Signaling pathway of NO-induced SMC relaxation in mouse small airways. Agonists, such as 5-HT, stimulate airway contraction by binding to their specific G protein–coupled membrane receptors (5-HT₂R) to stimulate the dissociation of their α subunit (G_qα). This G_qα activates phospholipase C β (PLCβ) to synthesize IP₃ that, in turn, activates SR IP₃Rs to release Ca²⁺ from the SR. The resulting high [Ca²⁺]_i inactivates the IP₃R, and the Ca²⁺ is pumped back into the SR by the SR/ER Ca²⁺ ATPase (SERCA), resulting in a decrease in [Ca²⁺]_i and reactivation of IP₃R. The cyclic release from and reuptake of Ca²⁺ into the SR results in Ca²⁺ oscillations. Ca²⁺ activates, via calmodulin, myosin light chain (MLC) kinase (MLCK) to phosphorylate myosin (MLC-P) and initiate SMC contraction. The simultaneous inactivation of MLC phosphatase (MLCP) by agonists, i.e., via Rho kinase (ROK), enhances MLC phosphorylation and SMC contraction. The relative activities of MLCK and MLCP determine the contractile state of the SMC. NO induces airway relaxation by the activation of sGC to synthesize cGMP from GTP. ODQ specifically inhibits sGC activity. The elevation of cGMP activates PKG, which inhibits the IP₃R, resulting in a lowering of the frequency of Ca²⁺ oscillations, deactivation of MLCK, and airway relaxation. The cGMP analogue Rp-8-pCPT-cGMPS specifically inhibits PKG activity. In addition, cGMP activates PDE-5 to convert cGMP to inactive GMP. ZAP inhibits PDE-5 to maintain high cGMP concentrations.