GI secretion

Question 1

1. Describe the mechanism of gastric acid generation and secretion, including the role of K+, Cl-/HC03, carbonic anhydrase and H+-K+ ATPase.

Answer 1

H/K ATPase on apical membrane pumps H into lumen > bicarb/Cl exchanger in basolateral membrane pumps bicarb into blood and Cl into cell > Cl accumulates in cell then is transported into cell by facilitated diffusion > water follows HCl into lumen by osmosis

Question 2

What kind of transport is the H/K ATPase, bicarb/Cl exchanger, and Cl facilitated diffusion?

Answer 2

H/K ATPase: primary active transport. bicarb/Cl exchanger: secondary active transport. Cl facilitated diffusion: passive transport

Question 3

What is alkaline tide

Answer 3

pH of venous blood leaving the stomach is high because of the bicarb transport from the cell into the blood

Question 4

Where is acid secreted in GI tract

Answer 4

Parietal cells in stomach (also secrete intrinsic factor) pump H via a H/K ATPase. Cl is pumped into cell from blood via Na/K/2Cl co transporter, then Cl is pumped out of cell into lumen via CFTR

Question 5

Function of acid in stomach

Answer 5

denatures proteins, activates pepsinogen, kills bacteria

Question 6

Defenses in stomach from acid

Answer 6

mucus layer from mucus cells (prostaglandins increase mucus production), bicarb from mucus cells, tight junctions btw cells prevent acid from infiltrating layers of wall, rapid cell turnover

Question 7

Describe B12 absorption

Answer 7

B12 binds R protein in stomach > pancreatic proteases remove R protein in duodenum >intrinsic factor from parietal cells binds B12 in duodenum > IF/ B12 complex binds receptor in terminal ileum for absorption

Question 8

Phases of HCl secretion

Answer 8

1. Interdigestive (basal) phase – between meals following circadian rhythm 2. Cephalic phase – mostly neural regulation. 3. Gastric phase – initially neural followed by endocrine (gastrin) and neural regulation. 4. Intestinal phase – mostly endocrine regulation

Question 9

3. Describe the modulation of gastric acid secretion throughout the day and night.

Answer 9

highest in the evening and lowest in the morning prior to waking due to basal phase of acid secretion

Question 10

describe cephalic phase of HCl secretion

Answer 10

Mediated by vagus nerve upon smell, sight, taste and swallowing of food- (1) release of ACh; (2) triggering of the histamine release from ECL cells; (3) release of gastrin-releasing peptide (GRP) from the vagal and enteric neurons (ENS); and (4) inhibition of somatostatin release from the delta cells (D cells) in the stomach.

Question 11

describe gastric phase of HCl secretion

Answer 11

First, the food distends the gastric mucosa, which activates a vagovagal reflex as well as local ENS reflexes. Second, partially digested proteins stimulate antral gastrin (G) cells, which release gastrin. The gastric phase accounts for about 50% to 60% of total acid secretion

Question 12

describe intestinal phase of HCl secretion

Answer 12

presence of amino acid and partially digested peptides in the proximal portion of the small intestine stimulates acid secretion by stimulating duodenal gastrin (G) cells to secrete gastrin.

Question 13

regulation of acid production

Answer 13

parietal cells undergo amplification of apical surface area with increased density of H/K ATPase. Ach and gastrin signal via Ca, and histamine signals via cAMP

Question 14

Direct vs indirect acid secretion stimulation

Answer 14

direct: Ach, gastrin and histamine directly stimulate parietal cell. Indirect: Ach and gastrin stimulate enterochromaffin-like cells, resulting in histamine secretion which then acts on parietal cells

Question 15

risk factors for peptic ulcer disease

Answer 15

NSAID use, tumors (Zollinger Ellison syndrome) and H pylori

Question 16

Function of cholera toxin

Answer 16

stimulates release of Cl into lumen, causing watery diarrhea

Question 17

Where is chloride secreted from

Answer 17

parietal cells in the crypts

Question 18

function and source of amylase

Answer 18

breaks down amylopectin (starch) into Maltose, Maltotriose & Alpha-limit Dextrin. major enzyme in saliva and pancreatic secretions

Question 19

Where are carbs digested

Answer 19

In mouth (saliva), but mainly at the surface of enterocytes in small intestine

Question 20

List enterocyte surface enzymes for polysaccharides

Answer 20

1. Isomaltase (alpha-dextrinase) – converts alpha-limit dextrins to glucose. 2. Maltase – converts maltose and maltotriose to glucose. 3. Lactase – converts lactose to glucose and galactose. 4. Sucrase – converts sucrose to glucose and fructose. 5. Trehalase – converts trehalose to glucose

Question 21

What is lactose inntolerance and what causes it

Answer 21

Missing the brush border enzyme, lactase. Causes gas and diarrhea due to bacterial digestion of lactose

Question 22

How are sugar monomers taken up by enterocytes

Answer 22

SGLT1: glucose/Na or galactose/Na co-transporter from lumen into cell. GLUT5: fructose uptake into cell. GLUT2: glucose/ galactose/ fructose uptake from cell into blood.

Question 23

Which transporter can operate in setting of secretory diarrhea

Answer 23

Even when cAMP is increased (ie. in cholera) SGLT1 still operates, so important for oral rehydration

Question 24

Regulation of carb absorption

Answer 24

Increased carb consumption upregulates transporters and increases uptake. Decreased carb consumption dowregulates transporters