6.4:
Adrenergic Receptors: β Subtype
β adrenergic receptors or β-adrenoceptors show a stronger response to ISO, compared to Adr and NA.
Upon stimulation, all β-adrenoceptors activate adenylyl cyclase leading to increased cAMP production and modulation of calcium channels.
These receptors are further classified into three subtypes based on various factors.
β1 receptors have almost equal affinity for both Adr and NA. They are located predominantly in cardiac tissue, lipocytes, and renal cells.
Their stimulation causes tachycardia, lipolysis, and renin production.
β2 receptors have a higher affinity for Adr than NA. They are located postsynaptically in the smooth muscles of the cardiovascular, respiratory, and genitourinary systems. They are responsible for causing bronchodilation, vasodilation, decreased peripheral resistance, relaxation of the bladder and gravid uterus.
β3 receptors show higher sensitivity to NA than Adr. They are located postsynaptically on the lipocytes and bladder. They regulate metabolic activity and lipolysis, and relax the bladder's detrusor muscle.
6.4:
Adrenergic Receptors: β Subtype
β-adrenoceptors have varied sensitivities towards adrenaline, noradrenaline, and isoprenaline. The order of agonist potency is as follows:
Isoprenaline > Adrenaline > Noradrenaline
Neurotransmitter binding to these receptors causes activation of adenylyl cyclase resulting in increased concentrations of cAMP and modulation of calcium ion channels within the cell. They are further classified into β1, β2, and β3 subtypes.
β1-adrenoceptors: β1-adrenoceptors have equal affinities for adrenaline and noradrenaline. They are located postsynaptically in cardiac and brain tissue, lipocytes, and presynaptically on cholinergic and adrenergic nerve terminals, as well as renal cells. Stimulation of β1-adrenoceptors causes increased myocardial contractility, tachycardia, lipolysis, and renin production.
β2-adrenoceptors: β2 receptors have a higher affinity for adrenaline and are located postsynaptically in the cardiac and smooth muscles of blood vessels. Their stimulation causes bronchodilation, vasodilation, decreased peripheral resistance, increased glucagon release and glycogenolysis in the liver and muscles, and uterine relaxation in females.
β3-adrenoceptors: β3 receptors show higher sensitivity to noradrenaline and are located postsynaptically on the heart and lipocytes and regulate lipolysis along with β1-adrenoceptors and metabolic activity.