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Background:Prior investigations have demonstrated that β2-adrenoceptor stimulation is ineffective in inhibiting synthesis of eicosanoids in human eosinophils. This effect has been postulated to relate to density or structural differences in the β2-adrenoceptor or its coupled G-protein. However, recent reports indicate that cAMP-specific PDE4 activity in eosinophils is 10-fold that of other inflammatory cells. We postulated that selective blockade of PDE4 in eosinophils would unmask the inhibitory effect of β2-adrenoceptor stimulation and that this inhibition would result from decreased phosphor-ylation of cytosolic group IV-PLA2(gIV-PLA2). Objective:To determine (a) whether PDE4 inhibition alone with rolipram blocked secretions of arachidonic acid (AA) and leukotriene C4(LTC4) caused by activation of eosinophils with formyl-met-leu-phe plus cytochalasin B (FMLP/B), (b) to determine if PDE4 inhibition plus β2-adrenoceptor agonist act additively to augment endogenous cAMP concentration, and (c) to determine the mechanism by which additive inhibition of AA and LTC4synthesis is regulated by cAMP. Methods:Human eosinophils were pretreated with buffer, salmeterol or rolipram (singly or combination) before FMLP/B activation. Release of AA and LTC4, intracellular cAMP concentration, and phosphorylation and activation of gIV-PLA2were determined. Results:Rolipram unmasked the inhibitory effect of β2-adrenoceptor stimulation with salmeterol and significantly attenuated the stimulated release of AA and subsequent LTC4. Inhibition corresponded to increased cAMP production caused by rolipram alone or rolipram plus salmeterol and blocked proportionately the phosphorylation and activation of gIV-PLA2in FMLP/B-activated eosinophils. Conclusions:Inhibition of PDE4 by rolipram unmasks β2-adrenergic blockade of LTC4synthesis caused by FMLP/B. (J Allergy Clin Immunol 2003;112:404-10.)