What is the mechanism of action of orceprenaline (metaproterenol) in treating respiratory conditions such as asthma or chronic obstructive pulmonary disease (COPD)?

Orciprenaline (Metaproterenol) Mechanism of Action

Orciprenaline (metaproterenol) is a selective β2-adrenergic receptor agonist that produces bronchodilation by directly stimulating β2-receptors on airway smooth muscle cells, leading to smooth muscle relaxation and airway opening.

Primary Mechanism: β2-Adrenergic Receptor Stimulation

• Orciprenaline selectively binds to and activates β2-adrenergic receptors located on bronchial smooth muscle 1
• This receptor activation triggers intracellular signaling cascades that result in smooth muscle relaxation and bronchodilation 1
• The drug demonstrates selectivity for β2-receptors over β1-receptors (cardiac), which provides bronchodilation with reduced cardiac side effects compared to non-selective agents like isoproterenol 1

Duration and Onset of Action

• Bronchodilation begins within 30 seconds of inhalation 1
• As a short-acting β2-agonist, metaproterenol produces significantly prolonged action on both small and large airways compared to non-selective agents 1
• The prolonged activity affects lung volumes and provides sustained bronchodilation 1

Clinical Relevance in Airway Disease

Effects on Different Airway Segments

• Metaproterenol produces bronchodilation in both small airways and large airways 1
• The drug effectively improves lung volumes (FVC and FEV1) in patients with asthma and COPD 2

Comparative Efficacy

• In asthmatic patients with severe airflow obstruction (FEV1 <50% predicted), metaproterenol increases FEV1 by approximately 33-49% from baseline 3
• In COPD patients, the drug increases FEV1 from baseline values, though the absolute improvement may be more modest due to the partially irreversible nature of COPD obstruction 2

Additional Mechanisms Beyond Bronchodilation

While the primary mechanism is direct smooth muscle relaxation, β2-agonists as a class may exert other clinically relevant effects 4:

• Inhibition of inflammatory mediator release from airway cells 4
• Stimulation of mucociliary transport 4
• Cytoprotection of respiratory mucosa 4
• Attenuation of neutrophil recruitment and activation 4
• Modulation of immune and endocrine responses that affect respiratory function 5

Important Clinical Caveats

• Metaproterenol is a short-acting β2-agonist, not a long-acting agent, and should be used for acute symptom relief rather than maintenance therapy 6
• The drug does not have anti-inflammatory effects and should not be used as monotherapy for persistent asthma or COPD 6
• Delivery method (metered-dose inhaler vs. jet nebulizer) does not significantly affect bronchodilator efficacy when proper technique is used 2, 3