53. Secretion and regulation of gastric juice

Question 1

Regions of the gastric mucosa:

Answer 1

• Aglandular region: microbial digestion (ruminants)
• Cardia: mucin – protection of gastric mucosa
• Fundus: hydrochloric acid and pepsinogen synthesis
• Pylorus: mucin (protection), pepsinogen (digestion), gastrin (regulation)

Question 2

There are three neurotransmitters or substances with hormonal activity which have an effect on the
parietal cells:

Answer 2

• 1. acetylcholine released from the parasympathetic vagus nerve,
• 2. gastrin, released by the G-cells,
• 3. histamine produced by H-cells, which can be found in the region of parietal cells

Question 3

There are three receptors on the parietal cells:

Answer 3

• muscarine type acetylcholine receptor (m-ACh),
• gastrin receptor
• H2 type receptor which binds histamine.

Question 4

Parietal cells – HCl production

Answer 4

The luminal surface of the parietal cells have canaliculi. This is the site of HCl secretion.

• H+/K+ pump is found at the luminal side and pumps H+ out of the cell, and K+ into the cell.
• The IC K+ level is kept by outflow of K through a conductive K+ ion channel.
• Water gives off H+, and OH- together with CO2 forms bicarbonate by the help of carbonic anhydrase.
• Cl- is taken in, and bicarbonate is transported out by an anion exchanger channel.
• Cl- is transported to luminal surface by a Cl- channel, and binds to H+ to form HCl.

Question 5

Regulation

Answer 5

• HCl production is influenced by neuronal and endocrine regulation
• There are 3 neurotransmitters or substances with hormonal activity which have an effect on the cell.
• The regulation of the parietal cells are divided into three phases: Cephalic phase, gastric phase, and intestinal phase

Question 6

Cephalic phase

Answer 6

Unconditional reflex mechanism

• Food in the mouth stimulates the tastebuds, visual, olfactory and auditory.
• Reaches the CNS via neuronal afferentation.
• Efferentaton via n.vagus will reach the stomach and initiate gastric juice secretion.
• G-cells also stimulated by acetylcholine, and produces gastrin which stimulated HCl again.

Question 7

Gastric phase:

Answer 7

• Gastric content stimulate stretch and chemo receptors in mucosa
• Mechanical effect: chief + parietal cells stimulated produces gastric juice
• chemical effect: G-cells activated, produces gastrin activates parietal cells
• At pH lower than 3, further gastrin secretion is inhibited.
• Gastrin stimulates H-cells to enhance histamine production

Question 8

Intestinal phase

Answer 8

• Chemical and mechanical effects of chyme (partially digested food) in duodenum activates secretion of secretin, CCK, GIP and VIP.
• These effects inhibits HCl production and gastric motility
• This ensures that the small intestine doesn’t get “overfilled” negative feedback