Selective inhibition of phosphodiesterase type IV suppresses the chemotactic responsiveness of rat eosinophils in vitro

Previous studies demonstrated that the selective inhibition of phosphodiesterase 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 chamber. 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 were obtained by lavage of the peritoneal cavity of normal Wistar rats and purified on Percoll gradients to 85-95% purity. Our results showed that PAF and leukotriene B-4 (0.001-10 mu M) elicited a concentration-dependent increase in eosinophil migration with maximal responses observed 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 association with elevation of eosinophil cyclic AMP, whereas the compounds milrinone and SK&F 94836 (type III selective) as well as zaprinast (type V selective) were ineffective. The beta(2)-adrenoceptor agonists salbutamol and salmeterol (1-100 mu M) did not alter the intracellular levels of cyclic AMP and also failed to inhibit the eosinophil response. Moreover, incubation of eosinophils with the adenylate cyclase activator forskolin (1-100 mu M), while inducing a discrete increase in cyclic AMP, markedly inhibited PAF- and leukotriene B-4-induced eosinophil chemotaxis. Eosinophils treated with a combination of individually inactive amounts of forskolin plus rolipram significantly inhibited the eosinophil migration elicited by PAF and leukotriene B-4, but did not change cyclic AMP baseline levels. Though only at the highest concentration tested (100 mu M), the analogue Bt(2) cyclic AMP abolished the eosinophil chemotaxis. Thus we conclude that the direct inhibitory effect of phosphodiesterase IV inhibitors on eosinophil chemotaxis may account for their suppressive activity on tissue eosinophil accumulation following antigen challenge.