BACKGROUND
Adrenergic receptors (ARs) (the term “adrenergic” reflects the alternative name for epinephrine: adrenaline) include five general types alpha1, alpha2, beta1, beta2 and beta3), which are found in different target tissues and differ in their affinities and responses to various agonists and antagonists. Each of the receptors consists 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.