Gastric Secretion: Johnson

Question 1

Identify the 5 secretory products of the stomach and their products.

Answer 1

H+: activates conversion of Pepsinogen—> Pepsin. Kills bacteria. Digests protein.
Pepsinogens: Pepsin digests proteins by cleaving interior peptide bonds.
Mucus: lubricates food and protects lining of stomach.
Intrinsic factor: Req’d for absorption of B12
Water: Dissolves and dilutes ingested mat’l

Question 2

Understand the mechanisms making it possible for the stomach to secrete 150mN of HCl

Answer 2

Upon stimulation, the lumenal surface area of the acid secreting cells increases by membrane fusion with sequestered tubulovesicles containing H+/K+ATPase pumps, that increase the acid production potentials. The lumenal surface area of the cells is already quite large, relative to its volume, because of the canaliculi (infoldings of the lumenal membrane).

The parietal cells also have a lot of mitochondria to supply the ATP req’d by the H+/K+ATPase.

Acid secretion steps:
H+ is actively pumped into the gastric lumen in exchange for K+ via H+/K+ATPase.
HCO3- diffuses into the blood in exchange for Cl-, raising the gastric mucosal venous blood pH (protective).
The movement of Cl- from blood to lumen against both electrical and chemical gradients is the result of excess OH- in the cell after the H+ has been pumped out. Now moving down its electrical gradient, the Cl- joins with H+ in the lumen to form HCl.

Question 3

Describe the electrolyte content of gastric secretion and how it varies with the rate of secretion.

Answer 3

As rate of secretion increases, [H+] and [Cl-] increase and [Na+] concentration decreases.
[K+] slightly increases

Solution is always hypotonic. Nears isotonicity as rate reaches maximum.
Sources vary. Johnson says oxytinic secretions are “approximately isosmotic with plasma at all rates”

Question 4

Identify the major stimulants of gastric secretion and explain their interactions.

Answer 4

Acetylcholine (vagus n.), gastrin (G cell), and histamine (histamine cell) stimulate gastric secretion.

Gastrin —> release of histamine from enterochromaffin-like cell.

Gastrin + histamine + Ach act on parietal cell to stimulate release of gastric secretions.

Question 5

Explain the various processes involved in the stimulation and inhibition of gastric acid secretion.

Answer 5

Cephalic phase: “it’s all in your head” accounts for 30-40% of gastric acid secretion at the beginning of a meal, BEFORE food is in stomach.
Smell/taste food—> Vagus (Ach)—> parietal cell and G cell (gastrin release)

Gastric phase: when food is actually in your stomach. Accounts for most of secretion.
Gastric distension—> Vagus (Ach)—> parietal cell and G cell
Vagus also inhibits somatostatin release, which allows for uninhibited gastrin release.

Stimulation:
Pyloric mucosa secretes gastrin (stim. secretion of HCl by parietal cells)
::The more you like the food you’re eating, the more acid is secreted.
::The more protein in the food, the more acid is secreted.
::Distension of the stomach as food fills it stimulates G cells to release gastrin —> parietal cells —> acid secretion

Inhibition:
::Gastric pH somatostatin –I gastrin release and also —I acid secretion directly
::Duodenal acid—> secretin production —I gastrin release and directly —I secretion of acid
::Hyperosmotic solutions in duodenum—I secretion of acid
Fatty acids in duodenum and jejunum —> GIP—I gastrin secretion and directly —I secretion of acid

Question 6

Describe the physiologic basis of gastric and duodenal ulcer dz.

Answer 6

Causes of gastric ulcers: 
defective gastric mucosa
H. pylori
low rates of acid "secretion"
NH4+ (inhibits HCO3- secretion) 

Causes of duodenal ulcers:
Incr. acid and pepsin
H. pylori
Incr. gastrin levels, inability of somatostatin to inhibit.
NH4+
Incr. maximal acid secretion, incr. parietal cell number

Question 7

Are the combined effects of gastrin and histamine on gastric acid secretion additive or multiplicative?
What is the significance of this?

Answer 7

Multiplicative, i.e. their combined result is potentiation. (much more than simply their added impacts, given independently)
Significance- allows for hormone conservation. You don’t have to secrete maximal levels of the independent hormones to get a maximal gastric acid secretion. Give a little of both and get a maximum result.
- combining gastrin/histamine +/- Ach also gives rise to potentiation

Question 8

What is the major stimulant for pepsinogen secretion by the chief cells?

Answer 8

Vagal stimulation via Ach.
The presence of H+ ions within the gastric lumen also helps by stimulating secretin secretion which stimulates pepsinogen secretion while also inhibiting gastric acid secretion. (not very significant in humans)

Question 9

What is needed to convert pepsinogen to pepsin?

Answer 9

H+ ions