Glucagon

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Glucagon is an important hormone involved in carbohydrate metabolism. Produced by the pancreas, it is released when the glucose level in the blood is low (hypoglycemia), causing the liver to convert stored glycogen into glucose and release it into the bloodstream. The action of glucagon is thus opposite to that of insulin, which instructs the body's cells to take in glucose from the blood in times of satiation.

History

In the 1920s, Kimball and Murlin studied pancreatic extracts and found an additional substance with hyperglycemic properties. They described glucagon in 1923. The amino acid sequence of glucagon was described in the late-1950s. A more complete understanding of its role in physiology and disease was not established until the 1970s, when a specific radioimmunoassay was developed.

Structure

Glucagon is a 29-amino acid polypeptide. Its primary structure in humans is: NH₂-His-Ser-Gln-Gly-Thr-Phe-Thr-Ser-Asp-Tyr-Ser-Lys-Tyr-Leu-Asp-Ser- Arg-Arg-Ala-Gln-Asp-Phe-Val-Gln-Trp-Leu- Met-Asn-Thr-COOH.

The polypeptide has a molecular weight of 3485 daltons.

Physiology

Production

The hormone is synthesized and secreted from alpha cells (α-cells) of the islets of Langerhans, which are located in the endocrine portion of the pancreas. In rodents, the alpha cells are located in the outer rim of the islet. Human islet structure is much less segregated, and alpha cells are distributed throughout the islet.

Regulatory mechanism

Increased secretion of glucagon is caused by:

• Decreased plasma glucose
• Increased catecholamines - norepinephrine and epinephrine
• Increased plasma amino acids (to protect from hypoglycemia if an all protein meal is consumed)
• Sympathetic nervous system
• Acetylcholine
• Cholecystokinin

Decreased secretion of glucagon (inhibition) is caused by:

• Somatostatin
• Insulin

Function

Glucagon ball and stick model, with the carboxyl terminus above and the amino terminus below

File:Glucagon rednblue.png

A microscopic image stained for glucagon.

Glucagon helps maintain the level of glucose in the blood by binding to glucagon receptors on hepatocytes, causing the liver to release glucose - stored in the form of glycogen - through a process known as glycogenolysis. As these stores become depleted, glucagon then encourages the liver to synthesize additional glucose by gluconeogenesis. This glucose is released into the bloodstream. Both of these mechanisms lead to glucose release by the liver, preventing the development of hypoglycemia. Glucagon also regulates the rate of glucose production through lipolysis.

Glucagon production appears to be dependent on the central nervous system through pathways which are yet to be defined. It has been reported that in invertebrate animals eyestalk removal can affect glucagon production. Excising the eyestalk in young crayfish produces glucagon-induced hyperglycemia.

• Increased free fatty acids and ketoacids into the blood
• Increased urea production

Mechanism of action

Glucagon binds to the glucagon receptor, a G protein-coupled receptor located in the plasma membrane. The conformation change in the receptor activates G proteins, a heterotrimeric protein with α, β, and γ subunits. The subunits breakup as a result of substitution of a GDP molecule with a GTP mol, and the alpha subunit specifically activates the next enzyme in the cascade, adenylate cyclase.

Adenylate cyclase manufactures cAMP (cyclical AMP) which activates protein kinase A (cAMP-dependent protein kinase). This enzyme in turn activates phosphorylase kinase, which in turn, phosphorylates glycogen phosphorylase, converting into the active form called phosphorylase A. Phosphorylase A is the enzyme responsible for the release of glucose-1-phosphate from glycogen polymers.

Pathology

Abnormally-elevated levels of glucagon may be caused by pancreatic tumors such as glucagonoma, symptoms of which include necrolytic migratory erythema (NME), reduced amino acids and hyperglycemia. It may occur alone or in the context of multiple endocrine neoplasia type 1.

Uses

An injectable form of glucagon is vital first aid in cases of severe hypoglycemia when the victim is unconscious or for other reasons cannot take glucose orally. The dose for an adult is typically 1 milligram, and the glucagon is given by intramuscular, intravenous or subcutaneous injection, and quickly raises blood glucose levels. Glucagon can also be administered intravenously at 0.25 - 0.5 unit.

Anecdotal evidence suggests a benefit of higher doses of glucagon in the treatment of overdose with beta blockers; the likely mechanism of action is the increase of cAMP in the myocardium, effectively bypassing the inhibitory action of the β-adrenergic second messenger system.

