GI: 346 - 348 Flashcards
Which cells make intrinsic factor?
Parietal cells (stomach)
What is the main role of intrinsic factor?
Binding protein for B12
Where is B12 taken up?
Terminal ileum
What happens as a result of autoimmune destruction of parietal cells?
Chronic gastritis and pernicious anemia
Which cells produce gastric acid?
Parietal cells (stomach)
What is the main effect of gastric acid?
Decrease stomach pH
How is gastric acid regulated?
Increased by: histamine, ACh, gastrin
Decreased by: somatostatin, GIP, prostaglandin, secretin
What is a gastrinoma?
Gastrin-secreting tumor that causes high levels of acid secretion and ulcers refractory to medical therapy
Which cells produce pepsin?
Chief cells (stomach)
What is the main action of pepsin?
Protein digestion
How is pepsin regulated?
Increased by vagal stimulation and local acid
What is the inactive form of pepsin and how is it activated?
Pepsinogen, activated by H+
Which cells in the GI tract secrete HCO3-?
Mucosal cells (stomach, duodenum, salivary glands, pancreas) and Brunner glands (duodenum)
What is the main function of HCO3- in the GI tract?
Neutralizes acid
How is HCO3- regulated in the GI tract?
Increased by pancreatic and biliary secretion with secretin
Where do you find the HCO3-containing mucus?
Covers the gastric epithelium
What are the 3 secretory cells found in the duodenum?
I cells (CCK), S cells (secretin), and K cells (GIP)
What are 3 secretory cells found in the antrum?
D cells (somatostatin), mucous cells, G cells (GRP)
In which part of the stomach will you find parietal cells and chief cells?
Body
What is the main mechanism by which gastrin promotes acid secretion?
Exerts effects on enterochromaffin-like (ECL) cells leading to histamine release rather than through its direct effect on parietal cells
Histamine goes on to increase intracellular levels of cAMP in the parietal cell –> drives H-K ATPase
Does atropine completely block vagal stimulation of gastrin release? Why or why not?
No, it blocks parietal cells but does not affect vagal stimulation of G cells since G cells use GRP instead of ACh
What does ACh bind on the gastric parietal cell?
M3 receptor
What does histamine bind on the gastric parietal cell?
H2 receptor
What does gastrin bind on the gastric parietal cell?
CCK-B receptor
What are two enzymes/compounds that antagonize the effects of ACh, gastrin, and histamine in the parietal cell?
- Prostaglandins/misopristol
2. Somatostatin
How do prostaglandins/misopristol and somatostatin exert their action on the parietal cell?
Via Gi protein to decrease the levels of cAMP
What class of drugs can inhibit release of H+ from parietal cells?
Proton pump inhibitors
Where are Brunner glands located?
Duodenal submucosa
What is the main function of Brunner glands?
Secrete alkaline mucus
What happens to Brunner glands in peptic ulcer disease?
Hypertrophy
Describe the tonicity/content of pancreatic secretions relative to the flow rate.
Isotonic fluid
At low flow: high Cl
At high flow: high HCO3
What are 4 enzymes that are secreted by the pancreas?
- Alpha amylase
- Lipase, phospholipase A, colipase
- Proteases
- Trypsinogen
What is the main role of alpha-amylase?
Starch digestion
Which pancreatic enzymes are important for fat digestion?
Lipase, phospholipase A, colipase
Give 4 examples of proteases secreted by the pancreas?
- Trypsin
- Chymotrypsin
- Elastase
- Carboxypeptidases
What form are pancreatic proteases secreted as?
Zymogens (proenzymes)
Describe the role of trypsinogen.
Once it is converted to its active form (trypsin) it can activate all of the other proenzymes and also cleave additional trypsinogen molecules into active trypsin (positive feedback loop).
How is trypsinogen initially converted to trypsin?
By enterokinase/enteropeptidase, a brush-border enzyme on the duodenal and jejunal mucosa
What is the only form of carbohydrate that can be absorbed by enterocytes?
Monosaccharides (glucose, galactose, fructose)
Describe the absorption of glucose, galactose, and fructose.
Glucose, galactose - taken up by SGLT1 (Na+ dependent)
Fructose - taken up by facilitated diffusion by GLUT5
All are transported to blood by GLUT2
What is the D-xylose absorption test used for?
Distinguishes GI mucosal damage from other causes of malabsorption
Where is iron absorbed and in what form?
Fe2+ in duodenum
Where is folate absorbed?
Jejunum and ileum
Where is B12 absorbed?
Terminal ileum along with bile acids (requires intrinsic factor)
What are Peyer patches and where are they found?
Unencapsulated lymphoid tissue found in lamina propria and submucosa of ileum
Which cells are found in Peyer patches?
- M cells - sample and present antigens to immune cells
2. B cells - stimulated in germinal centers differentiate into IgA secreting plasma cells
What happens to the B cells that become IgA secreting plasma cells?
Reside in the lamina propria
How is the IgA processed?
Receives protective secretory component and is then transported across epithelium to the gut to deal with intraluminal antigen
What are the components of bile?
- Bile salts
- Phospholipids
- Cholesterol
- Bilirubin
- Water
- Irons
Which enzyme catalyzes the rate-limiting step of bile synthesis?
Cholesterol 7alpha-hydroxylase
What are bile salts?
Bile acids conjugated to glycine or taurine to make them water soluble
What are 3 functions of bile?
- Digestion and absorption of lipids and fat-soluble vitamins
- Cholesterol excretion (body’s only means of eliminating cholesterol)
- Antimicrobial activity (via membrane disruption)
