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I am trying find explanation for these mechanisms

Tox plasmid -> exotoxin (enterotoxin) -> activation of adenylcyclase -> \up cAMP enterocytes -> release of H2O, electrolytes in gut lumen -> diarrhea.

Tox plasmid is a toxin generated by the genus enterobacteriaceae. I read this as that exotoxin Tox activates adenilciclase enzyme (protein) which increases the amount of cyclic AMP in the enterocytes, and releases water and electrocytes into gut lumen consequently resulting in diarrhea.

How does the activation of adenylcyclase lead to increased cAMP heparis and diarrhea?

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@Chris Thank you! I fixed that. –  Masi Feb 9 at 16:58

1 Answer 1

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The crypt cells of the gut can actively secrete electrolytes (ions) via special channel which finally leads to water secretion. This is usually tightly controlled, since uncontrolled loss of water is obviously not good for the cells. Have a look at this schematic figure, (from this webpage):

enter image description here

  • Sodium, Potassium and Chloride ions are co-transported into the cell, sodium and potassium are transported back out via other channels.
  • If the adenyl cyclase is activated (for example by bacterial toxins, cholera does this), then cAMP is made.
  • cAMP binds to the Cystic fibrosis transmembrane conductance regulator (CFTR)(this channel is also important for cystic fibrosis) and leads to the secretion of chloride ions from the cell. This builds up a negative potential across the membrane (positively charged ions are missing).
  • This leads to the secretion of sodium ions through the tight junctions, resulting in the netto secretion of NaCl.
  • The secretion of salt builds up and osmotic gradient across the membrane, which leads to water secretion and can also lead to hyponatraemia.

To overcome this problems, the so called "oral rehydration therapy" is used. The solution contains of an isotonic solution of sodium chloride and glucose. This is taken up by the sodium-glucose symporter (a special class of transporter, that transports two molecules in the same direction) and builds up a gradient which is directed in the opposite direction of the diarrhoea, helping to fight dehydration.

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What is netto secretion of NaCl? –  Masi Feb 9 at 17:56
    
I think the water goes inside cells wit the transport of Cl- ions. Then, NaCl accumulates in the cells. The gradient across the membrane may increase if the water does not follow enough Cl-. This then leads to the water secretion into the cells. Right? Since there is too much water in the intracellular space, more water stays in the extracellular space which then is released as diarrhea stools. Right? –  Masi Feb 9 at 18:08
    
No: Cl- ions are secreted through the CFTR channel. Then sodium is "secreted" through the tight junctions to neutralize the negative charge, which is building up on the outsite. The results in the net secretion of NaCl (which is of course still ionic and not there as a salt. –  Chris Feb 9 at 19:03

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