Epinephrine (Adrenaline)
Epinephrine (adrenaline; MW 183.21 Da) is the principal circulating catecholamine hormone of the stress response, secreted by adrenal medulla chromaffin cells within seconds of sympathetic splanchnic nerve activation. It is the terminal product of the Blaschko catecholamine biosynthetic pathway — norepinephrine methylated by PNMT (phenylethanolamine-N-methyltransferase), an enzyme uniquely induced by glucocorticoids from the adjacent adrenal cortex. Epinephrine's N-methyl group confers greater β₂ affinity than norepinephrine, explaining its unique combination of cardiac stimulation (β₁), bronchodilation (β₂), selective vasoconstriction (α₁ in skin/gut) + vasodilation (β₂ in muscle), and potent metabolic effects (hepatic glycogenolysis, adipose lipolysis). Pharmacologically, it is the first-line emergency drug for anaphylaxis and cardiac arrest, exploiting these wide-ranging adrenergic effects to simultaneously reverse vasodilation, bronchoconstriction, and cardiac arrest.
Structure & Chemistry
Epinephrine is a catechol (3,4-dihydroxybenzene) with an aminoethanol side chain bearing an N-methyl group — hence N-methylnorepinephrine. Key structural features: (1) catechol ring essential for adrenergic receptor binding, O-methylated by COMT at 3-OH → metanephrine; (2) chiral centre at the benzylic carbon (C-1 of side chain): natural L-(R)-isomer is ~10–100× more potent than D-(S)-isomer; (3) secondary amine (N-methyl) confers higher β₂ affinity than NE (primary amine) — explaining different adrenergic pharmacology at clinical doses.
Stored in chromaffin granules at ~0.5 M concentration (80% Epi, 20% NE in most human adrenal medullae), complexed with chromogranins A/B, ATP, enkephalins, and NPY. Cortisol from the intra-adrenal portal circulation (at 10–100× systemic concentration) induces PNMT expression — maintaining high Epi/NE ratio in chromaffin cells.
Blaschko Biosynthesis Pathway & Secretion
L-Phenylalanine
│ PAH (phenylalanine hydroxylase; liver, mainly)
▼
L-Tyrosine
│ TH (tyrosine hydroxylase) — RATE-LIMITING
│ Fe²⁺ / BH4 / O₂; inhibited by catecholamine products
▼
L-DOPA
│ AADC (aromatic L-amino acid decarboxylase; PLP cofactor)
▼
Dopamine [transported into vesicle by VMAT2 / SLC18A2]
│
│ DβH (dopamine β-hydroxylase) — INTRA-VESICULAR
│ Cu²⁺/ascorbate/O₂ cofactors
▼
Norepinephrine [exits vesicle into cytosol]
│
│ PNMT (phenylethanolamine-N-methyltransferase) — CYTOSOLIC
│ SAM (S-adenosyl-methionine) methyl donor
│ INDUCED by glucocorticoids (cortisol → GR activation in chromaffin cells)
▼
Epinephrine [re-enters storage in chromaffin granule]
SECRETION:
Splanchnic nerve AP → nAChR (α3β4) on chromaffin cell
│ Na⁺ influx → depolarisation → VGCCs open → Ca²⁺ influx
▼
Chromaffin granule exocytosis → Epi into adrenal vein → systemic circulation
Plasma t½ ~2 min (rapid tissue uptake + MAO/COMT degradation)
CATABOLISM:
├─ MAO (mitochondrial, MAO-A > B): Epi → deaminated → DOPGAL
├─ COMT (cytosolic, SAM-dependent): Epi → metanephrine (3-OH methylation)
└─ Sequential MAO + COMT → VMA (vanillylmandelic acid) + metanephrine
[Plasma/24h-urine fractionated metanephrines: most sensitive phaeochromocytoma test]
Adrenergic Receptor Pharmacology & Fight-or-Flight Effects
| Receptor | G-protein / 2nd messenger | Primary Tissue | Epinephrine Effect |
|---|---|---|---|
| α₁AR | Gq → PLC → IP3/DAG → ↑Ca²⁺ | Vascular smooth muscle (skin, gut, kidney), iris dilator, GI sphincters | Vasoconstriction in non-essential tissues → redirects blood to muscles/heart/brain; mydriasis; ↑sphincter tone |
| α₂AR | Gi → ↓cAMP; GIRK | Presynaptic sympathetic terminals, pancreatic β-cells, platelets | ↓NE release (presynaptic feedback); ↓insulin secretion (→ ↑blood glucose); platelet aggregation |
