Insulin, Glucagon and Somatostatin

874 Words2 Pages

Insulin, Glucagon and Somatostatin

The principal role of the pancreatic hormones is the regulation of

whole-body energy metabolism, principally by regulating the

concentration and activity of numerous enzymes involved in catabolism

and anabolism of the major cell energy supplies.

The earliest of these hormones recognized was insulin, whose major

function is to counter the concerted action of a number of

hyperglycemia-generating hormones and to maintain low blood glucose

levels. Because there are numerous hyperglycemic hormones, untreated

disorders associated with insulin generally lead to severe

hyperglycemia and shortened life span. Insulin is a member of a family

of structurally and functionally similar molecules that include the

insulin-like growth factors (IGF-1 and IGF-2), and relaxin. The

tertiary structure of all 4 molecules is similar, and all have

growth-promoting activities, but the dominant role of insulin is

metabolic while the dominant roles of the IGFs and relaxin are in the

regulation of cell growth and differentiation.

Insulin is synthesized as a preprohormone in the b cells of the islets

of Langerhans. Its signal peptide is removed in the cisternae of the

endoplasmic reticulum and it is packaged into secretory vesicles in

the Golgi, folded to its native structure, and locked in this

conformation by the formation of 2 disulfide bonds. Specific protease

activity cleaves the center third of the molecule, which dissociates

as C peptide, leaving the amino terminal B peptide disulfide bonded to

the carboxy terminal A peptide.

Insulin secretion from b cells is principally regulated by plasma

glucose levels, but...

... middle of paper ...

...s to

adenylate cyclase. The resultant increases in cAMP and PKA reverse all

of the effects described above that insulin has on liver. The

increases also lead to a marked elevation of circulating glucose, with

the glucose being derived from liver gluconeogenesis and liver

glycogenolysis.

Somatostatin, secreted by d cells of the pancreas, is a 14--amino acid

peptide, identical to somatostatin secreted by the hypothalamus. In

neural tissue somatostatin inhibits GH secretion and thus has systemic

effects. In the pancreas, somatostatin acts a paracrine inhibitor of

other pancreatic hormones and thus also has systemic effects. It has

been speculated that somatostatin secretion responds principally to

blood glucose levels, increasing as blood glucose levels rise and thus

leading to down-regulation of glucagon secretion.

More about Insulin, Glucagon and Somatostatin

Open Document