Glucagon Hormone

Glucagon Hormone is the hyperglycemic factor secreted by the alpha cells of the pancreas and L cells of GI tract. It has 29 amino acids in straight chain and synthesized as pre-proglucagon.

There is a considerable homology in the amino acid composition of glucagon of different species. Glucagon is also produced from other sites–from stomach gut glucagons (identical to pancreatic glucagons), and from small intestine glycentin (immunologically similar to pancreatic glucagons) are produced. This catabolic peptide hormone raise plasma sugar levels. Its secretion from the alpha cells of the pancreatic islets is directly stimulated by-

1. low plasma glucose concentrations: glucagon levels rise in the postabsorptive state some hours after a meal.
2. high levels of circulating amino acids. This may be important in the maintenance of normal plasma glucose levels during absorption of a protein meal, since amino acids also stimulate insulin secretion and this might cause hypoglycemia in the absence of any opposing action on carbohydrate metabolism.

Regulation of Secretion of Glucagon Hormone

Low blood glucose level stimulates glucagon synthesis and release. Glucogenic amino acids like serine, glycine, alanine are powerful activators of glucagon output.

Both insulin and glucagon work in-tantum to regulate blood glucose. Both sympathetic and parasympathetic systems of ANS, hypo­glycemia, gastrin, CCK, gastric inhibitory polypeptide, glucogenic amino acids, protein ingestion and exercise or stress stimulate glucagon release while increased levels of glucose and fatty acids and secretin are inhibitory to glucagon release.

Soma­tostatin is more inhibitory to glucagon than to insulin.

Physiological Effects of Glucagon

Glucagon has glycogenolytic, gluconeogenic, lipolytic and ketogenic effects.

Glucagon blocks the synthesis of glycogen by inhibiting the enzyme glycogen synthase. By activating phosphorylase, glucagon stimulates glycogenolysis and gluconeogenesis. It stimulates adenylcyclase activity in the liver cells and formation of cyclic AMP to activate protein kinase, a regulator protein. Protein kinase activates phosphorylase-b kinase that converts phosphorylase-b to phosphorylase-a and promotes the degradation of glycogen into glucose-1‑PO₄ which on dephosphorylation yields glucose in liver cells. It also promotes lipolysis.

Gluconeogenesis is enhanced. It decreases the activities of glycolytic enzymes. The ratio between phosphofructokinase and fructose 1-6-diphosphatase determines the relative rate of glycolysis and gluconeogenesis.

It also promotes lipolysis by stimulating hormone-sensitive lipase activity in the adipose tissue.

Glucagon secretion begins with ingestion of food and in­creases with the interval from food ingestion and declining blood glucose level. This helps the animal to mobilize energy stores for maintaining glucose homeostasis i.e., prevent postprandial hypogly­cemia.

In birds, glucagon has a predominant role than insulin on carbohydrate metabolism.

Glucagon is a ketogenic hormone, activates the hormone-sensi­tive adipose tissue lipase in adipose tissues to cause lipolysis. It inhibits Na⁺ resorption from the renal tubules causing natriuresis.