GLP-1, or glucagon-like peptide-1, is a native incretin hormone produced by enteroendocrine L-cells in the intestine and released into circulation after eating, where it acts on the GLP-1 receptor to stimulate glucose-dependent insulin secretion from pancreatic beta cells. It is studied as the central hormone in the “incretin effect,” the observation that oral glucose produces a greater insulin response than intravenous glucose at an equivalent blood concentration, and it underpins one of the most extensively characterised G protein-coupled receptor signalling pathways in metabolic research. This article reviews GLP-1’s structure, receptor mechanism, key published studies, research applications and the handling requirements relevant to a laboratory setting.
What is GLP 1?
GLP-1 is a peptide hormone derived from proteolytic processing of proglucagon, the precursor protein encoded by the glucagon gene, within intestinal L-cells. The predominant circulating bioactive forms are GLP-1(7-36)amide, a 30-residue peptide, and the closely related GLP-1(7-37), differing only by a C-terminal glycine in place of an amide group; the two forms are reported to have essentially identical biological potency. GLP-1(7-36)amide has a molecular weight of approximately 3,298 g/mol.
The glucagon gene and its proglucagon product were cloned and sequenced in the early 1980s, revealing two glucagon-like sequences within the precursor, subsequently named GLP-1 and GLP-2. The physiological significance of GLP-1 as a genuine incretin hormone in humans, rather than simply a predicted sequence from gene cloning, was established by Kreymann and colleagues, who reported in the Lancet in 1987 that infusing GLP-1(7-36)amide into human volunteers at a rate mimicking normal postprandial concentrations produced a significant rise in insulin and fall in glucose and glucagon, and that GLP-1 was more effective at stimulating insulin release than glucose-dependent insulinotropic peptide (GIP), the other principal incretin hormone (Kreymann et al., 1987, PubMed).
Mechanism of action
GLP-1 acts on the GLP-1 receptor (GLP-1R), a class B (secretin-like) G protein-coupled receptor expressed principally on pancreatic beta cells, as well as in the brain, heart, gastrointestinal tract and other peripheral tissues. Receptor engagement couples primarily to Gs proteins, activating adenylate cyclase and raising intracellular cyclic AMP, which in turn activates protein kinase A and the cAMP-regulated guanine nucleotide exchange factor Epac2.
The defining feature of GLP-1’s action on insulin secretion is its glucose dependence. GLP-1 receptor signalling alone, in the absence of elevated glucose, produces little or no insulin release; instead, it is reported to “prime” beta cells, rendering glucose-resistant cells responsive to subsequent glucose exposure, a phenomenon described in the literature as glucose competence. Mechanistically, GLP-1-stimulated cAMP signalling is reported to close ATP-sensitive potassium channels and enhance closure of these channels that have already been partially inactivated by glucose metabolism, depolarising the beta cell membrane and triggering calcium influx through voltage-gated calcium channels, which in turn triggers exocytosis of insulin-containing secretory granules.
Because this mechanism depends on ambient glucose concentration rather than acting independently of it, GLP-1 receptor activation is reported in the literature to carry a comparatively low intrinsic risk of hypoglycaemia relative to insulin secretagogues that act independently of glucose levels, a property that has made the GLP-1 signalling pathway of particular interest in metabolic research. GLP-1 receptor signalling in the brain and gastrointestinal tract is also reported to contribute to slowed gastric emptying and effects on appetite-related circuits, extending GLP-1’s research relevance beyond pancreatic beta cell biology alone.
What the research shows
Beyond the foundational human physiology work by Kreymann and colleagues described above, the molecular receptor underlying GLP-1’s action was identified by Thorens, who reported in the Proceedings of the National Academy of Sciences in 1992 the expression cloning of the GLP-1 receptor from a rat pancreatic islet cDNA library. The study reported that the cloned receptor bound GLP-1 with high affinity, was specifically coupled to activation of adenylate cyclase, and did not bind structurally related peptides including glucagon, GIP, vasoactive intestinal peptide or secretin, establishing GLP-1R as a distinct 463-amino-acid, seven-transmembrane receptor within the secretin-receptor family (Thorens, 1992, PMC).
The cellular mechanism underlying GLP-1’s glucose-dependent action was characterised by Holz, Kühtreiber and Habener, who reported in Nature in 1993 a study using patch-clamp electrophysiology on isolated pancreatic beta cells. The investigators found that a subpopulation of beta cells that did not respond to glucose alone became responsive, depolarising by 30 millivolts or more, when glucose was combined with GLP-1(7-37), a phenomenon the authors termed glucose competence. Pretreatment with GLP-1 was reported to render previously glucose-insensitive cells capable of subsequently responding to glucose, directly demonstrating at the single-cell level the synergistic, glucose-dependent mechanism underlying GLP-1’s insulinotropic action (Holz et al., 1993, PMC).
