glucagon

glucagon, hormone secreted by the α cells of the islets of Langerhans, specific groups of cells in the pancreas. It tends to counteract the action of insulin, i.e., it raises the concentration of glucose in the blood. Glucagon was first purified and crystallized in 1955; the amino acid sequence of this 29-amino acid polypeptide (see peptide) was published in 1956-57. One of the most important actions of glucagon is the promotion of glycogenolysis, i.e., the degradation of glycogen to glucose, in the liver. Glucagon stimulates adenyl cyclase, the enzyme that catalyzes the conversion of adenosine triphosphate to 3'5'-cyclic adenosine monophosphate (cyclic AMP).

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 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

References in pop culture

• American parodist "Weird Al" Yankovic released a song entitled Pancreas on his album Straight Outta Lynwood, which uses the lyric "Insulin, Glucagon, flowing from the Islets of Langerhans" as a repeated catchy chorus in the final minute of the song.

See also

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

References

Further reading