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  1. Structure
    It is a peptide having 15 amino acids. It is so named because it binds to guanylyl cyclase. Certain strains of E. coli which produce diarrohoea secrete an enterotoxin which has a structure close to guanlyin; this toxin acts on the intestinal guanylin receptor (see below) to produce diarrhoea.
  1. Site
    It is secreted by intestinal cells, from the pylorus to the rectum. 
  2. Mechanism of action
    Guanylin acts on guanylin receptors. It stimulates guanylyl cyclase this increases the concentration of intracellular cGMP this increases the activity of the cystic fibrosis-regulated chloride channel which causes increased secretion of chloride into the intestinal lumen
  3. Actions
    1. it increases chloride secretion into the intestinal lumen and thus may regulate fluid movement. It acts in the GIT in a paracrine fashion. 
    2. Guanylin receptors are also found in the kidneys, the liver, and the female reproductive tract; here, it may be act in an endocrine fashion to regulated fluid movement in these tissues. 

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