What is the mechanism of action of theophylline?

Theophylline primarily works by inhibiting phosphodiesterase, an enzyme responsible for the breakdown of cyclic adenosine monophosphate (cAMP). By inhibiting phosphodiesterase, theophylline increases the levels of cAMP within cells. This elevation in cAMP leads to multiple beneficial effects, particularly in the respiratory system. Increased cAMP in airway smooth muscle promotes bronchodilation, thereby facilitating easier airflow in conditions such as asthma and chronic obstructive pulmonary disease (COPD).

In addition to its role in airway dilation, the increase in cAMP also enhances the activity of protein kinase A (PKA), which subsequently leads to decreased release of inflammatory mediators from mast cells and also aids in reducing vascular permeability. The overall effect of theophylline makes it an effective bronchodilator, contributing to its use in treating respiratory conditions.

The other options, while they pertain to different mechanisms of action, do not apply to theophylline's pharmacology. Sodium channel inhibition, beta-adrenergic receptor blockade, and muscarinic receptor stimulation relate to different drugs used for various indications, none of which describe the action of theophylline.