Overview
Glucagon is the 29-amino-acid counter-regulatory hormone secreted by pancreatic islet α-cells in response to hypoglycaemia, fasting, exercise, amino acids, and sympathoadrenal activation. It is the primary driver of hepatic glucose mobilisation and ketogenesis, and thus essential for survival during fasting — a fact demonstrated by the lethal hypoglycaemia that follows pancreatectomy without exogenous glucagon replacement.
Glucagon is derived from proglucagon (180 aa), whose tissue-specific post-translational processing differs: in pancreatic α-cells, prohormone convertase 2 (PC2) yields glucagon + glicentin-related polypeptide (GRPP) + major proglucagon fragment (MPGF). In intestinal L-cells and brain neurons, PC1/3 yields GLP-1, GLP-2, and oxyntomodulin instead. This tissue-specific processing explains why GLP-1 receptor agonists (a therapeutic class targeting incretin signalling) have no direct hepatic glycogenolytic effects.
Hyperglucagonaemia is now recognised as co-driving hyperglycaemia in type 2 diabetes — glucagon is not normally suppressed post-prandially in T2D, resulting in inappropriate hepatic glucose output. GCGR antagonists, GLP-1 agonists (which suppress glucagon secretion), and dual/triple agonists (tirzepatide, retatrutide) target this axis.
Structure & Processing
| Proglucagon-derived peptide | Cell type | Convertase | Function |
|---|---|---|---|
| Glucagon (1–29) | Pancreatic α-cell | PC2 | Counter-regulation; hepatic glucose output; ketogenesis |
| GLP-1 (7–37 / 7–36-NH₂) | Intestinal L-cell, brainstem | PC1/3 | Incretin (↑glucose-dependent insulin secretion); satiety; gastric emptying delay |
| GLP-2 | Intestinal L-cell | PC1/3 | Intestinal epithelial proliferation; mucosal integrity; teduglutide analogue for short bowel syndrome |
| Oxyntomodulin | Intestinal L-cell | PC1/3 | Weak GCGR + GLP-1R agonist; satiety signal |
| Glicentin | Intestinal L-cell | PC1/3 | Intestinal mucosal function; contains GRPP + glucagon sequence |
Glucagon's N-terminal His1 is critical for GCGR activation — des-His1-glucagon is a competitive antagonist. Glucagon forms α-helical structure upon receptor binding. The GCGR is a class B GPCR with a large N-terminal extracellular domain (ECD) that provides high-affinity docking for the C-terminal portion of glucagon, while the N-terminal His1 of glucagon penetrates the transmembrane bundle to activate the receptor.
Mechanism — GCGR/cAMP/PKA Cascade
GCGR activates Gαs → adenylyl cyclase → cAMP → PKA. Hepatic PKA phosphorylates glycogen phosphorylase kinase (PhK), phosphorylase, and phosphofructokinase-2 (PFK-2), simultaneously accelerating glycogenolysis and gluconeogenesis while inhibiting glycogen synthesis and glycolysis. PKA also phosphorylates CREB → PEPCK and G6Pase gene transcription. In adipocytes, PKA-mediated phosphorylation of hormone-sensitive lipase (HSL) and perilipin-1 triggers lipolysis → FFA + glycerol released → hepatic ketogenesis.
