Epinephrine (adrenaline)
Also known as: adrenaline, catecholamine
The main catecholamine of the adrenal medulla (≈80 % of its output), released within seconds of sympathetic activation to prepare the body for fight or flight by acting on α- and β-adrenergic receptors.
Overview
Epinephrine is synthesised from tyrosine in chromaffin cells; its final step, methylation of norepinephrine by PNMT, depends on high cortisol from the adrenal cortex. It is released when preganglionic sympathetic fibres stimulate chromaffin cells via nicotinic receptors, and acts on all adrenergic receptors — with a higher affinity than norepinephrine for β2 receptors.
Receptors: α1 (Gq — vasoconstriction of skin and splanchnic vessels, pupillary dilator, sphincter contraction, hepatic glycogenolysis), α2 (Gi — inhibits insulin and neurotransmitter release), β1 (Gs — heart rate, contractility, renin release, lipolysis), β2 (Gs — bronchodilation, vasodilation in skeletal muscle, uterine relaxation, glycogenolysis, K⁺ uptake into cells, insulin and glucagon secretion), β3 (Gs — lipolysis, detrusor relaxation). Low concentrations favour β2 vasodilation (diastolic pressure may fall); high concentrations recruit α1 vasoconstriction.
Effects are rapid but brief: the plasma half-life is ≈2–3 min, as epinephrine is taken up into tissues and metabolised by COMT (to metanephrine) and MAO (ultimately to vanillylmandelic acid).
- Location
Source: chromaffin cells of the adrenal medulla. Targets: heart, blood vessels, bronchial smooth muscle, liver, adipose tissue, skeletal muscle, pancreatic islets, eye.
Function
- Increases heart rate, contractility and cardiac output.
- Redistributes blood to skeletal muscle and dilates the bronchi.
- Raises blood glucose (glycogenolysis, gluconeogenesis, ↓insulin, ↑glucagon) and free fatty acids.
- Increases metabolic rate, alertness and pupil size.
- Shifts K⁺ into cells (β2).
- Physiology
Class: catecholamine (tyrosine-derived amine). Receptors: α1 (Gq), α2 (Gi), β1/β2/β3 (Gs). Half-life ≈2–3 min. Metabolites: metanephrine, VMA.
- Regulation
Released by sympathetic preganglionic activity triggered by exercise, hypoglycaemia (a powerful stimulus), hypotension/haemorrhage, cold, pain and emotional stress. Cortisol is required for its synthesis (PNMT induction).
Clinical significance
Therapeutic adrenaline
Anaphylaxis: intramuscular adrenaline 0.5 mg (0.5 mL of 1 mg/mL) in adults into the anterolateral thigh — α1 reverses vasodilation and oedema, β2 bronchodilates and β effects stabilise mast cells. Cardiac arrest: 1 mg IV. Added to local anaesthetics to prolong action by vasoconstriction.
Hypoglycaemia unawareness
In long-standing diabetes, recurrent hypoglycaemia blunts the epinephrine response, abolishing warning symptoms (tremor, sweating, palpitations) and increasing the risk of severe hypoglycaemia.
Phaeochromocytoma
Adrenal tumours often secrete epinephrine (episodic palpitations, sweating, hyperglycaemia); extra-adrenal paragangliomas secrete mostly norepinephrine.
Compiled from standard anatomy and physiology references — see the Endocrine System sources. For education only; not medical advice.