α 1 -Adrenergic Receptors

α 1 -Adrenergic receptors (α 1 -ARs) are members of the biogenic amine family of G protein-coupled receptors (GPCRs). Like other members of the GPCR superfamily, the largest family of membrane receptors and possibly the largest gene family in the human genome, α 1 -ARs are long, single-chain polypeptides. Three subtypes have been identified by molecular cloning in several species, including humans. In addition to these subtypes, a putative fourth subtype, the α 1L -AR, which displays low affinity for prazosin and other α 1 -antagonists, has been identified by functional and not molecular studies. This subtype is postulated to mediate the contraction of prostate smooth muscle, but it remains unclear if it is a distinct subtype or merely a functional variant of one of the cloned α 1 subtypes—such as the α 1A splice variants—because all display some of the characteristics of an α 1L subtype when expressed in heterologous cell systems. Another molecular variant of the α 1A -AR is a coding region polymorphism that involves an arginine to cysteine substitution in an arginine-rich region of the C-terminal tail.