Learning Objective
By the end of this section, you should be able to identify the major adrenergic receptor subtypes (α₁, α₂, β₁, β₂, β₃), describe their primary tissue distribution, and predict their physiologic effects—a key skill for answering USMLE Step 1 pharmacology questions.
Tissue Distribution and Effects of Adrenergic Receptors
Adrenergic receptors are G-protein–coupled receptors activated by norepinephrine and epinephrine. Their effects depend on receptor subtype and tissue location.
α₁ Receptors
Major tissues
- Vascular smooth muscle
- Visceral smooth muscle
Physiologic effects
- Vasoconstriction (↑ peripheral vascular resistance, ↑ blood pressure)
- Smooth muscle contraction (e.g., GI and genitourinary sphincters)
α₂ Receptors
Major tissues
- Pancreatic β cells
- Presynaptic nerve terminals
- Salivary glands
Physiologic effects
- ↓ Insulin secretion
- ↓ Neurotransmitter release (negative feedback on norepinephrine)
- ↓ Salivary secretion
Step 1 tip: α₂ turns things down (inhibitory, Gi-coupled).
β₁ Receptors
Major tissues
- Heart
- Kidney (juxtaglomerular cells)
Physiologic effects
- ↑ Heart rate
- ↑ Contractility
- ↑ Renin secretion
Step 1 tip: β₁ = 1 heart, 1 kidney.
β₂ Receptors
Major tissues
- Bronchioles
- Cardiac muscle
- Liver
- Arterial smooth muscle
- Pancreas
Physiologic effects
- Bronchodilation
- ↑ Heart rate and contractility
- Glycogenolysis and glucose release
- Vasodilation (especially in skeletal muscle beds)
- ↑ Insulin secretion
Step 1 tip: β₂ relaxes smooth muscle and increases glucose availability.
β₃ Receptors
Major tissues
- Adipose tissue
Physiologic effects
- ↑ Lipolysis
Step 1 tip: β₃ = fat breakdown.
High-Yield Summary (Step 1)
- α₁: Vasoconstriction, smooth muscle contraction
- α₂: Inhibitory—↓ insulin, ↓ NE release
- β₁: Cardiac stimulation, ↑ renin
- β₂: Bronchodilation, vasodilation, ↑ glucose, ↑ insulin
- β₃: Lipolysis
