What second messenger is increased by glucagon signaling?

Glucagon signaling primarily leads to an increase in cyclic adenosine monophosphate (cAMP) as a second messenger. When glucagon, a hormone secreted by the pancreas, binds to its receptor on liver cells, it activates a G-protein coupled receptor (GPCR). This activation stimulates adenylate cyclase, the enzyme responsible for converting ATP to cAMP.

The increase in cAMP levels subsequently activates protein kinase A (PKA), which then initiates a cascade of phosphorylation events that promote gluconeogenesis and glycogenolysis in the liver. This effect is especially significant during fasting states, where glucagon plays a critical role in maintaining blood glucose levels.

In contrast, the other options represent different signaling pathways. cGMP is more commonly associated with pathways involving nitric oxide and certain hormones, IP3 (inositol trisphosphate) and DAG (diacylglycerol) are involved in signaling pathways associated with phospholipase C activation, which is important for hormones like oxytocin and catecholamines, but does not play a direct role in glucagon signaling. Therefore, the increase in cAMP is essential for understanding the metabolic effects of glucagon in the regulation of glucose metabolism.