GI: Intestinal Phase Flashcards
List some functions of the intestinal phase.
- secretion of biliary and pancreatic juices
- digestion and absorption
- large surface area
- control of the pylorus to regulate gastric emptying
- inhibits gastric secretion
What motility things need to occur to allow for gastric emptying?
- increased pressure in the proximal stomach
- increased contractions of the antrum
- relaxed pyloric sphincter
- relaxed duodenum
Describe feedback inhibition of gastric emptying.
- vagal afferents detect the presence of chyme and nutrients in the duodenum
- vagal efferents slow down gastric contractions and close up the pyloric sphincter
Describe the effect of fat in the duodenum on gastric emptying.
- fatty acids stimulate the secretion of CCK
- CCK inhibits gastric emptying and closes the pylorus to allow for proper digestion and absorption
- CCK allows pancreatic and biliary secretion release to help in fat digestion and absorption
Describe the effect of duodenal acidity on gastric emptying.
- proton receptors on the duodenal mucosa sense low pH of duodenal contents
- signal is sent through the myenteric plexus to the gastric smooth muscle
- slows down gastric contractions to allow time for pancreatic HCO3 secretions to neutralize acid in the duodenum
What factors contribute to the inhibition of gastric emptying?
- fat in the duodenum
- acid in the duodenum
- chyme in the duodenum
What is the largest contribution to enzymatic digestion?
exocrine pancreatic secretions (1L/day)
List the components of exocrine pancreatic secretions.
HCO3, aqueous solution
enzymes for macromolecular digestion
What is the purpose of HCO3 in the pancreatic secretions?
- neutralizes incoming acidic chyme
- allows for correct pH for pancreatic enzymes
- reduces risk for mucosal damage from the pepsins entering via chyme
What glands make up the endocrine pancreas? What do these glands secrete?
- islet of Langerhans’
- insulin, glucagon, somatostatin, pancreatic polypeptides
Describe the structure of endocrine pancreatic glands.
- acinus - blind end that secretes enzymes; made of acinar cells
- ductules - modify the secretions, adding HCO3 aqueous
What stimulates pancreatic gland secretion (specifically, each part of the gland)?
- acinus is stimulated by CCK
2. ductules are stimulated by secretin
Describe the cells that line the pancreatic glands.
- acinus = acinar cells
2. ductules = centroacinar cells, ductal cells
Describe the effect of decreased pH on the pancreatic secretions.
- activate S cells=> secretin
- stimulates ductule cells to release HCO3
- negative feedback once lumen pH reaches normal levels to stop secretin secretion
How does secretin work?
- increases cAMP in ductal cells
- opens CFTR Cl- channels
- ductal cells release Cl- into the ductal lumen
- stimulates Cl-/HCO3- antiporter => secretion of HCO3-, reuptake of Cl-
Where does HCO3 in pancreatic secretions come from?
- Na-HCO3 co-transporter Type 1 (NBC-1) brings Na and HCO3 into the ductal cells (basolateral membrane)
- Carbonic Anhydrase
Why does pancreatic venous blood get acidified?
Because H+ are produced from carbonic anhydrase and are transported to the blood via Na/H exchangers on the basolateral membrane
How does Cystic Fibrosis affect pancreatic secretions?
- mutated CFTR Cl- channel prevents secretion of Cl-
- secondary prevention of HCO3- secretion
- highly acidic contents in the glands leads to destruction of exocrine glands
- impaired pancreatic function
List the triggers of CCK release.
- fatty acids and amino acids bind to I cells
- fatty acids and amino acids bind to sensor paracrine cells that release CCK-Releasing Peptide
- Monitor Peptide released by acinar cells binds to I cells
Where is CCK released from?
intestinal I cells
What are the effects of CCK?
- binds to acinar cell CCK-1 receptor => increase intracellular Ca2+ => stimulates secretion of enzymes
- neural: stimulates vagovagal reflex => stimulates secretion of ACh, GRP, and VIP from ENS
Describe the mechanism of acinar enzymatic secretion.
- CCK, ACh, and GRP => increase intracellular calcium
- VIP and secretin => increase cAMP
- phosphorylation cascade
- zymogen granules fuse with acinar membrane
- exocytosis
- ductal secretions wash away enzymes out of the gland
How do pancreatic proteases get activated?
enterokinase activates trypsinogen to trypsin
trypsin then activates all the other proteases
What causes pancreatitis?
premature activation of proteases or trypsin is resistant to degradation
