GI secretions/digestions [4]

Question 1

What secretes Gastric acid?
What is GA and what does it do?
Is it passive or does it require E?

Answer 1

Secreted by parietal cells ( contains HCl and IF)

Kills bacteria
Begins protein digestion (denature prot + activates pepsinogen → pepsin)

E consumin processs (H+/K+ ATPase pumps across luminal surface against significant gradient)
- knows its getting E back via digestion

Question 2

4 phases of gastric secretion

Answer 2

basal phase + 3 assoc w/ eating

1. interdigestive (basal) phase
2. cephalic phase
3. gastric phase
4. intestinal phase

Question 3

interdigestive (basal) phase of gastric secretion

Answer 3

between meals, following circadian rhythm

• highest in the evening
• lowest in the morning prior to waking

Question 4

Cephalic phase of gastric secretion

Answer 4

neuronal control
- vagus

Cause:

1. release of ACh
2. trigger Histamine release from ECL cells
3. release of GRP form vagal and ENS
4. inhibition of somatostatin release form Delta cells

• 30% of acid secretions
  (note: somatostatin a hormone secreted in the pancreas and pituitary gland that inhibits gastric secretion - so you inhibit an inhibitor)

Question 5

gastric phase of gastric secretion

Answer 5

initially neural, followed by endocrine (gastrin) and neural reg.

1. Food enters stomach
2. distends gastric mucosa
3. activates vasovagalreflex + local ENS reflexes
4. partially digested proteins stimulate ANTRAL gastrin (G) cells
5. release gastrin (50-60% of total acid secretion)

Question 6

Intestinal phase of gastric secretion

Answer 6

mostly endocrine (largely inhibitory, telling us 2 stop prod acid)

1. presence of aa and partially digested peptides in proximal small intestine
2. stimulate acid secretion by stimulating DUODENAL gastrin (G) cells to secrete gastrin (5-10% of total acid secretion)

Question 7

How do parietal cell secretagogues induce acid secretion

Answer 7

ACH → M3 receptor → ↑ intracellular Ca2+
Histamine → H2 receptor → ↑ intracellular Ca2+
gastrin → CCK3 receptor → ↑cAMP

These receptors in the basolateral membrane of parietal cells, potentiate gastric acid secretion (on lumenal side) when stimulated concurrently

• one of the most E costly of any electrolyte transport process in body.
• Together, Ca2+ and cAMP activates protein kinases to phosphorylate H/K+ ATPase

Question 8

Direct/Indirect pathway of stimulating parietal cells to secrete gastric acid

Answer 8

direct pathway: ACh, His, Gastrin directly stim parietal cell

Indirect pathway: “ “ stimulate enterochromaffin like (ECL) cells to result in secretion of His → acts on parietal cells

Question 9

Mech of gastric acid generation + secretion.

Role of: K+, Cl/HCO3, carbonic anhydrase, H+K+ ATPase

Answer 9

1. H+ is actively transported across APICAL memb (←)
   in exchange for K+ by H/K ATPase (→)
   - when H+ moves out of cells (←) [HCO3-] in cells goes ↑
2. HCO3- is transported across BASOLATERAL memb (→)
   in exchange for Cl- (←)
3. Cl- that accumulates is transported across APICAL memb (←←) by facilitated diff
4. H2O follows Cl-
   - pH of venous blood leaving stomach is high (HCO3- transport, known as ALKALINE TIDE)

(NOTE: all of this depends on Na/K ATPase, creating electronegative gradient (inside more neg than outside)

Question 10

How does mucous serve as a protective barrier?

Answer 10

mucous secreted by goblet cells and mucous neck of the gland → form unstirred gel layer in which H2O is trapped + serves as neutralization zone for acid to be neutralized when diffused in

Question 11

Peptic ulcer disease risk factors

Answer 11

1. NSAIDs (block cox I → ↓ PG → ↓ mucous)
2. Tumors (ZE synd)
3. H. Pylori

Question 12

First step of carbohydrate digestion takes place in the ______ with the enzyme ______.

Final step in carbohydrate digestion occurs at _____ for all dietary carbs

Answer 12

Mouth - amylase

enterocyte surface

(note that amylase digests digest starch polysacc α-1,4 bonds to sugars that eventually → glucose)

Question 13

Digestion of proteins starts _______ from the enzyme, ______ and is completed in the ______ with the enzyme ______.

Answer 13

stomach - pepsin

small intestine - gastric, pancreatic, enterocyte brush border and cytoplasmic peptidases

Question 14

Only monomeric _____ can be absorbed

Answer 14

SUGARS!!!

unlike proteins

Question 15

Fat digestion starts in the mouth with the enzyme ____.

It ends in the ______.

Answer 15

fat - lingual lipase

duodenum/jejunum

Question 16

Brush border enzymes involved at enterocytes:
Maltase acts on \_\_\_\_\_ or \_\_\_\_\_ → \_\_\_\_\_\_\_
Isomaltase (SI) acts on \_\_\_\_\_\_ → \_\_\_\_\_\_\_
Lactase acts on \_\_\_\_ → \_\_\_\_\_+ \_\_\_\_\_
Sucrase acts on \_\_\_\_ → \_\_\_\_\_+ \_\_\_\_\_

Answer 16

Maltase: maltose or maltotriose → Glucose
Isomaltase (SI): alpha limit dextrin → Glucose
Lactase Lactose → Glucose + galactose
Sucrase: Sucrose → Glucose + fructose

Question 17

Predict small int. and colonic consequences of a deficiency in brush border enzyme, lactase following dairy product consumption.

Answer 17

1. unabsorbed lactose will draw water into intestinal lumen → osmotic diarrhea
2. Gut bacterial flora metabolizes unabsorbed lactose → gas (H+, methane, CO2)

Question 18

Apical, brush border, sugar monomer transporters of enterocytes
- regulates uptake of?

Answer 18

1. SGLT1: regulates uptake of glucose and galactose
2. GLUT5: regulates uptake of fructose
   (moves from lumen INTO cell)

• both are Na+ dependent (goes in with Na+ - cotransport)

Question 19

Basolateral, brush border, sugar monomer transporters of enterocytes
- regulates uptake of?

Answer 19

GLUT2: regulates glucose, galactose, and fructose
(moves OUT of cell into blood on basolateral side)

• not Na+ dependent

Question 20

Which of the brush border transporters can operate in the setting of secretory diarrhea like in cholera?

Answer 20

SGLT1
Na+ and Glucose → into cell
Important in oral rehydration