Peptide hormones: Pancreatic secretions – 1

The pancreas was first identified for western civilization by Herophilus (335–280 BC), a Greek anatomist and surgeon. Only a few hundred years later, Rufus of Ephesus, another Greek anatomist, gave the pancreas its name. The pancreas secrets three different peptide hormones. They are Insulin, Glucagon and Stomatostatin.

Peptide hormones: Pancreatic secretions - 1

Anatomy of Pancreas:

  • The pancreas (from pankreas: panG=all, kreasG=flesh); effering to the fleshy nature of the tissue) is both exocrine & endocrine gland.
  • It locates the below the stomach between the curve of duodenum & spleen.
  • It is a compact & lobulated organ.
  • The pancreas composed of two types of cell.
  • The pancreas secretes peptide hormones.
  • Pancreatic hormones are peptide hormones

 

1. Glandular cells (or) Acinar (or) Acinii (exocrine):

The cells make up the bulk of the pancreatic tissues & secrete digestive juices into the duodenum by the pancreatic duct.

2. Polygonal cells (or) islets of langerhans (or) islet tissue (endocrine):

Its secretion releases into the blood directly. These were discovered by “Langerhans” in 1867. The islets of Langerhans in mammals contain three major types of cells, they are a-cells, b-cells, g-cells.

  • alpha-cells –> Secretes – “Glucagon” hormones
  • beta-cells   –> Secretes – “Insulin” hormones
  • gamma-cells –> Secretes-“Somatostatin” hormone

The pancreas secretes three different hormones, namely,

  1. Insulin     
  2. Glucagon    
  3. Somatostatin

Peptide Hormone – Glucagon:

Chemistry of Glucagon:

  • First isolated in crystalline form by “Benhrens”& other.
  • This peptide hormone has a molecular weight of 3,485 and PI-8 and it has 29 amino acid residues.
  • “His” is the N-terminal amino acid and “Thr”, the C-terminal amino acid.
  • It contains no Cys, Pro, Ile, but Meth & Try are present in appreciable amounts.
What are peptide hormones? Pancreatic Hormones and its Structures

Mechanism of Action:

The peptide hormone Glucagon combines with a membrane bound receptor. The peptide hormone Glucagon activates the GDP-bound G-protein, by converting it into GTP form. The alpha sub unit of G-protein now dissociates from b and g sub units and a-sub unit binds to GTP. The GTP-G protein, in turn, activates adenyl cyclase. Thus, ATP is converted to cAMP. This combines with the regulatory sub unit of the protein kinase. So the catalytic sub unit is free to act. The active protein kinase will phosphorylate enzyme, so that glycogen phosphorylase is activated.

Secretion of Glucagon:

Low blood glucose increases secretion of glucagon. Most amino acids especially: “Arg” stimulates the secretion of glucagon. In stress insulin secretion is inhibited but glucagon secretion is stimulated.

Functions of Peptide hormone – Glucagon:

  • Glucagon stimulates glycogenolysis.
  • Glucagon acts through cAMP as its second messenger.
  • Glucagon stimulates gluconeogenesis by activation of pyruvate carboxylase.
  • Glucagon inhibits glucose oxidation by inhibiting pyruvate kinase.
  • It increases potassium release from liver.
  • In adipose tissue as well as in liver it increases the breakdown of lipids to fatty acids and glycerol.

Hyperglycemic agent:

Over doses of insulin, given to diabetic patients, often result in acute hypoglycemia. This may be cured by giving crystalline glucagon (in the form of glucagon hydrochloride) either intra-muscularly (or) intravenously. Response may be observed within 10 to 15 minutes after administration of 0.5 to 1.0 mg of glucagon.

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