Chemical structure and mechanism of action
In its chemical structure, the drug is very close to adrenaline. The main structural difference is the absence of hydroxyl groups in 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 is used: ephedrine hydrochloride.
Ephedrine is classified as a sympathomimetic (indirect adrenomimetic) because it has a mixed action with a pronounced predominance of the indirect component:
- Indirect action (primary): the drug promotes the active release of the neurotransmitter norepinephrine from varicose varicosities of sympathetic nerve fibers at the presynaptic level.
- Direct action (secondary): directly stimulates $\alpha$- and $\beta$-adrenoceptors, but significantly weaker than adrenaline.
The efficacy of the drug is strictly tied to norepinephrine stores in sympathetic nerve terminals. At the same time, ephedrine itself is characterized by high stability: it is resistant to the degrading action of monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT) enzymes.
Pharmacological effects
The drug's effects are divided into peripheral (largely similar to the effects of adrenaline) and central.
Peripheral effects:
- Cardiovascular system: increases heart rate and contractility, constricts blood vessels (including when applied to mucous membranes), and increases blood pressure (BP).
- Respiratory system: causes pronounced bronchodilation.
- Vision: dilates the pupils (mydriasis) without affecting accommodation.
- Gastrointestinal tract: inhibits intestinal motility.
- Metabolism and musculature: increases blood glucose levels (hyperglycemia develops) and increases skeletal muscle tone.
Central effects (CNS action): The drug stimulates the respiratory and vasomotor centers of the medulla oblongata. In addition, it has a moderate psychostimulant effect, which is weaker than that of amphetamine: it reduces the feeling of fatigue, decreases the need for sleep, and increases overall performance.
Comparison with Adrenaline and Tachyphylaxis
Despite the similarity of effects, the pharmacodynamics of ephedrine differs significantly from adrenaline:
- Activity: in terms of the pressor effect (ability to raise blood pressure), ephedrine is significantly inferior to adrenaline. To obtain a similar spike in blood pressure, the dose of ephedrine must be approximately 50 times higher.
- 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 sharply. This is due to the rapid depletion of norepinephrine stores in presynaptic terminals.
Clinical use and safety
Due to its wide range of effects, the drug is used in various fields of medicine:
- Pulmonology: for stopping (subcutaneously) and preventing (orally, as part of medications such as "Teofedrin", "Solutan", "Broncholitin") bronchial asthma attacks.
- Cardiology and intensive care: in acute arterial hypotension, for the prevention of vascular collapse during spinal and epidural anesthesia, and also to improve AV conduction in blocks.
- Allergology and ENT: in anaphylactic shock, serum sickness, and topically for rhinitis (vasoconstriction relieves edema and exudation).
- Other fields: as an antidote in poisoning with CNS depressants (sleeping pills, narcotics); in neurology for narcolepsy; in urology for nocturnal enuresis.
Pharmacokinetics: The drug is well and completely absorbed from the gastrointestinal tract, can be administered orally (before meals), subcutaneously, and intramuscularly. The half-life is 3–6 hours; it is excreted by the kidneys.
Side effects and contraindications: Excessive sympathetic stimulation can cause tachycardia, elevated blood pressure, ventricular fibrillation, insomnia, tremor, loss of appetite, vomiting, hyperhidrosis, and urinary retention. The drug is contraindicated in insomnia, uncompensated hypertension, atherosclerosis, ventricular fibrillation, hyperthyroidism, and pheochromocytoma. Due to its stimulating effect on the CNS, it is classified as a doping agent and is banned in sports.
Briefly: answers to topical questions
Which specific subtypes of alpha- and beta-adrenoceptors does ephedrine act on?
The source does not specify the exact subtypes of $\alpha$- and $\beta$-adrenoceptors. It is stated that ephedrine stimulates the same $\alpha$- and $\beta$-adrenoceptor subtypes as adrenaline, but more weakly. Ephedrine is a mixed adrenomimetic: the primary mechanism is the release of norepinephrine from varicose varicosities of sympathetic nerve fibers, and the secondary mechanism is the direct stimulation of adrenoceptors. Efficacy depends on norepinephrine stores in sympathetic nerve terminals.
With which drug groups does ephedrine clinically interact and remain incompatible?
Ephedrine interacts with glucocorticoids: it increases their metabolic clearance, thereby weakening the effect of glucocorticoids. Drugs that prevent the release of norepinephrine from adrenergic nerve terminals weaken the effect of ephedrine. Against the background of MAO blockade, nialamide enhances the pressor effect of sympathomimetics, including ephedrine, due to the accumulation of norepinephrine and its massive release.
Ephedrine acts like a "strict manager" who forces the "employees" (vesicles with norepinephrine) to report for an emergency shift. If you pull them too often (every 10–30 minutes), the employees will simply run out, resulting in tachyphylaxis.
Class and exam questions
Why is ephedrine classified as a sympathomimetic even though it also acts on receptors?
It is classified as an indirect adrenomimetic because the mechanism of neurotransmitter (norepinephrine) release dominates over the direct stimulating effect on receptors.
What causes the development of tachyphylaxis upon ephedrine administration?
Tachyphylaxis is caused by the fact that with frequent injections, norepinephrine stores in presynaptic terminals are depleted faster than new ones can be synthesized.
What is the mechanism of action of the drug in nocturnal enuresis?
Ephedrine works on two levels simultaneously: central (reduces sleep depth) and peripheral (increases bladder sphincter tone).
What else to study on this topic
- Features of ephedrine metabolism and resistance to MAO/COMT
- Comparative analysis of direct and indirect adrenomimetic action
- Mechanism of the psychostimulant action compared to amphetamine
- Use of combination drugs containing ephedrine in pulmonology
- Cardiovascular complications in case of overdose
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