[Characterization of platelet-activating factor (PAF)-receptors in neutrophilic and eosinophilic granulocytes]

It has recently been shown that the triazolodiazepine WEB 2086 inhibits the platelet-activating factor (PAF)-induced platelet aggregation, bronchoconstriction (J. Pharmacol. Exp. Therap. 1987; 241:974) and microvascular leakage (Br. J. Pharm. 1987; in press) in guinea pigs. In the present study we have determined the PAF antagonistic properties of WEB 2086 in human neutrophil and eosinophil leukocytes. In addition, we have characterized [3H] WEB 2086 as a radioligand in direct binding experiments with these cell types. PAF increased the beta-glucuronidase release from neutrophils with an EC50 of 138 nM. WEB 2086 caused a concentration-dependent inhibition of this PAF effect with an inhibition constant (Ki) of 11.9 nM. Binding of [3H] WEB 2086 to neutrophils was specific, reversible and saturable with a dissociation constant (KD) of 18.9 nM and a maximal number of binding sites of 40000 sites per cell. The characteristics of the binding sites were identical to those of PAF receptors. In human eosinophils PAF induced an increase in lucigenin-enhanced chemiluminescence with an EC50 of 2.5 nM. WEB 2086 competitively inhibited this response with a Ki of 16.4 nM. [3H] WEB 2086 bound to eosinophils with a KD of 18.5 nM, which is in good agreement with the Ki from the functional studies. Our results show that WEB 2086 is a potent and specific antagonist of PAF in human neutrophils and eosinophils. Also, [3H] WEB 2086 appears to be a suitable antagonist radioligand for labelling of PAF receptors in these inflammatory cells. WEB 2086 should be of value in further elucidation of the role of endogenous PAF in asthma through activation of inflammatory cells recruited to the lung.