Epinephrine
Epinephrine (adrenaline) is a catecholamine that functions as both a Neurotransmitter and a hormone, making it a classic example of a molecule bridging the nervous and endocrine systems.
Catecholamine biosynthesis pathway: tyrosine → DOPA → dopamine → norepinephrine → epinephrine (via PNMT)
Synthesis
Epinephrine is synthesized from tyrosine via the catecholamine pathway:
- Tyrosine → L-DOPA (tyrosine hydroxylase — rate-limiting step)
- L-DOPA → Dopamine (DOPA decarboxylase)
- Dopamine → Norepinephrine (dopamine β-hydroxylase)
- Norepinephrine → Epinephrine (PNMT — phenylethanolamine N-methyltransferase)
PNMT is expressed primarily in chromaffin cells of the adrenal medulla, which is why ~80% of adrenal catecholamine output is epinephrine, while in the CNS (where PNMT is sparse) norepinephrine dominates.
Dual Role: Neurotransmitter + Hormone
| Role | Site | Target |
|---|---|---|
| Hormone | Adrenal medulla → bloodstream | Systemic α/β-adrenoceptors |
| Neurotransmitter | Small CNS neuron populations | Local synaptic targets |
As a hormone, epinephrine is secreted by the adrenal medulla in response to sympathetic nervous system activation — it reaches organs that may not be directly innervated.
Fight-or-Flight Mechanism
Acute stress activates the hypothalamus → sympathetic preganglionic neurons → adrenal medulla → rapid epinephrine release into circulation. Effects are mediated by adrenergic receptors (α1, α2, β1, β2, β3):
- Heart: ↑ heart rate (β1), ↑ contractility → higher cardiac output
- Lungs: Bronchodilation (β2) → more O₂ available
- Blood vessels: Vasoconstriction in viscera/skin (α1), vasodilation in skeletal muscle (β2)
- Metabolism: Glycogenolysis in liver → ↑ blood glucose (β2); lipolysis in fat tissue
- Eyes: Pupil dilation (mydriasis, α1)
- GI/Urinary: Reduced peristalsis, sphincter contraction
Epinephrine vs. Noradrenaline
| Feature | Epinephrine | Norepinephrine |
|---|---|---|
| Primary source | Adrenal medulla | Sympathetic nerve terminals, locus coeruleus |
| β2 activity | High (bronchodilation, vasodilation in muscle) | Low |
| Heart rate effect | Strong ↑ | Modest (reflex bradycardia possible) |
| Role in CNS | Minor | Major (arousal, attention, mood) |
Clinical Use
- EpiPen (epinephrine auto-injector): First-line treatment for anaphylaxis — rapidly reverses bronchospasm, vasodilation, and hypotension via α1+β effects
- Cardiac arrest: IV epinephrine in ACLS protocols restores cardiac output
- Local anesthesia: Combined with local anesthetics to cause vasoconstriction, prolonging effect and reducing systemic absorption
- Asthma (historical): Now replaced by selective β2 agonists (e.g., salbutamol)
see also
Tags: HormoneNeurotransmitter science ai-generated
Superlink: 052 🫧Hormone und Neurotransmitter
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Erstellt: 01-09-22 11:19