Table 1.

Sympathetic Adrenergic and Parasympathetic Muscarinic Receptors: Response to Activation and Drug Effects.

Sympathetic adrenergic receptor	Response to activation	Drug effects
α1	• Arterial and arteriolar vasoconstriction • Pupillary dilation • Decreased gastrointestinal motility and sphincter contraction • Hepatic glycogenolysis and gluconeogenesis • Pro-inflammatory cytokine production • Central nervous systems effects including anorexia	Receptor Activation: • Norepinephrine, epinephrine (α1,α2) • Phenylephrine, midodrine (α1) • Clonidine, dexmedetomidine (α2)
α2	• Decreased norepinephrine release through autoreceptors (negative feedback) • Decreased acetylcholine release through parasympathetic heteroreceptors • Central nervous systems effects including sedation, analgesia, and down-regulation of sympathetic outflow (= hypotension, bradycardia)	Receptor Blockade: • Phentolamine, Phenoxybenzamine (α1, α2) • Doxazosin (α1)
	• Decreased gastrointestinal motility and gland secretion • Decreased insulin secretion • Coronary, renal, and skin vasoconstriction • Constriction of veins • Platelet aggregation • Monocyte-endothelial adhesion	Norepinephrine Reuptake Inhibition: • Cocaine, methylphenidate, amphetamines, tricyclic antidepressants.
ß1	• Increased heart rate, cardiac conductivity, and cardiac muscle contractility • Renin release from the kidney	Receptor Activation: • Norepinephrine, epinephrine (β1, β2) • Isoproterenol (β1, β2) • Dobutamine (β1) • Albuterol, terbutaline (β2)
ß2	• Bronchodilation • Decreased bronchial gland secretion • Potent coronary vasodilatation (exceeds α2 constriction effect)
	• Increased heart rate, cardiac conductivity, and cardiac muscle contractility • Bladder relaxation • Skeletal muscle, pulmonary, and visceral vasodilation • Immune modulation in lymphoid tissue	Receptor Blockade: • Propranolol (β1, β2) • Atenolol, esmolol, metoprolol, nadolol, timolol (β1 > β2)
ß3	• Lypolysis and thermogenesis in adipose tissue	Norepinephrine Reuptake Inhibition: • Cocaine, methylphenidate, amphetamines, tricyclic antidepressants.
Parasympathetic muscarinic receptor	Response to activation	Drug effects
M1	• Gastric acid secretion • Pancreatic amylase secretion • Cerebral vasoconstrictio	Receptor activation: • Acetylcholine (M1-M4) • Methacholine (M1-M4)
M2, M3	• Decreased heart rate, decreased cardiac conductivity • Pupillary and ciliary constriction • Lacrimal, salivary, nasopharyngeal, bronchial, and digestive gland secretion • Bronchial constriction • Increased gastrointestinal motility • Sphincter relaxation	Receptor Blockade: • Atropine, ipratropium, scopolamine (M1-M4) • Oxybutynin (M3)
M2, M4	• Decreased acetylcholine release through autoreceptors (negative feedback) • Decreased norepinephrine release through sympathetic heteroreceptors	Inhibition of Acetylcholinesterase: • Reversible: Edrophonium, neostigmine, physostigmine, pyridostigmine • Irreversible: Organophosphates

α₁ and ß₃ are activated by norepinephrine > epinephrine; α₂ and ß₁ are activated by norepinephrine = epinephrine; ß₂ are activated by epinephrine >> norepinephrine. Muscarinic receptors in the target organs (and the nicotinic receptors in the post-ganglionic neurons) are activated by acetylcholine (1).