Gastrin

Overview

Gastrin is a peptide hormone produced primarily by enteroendocrine G cells located in the pyloric antrum of the stomach. It is the principal hormonal regulator of gastric acid secretion and plays an important role in gastrointestinal mucosal growth and maintenance. Gastrin exists in multiple molecular forms arising from post-translational processing of a common precursor protein (preprogastrin), with gastrin-17 (G-17, "little gastrin") and gastrin-34 (G-34, "big gastrin") being the predominant circulating forms.

The physiological importance of gastrin has been recognized since the early 20th century, when John Edkins proposed in 1905 that a chemical substance in the gastric antral mucosa stimulated acid secretion. The complete amino acid sequence of gastrin was determined by Rodney Gregory and Hilda Tracy in 1964, and the minimal active fragment (the C-terminal tetrapeptide amide, Trp-Met-Asp-Phe-NH2) was subsequently identified. This C-terminal sequence is shared with Cholecystokinin (CCK), reflecting their evolutionary relationship as members of the gastrin-CCK peptide family.

Gastrin is not used therapeutically as an administered peptide, but its measurement in serum is a cornerstone of gastroenterological diagnosis, and its biology is central to understanding acid-related diseases, gastric neoplasia, and the pharmacology of acid-suppressive medications.

Structure and Molecular Forms

Gastrin-17 (G-17):

• Length: 17 amino acids
• C-terminal sequence: ...Trp-Met-Asp-Phe-NH2 (active pharmacophore)
• Post-translational modification: Tyrosine sulfation at position 12 (sulfated form, gastrin II, is the predominant bioactive species)
• Source: Predominantly antral G cells
• Half-life: ~7 minutes

Gastrin-34 (G-34):

• Length: 34 amino acids (N-terminally extended form of G-17)
• Source: Predominantly duodenal G cells
• Half-life: ~42 minutes (longer due to reduced hepatic clearance)

Other forms:

• Gastrin-71 ("component I"): Incompletely processed form
• Gastrin-14 ("mini-gastrin"): Truncated form
• Gastrin-6: Minimal bioactive fragment containing the C-terminal hexapeptide
• Progastrin and glycine-extended gastrin: Processing intermediates with distinct biological activities (trophic effects via non-CCK2 receptors)

The C-terminal amidation (Phe-NH2) is essential for binding to the CCK2 receptor. Non-amidated processing intermediates (progastrin, glycine-extended gastrin) have reduced CCK2 receptor affinity but exert distinct trophic effects through alternative receptors.

Receptor Pharmacology

Gastrin signals primarily through the CCK2 receptor (formerly called CCK-B or gastrin receptor):

CCK2 receptor:

• Type: Class A GPCR (rhodopsin family)
• Distribution: Gastric parietal cells, enterochromaffin-like (ECL) cells, gastric smooth muscle, pancreatic acinar cells, select CNS neurons
• Signaling: Gq-coupled; activates phospholipase C, generating IP3 and diacylglycerol (DAG), leading to intracellular calcium mobilization and protein kinase C (PKC) activation
• Affinity: G-17 and CCK-8 bind with approximately equal nanomolar affinity

Mechanism of Gastric Acid Stimulation

Gastrin stimulates acid secretion through both direct and indirect pathways:

Direct pathway:

• CCK2 receptors on parietal cells activate calcium/PKC signaling, stimulating the H+/K+-ATPase (proton pump) to secrete hydrochloric acid into the gastric lumen

Indirect (predominant) pathway:

1. Gastrin binds CCK2 receptors on ECL cells in the gastric corpus
2. ECL cells release histamine from intracellular granules
3. Histamine binds H2 receptors on adjacent parietal cells
4. H2 receptor activation (via Gs/cAMP) potently stimulates the proton pump

This indirect, histamine-dependent pathway accounts for the majority of gastrin-stimulated acid secretion and explains why H2 receptor antagonists (ranitidine, famotidine) effectively block gastrin-mediated acid output.