A substantial additional body of published research, spanning rodent and human studies through the 1990s and beyond, has examined GLP-1’s effects on insulin gene expression, beta cell proliferation and mass, gastric emptying, and its rapid inactivation in plasma by the enzyme dipeptidyl peptidase-4 (DPP-4), work that subsequently informed the development of DPP-4 inhibitors and longer-acting GLP-1 receptor agonist analogues as a distinct area of pharmaceutical research. Because this literature is extensive, readers wanting further primary detail on specific downstream effects should consult a review of GLP-1 and incretin biology rather than a single summarised study here.
Research applications
In the laboratory, GLP-1 is used mainly as a reference ligand for studying GLP-1 receptor signalling and glucose-dependent insulin secretion. Isolated pancreatic islet and beta cell line models, using techniques including patch-clamp electrophysiology, calcium imaging and insulin secretion assays, remain a standard approach for dissecting the cAMP, protein kinase A and Epac2-dependent signalling pathways established in the foundational mechanistic work described above.
Receptor pharmacology studies, using cell lines engineered to express the cloned human or rodent GLP-1 receptor, are used to characterise binding affinity and signalling efficacy for native GLP-1 and for synthetic GLP-1 receptor agonists and antagonists developed for comparison. Because native GLP-1 is rapidly degraded in plasma by DPP-4, it is also used as a reference compound in research examining peptide stability and in studies of DPP-4 inhibition, drawing on the enzymatic degradation pathway established in the broader incretin literature.
Rodent models are used to study GLP-1’s effects on glucose tolerance, insulin secretion, gastric emptying and beta cell mass in vivo, and GLP-1 receptor expression and signalling in the central nervous system is a further area of research interest, given the receptor’s documented distribution in brain regions involved in appetite-related and autonomic signalling circuits, separate from its pancreatic actions.
Purity, storage and handling
Because degradation accelerates once reconstituted, batch-level HPLC verification matters more here than with more stable peptides. UK laboratories sourcing GLP 1 should expect a batch-specific certificate of analysis and lyophilised storage at -20°C, and those comparing suppliers should look for the same documentation from any best peptides supplier UK they consider.
Reversed-phase HPLC separates the intact 30-residue peptide from truncated or deletion sequences and residual synthesis by-products, and research-grade material is commonly specified at 98% purity or above. Mass spectrometry alongside HPLC confirms identity against the expected mass of approximately 3,298 daltons for GLP-1(7-36)amide. Because native GLP-1 is intrinsically short-lived, being a substrate for DPP-4 in biological systems, researchers should distinguish this inherent enzymatic instability in cell-based or in vivo experiments from the separate question of chemical stability of the lyophilised and reconstituted peptide itself, which a certificate of analysis addresses. A batch-specific certificate of analysis, rather than a generic product datasheet, is what links a physical vial to those results and allows unexpected experimental data to be traced back to a defined lot.
Lyophilised GLP-1 should be kept sealed at -20°C for long-term storage, protected from moisture and light, and brought to room temperature before opening so that condensation does not enter the vial. Once reconstituted in sterile water or an appropriate buffer, the peptide should be divided into single-use aliquots, held at 2–8°C only for short working periods, and otherwise kept frozen, since repeated freeze–thaw cycles risk degrading the peptide. The tyrosine and tryptophan residues within the sequence are susceptible to photo-oxidation, so amber vials or foil-wrapped storage are sensible precautions during handling. Published reconstituted-stability data specific to synthetic GLP-1 are limited in the open literature, so laboratories should validate their own working window empirically, for example by re-running HPLC on aliquots at defined time points.
Frequently asked questions
What is GLP 1 used for in research?
It is used mainly as a reference ligand for studying GLP-1 receptor signalling, glucose-dependent insulin secretion, and the broader incretin pathway, in isolated pancreatic islet and cell line models as well as rodent studies of glucose tolerance and gastric emptying. It is not evaluated here as a treatment, and this article makes no claims about use in humans.
What is the difference between GLP-1(7-36)amide and GLP-1(7-37)?
Both are naturally occurring circulating forms of the hormone, differing only at the C-terminus, where GLP-1(7-36)amide carries a terminal amide group and GLP-1(7-37) carries a terminal glycine. Published pharmacology has reported that the two forms have essentially equivalent biological potency at the GLP-1 receptor.
Why is GLP-1’s action described as glucose-dependent?
Published electrophysiological research has shown that GLP-1 receptor activation alone produces little insulin release, but instead renders beta cells responsive to glucose, a phenomenon termed glucose competence. This means GLP-1’s insulinotropic effect depends on, and is amplified by, ambient glucose concentration rather than acting as an independent insulin secretagogue.
How should GLP 1 be stored once reconstituted?
Reconstituted GLP-1 should be divided into single-use aliquots, kept refrigerated only briefly and otherwise frozen, protected from light, and not subjected to repeated freeze–thaw cycles. Because specific published stability data are limited, laboratories should confirm their own storage window empirically, for example with HPLC comparisons of fresh and aged aliquots.
The GLP-1 discussed in this article is supplied for in-vitro and in-vivo laboratory research only. It is not intended for human or veterinary use, and nothing in this article should be read as guidance on administration to people or animals.