| β₁AR | Gs → ↑cAMP → PKA | SA node, AV node, ventricular myocardium, kidney JGA | ↑HR (chronotropy), ↑contractility (inotropy), ↑AV conduction (dromotropy), ↑renin; ↑cardiac output |
| β₂AR | Gs → ↑cAMP → PKA | Bronchi, skeletal muscle vasculature, hepatocytes, uterus, mast cells | Bronchodilation (key for anaphylaxis rescue), vasodilation in muscles, hepatic glycogenolysis; stabilises mast cells |
| β₃AR | Gs → ↑cAMP → PKA | Adipose (brown + white), urinary bladder detrusor | Lipolysis (↑FFA) + thermogenesis in brown adipose; bladder relaxation |
Mechanism of Action — Signal Transduction (β₁ and β₂ examples)
β₁AR CARDIAC PATHWAY (↑Contractility + ↑HR)
──────────────────────────────────────────────
Epi + β₁AR → Gαs → Adenylyl cyclase (AC5/AC6) → ↑cAMP → PKA
│
├─ Cav1.2 (L-type Ca²⁺ channel, β-subunit Ser1928) → ↑ICaL → ↑Ca²⁺ transient
├─ RyR2 (Ser2808) → ↑SR Ca²⁺ release → ↑contractility (positive inotropy)
├─ Phospholamban (PLN, Ser16) → ↓PLN inhibition of SERCA2a → faster SR reuptake
│ → ↑relaxation rate (positive lusitropy) + ↑SR Ca²⁺ loading
├─ Troponin I (Ser23/24) → ↓myofilament Ca²⁺ sensitivity → faster relaxation
└─ HCN4 (CNBD Ser) → cAMP shifts If activation +10 mV → faster diastolic depol → ↑HR
β₂AR BRONCHIAL PATHWAY (Bronchodilation)
──────────────────────────────────────────
Epi + β₂AR → Gαs → ↑cAMP → PKA
│
├─ MLCK phosphorylation + inactivation → smooth muscle relaxation
├─ BKCa (large-conductance Ca²⁺-activated K⁺ channels) → K⁺ efflux → hyperpolarisation
├─ PKA → β₂AR GRK2/3 phosphorylation → β-arrestin → receptor desensitisation
└─ Mast cell β₂AR → ↑cAMP → ↓IgE-triggered degranulation (anti-allergic)
α₁AR METABOLIC PATHWAY (Hepatic glycogenolysis)
──────────────────────────────────────────────────
Epi + α₁AR (hepatocyte) → Gq → IP3 → ↑Ca²⁺
[or via β₂AR → ↑cAMP → PKA on same hepatocyte]
│
▼
PKA / Ca²⁺-CaM → phosphorylase kinase → glycogen phosphorylase (active form)
│
▼ Glycogen → glucose-1-P → glucose-6-P → glucose → blood → ↑glycaemia
Physiological Roles — Fight-or-Flight Organ System Table
| System | Epi Effect | Receptor | Survival Purpose |
|---|---|---|---|
| Heart | ↑HR, ↑contractility, ↑CO, ↑conduction velocity | β₁AR | ↑Blood delivery to muscles and brain |
| Lungs | Bronchodilation, ↓mucus, ↓mast cell degranulation | β₂AR | Maximise O₂ intake; critical in anaphylaxis |
| Vascular (skin/gut) | Vasoconstriction | α₁AR | Redirect blood to muscles/brain; ↑MAP |
| Vascular (muscle) | Vasodilation at low Epi concentrations | β₂AR | ↑Skeletal muscle perfusion for fight/flight |
| Liver | Glycogenolysis → ↑blood glucose | α₁AR + β₂AR | Energy substrate mobilisation |
| Adipose | Lipolysis → ↑FFA + glycerol | β₁AR / β₃AR | Alternative energy substrate; glucose sparing |
| Pancreas | ↓Insulin (α₂), ↑Glucagon (β₂) | α₂AR (β-cells), β₂AR (α-cells) | Maintain hyperglycaemia for extended stress |
| Eyes | Mydriasis (pupil dilation) | α₁AR (iris dilator) | Improve visual field in threat response |
Pharmacology — Clinical Uses of Epinephrine
| Indication | Route / Dose | Mechanism Exploited | Rationale |
|---|---|---|---|
| Anaphylaxis | IM 0.3–0.5 mg (1:1000) anterior thigh; auto-injector (EpiPen) | β₂ bronchodilation; α₁ vasoconstriction → ↑BP; β₁ ↑HR + contractility; β₂ mast cell stabilisation → ↓further mediator release | Must be given BEFORE antihistamines and corticosteroids — Epi reverses all four major components of anaphylaxis; delay increases mortality |
| Cardiac arrest (ACLS) | IV 1 mg q3–5 min during CPR | α₁ peripheral vasoconstriction → ↑diastolic BP → ↑coronary and cerebral perfusion pressure during CPR compressions | Primary mechanism is NOT cardiac stimulation but α₁-mediated aortic diastolic pressure elevation; β effects may be harmful after ROSC |
| Severe asthma / status asthmaticus | SC 0.3 mg (1:1000); or nebulised adrenaline (croup) | β₂ bronchodilation + ↓mucosal oedema (α₁) | Second-line after SABA nebulisers; IM/SC route in extremis |