Trophic Effects

Gastrin is a potent growth factor for the gastrointestinal mucosa:

• ECL cell hyperplasia: Sustained hypergastrinemia drives ECL cell proliferation, which can progress to ECL cell carcinoid tumors in extreme cases
• Parietal cell mass: Gastrin maintains parietal cell mass and function
• Mucosal integrity: Gastrin promotes epithelial cell proliferation, angiogenesis, and wound healing in the gastric and intestinal mucosa
• Progastrin effects: Non-amidated precursors (progastrin, glycine-extended gastrin) stimulate colonic epithelial proliferation through distinct receptors, with potential implications for colorectal neoplasia

Regulation of Gastrin Secretion

Stimulators of gastrin release:

• Luminal peptides and amino acids (especially phenylalanine and tryptophan)
• Gastric distension (mechanical stretch of antral wall)
• Vagal stimulation (via gastrin-releasing peptide, GRP)
• Elevated gastric pH (loss of acid-mediated negative feedback)

Inhibitors of gastrin release:

• Luminal acid (pH < 3 activates Somatostatin release from D cells, which paracrinally inhibits G cells)
• Somatostatin (direct paracrine inhibition)
• Secretin
• GIP (glucose-dependent insulinotropic peptide)

This feedback system maintains gastric pH homeostasis: acid production stimulates somatostatin, which inhibits gastrin, reducing acid secretion.

Clinical Significance

Zollinger-Ellison Syndrome

Gastrinomas (gastrin-secreting neuroendocrine tumors) produce massive, unregulated gastrin secretion, causing severe peptic ulcer disease, diarrhea, and esophageal injury. Diagnosis relies on fasting serum gastrin levels (typically >1,000 pg/mL) and the Secretin stimulation test. Treatment includes proton pump inhibitors for acid control and surgical resection or systemic therapy for the tumor.

Hypergastrinemia from Proton Pump Inhibitors

Chronic PPI therapy suppresses gastric acid production, removing the negative feedback on G cells and causing compensatory hypergastrinemia. PPI-induced hypergastrinemia may reach 2-4 times normal levels and drives ECL cell hyperplasia. The clinical significance of long-term PPI-induced hypergastrinemia remains debated, with theoretical concerns about ECL cell carcinoid development in humans balanced against decades of clinical experience without significant increases in gastric carcinoid incidence.

Autoimmune Atrophic Gastritis

Autoimmune destruction of parietal cells eliminates acid production, removing the negative feedback brake on gastrin release and producing marked hypergastrinemia. This chronic ECL cell stimulation can progress to type I gastric carcinoid tumors, which are generally indolent.

Serum Gastrin Measurement

Fasting serum gastrin is measured to:

• Evaluate suspected Zollinger-Ellison syndrome
• Investigate refractory peptic ulcer disease
• Monitor patients with autoimmune atrophic gastritis
• Assess patients with gastric carcinoid tumors
• Distinguish causes of hypergastrinemia (PPI use, H. pylori infection, renal failure, G-cell hyperplasia)

Dosing Protocols

As an endogenous gastrointestinal hormone, gastrin is not typically administered exogenously in routine clinical practice. Pentagastrin (a synthetic gastrin analog) was historically used as a diagnostic agent for gastric acid secretion testing at a dose of 6 mcg/kg SC, but has been discontinued in most markets. Gastrin levels are measured clinically as a biomarker for gastrinoma (Zollinger-Ellison syndrome), atrophic gastritis, and pernicious anemia. The secretin stimulation test (see secretin) is the primary provocative diagnostic test for gastrinoma.

Research Directions

Ongoing research explores the role of gastrin and its precursors in gastrointestinal carcinogenesis, with particular attention to progastrin's trophic effects on colorectal epithelium. Anti-gastrin vaccines and CCK2 receptor antagonists have been investigated as adjunctive therapies in pancreatic and gastric cancer. The complex interplay between gastrin, the microbiome, and gastric carcinogenesis remains an active area of investigation.