130b/131b - Salivary Gland, Stomach, Pancreas Secretion

Question 1

List 3 mechanism that prevent the stomach from digesting itself

Answer 1

• Mucous barrier is impenetrable by acid
• Any acid that gets neutralized by bicarbonate
• Any acid that is not neutralized is washed away by blood flow

Question 2

What is the most powerful stimulator of gastrin secretion in the gastric phase?

Answer 2

Stretch

• Stretch
• -> Vagus nerve relesases GRP
• -> G cell stimulation
• -> Gastrin release
  
  • -> Histamine release from ECL cells -> HCl from parietal
  • -> HCl from parietal

Question 3

How does the composition of pancreatic secretion vary with flow rate?

Answer 3

• Low flow
  
  • Low bicarbonate, high Cl- (iso-osmotic with plasma)
• High flow
  
  • High bicarbonate, low Cl-

Higher flow = more stuff from stomach = need more HCO3-

Question 4

Parietal cell receptor and second messenger pathway activated by ACh?

• Receptor:
• 2nd Messenger:

Answer 4

• Receptor: M3 muscarinic
• 2nd Messenger: Ca2+/IP3

Question 5

Which hormone most strongly stimulates bicarbonate secretion from the pancreas?

Answer 5

Secretin (from S cells);
Released in response to HCl in the duodenum

• ACh from the enteric nervous system also plays a role

Question 6

What mechanisms convert pepsin to pepsinogen?

Does this vary depending on pH?

Answer 6

• At pH 3.0-5.0: hydrolysis
• At pH <3.5: Pepsin can cleave pepsinogen to pepsin
  
  • Self-activation

Question 7

How does gastric secretion vary based on flow rate?

Answer 7

• Low flow rate:
  
  • Surface epithelial cells secrete bicarbonate-enriched fluids
  • Less Cl- secreted
• High flow rate:
  
  • Secretion from parietal cells dominates; lots of HCl

Question 8

Salivary composition varies with secretory rate.

How will saliva compare to plasma at low secretory rates?

High secretory rates?

Answer 8

Saliva will always be hypotonic to plasma

• Low secretory rate = more hypotonic
• High secretory rate = more similar to plasma, but still hypotonic

*Note: secretions from salivary acinar cells are isotonic to plasma, but secretions are modified by ductal cells

(Ductal cells absorb Na+, Cl-, secrete K+, HCO3-)

Question 9

Which ions are secreted by salivary ductal cells?

Which ions are absorbed?

Answer 9

• Secreted = K+, HCO3-
• Absorbed = Na+, Cl-

Question 10

List 3 things that stimulate gastric acid secretion from the parietal cell

Answer 10

• Histamine from ECL cells
  
  • Stimulated by ACh and Gastrin; the indirect pathway
• Gastrin from G cells
  
  • Stimulated by GRP (released from CNX in response to stretch)
• ACh from the vagus nerve

Question 11

A patient is prescribe vitamin B12 shots for the rest of their life.

Loss of which cells in the GI tract might prompt this treatment?

Answer 11

Loss of parietal cells

Parietal cells secrete gastric acid and Intrinsic Factor (IF)

IF is needed for Vitamin B12 absorption

Possible causes of parietal cell loss:

Gastric bypass surgery, removal of the body and/or fundus of the stomach, chronic autoimmune gastritis

Question 12

Parietal cell receptor and second messenger pathway activated by histamine?

• Receptor:
• 2nd Messenger:

Answer 12

• Receptor: H2 histamine
• 2nd Messenger: cAMP

Question 13

What mechanism is responsible for the “alkaline tide” that occurs after a meal?

Answer 13

In response to a meal:

• H+/K+ ATPase is upregulated to pump H+ into the stomach lumen (Cl- follows)
• To maintain pH balance, OH- is sent to the interstitial space via HCO3-/Cl- exchanger

Remember, these gradients are set up by the Na+/K+ ATPase on the basolateral membrane

Question 14

What are the effects of secretin on:

• Gastric smooth muscle:
• Parietal cells:

Answer 14

• Gastric smooth muscle: relaxation
• Parietal cells: decreased HCl secretion

Remember, secretin is released in response to HCl in the duodenum

Production of secretin is proportional to acid load, not concentration

Question 15

What are the effects of CCK on the:

• Gallbladder:
• Pancreas:
• Stomach:
• Sphincter of Oddi:

Answer 15

• Gallbladder: Contraction to release bile
• Pancreas: Acinar secretion for digestive enzymes
• Stomach: Reduced emptying so the duodenum isn’t overwhelmed
• Sphincter of Oddi: Relaxation to allow bile and pancreatic secretions into the duodenum

Remember, CCK is released in response to nutrients in the duodenum

Question 16

Parietal cell receptor and second messenger pathway activated by gastrin?

• Receptor:
• 2nd Messenger:

Answer 16

• Receptor: CCK-B
• 2nd Messenger: Ca2+/IP3

Question 17

In which phase of digestion are pancreatic cells most stronlgy stimulated?

Answer 17

Intestinal phase

• Nutrients in the chyme stimulate S cells and I cells in the duodenum
  
  • S cells -> Secretin release -> HCO3- from ductal cells
  • I cells -> CCK release -> digestive enzymes from acinar cells

There is some stimulation from the vagus nerve during the cephalic and gastric phases

Question 18

Which hormone most strongly stimulates pancreatic acinar secretion?

Answer 18

CCK (from I cells)

• CCK is released in response to fatty acids and protein in the duodenum