Adrenergic Receptor - Subtypes

Subtypes

History: Raymond Ahlquist, Professor of Pharmacology in Georgia, US, published a paper concerning adrenergic nervous transmission in 1948 but its significance was largely ignored at that time. However, in 1954 he was able to incorporate his findings in a textbook, Drill's Pharmacology in Medicine, and thereby firmly establish the essential role played by alpha and beta receptor sites in the adrenaline/nor-adrenaline cellular mechanism. His discovery would revolutionise advances in pharmacotherapeutic research, allowing the selective design of specific molecules to target medical ailments rather than rely upon traditional research into the efficacy of pre-existing herbal medicines.

There are two main groups of adrenergic receptors, α and β, with several subtypes.

• α receptors have the subtypes α₁ (a G_q coupled receptor) and α₂ (a G_i coupled receptor). Phenylephrine is a selective agonist of the α receptor.
• β receptors have the subtypes β₁, β₂ and β₃. All three are linked to G_s proteins (although β₂ also couples to G_i), which in turn are linked to adenylate cyclase. Agonist binding thus causes a rise in the intracellular concentration of the second messenger cAMP. Downstream effectors of cAMP include cAMP-dependent protein kinase (PKA), which mediates some of the intracellular events following hormone binding. Isoprenaline is a non-selective agonist.