Incretin Pharmacology: GLP-1, GIP, Glucagon and Amylin Receptors

Oral glucose produces a larger insulin response than intravenous glucose at matched blood concentrations. The difference — the incretin effect — is mediated by gut hormones released in response to nutrients, principally GLP-1 and GIP.

Both act at class B G-protein-coupled receptors. Their insulinotropic action is glucose-dependent, which is pharmacologically important: the signal diminishes as glucose falls.

Native GLP-1 has a half-life of minutes, cleaved rapidly by dipeptidyl peptidase-4 at the penultimate alanine. Any useful analogue must solve this.

Two strategies dominate. Modify the cleavage site — exendin-4, the Gila monster peptide, does this naturally, which is why it was developed. Or add a fatty-acid chain promoting reversible albumin binding, which both shields the peptide and slows renal clearance: liraglutide and semaglutide take this route, with semaglutide extending duration from a day to a week.

Tirzepatide engages GIP as well as GLP-1 receptors. Retatrutide adds glucagon receptor activity to both. The rationale is complementary mechanisms: glucagon receptor agonism is associated in the literature with increased energy expenditure, a different lever from the satiety and insulinotropic effects of the other two.

The balance of potency between receptors is a deliberate design parameter, not an incidental property, and it is what makes these distinct compounds rather than variations on one.

Amylin is co-secreted with insulin and signals through calcitonin-receptor-based complexes — a different receptor family entirely. This is why amylin analogues such as cagrilintide are studied in combination with GLP-1 agonists rather than as alternatives to them.

Native human amylin aggregates into amyloid fibrils, which is why pramlintide exists with its rat-derived proline substitutions, and why long-acting amylin analogues are a formulation challenge as much as a pharmacological one.

The insulinotropic action of GLP-1 and GIP is glucose-dependent: the receptors potentiate insulin secretion when glucose is elevated and the signal falls away as glucose normalises. This is a property of the physiology rather than of any particular analogue, and it distinguishes incretin pharmacology from agents that drive insulin release regardless of prevailing glucose.

It also explains a persistent confusion in secondary writing. Effects on gastric emptying and on central satiety signalling are not glucose-dependent in the same way, so the overall profile of an incretin analogue is a composite of mechanisms that behave differently. Attributing everything a compound does to a single receptor action is almost always an oversimplification.

A separate class of agents raises endogenous incretin concentrations by inhibiting DPP-4, the enzyme that degrades native GLP-1, rather than supplying a receptor agonist. These are small molecules, not peptides, and they are frequently discussed alongside peptide analogues as though they were interchangeable.

They are not. A DPP-4 inhibitor raises native hormone to physiological concentrations; a receptor agonist produces sustained exposure well above them. The magnitude of effect differs accordingly, and literature about one should not be cited for the other. This is also a reminder that not everything in incretin pharmacology is a peptide — the compound class matters.

This field contains compounds at very different evidence tiers, and the distinction is routinely blurred. Semaglutide, tirzepatide, liraglutide and exenatide are approved medicines in various jurisdictions. Retatrutide and cagrilintide are investigational: studied in registered human trials, approved nowhere.

Each encyclopedia entry states this explicitly, because it is the fact that most changes how the rest of the page should be read.