Final answer:
Both cholera toxin and pertussis toxin lead to an increase in intracellular cAMP levels; cholera toxin by continuously activating adenylate cyclase, and pertussis toxin by preventing the inhibition of adenylate cyclase. Option C.
Step-by-step explanation:
Cholera and pertussis toxins both interfere with the normal functioning of G proteins within cells but have opposite effects on the concentration of intracellular cyclic AMP (cAMP).
The cholera toxin binds to G protein-coupled receptors (GPCRs) in the intestinal epithelial cells and ADP-ribosylates the α subunit of the stimulatory G protein (Gs), leading to the activation of adenylate cyclase which continuously produces cAMP.
This results in increased intracellular cAMP levels that cause electrolyte and fluid secretion into the intestinal lumen, leading to diarrhea.
In contrast, the pertussis toxin ADP-ribosylates the α subunit of the inhibitory G protein (Gi), which prevents it from interacting with its receptor.
This action inhibits adenylate cyclase from decreasing cAMP levels in response to normal regulatory signals, also leading to elevated intracellular cAMP levels. Despite affecting different G proteins, both toxins ultimately increase cAMP within the cell.
Thus, the correct answer to the effect of these toxins on intracellular cAMP concentration is C. They both tend to increase cAMP concentration.