Location of Beta-2 (β2) Receptors in the Body
Beta-2 (β2) adrenergic receptors are widely distributed throughout the body, with highest concentrations in the respiratory tract, vascular smooth muscle, skeletal muscle, and liver, where they mediate important physiological responses to catecholamines.
Distribution of Beta-2 Receptors by Body System
Respiratory System
- Airway smooth muscle: Highest concentration of β2 receptors in the body, mediating bronchodilation 1, 2
- Respiratory epithelium: Present on epithelial cells lining the airways 2
Cardiovascular System
- Vascular smooth muscle: Abundant in blood vessels, causing vasodilation when activated 1
- Heart: Present in cardiac tissue, though in lower concentrations than β1 receptors 1
Skeletal Muscle
- Widely expressed throughout skeletal muscle tissue 3
- Mediate metabolic effects including increased glycogenolysis and glucose uptake 1
Liver
- Present in hepatocytes, regulating glycogenolysis and gluconeogenesis 1
Adipose Tissue
- Found in adipocytes, where they regulate lipolysis 4
- Expression increases during adipocyte differentiation 4
Immune System
- Present on mast cells, T-cells, eosinophils, and macrophages 2
- Modulate inflammatory responses and mediator release 2
Eye
- Located in the ciliary processes, ciliary body, and iris 5
- Comprise approximately 90% of total β-adrenergic receptors in the human iris-ciliary body 5
Gastrointestinal System
- Present in the gut, primarily on smooth muscle cells 6
- Involved in regulation of gastrointestinal motility 6
Central Nervous System
- Found in various brain regions including cerebral cortex, hippocampus, striatum, and thalamus 6
- Higher functional activity in adolescent brains (ages 13-18) compared to adults 6
Clinical Significance
The widespread distribution of β2 receptors explains why β2-agonists and antagonists have diverse effects throughout the body:
- Respiratory effects: β2-agonists (like salbutamol/albuterol) are cornerstone treatments for asthma and COPD due to their bronchodilatory effects 3
- Cardiovascular effects: Can cause tachycardia and vasodilation when stimulated 1
- Metabolic effects: Influence glucose metabolism and lipolysis 4
- Anti-inflammatory effects: β2-agonists can inhibit release of pro-inflammatory mediators from various immune cells 2
Receptor Structure and Function
β2 receptors are G protein-coupled receptors that:
- Contain seven transmembrane domains 3
- Primarily couple to Gs proteins, activating adenylyl cyclase and increasing intracellular cAMP 3
- Undergo desensitization with prolonged exposure to agonists, which can affect clinical response 3
Genetic Variations
Several polymorphisms of the β2 receptor have been identified that can alter:
- Receptor expression levels
- Ligand binding properties
- Coupling efficiency to G proteins
- Susceptibility to desensitization 3
These genetic variations may explain individual differences in response to β2-agonist therapy in conditions like asthma.