NBME Form 16 Practice Test

When the Gs alpha subunit cannot hydrolyze GTP, it stays in the active GTP-bound state and continuously stimulates adenylate cyclase. This enzyme then converts ATP to cyclic AMP, so the intracellular level of cAMP rises. The rise in cAMP is what propagates the signal by activating protein kinase A, which phosphorylates various targets and alters cellular activity. So the component that increases is the second messenger cAMP. The increased activity of adenylate cyclase is the mechanism that raises cAMP, while phospholipase C is tied to other G protein pathways (not Gs), and while PKA activity also goes up downstream, the primary measurable increase is cAMP.