Hello. I'm currently reading about the signal transduction from G protein-coupled receptors in relation to the cAMP second messenger. But after this chapter, in every example of signal transduction, the book uses cAMP as a second messenger and activator of protein kinase A. They do this for the rest of the book. Not only in the chapter I'm currently reading. Does this mean that cAMP is the only sencond messenger we have, or is it used only in GPCR signal transduction? Or how does it work? Thank you.