GPCRs are cell surface receptors that detect extracellular signals and activate intracellular responses by coupling with G proteins. In unstimulated cells, the state of Ga is defined by its interaction with GDP, Gb/g, and a GPCR. Upon receptor stimulation by a ligand, Ga dissociates from the receptor and Gb/g, and GTP is exchanged for the bound GDP, leading to Ga activation. Ga goes on to activate other molecules in the cell. These effects include activating the MAPK and PI3K pathways, and inhibition of the Na+/H+ exchanger in the plasma membrane, etc.