Both epinephrine (a tyrosine derivative) and glucagon (a peptide hormone) increase glucose export from the liver into the bloodstream. Each ligand binds a different receptor, but both lead to an activation of PKA. How does this happen?

The answer is that the two receptors work as binders, thereby, they activate the same Ga subunit, Gsa, and this indirectly to the activation of PKA.

Explanation:

Note that, the action that occurred within the cell depends on heterotrimetric G protein that was activated by the receptor. In specificity, the receptors for glucagon and epinephrine interact with the intracellular Gsa subunit of a heterometric protein. Activation of Gsa leads to the same downstream which signal events, hence the same cellular responses despite the difference between the ligand-receptor interaction.