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 α1A, α1B, α1D and α2A, α2B, α2C subtypes, respectively, for a better understanding of drug-receptor selectivity.
