Gram-negative bacterium Vibrio cholera produces a protein (cholera toxin) that is responsible for cholera symptoms: extensive loss of water and Na through continuous diarrhea, which can be fatal. The toxin tightly binds specific sites on the plasma membrane of intestinal cells, where it prolongs the activation of adenylate cyclase (AC).

A. What effect does the cholera toxin have on cAMP levels in intestinal cells?

B. Based on the provided information: suggest a role for cAMP in intestinal cells under normal conditions.

A) It increases the cAMP levels in intestinal cells.

B) cAMP regulates absorption of NaCl and flux of chloride ions.

Explanation:

A) Choleragen, the toxin of cholera pathogen is an AB toxin. The A sub-unit of the toxin enters the intestinal epithelial cells and activates the enzyme adenylate cyclase which in turn leads to increased levels of cAMP in the small intestine.

B) The cAMP serves as one of the intracellular second messengers and regulates the absorption of NaCl as well as electrogenic secretion of chloride ions. Under normal conditions, the lower cAMP levels stimulate absorption of NaCl and inhibit secretion of chloride ions in small intestine.

Cholera toxin increases the cAMP concentration in small intestine and thereby inhibits absorption of neutral NaCl and facilitates the efflux of chloride ions. As a result, lots of fluids and electrolytes are lost from the body in form of diarrhea.