Classification of adrenotropic agents by mechanism of action
To understand the mechanism of action of adrenaline, it is necessary to review the general classification of drugs affecting adrenergic synapses. According to the mechanism of interference in synaptic transmission, three main groups of substances are distinguished.
The first group consists of direct-acting adrenomimetics. This includes adrenaline itself. These molecules have the ability to directly bind to and stimulate adrenoceptors on the membrane of target cells.
The second group comprises sympathomimetics, or indirect-acting agents. They do not stimulate receptors directly, but instead promote the release of endogenous noradrenaline from presynaptic nerve endings. A typical representative is ephedrine (ephedrine hydrochloride), which has a mixed action with a pronounced predominance of the indirect component.
The third group includes drugs of mixed action, which combine both of the mechanisms described above.
Receptor selectivity and the role of adrenaline
In addition to the mechanism of action, receptor selectivity is a critical criterion. Adrenoceptors are divided into alpha and beta types.
Epinephrine (adrenaline) is an α,β-adrenomimetic. It is a non-selective drug that stimulates absolutely all major receptor pools: α₁-, α₂-, β₁-, and β₂-adrenoceptors. It is precisely this broad spectrum that determines its powerful and diverse clinical effects.
By comparison, there are drugs with a narrower focus:
- α-adrenomimetics (stimulate predominantly α-receptors). For example, phenylephrine (mesaton).
- β-adrenomimetics (act on β-receptors). They can be non-selective, like isoprenaline (isadrin, isoproterenol), which affects β₁ and β₂, or selective, like dobutamine, which selectively stimulates only β₁-adrenoceptors.
Mechanism of action in anaphylactic shock
In clinical practice, adrenaline is the primary drug for treating acute life-threatening conditions, primarily anaphylactic shock. Anaphylaxis is accompanied by two critical manifestations: a sharp drop in blood pressure (hypotension) and severe bronchospasm.
The mechanism of action of epinephrine in anaphylaxis perfectly covers both of these pathological links:
- Relief of hypotension. The drug directly stimulates α-adrenoceptors located in blood vessels. This causes a pronounced pressor effect—blood vessels constrict, and blood pressure rises.
- Elimination of bronchial obstruction. At the same time, adrenaline acts as a potent symptomatic antiallergic agent due to the stimulation of bronchial β₂-adrenoceptors. Activation of these receptors leads to relaxation of the smooth muscles of the airways and relieves asphyxia.
To achieve the fastest possible clinical effect, adrenaline is administered intramuscularly or intravenously.
Comparative pharmacodynamics: Adrenaline and Noradrenaline
The closest analogue of adrenaline is norepinephrine (noradrenaline). Chemically, it fully corresponds to the natural neurotransmitter of the sympathetic nervous system. In medical practice, this drug is used as a salt—noradrenaline hydrogen tartrate.
Despite belonging to the same group of α,β-adrenomimetics, the action spectrum of noradrenaline has a critical difference: it exerts a direct stimulating effect on vascular α₁- and α₂-adrenoceptors, as well as cardiac β₁-adrenoceptors, but has virtually no effect on β₂-adrenoceptors.
This pharmacodynamic difference creates a different picture of the main effects:
- Effect on blood vessels and blood pressure. The mechanism of the pressor effect of noradrenaline consists of vasoconstriction due to the stimulation of α-receptors. This causes a pronounced but short-lived (a few minutes) increase in blood pressure. The main difference from epinephrine is that noradrenaline lacks a subsequent blood pressure drop phase. This is precisely explained by the absence of stimulation of vascular β₂-receptors, which are responsible for vasodilation.
- Effect on heart rate. Although noradrenaline has a direct stimulating effect on cardiac β₁-receptors, a paradoxical effect is observed in practice. Due to the sharp spike in blood pressure, reflex bradycardia occurs.
Adrenaline is a universal drug: it stimulates all 4 receptors (α₁, α₂, β₁, β₂). Noradrenaline "forgot" about β₂, so it does not dilate bronchi and blood vessels.
Class and exam questions
Why is adrenaline the drug of choice in anaphylactic shock?
Because it simultaneously eliminates two main life-threatening symptoms: it increases blood pressure (by constricting blood vessels via α-receptors) and relieves bronchospasm (by relaxing the smooth muscles of the bronchi via β₂-receptors).
What is the main difference between the action of noradrenaline and adrenaline?
Noradrenaline has virtually no effect on β₂-adrenoceptors. Because of this, the blood pressure reduction phase does not occur after its administration, as there is no β₂-mediated vasodilation.
Why does bradycardia occur upon administration of noradrenaline if it stimulates cardiac β₁-receptors?
Bradycardia is reflex in nature. It occurs as the body's response to a sharp and pronounced increase in blood pressure caused by potent stimulation of vascular α-adrenoceptors.
Which pharmacological group based on the mechanism of action does ephedrine belong to?
Ephedrine belongs to sympathomimetics (mixed-action drugs with a predominance of the indirect component). It promotes the release of noradrenaline from presynaptic terminals.
What else to study on this topic
- Pharmacodynamics of selective β₁-adrenomimetics (dobutamine)
- Mechanism of action of non-selective β-adrenomimetics (isoprenaline)
- Features of the use of α-adrenomimetics (phenylephrine)
- Mechanism of indirect action of sympathomimetics using the example of ephedrine
Master the entire topic in the «Pharmacology» course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on the "Mechanism of action of adrenaline" and another 5,304 pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 116 video lessons on the subject
Other topics in the section "Agents Regulating Immune Processes"
Free video lessons: pharmacologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.