Alan B.R. Thomson, MD, PhD, and Patrick Tso, PhD The digestion and absorption of nutrients are both neurally and hormonally regulated. TABLE 7-1 The Endocrine Cells of the GI Tract GI, Gastrointestinal; HCI, hydrochloric acid.
Overview of Digestion and Absorption
General Structure and Function of the Gi Tract
General Mechanisms for Regulation of Gi Tract Function
GI Hormones
CELL NAME
HORMONE
LOCATION
STIMULUS FOR SECRETION
MAIN FUNCTION
G cells
Gastrin
Stomach, mainly pyloric region
Protein in gastric lumen
Stimulates gastric HCl secretion; stimulates pepsinogen secretion; increases gastric motility
X/A-like cells
Ghrelin
Stomach
Fasting
Stimulates gastric emptying; acts via the central nervous system to stimulate appetite
Enterochromaffin-like cells (ECL cells)
Histamine
Stomach
Gastrin
Acts on G cells to stimulate gastrin secretion
D cells in stomach and intestinal mucosa, as well as in pancreatic islets and enteric neurons
Somatostatin
Stomach,
intestines, and
pancreas
Intake of fat, protein, or glucose; presence of HCl in stomach lumen
Inhibits hormone release from GI tract and pancreas; acts as an enterogastrone to inhibit gastric acid secretion
Enterochromaffin cells (EC cells)
Serotonin
Stomach and intestines
Small intestinal release: intraluminal fluids, nutrients, acid, amino acids, hypertonic and hypotonic solutions;Colonic release: short-chain fatty acids
Stimulates both intrinsic and extrinsic nerves
I cells
Cholecystokinin (CCK)
Duodenum and jejunum
Partially hydrolyzed fat or protein in the duodenum/jejunum; acidic pH of chyme from the stomach entering duodenum
Stimulates contraction of gallbladder; stimulates release of enzymes by exocrine pancreas; inhibits gastric emptying
S cells
Secretin
Duodenum and jejunum
Acidic pH of chyme from the stomach entering duodenum
Stimulates release of bicarbonate-rich secretion by pancreas; inhibits HCl secretion by stomach; decreases gastric motility
K cells
Gastric inhibitory polypeptide (GIP)
Duodenum and proximal jejunum
Food in the small intestine
Stimulates pancreas to secrete insulin
L cells
Glucagon-like peptide-1 (GLP1)Glucagon-like peptide-2 (GLP2)
Small intestine
Food in the small intestine
GLP-1 stimulates pancreas to secrete insulinGLP-2 stimulates nutrient absorption and has trophic effects on the gut mucosa
M cells
Motilin
Duodenum and jejunum
Periodic and recurrent secretion pattern synchronized with phasic contractions during fasting
Stimulates GI motility, regulates phasic contractions
The Upper Gi System
Overview of Digestion and Absorption
