Chemical Structure and Mechanism of Action
Structurally, the drug is very close to epinephrine. The primary structural difference is the absence of hydroxyl groups on the aromatic ring (phenyl ring). The natural alkaloid is a levorotatory isomer, whereas the synthetic analogue is a racemic mixture of levo- and dextrorotatory isomers. In medical practice, the salt ephedrine hydrochloride is used.
Ephedrine is classified as a sympathomimetic (indirect-acting adrenomimetic) because it possesses a mixed mechanism of action with a clear predominance of the indirect component:
- Indirect action (primary): The drug promotes the active release of norepinephrine from varicosities of sympathetic nerve fibers at the presynaptic level.
- Direct action (secondary): It directly stimulates $\alpha$- and $\beta$-adrenoceptors, though significantly weaker than epinephrine.
The efficacy of the drug is strictly dependent on the stores of norepinephrine in sympathetic nerve terminals. At the same time, ephedrine itself is highly stable: it is resistant to degradation by monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT).
Pharmacological Effects
The drug's effects are divided into peripheral (largely analogous to epinephrine) and central.
Peripheral Effects:
- Cardiovascular system: Increases heart rate and myocardial contractility, constricts blood vessels (including when applied topically to mucous membranes), and raises blood pressure (BP).
- Respiratory system: Produces pronounced bronchodilation.
- Eyes: Causes pupil dilation (mydriasis) without affecting accommodation.
- Gastrointestinal tract: Inhibits intestinal peristalsis.
- Metabolism and musculature: Elevates blood glucose levels (hyperglycemia) and increases skeletal muscle tone.
Central Effects (CNS Impact): Stimulates the respiratory and vasomotor centers of the medulla oblongata. Additionally, it exhibits a moderate psychostimulatory effect—weaker than that of amphetamine—which reduces fatigue, decreases the need for sleep, and enhances overall performance.
Comparison with Epinephrine and Tachyphylaxis
Despite similarities in their effects, the pharmacodynamics of ephedrine differ significantly from epinephrine:
- Potency: In terms of its pressor effect (ability to raise blood pressure), ephedrine is substantially inferior to epinephrine. To achieve an equivalent blood pressure spike, the dose of ephedrine must be approximately 50 times greater.
- Duration: The effect lasts much longer—from 1 to 1.5 hours.
A specific feature of ephedrine is the phenomenon of tachyphylaxis (rapid tolerance). If the drug is administered repeatedly at short intervals (every 10–30 minutes), its pressor effect drops precipitously. This is due to the rapid depletion of norepinephrine stores in the presynaptic terminals.
Clinical Application and Safety
Due to its broad spectrum of effects, the drug is used in various medical fields:
- Pulmonology: For aborting (subcutaneously) and preventing (orally, in combination products) bronchial asthma attacks.
- Cardiology and Critical Care: For acute hypotension, prevention of vascular collapse during spinal and epidural anesthesia, and improving atrioventricular conduction in heart blocks.
- Allergology and ENT: In anaphylactic shock, serum sickness, and topically for rhinitis (vasoconstriction reduces edema and exudation).
- Other fields: As an antidote for poisoning by CNS depressants (sedatives, narcotics); in neurology for narcolepsy; in urology for nocturnal enuresis.
Pharmacokinetics: The drug is well and completely absorbed from the gastrointestinal tract and can be administered orally (before meals), subcutaneously, and intramuscularly. The elimination half-life is 3–6 hours, and it is excreted by the kidneys.
Adverse Effects and Contraindications: Excessive sympathetic stimulation can cause tachycardia, hypertension, ventricular fibrillation, insomnia, tremor, anorexia, vomiting, hyperhidrosis, and urinary retention. The drug is contraindicated in insomnia, uncompensated hypertension, atherosclerosis, ventricular fibrillation, hyperthyroidism, and pheochromocytoma. Due to its CNS-stimulating properties, it is classified as a doping agent and banned in sports.