Specific high affinity binding of platelet activating factor to intact human blood neutrophils and eosinophils.

Neutrophils and eosinophils are involved in various inflammatory reactions such as leukocyte migration, adherence and phagocytosis. A regulation of platelet-activating factor (PAF) receptors in intact human blood neutrophils and eosinophils is clinically important. Intact human blood neutrophils and eosinophils prepared under sterile conditions specifically bound [3H]PAF in the presence of fatty acid-free serum albumin (0.25% BSA). Excess unlabeled PAF (500 nM) or the specific PAF receptor antagonist WEB 2086 (1 microM) inhibited the [3H]PAF binding. PAF receptors on the surface of intact blood neutrophils and eosinophils had high affinity Kd values of 0.55 and 2.3 nM at 4 degrees C. The Bmax values were 200 fmol/2.5 x 10(6) neutrophils and 26 fmol/2.5 x 10(5) eosinophils. PAF receptors on the outer plasma membranes were functionally relevant as high dose PAF displaced WEB 2086 after 3 min preincubation mediating maximal cytosolic [Ca2+]i flux. High doses of PAF or phorbol myristate acetate (PMA) downregulated neutrophils and a low dose PAF decreased the specific [3H]PAF binding to eosinophils determined with WEB 2086 at 20 degrees C. Only neutrophils were significantly upregulated by low dose PAF (5 nM), lyso PAF or low dose PMA (1 nM). Up- and downregulation by PAF itself of neutrophil and eosinophil PAF receptors might explain their desensitization and some clinical controversy concerning the role of PAF in inflammatory and allergic diseases. The latter hypothesis would lead to a novel combination of antagonists against PAF receptors and PAF production.