Adrenergic receptors (ARs) (the term "adrenergic" reflects the alternative name for epinephrine, adrenaline) include four general types (a1, a2, b1 and b2) which are found in different target tissues and differ in their affinities and responses to various agonists and antagonists. cDNA clones have been isolated for all of the major AR subtypes and a number of closely related receptors have been identified by this approach. Each of the receptors have been shown to consist of single polypeptide chains which transverse the plasma membrane seven times, presumably forming a bundle of helices within the membrane. These transmembrane regions are hydrophobic and are interconnected by extracellular and intracellular hydrophilic loops. The coupling of ARs to specific intracellular effectors is mediated through diverse heterotrimeric G proteins and is regulated by G protein-coupled receptor kinases (GRKs), cAMP-dependent protein kinase A and protein kinase C directed phosphorylation. b2-adrenergic receptors bind cathecholamines (epinephrine, norepinephrine) and influence development, behavior, cardiac function, smooth muscle tone, and metabolism. b2-AR signaling complexes can contain C L-type calcium channel Ca(V)1.2, G protein, adenylyl cyclase, cAMP-dependent kinase, and PP2A phosphatase.