What effect does pertussis toxin have on Gi?

Pertussis toxin, produced by Bordetella pertussis, has a specific effect on the Gi protein that leads to its inactivation via ADP-ribosylation. This toxin modifies the Gi protein by attaching an ADP-ribose moiety to it, which inhibits its ability to inhibit adenylate cyclase. Normally, Gi plays a role in reducing cAMP levels in cells, and when pertussis toxin ADP-ribosylates Gi, it effectively prevents Gi from functioning properly.

As a result, the downstream effect of this inactivation is an increase in intracellular cAMP levels, which leads to various physiological responses including increased mucus production and inflammation in the respiratory tract. This mechanism is critical in understanding how pertussis toxin contributes to the pathophysiology of whooping cough, as the disruption of normal Gi signaling pathways is a key factor in the disease process.

The other options do not accurately describe the action of pertussis toxin on Gi.