Once produced, the intracellular second messengers activate protein kinases, and these enzymes begin adding phosphate groups to target proteins. IP₃ triggers the release of Ca²⁺ from the endoplasmic reticulum (ER). The Ca²⁺ then binds to calmodulin. This complex along with DAG binds to and activates protein kinase C. Unlike IP₃ and DAG, cAMP binds directly to protein kinases and stimulates the activity of these enzymes.

Once a cell responds to a hormone, how is the cell deactivated?

1. The GTP that is associated with the G protein is hydrolyzed to GDP. This inactivates the G protein, thereby dampening the response.
2. Ca²⁺ is quickly resorbed by the endoplasmic reticulum, preventing the calmodulin-Ca²⁺ complex from forming.
3. cAMP is rapidly degraded by phosphodiesterase into AMP.
4. Once the cell is reset, it is able to respond again if additional hormone molecules are present and bind to the receptor.