Adrenergic receptors (Alpha & Beta) and their distribution

• Adrenergic receptors are G-protein-coupled receptors (GPCRs) that mediate the effects of norepinephrine and epinephrine.
• Adrenergic receptors are classified into Alpha (α) and Beta (β) receptors, with further subtypes.

Alpha-Adrenergic Receptors (α-Receptors)

• These receptors mainly mediate vasoconstriction and smooth muscle contraction.

1. α1-Receptors (Gq-coupled)

   • Mechanism:

     • Activate phospholipase C (PLC) → Increase IP3 & DAG → Increase intracellular Ca²⁺

   • Effects:

     • Vasoconstriction → Increased blood pressure (BP)
     • Mydriasis (pupil dilation)
     • Contraction of bladder sphincter

   • Location:

     • Blood vessels (vasoconstriction)
     • Eye (dilator pupillae muscle)
     • Prostate and urinary sphincter

2. α2-Receptors (Gi-coupled)

   • Mechanism:

     • Inhibit adenylyl cyclase (AC) → Decrease cAMP

   • Effects:

     • Inhibits norepinephrine release (negative feedback)
     • Reduces insulin secretion
     • Lowers blood pressure (central effect)

   • Location:

     • Presynaptic nerve terminals (inhibits NE release)
     • Pancreas (reduces insulin release)
     • CNS (sedative effects)

Beta-Adrenergic Receptors (β-Receptors)

• These receptors mainly mediate vasodilation, cardiac stimulation, and metabolic effects.

1. β1-Receptors (Gs-coupled)

   • Mechanism:

     • Activate adenylyl cyclase (AC) → Increase cAMP → Increase PKA activity

   • Effects:

     • Increase heart rate (chronotropic effect)
     • Increase cardiac contractility (inotropic effect)
     • Increase renin secretion (increases BP)

   • Location:

     • Heart (SA node, AV node, myocardium)
     • Kidneys (juxtaglomerular cells → renin release)

2. β2-Receptors (Gs-coupled)

   • Mechanism:

     • Increase cAMP → Activate PKA

   • Effects:

     • Bronchodilation (relaxation of airway smooth muscles)
     • Vasodilation (skeletal muscle)
     • Glycogenolysis (glucose release)

   • Location:

     • Lungs (bronchodilation)
     • Skeletal muscles (vasodilation)
     • Liver (glycogenolysis)

3. β3-Receptors (Gs-coupled)

   • Mechanism:

     • Increase cAMP

   • Effects:

     • Lipolysis (fat breakdown)
     • Thermogenesis in brown fat

   • Location:

     • Adipose tissue (fat metabolism)
     • Bladder (relaxes detrusor muscle)

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