Differential effects of the PAF receptor antagonist UK‐74,505 on neutrophil and eosinophil accumulation in guinea‐pig skin

Abstract

1. The effect of the dihydropyridine, platelet activating factor (PAF) receptor antagonist, UK-74,505, on leucocyte accumulation and oedema formation in guinea-pig skin was investigated. The inflammatory reactions studied were elicited by exogenous mediators, a passive cutaneous anaphylactic (PCA) reaction and zymosan particles. 2. Leucocyte accumulation and oedema formation were measured as the local accumulation of i.v. administered 111In-labelled neutrophils or eosinophils together with 125I-labelled albumin. UK-74,505 was either administered i.v. or used to pretreat the radiolabelled leucocytes in vitro prior to their last wash and injection into recipient animals. 3. In vitro, UK-74,505 inhibited PAF-induced elevations in cytoplasmic levels of Ca2+ ([Ca2+]i) in fura-2-loaded guinea-pig neutrophils and eosinophils with IC50 values of 10(-9) M and 7 x 10(-9) M respectively. Neutrophils and eosinophils pretreated with 10(-7) M and 10(-6) M UK-74,505 respectively, and maintained at 37 degrees C, were unresponsive to PAF for the 4 h period investigated. 4. In vivo, using 2 h test periods, i.v. UK-74,505 (0.5 and 2.5 mg kg-1) inhibited the accumulation of 111In-neutrophils, 111In-eosinophils and oedema formation induced by intradermal PAF, but had no effect on responses elicited by leukotriene B4 (LTB4) and zymosan-activated plasma (ZAP, used as a source of C5a des Arg). UK-74,505 (2.5 mg kg-1) was also without an effect on response induced by a PCA reaction but significantly suppressed the 111In-eosinophil accumulation following the intradermal administration of zymosan particles. The 111In-neutrophil accumulation induced by zymosan particles was not, however, affected by UK-74,505. 5. In a second series of in vivo experiments, "'In-leucocytes were pretreated in vitro with UK-74,505 prior to their last wash and injection into recipient animals. Radiolabelled neutrophils, and eosinophils were pretreated with 10-7 M and 10-6 M UK-74,505 respectively, concentrations previously shown to block the leucocyte responses to PAF in vitro for up to 4 h. The in vitro pretreatment of the cells with the PAF antagonist, whilst not affecting the responses to intradermally-injected PAF, suppressed the"'In-eosinophil accumulation response induced by zymosan particles.6. The results of this study indicate that PAF is not involved in neutrophil accumulation, eosinophil accumulation and oedema formation induced by LTB4, ZAP and a PCA reaction. Endogenous PAF does, however, appear to have a role in zymosan-induced eosinophil accumulation but not neutrophil accumulation, suggesting the existence of different inflammatory pathways in the induction of neutrophil and eosinophil accumulation in vivo. Furthermore, while leucocyte accumulation induced by exogenous PAF does not appear to involve leucocyte PAF receptors, the mechanism by which endogenous PAF mediates the zymosan-induced eosinophil accumulation appears dependent on the expression of PAF receptors on eosinophils.