┌──────────────────────────────────────────────────────────────────────┐ │ GLUCAGON SIGNALLING CASCADE (hepatocyte) │ │ │ │ ↓Glucose → α-cell KATP channels close (paradox: low glucose │ │ → depolarisation → Ca²⁺ → glucagon exocytosis) │ │ │ │ Glucagon (portal blood) → GCGR (class B GPCR, hepatocyte) │ │ │ │ │ Gαs activation → Adenylyl cyclase → cAMP ↑↑ │ │ │ │ │ PKA (Protein kinase A) activated │ │ │ │ │ ┌────┴──────────────────────────────────────────────┐ │ │ │ │ │ │ │ │ Glycogenolysis Gluconeogenesis Glycolysis↓ Lipid │ │ ────────────── ─────────────── ────────── ──────── │ │ PhK→P → GlyP-b→a PFK-2 Ser32→P PFK-2 Ser32→P HSL-P │ │ (phosphorylase (bisphosphatase (phosphatase (lipolysis) │ │ active form) active → active → │ │ ↓F-2,6-BP) ↓F-2,6-BP │ │ Glycogen→G1P PEPCK + G6Pase ↓glycolysis FFA release │ │ →G6P→glucose gene expression flux → ketogenesis │ │ (via G6Pase) via CREB (TORC2 │ │ nuclear entry) │ │ │ │ Net hepatic output: │ │ GLUCOSE ↑↑ (glycogenolysis + gluconeogenesis) │ │ KETONE BODIES ↑ (FFA → β-oxidation → acetyl-CoA → HMG-CoA → │ │ acetoacetate → β-OHB; insulin-suppressed) │ │ │ ├──────────────────────────────────────────────────────────────────────┤ │ GLUCAGON-INSULIN COUNTERBALANCE │ │ │ │ HIGH GLUCOSE state: │ │ → ↑Insulin (β-cell) → GCGR expression ↓, Gαi coupling ↑ │ │ → ↑Insulin → PDE3B activated → cAMP degraded → PKA ↓ │ │ → α-cell paracrine: insulin via δ-cell somatostatin → ↓glucagon │ │ │ │ T2D state (hyperglucagonaemia): │ │ → Inappropriate α-cell glucose sensing; zinc-glucagon │ │ co-secretion blunted; α-cell GLP-1R signalling impaired │ │ → Excess hepatic glucose output → fasting hyperglycaemia │ └──────────────────────────────────────────────────────────────────────┘
- Low blood glucose (or amino acids, sympathetic activation) → α-cell KATP closes → depolarisation → voltage-gated Ca²⁺ entry → glucagon exocytosis into portal blood.
- Glucagon → GCGR → Gαs → adenylyl cyclase III/IV → cAMP ↑ → PKA holoenzyme dissociates: regulatory (R) subunits bind cAMP, catalytic (C) subunits phosphorylate targets.
- Glycogenolysis: PKA → phosphorylase kinase → glycogen phosphorylase b → a → glycogen → G1P → G6P → glucose (via G6Pase, liver-specific).
- Gluconeogenesis: PKA → CREB (Ser133 phosphorylation) → TORC2 nuclear translocation → PEPCK1 and G6Pase gene transcription; FOXO1 also activated → amplifies gluconeogenic programme.
- Lipid mobilisation: glucagon → adipocyte GCGR (lower expression but functional) → PKA → HSL-Ser563/Ser659/Ser660 phosphorylation + perilipin-1 phosphorylation → lipase accessibility to lipid droplet → FFA + glycerol release.
- Ketogenesis: hepatic FFA → β-oxidation → acetyl-CoA; glucagon simultaneously depresses malonyl-CoA (via ↓ACC2) → CPT1 disinhibited → mitochondrial FFA import → HMG-CoA → acetoacetate + β-OHB.
Physiological Roles
| Context | Role |
|---|---|
| Fasting (overnight) | Maintains glucose ≥3.5 mmol/L; glycogenolysis for first 8 h, then gluconeogenesis (alanine, lactate, glycerol) via PEPCK/G6Pase |
| Exercise | Sympathetic activation + ↓portal insulin/glucose → glucagon pulse; hepatic glucose output matches muscle uptake; prevents exercise-induced hypoglycaemia |
| Protein meal | Amino acids (Arg, Lys) depolarise α-cells → glucagon; counteracts insulin secretion to prevent hypoglycaemia after high-protein low-carb meal |
| Hypoglycaemia | First-line defence below 3.5 mmol/L; emergency kit glucagon (1 mg IM/SC) raises glucose ~3–4 mmol/L within 15 min; impaired in long-standing T1D (α-cell glucagon response defective) |
| Starvation / ketosis | Sustained low insulin/elevated glucagon → ketogenesis; ketone bodies spare glucose for brain; glucagon:insulin ratio drives metabolic state |
Pharmacology & Clinical Use
Glucagon emergency kits — traditional IM powder reconstitution (Glucagen, Novo Nordisk); now replaced by nasal powder (Baqsimi: 3 mg intranasal; GCGR-mediated hepatic glucose output within 10 min) and intradermal autoinjector (Gvoke, dasiglucagon). Standard of care for severe hypoglycaemia in T1D when IV access unavailable.
