Inhibition of FMLP-stimulated neutrophil chemiluminescence by blood platelets increased in the presence of the serotonin-liberating drug chloroquine
Introduction: Previously, we reported that human blood platelets significantly decreased the concentration of reactive oxygen species (chemiluminescence) produced by Ca2+-ionophore-stimulated neutrophils and that the reduction was partially mediated by serotonin liberated from platelets during their activation. The aim of the present study was to investigate whether platelet inhibition can occur independently of serotonin liberation and whether it can be pharmacologically enhanced. Materials and methods: Chemiluminescence was measured after stimulation of human neutrophils with N-formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP) in the presence of luminophore luminol. Concentration of platelet serotonin was estimated fluorometrically. Results: Platelets, added to neutrophils in the physiological cell ratio 50:1, decreased neutrophil chemiluminescence by 47%. The inhibition was not accompanied with liberation of platelet serotonin and rose after addition of chloroquine to platelet-neutrophil samples. In the absence of platelets, this drug did not affect neutrophil chemiluminescence. Chloroquine actively liberated serotonin; amine concentrations found in platelet supernatants were sufficient to inhibit neutrophil chemiluminescence. Conclusions: The presented results indicate that unstimulated platelets decreased neutrophil chemiluminescence by a serotonin-independent mechanism, yet their inhibitory effect could be enhanced pharmacologically through chloroquine-induced serotonin liberation. (C) 2003 Elsevier Science Ltd. All rights reserved.