Adrenergic receptors (also called adrenoceptors) are the receptors for the catecholamines adrenaline and noradrenaline (called epinephrine and norepinephrine in the United States). Adrenaline and noradrenaline play important roles in the control of blood pressure, myocardial contractile rate and force, airway reactivity, and a variety of metabolic and central nervous system functions.

Adrenergic receptors are members of the G-protein coupled receptor (GPCR) superfamily of membrane proteins. They share a common structure of seven putative transmembrane domains, an extracellular amino terminus, and a cytoplasmic carboxyl terminus.

Adrenoceptors are divided into three types: α₁, α₂ and β adrenoceptors. Each type is further divided into at least three subtypes: α_1A, α_1B, α_1D, α_2A, α_2B, α_2C, β₁, β₂, β₃.^1,2 They are expressed in nearly all peripheral tissues and in the central nervous system.^1,2

All α₁-adrenoceptors (α₁-ARs) activate phospholipases C and A2.³ In addition to mobilizing intracellular calcium, the α₁-ARs have also been shown to activate calcium influx via voltage-dependent and -independent calcium channels.⁴

The α_1A-ARs have at least nine known alternative spliced isoforms. They are the most abundant adrenergic receptor in the brain, and are considered postsynaptic receptors. α_1A-ARs are also expressed in the heart, liver and prostate.