SELECTIVE-INHIBITION OF PHOSPHODIESTERASE TYPE-IV SUPPRESSES THE CHEMOTACTIC RESPONSIVENESS OF RAT EOSINOPHILS IN-VITRO

Previous studies demonstrated that the selective inhibition of phospho diesterase type IV suppresses antigen-induced eosinophil infiltration and also downregulates certain eosinophil functions assessed in vitro. In the current study, we compared the effect of selective inhibitors of phosphodiesterase IV with the effect of phosphodiesterase III and V inhibitors, focusing on eosinophil chemotaxis stimulated by platelet- activating factor (PAF) and leukotriene B-4 in a modified Boyden chamb er. The effect of beta(2)-adrenoceptor agonists and forskolin as well as the analogue N6-2'-O-dibutyryladenosine 3':5'-cyclic monophosphate (Bt, cyclic AMP) was also determined. For this purpose eosinophils wer e obtained by lavage of the peritoneal cavity of normal Wistar rats an d purified on Percoll gradients to 85-95% purity. Our results showed t hat PAF and leukotriene B-4 (0.001-10 mu M) elicited a concentration-d ependent increase in eosinophil migration with maximal responses obser ved at 1 mu M and 0.1 mu M respectively. Pre-incubation with the type IV phosphodiesterase inhibitor, rolipram (1-100 mu M), suppressed the chemotactic response triggered by PAF and leukotriene B-4, in associat ion with elevation of eosinophil cyclic AMP, whereas the compounds mil rinone and SK&F 94836 (type III selective) as well as zaprinast (type V selective) were ineffective. The beta(2)-adrenoceptor agonists salbu tamol and salmeterol (1-100 mu M) did not alter the intracellular leve ls of cyclic AMP and also failed to inhibit the eosinophil response. M oreover, incubation of eosinophils with the adenylate cyclase activato r forskolin (1-100 mu M), while inducing a discrete increase in cyclic AMP, markedly inhibited PAF- and leukotriene B-4-induced eosinophil c hemotaxis. Eosinophils treated with a combination of individually inac tive amounts of forskolin plus rolipram significantly inhibited the eo sinophil migration elicited by PAF and leukotriene B-4, but did not ch ange cyclic AMP baseline levels. Though only at the highest concentrat ion tested (100 mu M), the analogue Bt(2) cyclic AMP abolished the eos inophil chemotaxis. Thus we conclude that the direct inhibitory effect of phosphodiesterase IV inhibitors on eosinophil chemotaxis may accou nt for their suppressive activity on tissue eosinophil accumulation fo llowing antigen challenge.