Activities of the Gastrointestinal Tract • Motility • Secretion • Digestion • Absorption
Criteria for GI Hormones • a physiologic stimulus in one part of the GI tract produces a response in another part • the response must be independent of the nervous system • the physiologic response can be duplicated with an “extract” from the stimulus site • the substance must be isolated, purified, identified chemically, and synthesized
Gastrin • Stimulates gastric acid secretion and growth of gastric oxyntic gland mucosa. • Released from the gastric antrum and perhaps the duodenum. • Release is stimulated by peptides, amino acids, gastric distention and stimulation of the vagus.
Cholecystokinin (CCK) • Stimulates gallbladder contraction, pancreatic enzyme and bicarbonate secretion, and growth of the exocrine pancreas • Inhibits gastric emptying • Released from the duodenum and jejunum • Release is stimulated by peptides, amino acids and >8C fatty acids, and to a lesser extent by acid
Secretin • Stimulates pancreatic bicarbonate secretion, biliary bicarbonate secretion, growth of the exocrine pancreas, and pepsin secretion • Inhibits gastric acid secretion and the trophic effect of gastrin • Secreted by the duodenum • Secretion is stimulated by acid and to a lesser extent fat
Gastric Inhibitory Peptide (GIP) • Stimulates insulin release • Inhibits gastric acid secretion • Secreted by the duodenum and jejunum • Secretion is induced by glucose, amino acids and fatty acids
Motilin • Stimulates gastric and intestinal motility • Released by the duodenum and jejunum • Release is stimulated by nerves, fat and acid
Candidate Hormones • Pancreatic polypeptide-a 36 amino acid peptide released from the pancreas in response to all three food stuffs • Peptide YY-a 36 amino acid peptide released from the ileum and colon in response to meals, especially fat • Enteroglucagon-is present in the distal small intestine and released into the bloodstream
Paracrines • Act on cells in the immediate vicinity, not through the blood stream. • Somatostatin • Histamine
Somatostain • Inhibits gastrin release, release of other peptide hormones and gastric acid secretion. • Released from the GI mucosa and the pancreatic islets. • Release is induced by acid. • Release is inhibited by vagal stimulation.
Histamine • Stimulates gastric acid secretion. • Released from oxyntic gland mucosa and enterochromaffin-like (ECL) cells. • Released in response to gastrin.
Neurocrines • Released from neurons by an action potential, then diffuse across the synaptic cleft to the target tissue • Vasoactive intestinal peptide (VIP) • Bombesin or gastrin-releasing peptide (GRP) • Enkephalins
Vasoactive Intestinal Peptide • Stimulates intestinal and pancreatic secretion. • Relaxes sphincters and gut circular smooth muscle in general. • Released by cells in the mucosa and smooth muscle of the GI tract.
Bombesin or Gastrin-releasing peptide • Stimulates gastrin release. • Released by cells in the gastric mucosa.
Enkephalins • Stimulates smooth muscle contraction. • Inhibits intestinal secretion. • Released from cells in the GI mucosa and smooth muscle.
Smooth Muscle Cells • They are smaller than skeletal muscle cells and long, narrow and spindle-shaped. • Cells that belong to a bundle are functionally coupled by gap junctions. • There are no sacromeres, therefore no striations. • The ratio of thin to thick filaments is 15:1.
Robert D. Specian, Ph.D. • firstname.lastname@example.org