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Organization and control of the gastrointestinal system. Digestive processes – motility, secretion, digestion & absorption Phases – cephalic, gastric, intestinal & interdigestive period Contacts external environment Excretes steroid metabolites
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Hormones are secreted from Enteroendocrine Cells dispersed along the GI tract
Many different length peptide fragments with same action but different half lives.
Sulfated tyrosine 12 and amidation of carboxyl terminal phenylalanine 17 are equally active.
released from the G-cells in lateral walls of glands in the gastric antral mucosa
endogenous stimulus for release
protein digestion products: small peptides and amino acids
especially phenylalanine, tryptophan
distention of stomach-vagovagal and local reflexes
exogenous stimulus for release
calcium, caffeinated coffee, wine
inhibition of release
luminal acidity (pH < 3) by direct inhibition of G cells through somatostatin production
stimulation of gastric acid secretion
stimulates parietal cells > acid
stimulates enterochromaffin-like cells (ECL) > histamine -> parietal cells -> acid
gastrin potency is 1500 times greater than histamine
stimulates growth of the oxyntic glands, mucosa of the stomach, duodenal mucosa, colonic mucosa
atrophy of these tissues occur following antral resection
hypertrophy occurs with gastrin-producing tumors
Zollinger-Ellison syndrome – gastrin producing tumors (gastroma) of pancreas or duodenum that produce ulcers, diarrhea and steatorrhea. High serum gastrin and no acid response to gastrin infusion is diagnostic.
Released in response to all three classes of foodstuffs
- hydrolyzed fats, amino acids, glucose
Paracrines - released by secretory cells near target cells – reach nearby target cells by diffusion through interstitial fluid.
•Gastrointestinal hormones are found in endocrine cells scattered over large areas of mucosa. They are released by chemicals found in food, neural activity, or physical distension and may stimulate or inhibit several processes.
• The primary gastrointestinal tract hormones: gastrin, cholecystokinin, secretin, GIP, and motilin, but Peptide YY is also a potent inhibitor of gastric function.
Proglucagon is processed to fragments in the upper GI tract (glucagon), and lower GI tract to GLP-1 which stimulates insulin release even before blood glucose is increased.
• Paracrine agents are: Somatostatin and histamine.
• Three peptides function as neurocrines: VIP, bombesin (GRP), and enkephalins.