## Theophylline: Mechanism of Bronchodilation **Key Point:** Theophylline is a methylxanthine that produces bronchodilation through dual mechanisms: phosphodiesterase (PDE) inhibition and non-selective adenosine receptor antagonism. ### Primary Mechanisms 1. **Phosphodiesterase Inhibition** - Inhibits PDE (particularly PDE3 and PDE4) - Increases intracellular cAMP in airway smooth muscle - cAMP → smooth muscle relaxation → bronchodilation 2. **Adenosine Receptor Antagonism** - Non-selective blockade of adenosine A₁ and A₂ receptors - Adenosine normally causes bronchoconstriction via A₁ receptors - Antagonism relieves this bronchoconstrictor effect ### Secondary Effects - Increases diaphragmatic contractility (respiratory muscle stimulation) - Anti-inflammatory effects (mild) - Increases mucociliary clearance **High-Yield:** Theophylline has a narrow therapeutic window (10–20 μg/mL); toxicity occurs at levels >20 μg/mL, causing arrhythmias, seizures, and GI distress. **Warning:** ~~Theophylline is a selective beta-2 agonist~~ — it is NOT; beta-2 agonists work via G-protein coupled receptor signalling, not PDE inhibition. **Clinical Pearl:** Theophylline is rarely used as monotherapy in modern asthma/COPD management due to narrow therapeutic index and the availability of safer alternatives (ICS, LABAs, anticholinergics).
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