GCGR antagonists — LGD-6972, MK-0893: block glucagon action → reduce fasting glucose in T2D, but cause compensatory hyperglucagonaemia → α-cell hyperplasia; hepatic side effects (hepatic lipid accumulation). Development largely discontinued.
GLP-1/GCGR dual agonists — cotadutide, survodutide: balance GLP-1 (satiety, insulin secretion) with GCGR (energy expenditure, lipolysis); under investigation for NASH and obesity. Retatrutide (GLP-1/GIP/GCGR triple agonist): Phase 3 trials for obesity — up to 24% body weight reduction, partly via GCGR-mediated ↑energy expenditure and hepatic fat reduction.
Glucagon in clinical imaging — IV glucagon (0.5–2 mg) reduces gastric and bowel motility during upper GI endoscopy, CT colonography, and MRCP — smooth muscle relaxation via GCGR/cAMP/PKA → smooth muscle hyperpolarisation.
Pathology
| Condition | Glucagon Status | Key Features |
|---|---|---|
| T2D hyperglucagonaemia | Elevated (fasting + post-prandial) | Failure of post-prandial glucagon suppression; hepatic glucose output continues after meals; 20–30% of post-prandial glucose excursion attributed to excess glucagon |
| Glucagonoma | Very high (tumour secretion) | 4Ds: Dermatitis (necrolytic migratory erythema), Diabetes, DVT, Depression; also weight loss, glossitis; somatostatin analogue (octreotide) suppresses tumour glucagon; surgical resection |
| T1D: impaired counter-regulation | Deficient response to hypoglycaemia | Loss of intra-islet paracrine signals; α-cells no longer sense low glucose → glucagon response impaired after 5+ years of T1D → severe hypoglycaemia risk |
| Multiple endocrine neoplasia type 1 (MEN1) | Variable | MEN1 (menin) mutation → pancreatic neuroendocrine tumours including glucagonoma; glucagon part of MEN1 tumour spectrum |
| Post-bariatric hypoglycaemia | Inappropriate post-prandial suppression | Roux-en-Y bypass → exaggerated GLP-1 → suppressed glucagon → post-prandial hypoglycaemia (distinct from dumping) |
Connections
Glucagon is the direct counterpart to insulin — their molar ratio in portal blood determines hepatic metabolic state (glycogen synthesis vs breakdown). cAMP/PKA directly phosphorylates and inactivates AMPK (Ser485 on α-subunit) — thus glucagon opposes AMPK-driven anabolic/fasting adaptations. Glucagon stimulates hepatic lipolysis complementing adipose lipolysis via HSL. GLP-1 (same proglucagon gene) suppresses glucagon secretion from α-cells via paracrine D-cell somatostatin.
References
- Unger RH, Cherrington AD (2012). Glucagonocentric restructuring of diabetes: a pathophysiologic and therapeutic makeover. J Clin Invest 122:4–12.
- Bæk MH et al. (2021). Structure of glucagon receptor in complex with a glucagon analogue. Nature 546:256–260.
- Müller TD et al. (2023). Glucagon-like peptide 1 (GLP-1). Mol Metab 30:72–130.
- Wewer Albrechtsen NJ et al. (2019). The liver–alpha cell axis and type 2 diabetes. Endocr Rev 40:1353–1366.
- Jørgensen NB et al. (2012). Exaggerated glucagon-like peptide 1 response is important for improved β-cell function and glucose tolerance after Roux-en-Y gastric bypass. Diabetes 62:3044–3052.
- Hædersdal S et al. (2023). Retatrutide, a GIP, GLP-1 and glucagon receptor agonist, for people with type 2 diabetes: a randomised, double-blind, placebo and active-controlled, parallel-group, phase 2 trial. Lancet 402:2271–2284.