Figure 1

Indirect and direct relaxation of smooth muscle. Muscarinic antagonists (X) block M₃ receptors to prevent binding of acetylcholine (ACh), indirectly stimulating smooth muscle relaxation via inhibition of bronchoconstriction. β₂-agonists interact with β₂ receptors (β₂R) to activate coupling of the stimulatory G protein (G_s) with adenylcyclase (AC). This leads to enhanced production of cyclic adenosine monophosphate (cAMP) which activates protein kinase A (PKA) and results in smooth muscle relaxation. β₂-agonists may also interact with presynaptic β₂Rs at parasympathetic ganglia, modulating parasympathetic neurotransmission.