Mechanism of action of theophylline –
**Core Concept**
Theophylline is a methylxanthine derivative used in the management of chronic obstructive pulmonary disease (COPD) and asthma. It acts by inhibiting phosphodiesterase enzymes, leading to an increase in intracellular cyclic adenosine monophosphate (cAMP) levels, which in turn relaxes smooth muscle and increases airway diameter.
**Why the Correct Answer is Right**
Theophylline's mechanism of action primarily involves the inhibition of phosphodiesterase type 3 (PDE3) and to a lesser extent, phosphodiesterase type 4 (PDE4). This inhibition results in the accumulation of cAMP within airway smooth muscle cells, which triggers the relaxation of airway smooth muscle and leads to bronchodilation. Additionally, theophylline also exhibits adenosine receptor antagonism, which contributes to its bronchodilatory effect.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as theophylline does not primarily act by blocking histamine receptors (H1 receptors).
**Option B:** This option is incorrect as theophylline does not primarily act by inhibiting the enzyme monoamine oxidase (MAO).
**Option C:** This option is incorrect as theophylline does not primarily act by stimulating the enzyme adenylate cyclase.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that theophylline has a narrow therapeutic index, meaning the difference between an effective dose and a toxic dose is small. Regular monitoring of serum theophylline levels is crucial to avoid toxicity, which can manifest as seizures, arrhythmias, and other systemic effects.
**Correct Answer: D. Inhibition of phosphodiesterase enzymes leading to increased intracellular cAMP levels.**