Stimulation of beta-2 adrenergic receptors in bronchial smooth muscle leads to bronchodilation. The intracellular signaling cascade involves:

• A Gq-protein → phospholipase C → IP3 → intracellular Ca2+ release → calmodulin activation → MLCK activation
• B Gs-protein → adenylyl cyclase → cAMP → PKA → phosphorylation of myosin light-chain kinase (MLCK) reducing its activity → reduced cross-bridge formation ✓
• C Gi-protein → decreased cAMP → protein kinase C activation → smooth muscle contraction
• D Beta-2 receptor directly gates a K+ channel causing hyperpolarization and relaxation

Explanation

Beta-2 adrenergic receptors are coupled to Gs (stimulatory G-protein), which activates adenylyl cyclase to convert ATP to cAMP. Elevated cAMP activates protein kinase A (PKA), which phosphorylates multiple targets: (1) it phosphorylates and inactivates myosin light-chain kinase (MLCK), reducing the phosphorylation of myosin light chain and decreasing cross-bridge cycling; (2) it promotes Ca2+ reuptake into the sarcoplasmic reticulum by phosphorylating phospholamban; (3) it activates large-conductance K+ channels (BKCa), causing hyperpolarization. The net result is smooth muscle relaxation and bronchodilation. Beta-1 receptors in the heart use the same Gs-cAMP-PKA cascade to increase contractility (positive inotropism).

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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