PDE4A and asthma: Since PDE4D has previously been shown to be the specific PDE4 subtype involved in the modulation of cAMP production in ASM cells [25], we tested the hypothesis that the enhanced activity is due to increased protein expression by measuring the amount of PDE4D by western blots, and found significantly increased expression in ASM cells from patients with asthma compared to patients without asthma (Fig. 4A and B).