Insulin is the most abundant endocrine secretion. Its principal effects are on the liver, skeletal muscle, and adipose tissue. Insulin has multiple individual actions in each of these tissues. In general, insulin stimulates  uptake of glucose from the circulation. Specific cell membrane glucose transporters are involved in this process.  storage of glucose by activation of glycogen synthase and subsequent glycogen synthesis.  phosphorylation and use of glucose by promoting its gly colysis within cells.
Absence or inadequate amounts of insulin lead to elevated blood glucose levels and the presence of glucose in the urine, a condition known as diabetes mellitus. Reduced expression of insulin and insulin growth factors in the central nervous system (CNS) has recently been linked to Alzheimer’s disease (Folder 18.3).
In addition to its effects on glucose metabolism, insulin stimulates glycerol synthesis and inhibits lipase activity in adipose cells. Circulating insulin also increases the amount of amino acids taken up by cells (which may involve cotransport with glucose) and inhibits protein catabolism.
Glucagon, secreted in amounts second only to insulin, increases blood glucose levels.
chapter 18 Digestive System III: Liver, Gallbladder, and Pancreas PANCR EAS 651
FIGURE 18.24 • Photomicrographs of islets of Langerhans. a. In this routine H&E preparation, it is difficult to identify specific islet cell types without special stains. At best, one can identify small cells (arrows) at the periphery of the islet that are probably A cells. 360. b. This photo micrograph shows an islet of Langerhans stained with a special Grimelius silver stain that reacts with glucagonsecreting cells. The silver-impregnated A cells are arranged around the periphery of the islet. 360.
The actions of glucagon are essentially reciprocal to those gluconeogenesis, mobilizes fats from adipose cells, and of insulin. Glucagon stimulates release of glucose into the stimulates hepatic lipase. bloodstream and stimulates gluconeogenesis (synthesis
Somatostatin inhibits insulin and glucagon secretion.
of glucose from metabolites of amino acids) and glycogenolysis (breakdown of glycogen) in the liver. Somatostatin is secreted by the D cells of the islets. It is Glucagon also stimulates proteolysis to promote identical to the hormone secreted by the hypothalamus that regulates somatotropin (growth hormone) release from the anterior pituitary gland. Although the precise role of somatostatin in the islets is unclear, it has been shown to inhibit both insulin and glucagon secretion.
TABLE Principal Cell Types in Pancreatic Islets18.2 Cell Type % Cytoplasmic Staining with Mallory-Azan Product Granules (TEM) A 15–20 Red Glucagon About 250 nm; dense, eccentric core surrounded by light substance B 60–70 Brownish orange Insulin About 300 nm; many with dense, crystalline (angular) core surrounded by light substance D 5–10 Blue Somatostatin About 325 nm; homogeneous matrix
FIGURE 18.25 • Diagram of an islet of Langerhans stained by the Mallory-Azan method. A cells display red cytoplasmic staining, B cells (comprising most of the islet cells) display brownish-orange staining, and D cells show a blue cytoplasm.
The molecular characteristics of the major and some minor islet hormones are summarized in Table 18.4.
Regulation of Islet Activity