Cholera is the leading cause of death in children in developing countries, characterized by massive diarrhea. The effects stem from an extracellular toxin that the bacterium Vibrio cholerae releases into the gut. The toxin acts on a __________ that overstimulates __________ which causes a cascade of events that leads to electrolytes leaving the cell and causing water to follow them into the lumen of the intestine, resulting in severe diarrhea.