Glucagon acts very quickly: common side effects include headache and nausea.

Drug interactions: Glucagon interacts only with oral anticoagulants increasing the tendency to bleed.

Media

File:Glucagon stereo animation.gif

Rotating stereogram animation of glucagon. (1.70 MB, animated GIF format).

See also

• Insulin
• Diabetes mellitus
• Proglucagon
• Glucagon-like peptide-1
• Glucagon-like peptide-2
• Islets of Langerhans
• Pancreas

Further reading

• Kieffer TJ, Habener JF (2000). The glucagon-like peptides. Endocr. Rev. 20 (6): 876–913.
• Drucker DJ (2003). Glucagon-like peptides: regulators of cell proliferation, differentiation, and apoptosis. Mol. Endocrinol. 17 (2): 161–71.
• Jeppesen PB (2004). Clinical significance of GLP-2 in short-bowel syndrome. J. Nutr. 133 (11): 3721–4.
• Brubaker PL, Anini Y (2004). Direct and indirect mechanisms regulating secretion of glucagon-like peptide-1 and glucagon-like peptide-2. Can. J. Physiol. Pharmacol. 81 (11): 1005–12.
• Baggio LL, Drucker DJ (2005). Clinical endocrinology and metabolism. Glucagon-like peptide-1 and glucagon-like peptide-2. Best Pract. Res. Clin. Endocrinol. Metab. 18 (4): 531–54.
• Holz GG, Chepurny OG (2006). Diabetes outfoxed by GLP-1?. Sci. STKE 2005 (268): pe2.
• Dunning BE, Foley JE, Ahrén B (2006). Alpha cell function in health and disease: influence of glucagon-like peptide-1. Diabetologia 48 (9): 1700–13.
• Gautier JF, Fetita S, Sobngwi E, Salaün-Martin C (2005). Biological actions of the incretins GIP and GLP-1 and therapeutic perspectives in patients with type 2 diabetes. Diabetes Metab. 31 (3 Pt 1): 233–42.
• De León DD, Crutchlow MF, Ham JY, Stoffers DA (2006). Role of glucagon-like peptide-1 in the pathogenesis and treatment of diabetes mellitus. Int. J. Biochem. Cell Biol. 38 (5-6): 845–59.
• Beglinger C, Degen L (2007). Gastrointestinal satiety signals in humans--physiologic roles for GLP-1 and PYY?. Physiol. Behav. 89 (4): 460–4.
• Stephens JW, Bain SC (2007). Safety and adverse effects associated with GLP-1 analogues. Expert opinion on drug safety 6 (4): 417–22.
• Orskov C, Bersani M, Johnsen AH, et al. (1989). Complete sequences of glucagon-like peptide-1 from human and pig small intestine. J. Biol. Chem. 264 (22): 12826–9.
• Drucker DJ, Asa S (1988). Glucagon gene expression in vertebrate brain. J. Biol. Chem. 263 (27): 13475–8.
• Novak U, Wilks A, Buell G, McEwen S (1987). Identical mRNA for preproglucagon in pancreas and gut. Eur. J. Biochem. 164 (3): 553–8.
• White JW, Saunders GF (1986). Structure of the human glucagon gene. Nucleic Acids Res. 14 (12): 4719–30.
• Schroeder WT, Lopez LC, Harper ME, Saunders GF (1984). Localization of the human glucagon gene (GCG) to chromosome segment 2q36----37. Cytogenet. Cell Genet. 38 (1): 76–9.
• Bell GI, Sanchez-Pescador R, Laybourn PJ, Najarian RC (1983). Exon duplication and divergence in the human preproglucagon gene. Nature 304 (5924): 368–71.
• Kärgel HJ, Dettmer R, Etzold G, et al. (1982). Action of rat liver cathepsin L on glucagon. Acta Biol. Med. Ger. 40 (9): 1139–43.
• Wayman GA, Impey S, Wu Z, et al. (1994). Synergistic activation of the type I adenylyl cyclase by Ca2+ and Gs-coupled receptors in vivo. J. Biol. Chem. 269 (41): 25400–5.
• Unson CG, Macdonald D, Merrifield RB (1993). The role of histidine-1 in glucagon action. Arch. Biochem. Biophys. 300 (2): 747–50.

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Target-derived NGF, BDNF, NT-3

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