Figure 3: Inhibition of PDE3- (a, b) or PDE4 (c, d)-induced cAMP hydrolysis by S-petasin (a, c), and the reference drugs, milrinone (b) and Ro 20-1724 (d). The activities of PDE3 and PDE4 in the presence of various concentrations of S-petasin or reference drugs, and the substrate (cAMP) were plotted according to the Lineweaver-Burk analysis. The K i value was determined from the equation of the apparent K m as a function of the inhibitor concentration (inset). Each value represents the mean ± SEM (n = 4).