Pancreas

The pancreas is an exocrine gland and an endocrine gland. The exocrine tissue produces a bicarbonate solution and digestive enzymes. These substances are transported to the small intestine where they play a role in the chemical digestion of food. These functions are fully discussed in Chapter 18 on the digestive system.

Scattered throughout the pancreas and surrounded by exocrine cells are small clusters of endocrine cells referred to as the islets of Langerhans. These islets make up only 2 to 3% of the mass of the pancreas; however, their blood supply has been modified so that they receive 5 to 10 times more blood than the exocrine pancreas. Furthermore, this blood carrying the pancreatic hormones is then transported through the hepatic portal vein and delivered directly to the liver where the hormones, in a relatively high concentration, carry out many of their metabolic effects. The most important hormones produced by the pancreas that regulate glucose metabolism are insulin and glu-cagon.

Insulin. Insulin is a peptide hormone produced by b-cells of the islets of Langerhans. It is an important anabolic hormone secreted at times when the concentration of nutrient molecules in the blood is high, such as periods following a meal. Its overall effects include allowing the body to use carbohydrates as an energy source and to store nutrient molecules. Specifically, insulin exerts its important actions on the following tissues:

• Increase in glucose uptake

• Increase in glycogenesis (formation of glycogen, the storage form of glucose)

• Increase in lipogenesis (formation of triglycerides, the storage form of lipids)

• Adipose tissue

• Increase in glucose uptake

• Increase in free fatty acid uptake

• Increase in lipogenesis

• Increase in glucose uptake

• Increase in glycogenesis

• Increase in amino acid uptake

• Increase in protein synthesis

Insulin is the only hormone that lowers blood glucose (epinephrine, growth hormone, cortisol, and glucagon increase blood glucose). It does so by stimulating the uptake of glucose from the blood into the liver, adipose tissue, and muscle. This glucose is first used as an energy source and then stored in the form of glycogen in the liver and in muscle. Excess glucose is stored as fat in adipose tissue.

Insulin also plays a role in fat metabolism. In humans, most fatty acid synthesis takes place in the liver. The mechanism of action of insulin involves directing excess nutrient molecules toward metabolic pathways leading to fat synthesis. These fatty acids are then transported to storage sites, predominantly adipose tissue. Finally, insulin stimulates the uptake of amino acids into cells where they are incorporated into proteins.

The secretion of insulin from the pancreas is regulated primarily by the circulating concentration of glucose. When serum glucose increases, secretion of insulin is stimulated; when it decreases, insulin secretion is inhibited. Secretion typically begins to increase within 10 minutes following the ingestion of food and reaches a peak in 30 to 45 minutes. This increased insulin stimulates the uptake of glucose into the body's cells and lowers serum glucose levels back to normal. Other factors affecting insulin secretion include: circulating amino acids and free fatty acids; several gastrointestinal hormones, including gastrin, secretin, and cholecystokinin; and the para-sympathetic nervous system. Each of these factors stimulates the secretion of insulin. Sympathetic nervous stimulation inhibits insulin secretion.

Glucagon. Also a peptide hormone, glucagon is produced by a-cells of the islets of Langerhans. The overall effects of glucagon include:

• Increase in hepatic glucose production

• Glycogenolysis

• Gluconeogenesis

• Stimulation of lipolysis in the liver and in adipose tissue

The effects of glucagon on glucose metabolism are generally opposite to those of insulin. Acting primarily on the liver, glucagon stimulates glyco-genolysis (breakdown of glycogen, the storage form of glucose) and gluco-neogenesis, which increase blood glucose levels. This hormone also stimulates lipolysis, which increases the circulating concentration of free fatty acids. These molecules may then be used as an alternative energy source by muscle or serve as gluconeogenic substrates in the liver. Finally, glucagon stimulates the hepatic uptake of amino acids, which also serve as substrates for gluconeogenesis.

Factors that stimulate glucagon secretion include: a decrease in blood glucose; an increase in blood amino acids; sympathetic nervous stimulation; stress; and exercise. Factors that inhibit glucagon secretion include insulin and an increase in blood glucose. Table 10.2 summarizes the major functions of the hormones discussed in this chapter.

Supplements For Diabetics

Supplements For Diabetics

All you need is a proper diet of fresh fruits and vegetables and get plenty of exercise and you'll be fine. Ever heard those words from your doctor? If that's all heshe recommends then you're missing out an important ingredient for health that he's not telling you. Fact is that you can adhere to the strictest diet, watch everything you eat and get the exercise of amarathon runner and still come down with diabetic complications. Diet, exercise and standard drug treatments simply aren't enough to help keep your diabetes under control.

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