What are the 4 GI peptide hormones?
gastrin, secretin, CCK, glucose-dependent-inhibitory peptide (GIP)
Gastrin
- secreted by G cells of tummy (antrum)
- stomach distention/vagal stimulation via GRP (gastrin releasing peptide) cause secretion
- increase gastric H+ secretion (directly on parietal cell for HCl and indirectly ECL cell for HA)
- peptide hormone
cholecystokinin
- secreted by I cells of duodenum/jejunum
- secreted in response to fats
- increase gallbladder contraction, pancreatic enzyme/bicarbonate secretion
- decreases gastric emptying
- peptide hormone
secretin
- secreted by S cells of duodenum
- secreted when chyme leaves tummy
- increases bicarb/pancreatic fluid sectretion
- inhibits gastrin (therefore, stops acidic secretions), stops gastric emptying
- peptide hormone
glucose-dependent insulinomic peptide (GIP)
- secreted by K cells of duodenum/jejunum
- secreted in response to fat/carbs
- stimulates insulin secretion from pancreas
- inhibits HCl secretion by parietal cells
- peptide hormone
What does VIP do?
- neurocrine of mucosa/smooth muscle
- relaxes smooth muscle
- increases intestinal secretions
- increases pancreatic secretions
What are the GI paracrines?
somatostatin, serotonin, histamine
serotonin
- enterochromaffic cells of intestine
- respond to distension by exciting ENS to increase motility and secretions
somatostatin
- D cells of tummy produce it
- inhibits pancreatic and gastric secretions, motility
- can be endocrine or paracrine
histamine
- released by EC-like cells in tummy
- stimulates HCL secretion (H2 receptors)
mucus neck cells, cardiac glands (these only near esophagus)
- secrete mainly mucus
- in antrum of tummy
peptic (chief) cells
- secrete pepsinogen (cleaved to pepsin at low pH) and gastric lipase
- in body of tummy
parietal (oxyntic, acid-forming) cells
- secrete intrinsic factor (cyanocobalamin reab. in ileum) and HCl
- in body of tummy
HCO3-/Cl- exchanger (no energy!!)
bicarb from carbonic anhydrase sent to blood, Cl- enters from blood
H+/K+ ATPase
H+ from carbonic anhydrase into tummy lumen (digestion), K+ into cell
What does prolonged vomiting cause?
hypokalemia (kidney response), alkalosis (lost HCl), dehydration (lost volume/H2O)
What stimulates acid secretion?
histamine, vagus, gastrin, caffeine, insulin, stress
What inhibits acid secretion?
somatostatin, GIP (gastric inhibitory peptide), secretin, glucose insulinotropic peptide
Whom does the vagus stimulate?
- parietal cells, ECL cells, D cells (directly via ACh)
- G cells (via GRP)
- D cells (inhibits somatostatin via ACh)
What’s in saliva normally?
- hypotonic (less osmolarity than plasma)
- alpha-amylase, lingual lipase
- high K+, high bicarb
- low Na+, low Cl-
- uses Cl-/bicarb, K+/H+, Na+/H+ transporters
What about high flow rates?
- less time for ion reabsorption
- bicarb higher, K+ low
- saliva more like plasma
What about low flow rates?
- more time for ion reabsorption
- less Na+/Cl-
- more K+, lower bicarb
What’s in pancreatic secretions?
- lots of bicarb to neutralize acidic chyme (CCK/secretion increase bicarb secretion)
- isotonic secretion
