High concentrations of histamine stimulate equine polymorphonuclear neutrophils to produce reactive oxygen species.

Objective and Design: Because high concentrations of histamine are locally released in inflammation, we investigated the effects of supraphysiological doses of histamine on the production of reactive oxygen species (ROS) by neutrophils. ¶ Materials and Methods: Isolated equine neutrophils were activated by 10^-4 to 5 × 10^-3 M histamine. The production of ROS and free radicals was estimated by luminol-enhanced chemiluminescence (CL) and electron spin resonance (ESR) with spin trapping technique. In this model of histamine-stimulated neutrophils, we tested the antagonists of H1 and H2 histamine receptors, the role of Ca²⁺ and Mg²⁺, the role of staurosporine and pertussis toxin (inhibitors of protein kinase C and proteins G) and the effects of superoxide dismutase, catalase, hydroxyl radical scavengers (phenylalanine and mannitol) and N^G-monomethyl-L-arginine (L-NMMA), inhibitor of NO-synthase. ¶ Results: Histamine (from 10^-5 to 10^-3 M) stimulated neutrophils to produce CL and ESR signals characterized by spin adducts of superoxide anion and/or hydroxyl radicals. The CL response was inhibited by 10^-4 and 10^-3 M H1 receptor antagonists (promethazine, pyrilamine, and diphenhydramine), by Ca²⁺ and Mg²⁺ depletion and by 10 nmoles staurosporine. CL was partially inhibited by pertussis toxin (4 μg/ mL). The ESR signals were practically suppressed by pyrilamine (an H1 receptor antagonist) and superoxide dismutase, and partially inhibited by catalase, hydroxyl radical scavengers and L-NMMA (respectively 59, ± 30% and 68% inhibition). ¶ Conclusions: High concentrations of histamine stimulated the neutrophils to product ROS and free radicals via H1 receptors and the NADPH-oxidase pathway. [ABSTRACT FROM AUTHOR]