GI secretion and digestion

Question 1

Secretion – General Properties

Answer 1

Tightly regulated processes (receptors)

Enzymes are usually released in their pro-enzyme or zymogen form

Involved in both the maintenance and integrity of the inner surface of the GI tract (Cl-, mucus, HCO3-, etc) in addition to digestive processes

Chloride secretion via a CFTR channel is one mechanism for water/ion secretion in the small intestine and colon

Question 2

Acid secretion (purpose, what factors are released/activated, what drives it?)

Answer 2

Kills bacteria (disinfects food at pH 1.0)

Begins protein digestion - denatures proteins and activates pepsinogen (active form = pepsin)

Acid producing parietal cells also secrete intrinsic factor when secreting acid (vitamin B12 absorption)

Energy consuming process – H+/K+ ATPASE pumps across the luminal surface against a significant gradient

Question 3

Defences in stomach

Answer 3

HCl secretion is risky, so the body has mechanisms in place to confine acidity to the stomach as much as possible

Mucus layer and alkaline (HCO3-) layer at the cell surface (surface mucus cells) protects the stomach lining – prostaglandins can increase mucus production

Tight junctions between cells prevent acid from infiltrating the layers of the wall

Rapid cell turnover maintains surface integrity

Question 4

Parietal cell

Answer 4

Produces Intrinsic Factor and HCl

Question 5

Vit B12

Answer 5

Important for RBC production

Binds salivary R protein in stomach

Pancreatic proteases remove R protein in duodenum

IF from stomach then binds B12 in duodenum

IF/B12 complex binds to receptor in terminal ileum for absorption (receptor is for IF)

Question 6

Phases of HCl Secretion

Answer 6

Interdigestive (basal) phase – between meals following circadian rhythm (highest in the evening and lowest in the morning prior to waking

Cephalic phase – mostly neural regulation

Gastric phase – initially neural followed by endocrine (gastrin) and neural regulation

Intestinal phase – mostly endocrine regulation

Question 7

How do the three parietal cell secretagogues induce acid secretion?

Answer 7

structural changes push pumps to surface to secrete acid

increased vagal release of ACh (and possibly VIP)

Question 8

Describe the mechanism of gastric acid generation and secretion, including the role of K+, Cl /HCO3, carbonic anhydrase and H+-K+ ATPase

Answer 8

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Question 9

Peptic ulcer disease risk factor

Answer 9

NSAID use

Tumors (Zollinger Ellison Syndrome)

Helicobacter pylori

Question 10

Digestion

Answer 10

Process of breaking down food into an absorbable form

Some digestion occurs in the mouth and stomach but most of the final digestion occurs in the intestinal lumen or at the surface of the absorptive cells (enterocytes)

Question 11

Absorption

Answer 11

Process by which molecules of food are transported across the enterocyte membrane (transcellular/cellular) or between cells (paracellular) and into the blood or lymph

Question 12

Dietary carbs

Answer 12

Only simple monomeric sugars can be absorbed

Polymers can’t

Question 13

Amylase

Answer 13

digests carbs at the alpha 1,4 bonds

major enzyme in saliva and pancreatic secretions

Question 14

Enterocyte Surface Enzymes that Covert Small Polysaccharides to Sugar Monomers

Answer 14

Isomaltase (alpha-dextrinase) – converts alpha-limit dextrins to glucose

Maltase – converts maltose and maltotriose to glucose

Lactase – converts lactose to glucose and galactose

Sucrase – converts sucrose to glucose and fructose

Trehalase – converts trehalose to glucose

Question 15

Lactose intolerance

Answer 15

Missing brush border enzyme, lactase

Causes gas and diarrhea due to colonic bacterial digestion of lactose

Areas where dairy is not part of staple have a higher prevalence

Question 16

Enterocyte uptake of sugar monomers

Answer 16

Sodium transporters

Glucose binds, facilitating binding of Na, undergoes another conformational change that brings the Na into the cell, which causes another transformational change releasing the glucose

then transporter faces outside again and repeats

Question 17

SGLT1 Transporter

Answer 17

Requires sodium as a co-transporter

Transports glucose & galactose across the apical membrane of the enterocyte

can operate in the setting of secretory diarrhea (increased cAMP/cholera) so is important in oral rehydration

Question 18

GLUT 2

Answer 18

Not Na+ dependent

Transporter at basolateral surface that transports glucose & galactose and fructose

Question 19

GLUT 5

Answer 19

Fructose transport across the apical surface

Sodium independent

Question 20

Regulation of Carbohydrate Absorption

Answer 20

Increased carbohydrate consumption upregulates transporters and increases the uptake of simple sugars

Decreased carbohydrate consumption downregulates transporters and decreases the uptake of simple sugars