Structure, Sythesis & Transport
  • A peptide hormone made of 51 amino acids
    • Formed of 2 chains (α and β) connected by disulphide bridges
  • Synthesised as proinsulin with chains joined by C-peptide
    • C-peptide excised by endopeptidases in the endoplasmic reticulum of β cells prior to release
    • C-peptide is 31 amino acid sequence has no known biological function
    • Can be used as marker of endogenous insulin secretion
  • Stored in secretory granules and is secreted by the cells via exocytosis
  • It enters the portal system with half cleared by first pass through the liver
  • Circulates entirely in an unbound form
  • Key to glucose homeostasis - 'hypoglycaemic anabolic hormone':
  • Liver:
    • Promotes glycolysis and storage of glucose as glycogen (glycogenesis)
    • Promotes conversion of glucose to triglycerides
  • Muscle
    • Promotes the uptake of glucose and its storage as glycogen
  • Adipose tissue:
    • Promotes uptake of glucose and its conversion to triglycerides for storage.
Mechanism of Action
  • Binds to a specific receptor tyrosine kinase on the plasma membrane
  • Increases its activity to of numerous numerous regulatory enzymes and promotes glucose transport in to cells by increasing membrane permeability

Stimulated by:

  • Raised plasma glucose
  • Raised plasma fatty acid & ketone bodies
  • Vagal nerve stimulation
  • Amino acids
  • Gut hormones (gastrin, secretin, CCK, and GIP)

Inhibited by:

  • Sympathetic stimulation
  • Somatostatin