Figure 4: General scheme of intracellular mechanisms of cholinergic and adrenergic stimulation of airway myocyte. Stimulation of muscarinic receptor 3 (M3) activates phospholipase C (PLC) and Ins3P production and Ca2+ release from the sarcoplasmic reticulum (SR), with little extracellular Ca2+ influx. Ca2+ binds to calmodulin (CaM) and activates myosin light chain kinase (MLCK) that phosphorylates the regulatory myosin light chain (M-P) leading to contraction. Additionally, M3 receptor stimulation activates Rho Kinase (RhoK) and protein kinase C (PKC) that inhibit myosin light chain phosphatate (MLCP), resulting in increased myosin phosphorylation and contraction. Stimulation of β2 adrenoceptor activates adenylyl cyclase and subsequent cyclic AMP production (cAMP), which activates protein kinase A (PKA). PKA inhibits MLCK and hence contrac tion and, additionally, may reduce Ca2+ release from the SR.