Gs / Gi Pathway
The Gs and Gi pathways are important cell signalling pathways activated when a ligand binds to a G-protein coupled receptor, with a variety of downstream effects. These pathways both operate through the second messenger cAMP.
When a ligand binds to a Gs-coupled receptor, the Gs subunit is activated and stimulates the synthesis of cAMP. Conversely if a Gi subunit is activated, it inhibits the synthesis of cAMP. cAMP in turn stimulates PKA, an important kinase enzyme with two downstream effects. First, PKA inhibits MLCK by phosphorylating it, and second, PKA increases calcium levels in the heart by phosphorylating calcium channels.