| Local anaesthesia adjunct | Added to lignocaine (1:100,000–1:200,000) | α₁ local vasoconstriction → ↓anaesthetic absorption → prolonged/intense block + haemostasis | Contraindicated in digits, penis, ear — end arteries → risk of ischaemic necrosis |
| Cardiogenic shock (refractory) | IV infusion 0.01–1 mcg/kg/min | Low dose: β₁/β₂ dominates → ↑CO; high dose: α₁ adds vasoconstriction → ↑MAP but ↑afterload | Used when NE + dobutamine insufficient; risk: arrhythmia, ↑myocardial O₂ demand, organ ischaemia |
| Glaucoma (as dipivefrin prodrug) | Topical eye drops | Penetrates cornea → hydrolysed → Epi; ↑aqueous outflow (β₂) + ↓production (α₂) | Largely superseded by prostaglandin analogues and β-blockers (timolol) |
Pathology
| Condition | Epi Perturbation | Mechanism | Diagnosis / Treatment |
|---|---|---|---|
| Phaeochromocytoma | ↑Epi ± NE (paroxysmal or sustained) | Adrenal medullary chromaffin tumour (90% benign); constitutive or paroxysmal Epi/NE secretion; "rule of 10s" — 10% bilateral, 10% extra-adrenal, 10% malignant, 10% paediatric | Dx: plasma/24h urine fractionated metanephrines (metanephrine + normetanephrine, most sensitive test); imaging CT/MRI; Tx: phenoxybenzamine (irreversible α-block) FIRST, then add β-blocker; surgical adrenalectomy (never β-block first — unopposed α → hypertensive crisis) |
| Anaphylaxis | Epi is the antidote, not the cause | IgE → mast cell → histamine, leukotrienes, tryptase → vasodilation, bronchospasm, laryngeal oedema; Epi reverses all components | IM epinephrine 0.3–0.5 mg (1:1000) anterior thigh; repeat q5-15 min if needed; IV fluids; adjuncts: H1+H2 antihistamines, corticosteroids, β₂ inhaler |
| Hypoglycaemia counter-regulation | ↓Blood glucose triggers Epi release → glycogenolysis + gluconeogenesis | Hypothalamus → splanchnic nerve → chromaffin cells → Epi surge → hepatic glucose output + glucagon (β₂ on α-cells) | HAAF (hypoglycaemia-associated autonomic failure) in T1DM: repeated hypos → blunted Epi counter-regulatory response → hypoglycaemia unawareness → dangerous episodes |
| Takotsubo (Stress) Cardiomyopathy | Acute massive Epi surge → apical stunning | Intense psychological or physical stress → massive sympathetic activation → β₁/β₂ AR-mediated apical cardiomyocyte toxicity (direct cAMP/Ca²⁺ overload); apical sparing of β-AR density plays role | Supportive care; β-blockers; usually reversible; recurrence ~10%/year; not benign — acute heart failure, arrhythmia |
Phaeochromocytoma — alpha-before-beta rule: Tumours secreting predominantly Epi cause both ↑HR and ↑BP. NE-secreting tumours cause ↑BP + reflex bradycardia (baroreflex). In both, NEVER start a β-blocker first — blocking β₂-mediated vasodilation without α₁ block leaves unopposed α₁ vasoconstriction → extreme ↑↑ BP → hypertensive crisis. Alpha-block (phenoxybenzamine, irreversible) first — wait ~10–14 days for full effect + volume expansion (as α-block allows volume to expand) — then add β-blocker. Surgical resection is definitive treatment.
Connections
References
- Berg JM, Tymoczko JL, Stryer L. Biochemistry. 9th ed. W.H. Freeman; 2019. Macmillan Learning
- Blaschko H. The specific action of L-DOPA decarboxylase. J Physiol. 1939;96(1):50-51. doi:10.1113/jphysiol.1939.sp003748 — classic Blaschko pathway paper
- Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 7th ed. W.W. Norton; 2022. NCBI Bookshelf
- Kemp SF, Lockey RF, Simons FE. Epinephrine: the drug of choice for anaphylaxis — a statement of the World Allergy Organization. Allergy. 2008;63(8):1061-1070. doi:10.1111/j.1398-9995.2008.01716.x
- Lenders JW et al. Phaeochromocytoma. Lancet. 2005;366(9486):665-675. doi:10.1016/S0140-6736(05)67139-5 — diagnosis and management review