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. The adrenoceptors are expressed in nearly all peripheral tissues and in the central nervous system^1,2.
 
α₁-Adrenergic receptors play an important role in the physiological response to epinephrine and norepinephrine, particularly in the cardiovascular system. All three cloned α₁ receptors (α_1A, α_1B, and α_1D) couple to G_q/11. While their cellular distribution is clear, their functional role is less so due to a lack of specific agonists/antagonists³.
 
As these receptors lack specific agonists/antagonists, knock-out (KO) studies in mice have shed significant light on their cellular role. Individual α₁-adrenoceptor KO mice do not display physical abnormalities in general. Similar KO studies indicate that all α₁ subtypes seem to be involved in the regulation of blood pressure³. Also, studies indicate that α_1D-adrenoceptor seems to play an important role in aortic contraction⁴, renal function, nociception, basal locomotor activity³ as well as bladder contraction⁵.