Gastric Secretions

Question 1

Oxyntic (gastric) glands

Answer 1

• mucous neck cells that secrete mucus and some pepsinogen
• peptic (chief) cells that secrete pepsinogen
• parietal (oxyntic) cells: HCl and intrinsic factor

Question 2

Pyloric glands

Answer 2

• primarily mucus
• small amounts of pepsinogen and large amounts of thin mucus
• gastrin

Question 3

*Actions of gastrin

Answer 3

Secreted by G cells in response to proteins in the gastric lumen

• stimulates secretion of histamine by enterochromaffin-like cells
• histamine then acts on parietal cells stimulating acid secretion
• gastrin also has trophic effects on enterochromaffin and parietal cells

Question 4

*Increased gastrin =

Answer 4

Increased gastric acid

Question 5

Pepsinogen secretion

Answer 5

Secreted pepsinogen is an inactive precursor

• activated by HCl to form pepsin
• catalyzed by presence of pepsin
• HCl is required for proteolytic activity of pepsin

Question 6

Regulation of pepsinogen secretion

Answer 6

Release from peptic cells is stimulated by Ach release or presence of acid
- rate of release strongly influenced by amount of acid in the stomach

Question 7

Acid secretion

Answer 7

Stimulated parietal cells secrete acid solution

• isotonic to body fluids
• pH: 0.8
• requires 1500 calories per liter of gastric juice

Question 8

Mechanisms of acid secretion: process 1

Answer 8

Chloride is actively transported from cytoplasm to lumen, while Na is actively transported in the opposite direction
- causes negative potential in lumen = K diffuses out of cell into lumen

Question 9

Mechanisms of acid secretion: process 2

Answer 9

H2O dissociated via carbonic anhydrase

• H ions actively secreted into lumen in exchange for potassium
• catalyzed by H K ATPase pump
• remaining Na in lumen is actively absorbed, allowing H to take its place

Question 10

Mechanisms of acid secretions: process 3

Answer 10

CO2 combines with hydroxyl ions in cytoplasm to form bicarb

- bicarb diffuses out of cell into ECF in exchange for chloride ions

Question 11

Mechanisms of acid secretions: process 4

Answer 11

Water passes into lumen by osmosis following ions

Question 12

Final secretion contains

Answer 12

• HCl
• KCl
• small amount of NaCl

Question 13

*Hydrogen-potassium ATPase

Answer 13

Unique to parietal cell

• pumps H against an enormous concentration gradient (3 million to one)
• exists, preformed in cytoplasm of cell until needed
• transported to cell membrane upon stimulation of the parietal cell

Question 14

Neural and hormonal control

Answer 14

• Ach: acts on all cell types in gastric glands
• gastrin: secreted by G cells in pyloric glands and acts on parietal cells in gastric glands
• histamine: acts on parietal cells, secreted by enterochromaffin-like cells

Question 15

Control of acid secretion

Answer 15

Parietal cells are only source of HCl, closely associated with enterochromaffin-like cells which lie adjacent to parietal cells in gastric glands
- rate of acid secretion is related to the rate of histamine secretion

Question 16

Enterochromaffin-like cells are stimulated by ____

Answer 16

• gastrin secreted by pyloric glands in response to proteins

- acetylcholine released from vagus nerve

Question 17

Activation of what receptors leads to production of gastric acid?

Answer 17

• acetylcholine (M3)
• histamine (H2)
• gastrin (G)

Question 18

What contributes to complete blockade of acid secretion?

Answer 18

H-K pump (proton pump) blockade

- drug of choice for reduction of acid secretion

Question 19

Delayed effect

Answer 19

Complete pump inhibition may take up to 4 days

- include histamine blockers for first few days of therapy

Question 20

When pH drops below 3.0

Answer 20

• gastrin secreted by G cells is stopped
• gastrin mediated HCl release is reduced
• reduction in acid (raising pH) slows pepsinogen release

Question 21

If acid production is blocked?

Answer 21

• feedback inhibition is lost
• gastrin release continues
• may lead to hypertrophy of parietal and enterochromaffin-like cells

Question 22

*Cephalic phase (neural stimulation)

Answer 22

• release of HCl by parietal cells
• release of gastrin by G cells
• gastrin subsequently acts on parietal cells increasing acid secretion

Question 23

*Cephalic phase is the ______ response to food

Answer 23

Brain

• sight, smell, thought, taste, intensity increases with appetite
• mediated by neural signals: transmitted via vagus nerve, responsible for 20% of gastric secretion

Question 24

Gastric phase

Answer 24

Presence of food in the stomach

• stimulates vagovagal and enteric reflexes
• stimulates gastrin mechanism
• accounts for 70% of gastric secretion

Question 25

Intestinal phase

Answer 25

Presence of food in proximal SI

• releases small amounts of gastrin from duodenum
• leads to a weak stimulus for secretion

Question 26

*Inhibition of gastric secretion intestinal factors

Answer 26

Following the weak intestinal phase the intestines often inhibit gastric secretion
- presence of chyme may lead to an inhibitory enterogastric reflex

Question 27

*Enterogastric reflex

Answer 27

• distention of small bowel
• presence of acid in duodenum
• presence of protein breakdown products
• mucosal irritation

Question 28

Presence of acid, fat, protein breakdown products, hyper/hypo osomolar fluids, or any irritation leads to release of

Answer 28

Secretin: opposes gastric secretion

• gastric inhibitory peptide
• vasoactive intestinal polypeptide
• somatostatin