Gastrin

Physiology

• Gastrin is a peptide hormone that stimulates secretion of gastric acid by Parietal cells of the stomach and aids in gastric motility.
• It is released by G cells in the pyloric antrum of the stomach, duodenum, and the pancreas.
• Gastrin is released in response to certain stimuli including
  • Stomach antrum distension
  • Vagal stimulation
  • Presence of partially digested proteins, especially amino acids, in the stomach. Aromatic amino acids are particularly powerful stimuli for gastrin release.
  • Hypercalcemia
• Gastrin release is inhibited by
  • Presence of acid in stomach
  • Somatostatin

Function

G cell is visible near bottom left, and gastrin is labeled as the two black arrows leading from it. Note: this diagram does not illustrate gastrin's stimulatory effect on ECL cells. The presence of gastrin stimulates parietal cells of the stomach to secrete hydrochloric acid (HCl)/gastric acid. This is done both directly on the parietal cell^{[failed verification]} and indirectly via binding onto CCK2/gastrin receptors on ECL cells in the stomach, which then responds by releasing histamine, which in turn acts in a paracrine manner on parietal cells stimulating them to secrete H+ ions. This is the major stimulus for acid secretion by parietal cells.

Along with the above-mentioned function, gastrin has been shown to have additional functions as well:

• Stimulates parietal cell maturation and fundal growth.
• Causes chief cells to secrete pepsinogen, the zymogen (inactive) form of the digestive enzyme pepsin.
• Increases antral muscle mobility and promotes stomach contractions.
• Strengthens antral contractions against the pylorus, and relaxes the pyloric sphincter, which increases the rate of gastric emptying.
• Plays a role in the relaxation of the ileocecal valve.
• Induces pancreatic secretions and gallbladder emptying.
• May impact lower esophageal sphincter (LES) tone, causing it to contract, - although pentagastrin, rather than endogenous gastrin, may be the cause.
• Gastrin contributes to the gastrocolic reflex.

Role in disease

In the Zollinger–Ellison syndrome, gastrin is produced at excessive levels, often by a gastrinoma gastrin-producing tumor, mostly benign of the duodenum or the pancreas. To investigate for hypergastrinemia high blood levels of gastrin, a "pentagastrin test" can be performed.

In autoimmune gastritis, the immune system attacks the parietal cells leading to hypochlorhydria low stomach acid secretion. This results in an elevated gastrin level in an attempt to compensate for increased pH in the stomach. Eventually, all the parietal cells are lost and achlorhydria results leading to a loss of negative feedback on gastrin secretion. Plasma gastrin concentration is elevated in virtually all individuals with mucolipidosis type IV (mean 1507 pg/mL; range 400-4100 pg/mL) (normal 0-200 pg/mL) secondary to a constitutive achlorhydria. This finding facilitates the diagnosis of patients with this neurogenetic disorder. Additionally, elevated gastrin levels may be present in chronic gastritis resulting from H pylori infection.