Adrenal Medullary hormones (Epinephrine and Nor-Epinephrine)

Adrenal Medullary Hormones

1. Chemistry

  • Belong to the catecholamine group (derived from the amino acid tyrosine).

  • Water-soluble hormones.

  • Stored in secretory vesicles (chromaffin granules) inside adrenal medulla.

  • Act via adrenergic receptors (α and β receptors) on target tissues.

2. Secretion

  • Secreted by chromaffin cells of the adrenal medulla.

  • Stimulated by sympathetic nervous system (fight-or-flight response).

  • Normal ratio: about 80% epinephrine, 20% norepinephrine.

  • Triggered by stress (fear, anger, hypoglycemia, exercise, trauma, pain).

3. Functions

Epinephrine (Adrenaline)

  • Acts mainly on β-adrenergic receptors.

  • Metabolic effects:

    • ↑ Blood glucose (stimulates glycogen breakdown in liver and muscle).

    • ↑ Lipolysis (fat breakdown).

  • Cardiovascular effects:

    • ↑ Heart rate (positive chronotropic effect).

    • ↑ Cardiac output.

    • Dilates skeletal muscle blood vessels (β₂ effect).

  • Respiratory effects:

    • Bronchodilation (helps in asthma, anaphylaxis).

Norepinephrine (Noradrenaline)

  • Acts mainly on α-adrenergic receptors.

  • Vasoconstriction (raises blood pressure).

  • Slight effect on heart (less than epinephrine).

  • Maintains vascular tone at rest (basal secretion).

4. Regulation

  • Controlled by sympathetic nervous system.

  • Preganglionic sympathetic fibers (cholinergic) release acetylcholine, which stimulates chromaffin cells to release catecholamines.

  • Stress (physical, emotional, hypoglycemia, cold exposure, exercise) → increases secretion.

  • Short half-life (about 2 minutes) → metabolized quickly in liver and kidney.

  • Inactivated by enzymes:

    • COMT (Catechol-O-Methyltransferase)

    • MAO (Monoamine Oxidase)

✅ In summary:

  • Epinephrine → “emergency hormone,” increases heart rate, bronchodilation, boosts glucose & energy.

  • Norepinephrine → “pressor hormone,” mainly increases blood pressure by vasoconstriction.

  • Together, they prepare the body for fight-or-flight response.

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