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- Identify the principal hormones secreted from the endocrine pancreas, their cells of origin, and their chemical nature.
- Understand the nutrient, neural, and hormonal mechanisms that regulate pancreatic hormone release.
- List the principal target organs for insulin and glucagon action and their major physiologic effects.
- Identify the time course for the onset and duration of the biologic actions of insulin and glucagon.
- Identify the disease states caused by oversecretion, undersecretion, or decreased sensitivity to insulin, and describe the principal manifestations of each.
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The pancreas is a mixed exocrine and endocrine gland that plays a central role in digestion and in the metabolism, utilization, and storage of energy substrates. This chapter focuses on the endocrine function of the pancreas through the release of insulin and glucagon and the mechanisms by which these hormones regulate events essential to maintaining glucose homeostasis. Maintenance of glucose homeostasis is similar to the maintenance of calcium balance discussed in Chapter 5, in which several tissues and hormones interact in the regulatory process. In the case of glucose, the process involves a regulated balance among hepatic glucose release (from glycogen breakdown and gluconeogenesis), dietary glucose absorption, and glucose uptake and disposal by skeletal muscle and adipose tissue. The pancreatic hormones insulin and glucagon play central roles in regulating each of these processes, and their overall effects are in part modified by other hormones such as growth hormone, cortisol, and epinephrine. In addition to secreting insulin and glucagon, the endocrine pancreas also secretes somatostatin, amylin, and pancreatic polypeptide.
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The pancreas is a retroperitoneal gland divided into a head, body, and tail that is located near the duodenum. Most of the pancreatic mass is composed of exocrine cells that are clustered in lobules (acini) divided by connective tissue and connected to a duct that drains into the pancreatic duct and into the duodenum. The product of the pancreatic exocrine cells is an alkaline fluid rich with digestive enzymes, which is secreted into the small intestine to aid in the digestive process. Embedded within the acini are richly vascularized, small clusters of endocrine cells called the islets of Langerhans, in which 2 endocrine cell types (β and α) predominate. The β-cells constitute most of the total mass of endocrine cells, and their principal secretory product is insulin. The α-cells account for approximately 20% of the endocrine cells and are responsible for glucagon secretion. A small number of δ-cells secrete somatostatin, and an even smaller number of cells secrete pancreatic polypeptide. The localization of these cell types within the islets has a particular pattern, with the β-cells located centrally, surrounded by α- and δ-cells.
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The arterial blood supply to the pancreas is derived from the splenic artery and the superior and inferior pancreaticoduodenal arteries. Although islets represent only 1%–2% of the mass of the pancreas, they receive approximately 10%–15% of the pancreatic blood flow. The rich vascularization by fenestrated capillaries allows ready access to the circulation for the ...