Gastrin-releasing peptide (GRP): gut hormone that triggers digestion

What this is

Gastrin-releasing peptide (GRP) is a signaling molecule produced in the nervous system and gut that tells the stomach to release gastrin, a hormone that drives acid secretion and digestive enzyme output. It is the mammalian counterpart of bombesin, a peptide first discovered in frog skin, and belongs to the bombesin-like peptide family found across vertebrates. GRP acts locally in the stomach wall, the pancreas, and the brain, making it a central coordinator of digestive responses to a meal.

History

GRP was first isolated from porcine non-antral gastric tissue in 1979 by McDonald and colleagues, who characterized it by using gastrin release as the biological readout (McDonald et al., Biochemical and Biophysical Research Communications, 1979). The identification connected a decades-old observation — that extracts of frog skin (bombesin) triggered gastric hormone release in mammals — to a native mammalian peptide with the same core activity. Follow-on work from the same group documented GRP's actions on the endocrine pancreas in detail, establishing that it stimulates insulin and other pancreatic hormone secretion (McDonald et al., Annals of the New York Academy of Sciences, 1988).

What it does

GRP acts on the gastrointestinal tract and pancreas to coordinate the hormone and enzyme response to food. In the stomach, it triggers gastrin release, which in turn drives acid secretion. In the pancreas, it stimulates the release of insulin and other endocrine hormones. In isolated perfused pancreas preparations, intra-arterial GRP infusion significantly increased secretion of fluid, bicarbonate, and protein. Beyond digestion, GRP is expressed in neurons of the suprachiasmatic nucleus, where it contributes to synchronizing cellular circadian rhythms. Vagal nerve activity modulates GRP, gastric somatostatin, and gastrin secretion in concert, placing GRP at the intersection of neural and hormonal gut control.

Evidence

• Human: Wood and colleagues (1983) reported that intravenous GRP produces a broad gastrointestinal and pancreatic hormone response in humans, confirming in-vivo activity consistent with the porcine isolation studies.
• Animal: In isolated perfused pig and rat pancreas models, GRP infusion increased fluid, bicarbonate, and protein secretion. McDonald and colleagues (1988) characterized the full endocrine pancreatic response to GRP in animal preparations.
• In vitro: GRP drives gastrin release from isolated gastric tissue; this activity served as the original bioassay for its isolation (McDonald et al., 1979).

Known effects

• Gastrin release — Documented in both animal preparations and human studies; the defining activity for which GRP was named.
• Pancreatic hormone secretion (insulin and others) — Demonstrated in isolated perfused pancreas models and in conscious animal studies.
• Exocrine pancreatic secretion — Increased fluid, bicarbonate, and protein output observed in isolated pig and rat pancreas preparations.
• Circadian rhythm modulation — GRP promotes suprachiasmatic nucleus cellular rhythmicity, particularly in the absence of VIP-VPAC2 receptor signaling.
• Satiety signaling — GRP has been studied as a satiety-related neuropeptide, consistent with its role in meal-initiated gut signaling.

Regulatory status

• US: GRP itself is not an approved drug. Synthetic GRP analogs and radiolabeled bombesin derivatives targeting the GRP receptor (GRPR) are under clinical investigation for cancer imaging and therapy.
• Research use: GRP is widely used as a research tool to study gastroentero-pancreatic hormone release, pancreatic physiology, and circadian rhythm signaling.

Mechanism

GRP acts by binding to the gastrin-releasing peptide receptor (GRPR, also known as bombesin receptor subtype 2), a class A G-protein-coupled receptor. Activation triggers downstream signaling that stimulates secretory cells in the gastric antrum and endocrine pancreas. The sequence stored here (APVSVGGGTVLAKMYPRGNHWAVGHLM, 27 residues) represents the C-terminal biologically active fragment; the full-length human prepro-GRP gives rise to several processed forms, with the C-terminal decapeptide GRP(18–27) retaining receptor-binding activity. The card's annotated target (CCKAR) reflects the broader gastric satiety signaling context in which GRP research is situated, though GRP's direct receptor is GRPR.

Radiolabeled GRPR-targeting analogs — including bombesin-derived compounds conjugated to chelators for PET or radiotherapy — are an active area of translational research, with GRPR overexpression documented in prostate cancer, breast cancer, and gastrointestinal stromal tumours.

Open questions

• The relative contributions of GRP-GRPR signaling vs. CCK-CCKAR signaling to post-prandial pancreatic output remain incompletely resolved in human physiology.
• GRPR-targeted radionuclide therapy for solid tumours (prostate, breast, GIST) is under clinical investigation; head-to-head comparisons with established somatostatin analog-based approaches are lacking.
• The role of central GRP in circadian timing — particularly its interaction with VIP in the suprachiasmatic nucleus — is not fully characterized.
• Oral or non-injectable delivery formats for GRPR-targeted diagnostics have not been validated.