, cholecystokinin, and stress and is inhibited by glucose, secretin, insulin, and somatostatin.
Somatostatin release is enhanced by almost every factor that increases insulin secretion.
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