GIT Hormones | Gastrointestinal system | Physiology

More than 30 peptide hormone genes are known to be expressed throughout the digestive tract, which makes the gut the largest endocrine organ in the body. These hormones may act in an endocrine (enter the bloodstream and act at sites distant from the site of secretion), paracrine (act locally) , autocrine (act on self) or neurocrine (produced by GIT neurons) fashion. Many GIT hormones enter the portal circulation first before reaching the systemic circulation which takes them to their target organ.

Neurocrines are secreted by enterochromaffin cells of the GIT. Acetylcholine, norepinephrine, VIP, GRP, enkephalins, neuropeptide Y and substance P are the GIT neurocrines. Some neurocrines act as neuromodulators i.e. they modulate the activity of neurotransmitters. ACh causes contraction of smooth muscle in the wall and relaxation of sphincters (makes you go) and increases all GIT secretions. Norepinephrine causes relaxation of smooth muscle in the wall (flight and fright hormone, makes you hold) and contraction of sphincters. Galanin is a newly discovered neuromodulator that affects GIT motility and secretions. It is predominantly inhibitory and acts through G-protein coupled receptors.

Important GIT hormones and their functions

Hormone	Stimuli for secretion	Effect
Gastrin*	Peptides and amino acids especially tryptophan and phenylalanine; distension of stomach; GRP	Increased gastric acid secretion by parietal cells; stimulates growth of gastric and intestinal mucosa
CCK or cholecystokinin	Peptides, amino acids, monoglycerides	Increased pancreatic enzyme and HCO3- secretion; contraction of gallbladder and relaxation of the sphincter of Oddi; slows gastric emptying; stimulates growth of exocrine pancreas and gallbladder
Secretin**	H+ and fatty acids	Increased pancreatic HCO3- secretion;decreased gastric acid secretion; inhibits gastrin
Motilin	Released by enterochromaffin cells in fasting	Increases GIT motility by stimulating the migratory motor complexes.
GIP or glucose dependent insulinotropic peptide or gastric inhibitory peptide	Oral glucose, amino acids and fatty acids	Increased secretion of Insulin; decreased gastric H+ secretion; satiety; slows gastric emptying
GLP 1 or glucagon like peptide 1	Hexoses and fat	Decreases gastric emptying; satiety; increases sensitivity of beta cells of the pancreas to glucose; inhibits glucagon secretion; lowers blood glucose levels;
Enteroglucagon	Low blood glucose	Stimulates glycogenolysis and gluconeogenesis in the liver.
Pancreatic polypeptide	Ingestion of a meal	Inhibits pancreatic secretion of HCO3-; satiety
Somatostatin	Decreased luminal pH	Inhibits other GIT hormones and inhibits H+ secretion
Histamine	Gastrin	Stimulates H+ secretion by gastric parietal cells
VIP or vasoactive intestinal peptide	Enterochromaffin cell stimulation	Relaxation of smooth muscle in GIT; increased intestinal and pancreatic secretion;
GRP or gastrin releasing peptide or Bombesin	Enterochromaffin cell stimulation	Increases secretion of gastrin
Enkephalins (opiates)	Enterochromaffin cell stimulation	Smooth muscle contraction; decreased secretion
Substance P	Enterochromaffin cell stimulation	Contraction of smooth muscle; increased secretion of saliva
Neuropeptide Y	Enterochromaffin cell stimulation	Relaxation of smooth muscle; decreased intestinal secretion
Serotonin	Enterochromaffin cell stimulation	Inhibits gastric acid secretion; stimulates gastric and colonic mucus production; modulates GI smooth muscle activity and blood flow
Ghrelin	Fasting; vagal stimulation	Stimulation of appetite; increases food intake; modulates gastric acid secretion and motility; modulates pancreatic endocrine and exocrine secretions; increases GH secretion.

*Gastrin levels will increase in Zollinger-Ellison syndrome, H.pylori infections and chronic use of H2 blockers and proton pump inhibitors.

**Action of secretin is important as pancreatic lipases are active only at alkaline pH.