Atropine does what to parietal cells
Atropine blocks vagal stimulation of parietal cells but vagal stimulation of G cells is unaffected as a different transmitter is used (not ACh)
Vagus stimulation of parietal cells
Stimulates ACh --> M3 receptor --> Gq --> IP3/Ca2+ --> H+/K+ ATPase --> H+ seceretion
Stimulates GRP --> +G cells --> Gastrin --> CCKb receptor --> Gq --> IP3/Ca2+ --> H+/K+ ATPase --> H+ secretion
ECL cells mechanism on parietal cells
Gastrin stimulates ECL --> histamine --> H2 receptor --> activate cAMP --> activate H+/K+ ATPase --> H+ secretion
H2 blockers inhibit histamine from activating H2 receptors
Prostaglandins/misoprostol mechanism on parietal cells
PG/misoprostol --> Gi --> cAMP inhibited --> H+/K+ ATPase inhibited
Somatostatin mechanism on parietal cells
Somatostatin --> Gi --> inhibitory cAMP --> inhibit H+/K+ ATPase
Proton Pump Inhibitor
Inhibits H+/K+ ATPase and H+ secretion into gastric lumen
Located in duodenal submucosa. Secrete alkaline mucus. Hypertrophy seen in peptic ulcer disease.
Low flow --> high Cl-
High flow --> high HCO3-
Secreted in active form
Lipase, phospholipase A, colipoase
Includes trypsin, chymotrypsin, elastase, carboxypeptidases
Secreted as proenzymes also known as zymogens
Converted to active enzymes trypsin --> activation of other proenzymes and creation of more trypsinogen (positive feedback loop)
Converted to trypsin by enterokinase/enteropeptidase, an enzyme secreted from duodenal mucosa
Starts digestion, hydrolyzes alpha-1,4 linkages to yield disaccharides (maltose and alpha-limit dextrins)
Highest concentration in duodenal lumen, hydrolyzes starch to oligosaccharides and disaccharides
At brush border of intestine, the rate-limiting step in carbohydrate digestion, produce monosacchardies from oligo- and disaccharides
Only monosaccharides (glucose, galactose, fructose) are absorbed by enterocytes. Glucose and galactose are taken up by SGLT1 (Na+ dependent). Fructose is taken up by facilitate diffusion by GLUT-5. All are transported to blood by GLUT-2.
D-xylose absorption test: distinguishes GI mucosal damage from toher causes of malabsorption
Absorbed as Fe2+ in duodenum
Absorbed in jejunum
Absorbed in terminal ileum along with bile acids, requires intrinsic factor
Unencapsulated lymphoid tissue found in lamina propria and submucosa of ileum.
Contained specialized M cells that take up antigen.
Peyer's patches B cells
B cells stimulated in germinal centers of Peyer's patches differentiate into IgA-secreting plasma cells, which ultimately reside in lamina propria. IgA receives protective secretory component and is then transported across the epithelium to the gut to deal with intraluminal antigen.
Think of IgA, the Intra-gut Antibody. And always say "secretory IgA"
Composed of bile salts (bile acids conjugated to glycine or taurine, making them water soluble), phospholipids, cholesterol, bilirubin, water, and ions.
What catalyzes rate-limiting step of bile salts?
Cholesterol 7alpha-hydroxylase catalyzes rate-limitng step.
Function 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)
Product of heme metabolism.
Removed from blood by liver, conjugated with glucuronate and excreted in bile.
Conjugated with glucuronic acid - water soluble
Unconjugated - water insoluble
Macrophage role in breakdown of RBC
Macrophage breakdown RBC --> heme --> unconjugated bilirubin (indirect bilirubin - water insoluble)
Unconjugated bilirubin enters bloodstream by
Binding to albumin to form unconjugated bilirubin-albumin complex
Conjugated bilirubin forms from unconjugated by
UDP-glucuronosyl-transferase in the liver to form Conjugated bilirubin (water soluble)
Conjugated bilirubin to Urobilinogen
Gut bacteria take conjugated to urobilinogen
80% of urobilinogen to excreted in feces as stercobilin, which gives characteristic color of stool
20% of urobilinogen to enterohepatic circulation --> 90% to liver and 10% to kidney --> excreted in urine as urobilin which gives characteristic color of urine