Adrenergic receptors (also known as adrenoceptors, ARs) belong to the guanine nucleotide-binding G protein–coupled receptor (GPCR) superfamily, and are membrane receptors that activate heterotrimeric G proteins following the binding of a ligand. GPCRs consist of one extracellular N-terminal domain, seven membrane-spanning domains, three intra- and three extracellular loops, and one intracellular C-terminal tail. These heptahelical trans-membrane sensors account for approximately 4% of the total protein-coding genome and are considered the most important drug targets in medicine and physiology. G proteins typically stimulate (via Gs protein) or inhibit (via Gi protein) the enzyme adenylyl-cyclase or activate (via Gq protein) phospholipase C (PLC).
