Use of Electron Microscopy in the Detection of Adrenergic Receptors

Adrenergic receptors (ARs) belong to a superfamily of the G-protein-coupled receptors and are categorized by their binding to endogenously occurring catecholamines, i.e., norepinephrine and epinephrine. Adrenergic receptors are classified into three groups (α₁ -, α₂ -, and β-ARs), each of which is further divided into three subtypes. The α₁ - (α_1A -, α_1B -, and α_1D -ARs) couple with G_q family of G-proteins (G₁₁ , G₁₄ , G₁₅ , and G₁₆ ) and result in activation of phospholipase C-βs that liberate two second messengers, diacylglycerol and inositol-1,4,5-trisphosphate. The three subtypes of α₂ -ARs are designated α_2A -, α_2B -, and α_2C -AR. On binding with agonists, α₂ -AR inhibit adenylyl cyclase and calcium channels, but activate potassium channels through coupling to the G_i family of G-proteins (G_i1 , G_i2 , G_i3 , and G_o ). Finally, the three groups of β-AR are designated β₁ -, β₂ -, and β₃ -AR: these increase the intracellular cAMP content by activating G_s , which is coupled to the enzyme, adenylyl cyclase (1 ).