Making the messenger
When a Gs protein is switched on, it travels to the membrane enzyme adenylyl cyclase and turns it on. Adenylyl cyclase takes ATP — the cell's energy currency — and snips it into cyclic AMP (cAMP), a small ring-shaped molecule that diffuses freely through the cytoplasm. cAMP is the classic second messenger: it carries the receptor's message deep into the cell.
The opposing Gi protein does the reverse: it dampens adenylyl cyclase, so cAMP levels fall. Because Gs and Gi push the same enzyme in opposite directions, the cell's cAMP level at any moment is a running balance of stimulating and inhibiting signals arriving at its surface.
From messenger to action: protein kinase A
cAMP itself does not do the cell's work — it has one main job: to wake up protein kinase A (PKA). A kinase is an enzyme that attaches a phosphate group onto other proteins, and that little tag flips those proteins on or off. When cAMP rises, it releases PKA's active core, which then phosphorylates a long list of target proteins — the true effectors of the cell.
In a heart cell, PKA phosphorylates calcium channels and pumps so the cell contracts harder and faster — which is exactly what an agonist at the beta-adrenoceptor (a Gs-linked GPCR) achieves. In a fat cell, the same PKA tells the cell to break down stored fat. One messenger, one kinase, but many tissue-specific outcomes, because each cell type owns a different set of target proteins.
Agonist -> beta-adrenoceptor -> Gs ON
Gs -> adenylyl cyclase ON
ATP -> cAMP (second messenger, rises)
cAMP -> protein kinase A ON
PKA adds phosphate to target proteins
=> stronger, faster heart contraction
(Gi pathway runs the same line in reverse: cAMP falls)