Isoprenaline

Q: What are the possible routes of administration for isoprenaline?

Possible routes of administration include: Sublingual route — tablets placed under the tongue are used to enhance atrioventricular conduction in AV block.Inhalation route — aqueous inhalation solutions are used in pulmonology as a bronchodilator.Lozenges / sublingual tablets are also utilized in respiratory therapy.

Isoprenalinum

For medical students2 min readUpdated 2026-10-10

Isoprenaline is a direct, non-selective $\beta$-adrenoceptor agonist that stimulates both $\beta_1$ and $\beta_2$ receptors. It is used in medical practice as a bronchodilator and to improve atrioventricular conduction.

Chemical StructureN-isopropylnoradrenaline (isoproterenol)

MechanismDirect non-selective $\beta$-adrenoceptor agonist

RisksMarked tachycardia and high risk of arrhythmias

Clinical UseBronchodilator and for conduction disorders

Chemical Structure and Pharmacological Properties

Chemically, isoprenaline (also known as isoproterenol) is N-isopropylnoradrenaline.

The core feature of its mechanism of action is that it acts as a direct non-selective $\beta$-adrenoceptor agonist. This means it simultaneously and potently stimulates both receptor subtypes:

$\beta_1$ receptors (located predominantly in the heart);
$\beta_2$ receptors (mediating the bronchodilatory effect).

Clinical Applications

The medical use of isoprenaline encompasses two main directions:

In pulmonology: the drug is used as a bronchodilator (delivered via aqueous inhalation solutions and sublingual/lozenges tablets).
For conduction disorders: the drug is prescribed to improve AV conduction in atrioventricular block (administered via sublingual tablets).

However, in modern clinical practice, it has been largely superseded as a bronchodilator by newer selective $\beta_2$-agonists due to a more favorable safety profile.

Adverse Effects and Cardiac Risks

Because isoprenaline non-selectively stimulates all beta-adrenoceptors, its adverse effects fall into two categories:

Effects resulting from $\beta_2$ receptor activation are similar to those of salbutamol.
Cardiac adverse effects from $\beta_1$ receptor stimulation are significantly more pronounced than those seen with selective agents.

Key cardiovascular risks:

Marked tachycardia.
Significant increase in myocardial oxygen demand.
High probability of developing arrhythmias.

Development of Tolerance

With prolonged use of any $\beta$-agonist (both selective and non-selective), tolerance inevitably develops—a gradual reduction in therapeutic response at a previously effective dose (tachyphylaxis).

Underlying mechanisms include:

Desensitization (reduced sensitivity) of the receptor apparatus.
Downregulation of receptor density and total number on cell membranes.

Process dynamics:

Tolerance develops first to various adverse effects (such as tremor or tachycardia).
A decrease in the therapeutic (bronchodilatory) effect occurs somewhat later.

Mnemonic

ISO-PRÉN-aline: PREN-ates the heart to tachycardia and arrhythmias through a non-selective hit on both types of beta receptors.

Frequently asked questions

What are the possible routes of administration for isoprenaline?

Possible routes of administration include:

Sublingual route — tablets placed under the tongue are used to enhance atrioventricular conduction in AV block.
Inhalation route — aqueous inhalation solutions are used in pulmonology as a bronchodilator.
Lozenges / sublingual tablets are also utilized in respiratory therapy.

Which adverse effects of isoprenaline are mediated by beta-2 adrenoceptor stimulation?

Adverse effects mediated by $\beta_2$-adrenoceptor stimulation are described as analogous to the effects of salbutamol.

Additionally, with $\beta$-agonists, tolerance develops first to side effects such as tremor and tachycardia upon prolonged use.

What is the mechanism of action of isoprenaline?

Isoprenaline is a direct non-selective $\beta$-adrenoceptor agonist. It stimulates both $\beta_1$ and $\beta_2$ adrenoceptors to a similar degree.

Why has isoprenaline been replaced by modern bronchodilators?

As a bronchodilator, it is inferior to modern selective $\beta_2$-agonists due to an unfavorable safety profile and prominent cardiac adverse effects.

What is the basis of tolerance to $\beta$-agonists?

Tolerance develops with prolonged drug use and presents as a diminished response to the same dose. This occurs due to receptor desensitization and a reduction in receptor density on cell membranes.

Go deeper

Role of Gq proteins and smooth muscle contraction
Formulations for sublingual and inhalation administration
Comparison of safety profiles with salbutamol
Molecular mechanisms of receptor desensitization
Pharmacokinetic features and chemical structure of N-isopropylnoradrenaline