A blood glucose level above the normal 70 mg/100 mL (70 mg/dL) stimulates release of insulin from beta cells, leading to uptake and storage of glucose by liver and muscle. The resultant decrease in the blood glucose level stops insulin secretion. Some amino acids also stimulate insulin secretion, either alone or in concert with elevated blood glucose levels. Increased blood fatty acid levels also stimulate insulin release, as do circulating gastrin, CCK, and secretin. CCK and glucagon, released in the islet by the A cells, act as paracrine secretions to stimulate B cell secretion of insulin.
Blood glucose levels below 70 mg/100 mL stimulate release of glucagon; blood glucose levels significantly above 70 mg/100 mL inhibit glucagon secretion. Glucagon is also released in response to low levels of fatty acids in the blood. Insulin inhibits release of glucagon by A cells, but because of the cascading circulation in the islet (explained in a following paragraph), this inhibition is effected by a hormonal action of insulin carried in the general circulation.
The islets have both sympathetic and parasympathetic innervation. About 10% of the islet cells have nerve endings directly on their plasma membrane. Well-developed gap junctions are located between islet cells. Ionic events triggered by synaptic transmitters at the nerve endings are carried from cell to cell across these junctions. Autonomic nerves may have direct effects on hormone secretion by A and B cells.
Parasympathetic (cholinergic) stimulation increases secretion of both insulin and glucagon; sympathetic (adrenergic) stimulation increases glucagon release but inhibits insulin release. This neural control of insulin and glucagon may
TABLE Minor Cell Types in Pancreatic Islets 18.3 Cell Type Secretion Location (in Addition to Islet) Actions PP cell (F cell)a Pancreatic polypeptide Stimulates gastric chief cells, inhibits bile secretion and intestinal motility, inhibits  pancreatic enzymes and HCO3 secretion D 1 cell Vasoactive intestinal peptide (VIP) Also in exocrine acini and duct epitheliumb Similar to those of glucagon (hyperglycemic and glycogenolytic); also affects secretory activity and motility in gut; stimulates pan-creatic exocrine secretion EC cella Secretin, motilin, substance P Also in exocrine acini and duct epitheliumb Secretin: acts locally to stimulate HCO3 secretion in pancreatic juice and pancreatic enzyme secretion Motilin: increases gastric and intestinal motility Substance P: has neurotransmitter properties Epsilon cell Ghrelin Epithelium lining the fundus of the stomachc Stimulate appetite aPP, protein polypeptide; EC, enterochromaffin cell. bThis localization further emphasizes the ontogeny of the pancreas from the embryonic gut. cGhrelin is produced in the stomach by P/D1 cells.
Digestive System III: Liver, Gallbladder, and Pancreas  PANCR EAS
FIGURE 18.26 • Electron micrograph of pancreatic islet cells. The portion of the cell in the upper part of the illustration is an A cell. It contains characteristic granules (arrows) showing a dense spherical core surrounded by a clear area and then a membrane. This cell also displays a characteristically well-developed Golgi apparatus. The cell in the bottom of the illustration is a D cell. It contains numerous membrane-bounded granules of moderately low density (arrowheads). 15,000.