SN3
SHORT NOTE
5M
★ High-yield
Describe the hormones of the adrenal medulla and their actions. Differentiate adrenaline from noradrenaline.
Nature and Source
The adrenal medulla is a modified sympathetic ganglion; its chromaffin cells are postganglionic neurones without axons, innervated directly by preganglionic sympathetic fibres from T5–T11.
- Secretes adrenaline 80% and noradrenaline 20%
- Also dopamine and enkephalins
- Adrenaline is formed from noradrenaline by PNMT, an enzyme induced by cortisol arriving from the cortex through the portal vessels — hence adrenaline is made only in the medulla
Synthesis
Tyrosine → Tyrosine hydroxylase (rate-limiting) → dopa → Dopamine → Noradrenaline → PNMT → Adrenaline
- Stored in chromaffin granules; released by exocytosis, which is Ca2+ dependent
- Metabolised by MAO and COMT → excreted as VMA and metanephrines
- Plasma half-life only 1–3 minutes
Actions — the "fight or Flight" Response
| System | Effect | Receptor |
|---|---|---|
| Heart | ↑ Rate, force, conduction, excitability | β1 |
| Blood vessels | Constriction in skin and viscera (α1); dilatation in skeletal muscle (β2) | α, β2 |
| Bronchi | Bronchodilatation | β2 |
| Metabolism | ↑ Glycogenolysis, gluconeogenesis, lipolysis → hyperglycaemia | β2, β3 |
| Eye | Mydriasis | α1 |
| GI, bladder | Relaxation of wall, contraction of sphincters | α, β2 |
| CNS | Alertness, anxiety | — |
| BMR | ↑ Up to 100% | — |
Adrenaline VS Noradrenaline
| Feature | Adrenaline | Noradrenaline |
|---|---|---|
| Receptors | α and β (both) | Chiefly α |
| Systolic BP | ↑ | ↑ |
| Diastolic BP | ↓ or unchanged | ↑ |
| Peripheral resistance | ↓ | ↑↑ |
| Heart rate | ↑ | ↓ (reflex bradycardia) |
| Cardiac output | ↑ | Unchanged or ↓ |
| Metabolic effect | Powerful | Weak |
| Bronchodilatation | Marked | Slight |
Applied Aspects
- Phaeochromocytoma — catecholamine-secreting tumour; episodic hypertension with headache, palpitations and sweating. Diagnosed by urinary VMA and metanephrines. The "rule of 10" — 10% bilateral, 10% extra-adrenal, 10% malignant, 10% familial
- α-blockade must precede β-blockade in phaeochromocytoma, or unopposed α action causes a hypertensive crisis
- Adrenaline is life-saving in anaphylaxis — it reverses bronchospasm and hypotension simultaneously
- The adrenal medulla is not essential to life, unlike the cortex — the sympathetic nerves can compensate
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