Figure 7.15 A Cascade of Reactions Leads to Altered Enzyme Activity Liver cells respond to epinephrine by activating G proteins, which in turn activate the synthesis of the second messenger cAMP. Cyclic AMP initiates a protein kinase cascade, greatly amplifying the epinephrine signal, as indicated by the bold, black numbers. The cascade both inhibits the conversion of glucose to glycogen and stimulates the release of previously stored glucose.