Cholera toxin crosses the membrane of an intestinal cell and permanently activates a G protein.
The cyclic AMP activates a chloride channel and large amounts of Cl, bicarbonate, Na and K are secreted into the intestinal lumen. This raises the lumen osmotic pressure. There is now a large osmotic pressure gradient between the cells and the lumen, and this causes large amounts of water to flow into the lumen. This produces diarrhea and dehydrates the body. Both substances must be present in the lumen at the same time.