Mechanism of Glucagon

Glucagon binds to high-affinity G protein–coupled receptors (GPCR) on the cell membrane of hepatocytes. The GPCR for glucagon is distinct from the GPCR that bind epinephrine. [Note: Glucagon receptors are not found on skeletal muscle.] Glucagon binding results in activation of adenylyl cyclase in the plasma membrane (Fig. 1). This causes a rise in cyclic AMP (cAMP), which, in turn, activates cAMP-dependent protein kinase A and increases the phosphorylation of specific enzymes or other proteins. This cascade of increasing enzymic activities results in the phosphorylation-mediated activation or inhibition of key regulatory enzymes involved in carbohydrate and lipid metabolism. 
[Note: Glucagon, like insulin, affects gene transcription. For example, glucagon induces expression of phosphoenolpyruvate carboxykinase.]

Figure 1: Mechanism of action of glucagon. [Note: For clarity, G-protein activation of adenylyl cyclase has been omitted.] R = regulatory subunit; C = catalytic subunit; cAMP = cyclic adenosine monophosphate; ADP = adenosine diphosphate; = phosphate.