Epinephrine (Adrenaline)

1. Overview

Epinephrine, commonly known as adrenaline, is a powerful catecholamine hormone and neurotransmitter that plays a central role in the body’s acute stress ("fight or flight") response. It is released rapidly during physical or emotional stress and has widespread effects on multiple organ systems. Epinephrine increases heart rate, blood pressure, airway dilation, and blood glucose levels to prepare the body for immediate action. It is also used therapeutically in emergency medicine, particularly in cases of anaphylaxis and cardiac arrest.

2. Location

Epinephrine is synthesized and secreted by the chromaffin cells of the adrenal medulla, which is the inner region of the adrenal gland. The adrenal glands are located atop each kidney. The secretion of epinephrine is stimulated by sympathetic preganglionic neurons through cholinergic signaling. Though produced in the adrenal medulla, epinephrine acts as both a hormone (via the bloodstream) and a neurotransmitter in the central and peripheral nervous systems.

3. Structure

Epinephrine is a catecholamine, derived from the amino acid tyrosine. Its chemical formula is C9H13NO3. Structurally, it contains a catechol group (a benzene ring with two hydroxyl groups) and an amine group, which make it highly polar and water-soluble. It is synthesized from norepinephrine by the action of the enzyme phenylethanolamine N-methyltransferase (PNMT) in the adrenal medulla.

4. Function

Epinephrine acts through adrenergic receptors (α and β subtypes) to produce rapid physiological changes in response to stress:

• Cardiovascular system: Increases heart rate (positive chronotropy), contractility (positive inotropy), and cardiac output. It also induces vasoconstriction or vasodilation depending on receptor subtype and tissue.

• Respiratory system: Relaxes bronchial smooth muscle via β2-adrenergic receptors, leading to bronchodilation.

• Metabolism: Stimulates glycogenolysis in the liver and lipolysis in adipose tissue, raising blood glucose and free fatty acid levels.

• Ocular effects: Causes pupil dilation (mydriasis) via α1-adrenergic stimulation.

• Gastrointestinal system: Reduces GI motility and secretion during stress.

5. Physiological role(s)

Epinephrine is critical for survival in situations requiring rapid response and energy mobilization:

• Fight or flight response: Epinephrine prepares the body for rapid action during acute stress by increasing oxygen and energy delivery to muscles and vital organs.

• Blood pressure regulation: In low-volume states (e.g., shock), it helps restore blood pressure by increasing vascular tone and cardiac output.

• Energy mobilization: Enhances glucose availability by stimulating gluconeogenesis, glycogenolysis, and lipolysis.

• Respiratory efficiency: Bronchodilation improves oxygen intake, especially important during physical exertion or allergic reactions.

• Homeostatic support during exercise: Maintains blood glucose and circulatory dynamics during prolonged physical activity.

6. Clinical Significance

Epinephrine has critical diagnostic and therapeutic applications and is associated with various clinical conditions:

• Anaphylaxis:

  • Epinephrine is the first-line treatment for anaphylaxis. It reverses airway constriction, raises blood pressure, and reduces edema through vasoconstriction and bronchodilation.

  • Administered intramuscularly via auto-injectors (e.g., EpiPen).

• Cardiac Arrest:

  • Used during advanced cardiac life support (ACLS) protocols to restore circulation by increasing coronary and cerebral perfusion.

• Asthma and bronchospasm:

  • Emergency inhaled or injected epinephrine can relieve acute severe bronchospasm via β2-mediated bronchodilation.

• Pheochromocytoma:

  • A catecholamine-secreting tumor of the adrenal medulla that may release excess epinephrine, causing episodic hypertension, tachycardia, palpitations, and sweating.

• Drug interactions and synthetic analogs:

  • Sympathomimetic drugs mimic epinephrine’s effects; beta-blockers may antagonize them.

  • Epinephrine is used in local anesthesia to prolong action and reduce bleeding via vasoconstriction.