6.3 : Adrenergic Receptors: ɑ Subtype

Adrenoceptors are classified into α and ꞵ classes based on their potencies to catecholamine agonists. α-adrenoceptors show the following order of catecholamine potency:

Adrenaline ≥ Noradrenaline >> Isoprenaline

α-adrenoceptors are further divided into α₁ and α₂-adrenoceptors.

α₁-Adrenoceptors: These receptors are located postsynaptically on the effector organs and cause constriction of smooth muscle mediated by activation of phospholipase C—inositol-1,4,5-trisphosphate (IP₃)—diacylglycerol (DAG) pathway.

α₂-Adrenoceptors: α₂-adrenoceptors are predominantly located on the presynaptic nerve endings, which control noradrenaline release. They are also found on presynaptic parasympathetic neurons. Noradrenaline released from a presynaptic sympathetic interacting with these receptors causes inhibition of ACh release. In contrast to α₁ receptors, activation of α₂ receptors causes inhibition of adenylyl cyclase, decreased intracellular cAMP levels, and closure of ion channels.

Further subdivisions: The α₁ and α₂ receptors are further classified into α_1A, α_1B, α_1D and α_2A, α_2B, α_2C subtypes, respectively, for a better understanding of drug-receptor selectivity.