Figure 9: Mechanisms of action of S-petasin. S-Petasin mainly inhibits PDE3/4 activities and results in increase of cAMP, which activates cAMP-dependent protein kinase (PKA) and increases calcium extrusion from intracellular space and uptake to sarcoplasmic reticula (SR). S-Petasin was also reported to inhibit voltage-dependent calcium channels (VDCCs). Therefore, S-petasin largely decreases the concentration of intracellular calcium ([Ca2+]i) and results in trachea-bronchial smooth muscle relaxation. The increased cAMP also has anti-inflammatory and immunoregulatory effects. AC, adenylate cyclase. Up and down arrows indicate increases and decreases, respectively.