6.3:
Adrenergic Receptors: ɑ Subtype
ɑ-adrenoceptors show higher affinity to Adr and NA than ISO.
They are classified into ɑ1 and ɑ2 subtypes based on various factors.
Upon stimulation, ɑ1 receptors activate phospholipase C , releasing IP3 and DAG as secondary messengers.
They are present in the postsynaptic effector organs, particularly the smooth muscles of the cardiovascular and GI systems, and are responsible for vasoconstriction, increased blood pressure, and GI muscle relaxation.
ɑ2 receptor stimulation inhibits adenylyl cyclase, decreases cAMP production, and modulates ion channels.
They are predominantly present in autonomic nerve terminals, pancreatic beta cells, vascular smooth muscles, and platelets. They affect the release of autonomic neurotransmitters and insulin, vascular smooth muscles contraction and platelet aggregation.
ɑ1 and ɑ2 receptors are subdivided into three subtypes based on subtype-selective drugs. For instance, tamsulosin—used for treating prostatic hyperplasia—preferentially inhibits ɑ1-A receptors in the prostate gland over ɑ1-B receptors in blood vessels, so they have fewer cardiovascular side effects.
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 α1A, α1B, α1D and α2A, α2B, α2C subtypes, respectively, for a better understanding of drug-receptor selectivity.