Muscarinic M3 receptor activation in bronchial smooth muscle results in bronchoconstriction. Which second messenger system is primarily responsible for this effect?

Explanation

## Muscarinic M3 Receptor Signaling in Airway Smooth Muscle ### G-Protein Coupling and Phospholipase C Activation **Key Point:** Muscarinic M3 receptors are coupled to Gq/11 proteins, which activate phospholipase C (PLC). PLC cleaves PIP2 into two second messengers: inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). ### IP3-Mediated Calcium Release and Contraction 1. IP3 binds to IP3 receptors on the sarcoplasmic reticulum 2. Calcium is released into the cytoplasm 3. Increased intracellular Ca²⁺ binds to calmodulin 4. Ca²⁺-calmodulin complex activates myosin light chain kinase (MLCK) 5. MLCK phosphorylates myosin light chains → cross-bridge cycling → bronchoconstriction ### DAG-Mediated Protein Kinase C Activation **High-Yield:** DAG remains membrane-bound and activates protein kinase C (PKC), which phosphorylates various contractile and regulatory proteins, potentiating and sustaining the contractile response. ### Contrast with Beta-2 Adrenergic Receptors (Bronchodilation) | Receptor | G-Protein | Second Messenger | Effect | |----------|-----------|------------------|--------| | M3 (muscarinic) | Gq/11 | ↑ IP3/DAG | Bronchoconstriction | | β2 (adrenergic) | Gs | ↑ cAMP | Bronchodilation | **Mnemonic:** **Gq-PLC-IP3-Ca²⁺-CONTRACTION** — remember that Gq proteins activate the phospholipase C cascade, leading to calcium mobilization and smooth muscle contraction. ### Clinical Pearl Anticholinergic agents (e.g., ipratropium, tiotropium) block M3 receptors in the airways, preventing IP3-mediated calcium release and providing bronchodilation in COPD and asthma — this is why they are effective in airway disease despite being older agents.