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Hormone Regulation of the GI System
Here we’ll learn about hormonal control of gastrointestinal functions, including regulation of contraction and relaxation of smooth muscle walls and sphincters, enzyme and bicarbonate secretion, and growth of the GI tract.
There are three categories of hormones that regulate the GI system:
Endocrine hormones are secreted by enteroendocrine cells into the blood stream. These hormones travel through the hepatic portal circulation, pass through the liver, and enter the systemic circulation before reaching receptors on their target cells. Many of the hormones we learn about in this tutorial are endocrine hormones.
Paracrines diffuse into the nearby extracellular fluid and act on the receptors of local target cells. Somatostatin and histamine, which influence on gastric acid secretion, are examples of paracrines.
Neurocrines are released by postganglionic neurons of the enteric nervous system upon action potentials and act on target cells.
Nonpeptide examples include acetylcholine and norepinephrine; peptide examples include vasoactive intestinal peptide and gastrin-releasing peptide.
Gastrin is produced by G-cells of the stomach and duodenum in response to amino acids and fatty acids in the duodenum, distention of the stomach, and vagal nerve stimulation.
Gstrin is an endocrine hormone that increases H+ secretions by parietal cells in the stomach, which play an important role in protein digestion.
CCK (cholecystokinin) is secreted by I-cells in the small intestine in response to the presence of small peptides, amino acids, and fatty acids in the duodenum.
CCK is an endocrine hormone that acts on both the gallbladder and the pancreas: it triggers gallbladder contraction and opening of the Sphincter of Oddi so that bile can enter the small intestine, and it triggers pancreatic enzyme secretions.
Recall that bile helps break down lipids, and that and pancreatic juice helps break down proteins, carbohydrates, and lipids.
Secretin, another endocrine hormone, is secreted by S-cells in response to H+ and fatty acids. Secretin has several roles: it inhibits gastrin and hydrogen secretion in the stomach, and it stimulates biliary and pancreatic secretion of bicarbonate. Recall that bicarbonate neutralizes acidic chyme in the intestines.
Incretins
The next two hormones are incretins – they stimulate insulin secretion from pancreatic beta cells.
GIP (gastric inhibitory peptide) is secreted by K-cells in the small intestine in response to glucose, fatty acids, and amino acids.
GLP-1 (glucagon-like peptide 1) is secreted by L cells in the small intestine in response to the presence of glucose (and other hexoses) and fat.
In addition to stimulating beta-cell insulin release, GLP-1 also increases beta-cell glucose sensitivity, and decreases gastric emptying, which allows for more time for lipid digestion and promotes satiety.
Thus, GLP-1 receptor agonists are prescribed to diabetic and/or obese patients to help manage blood-glucose levels, reduce food intake, and facilitate weight loss.
Motilin is secreted by cells in the duodenum in response to food ingestion; it stimulates the migrating motility complex and is not shown in our diagram.
Endocrine, Paracrine, Neurocrine
Key Hormones