Hydrogen peroxide release by rat peritoneal macrophages in the presence and absence of tumor cells

The ability of mineral oil-elicited rat peritoneal macrophages to release hydrogen peroxide (H₂O₂) to the extracellular medium was measured in the presence and absence of rat lymphoma cells grown in tissue culture, and in the presence of phorbol myristate acetate (PMA). Horseradish peroxidase (HRP)-catalyzed oxidation of scopoletin or phenol red was used to measure H₂O₂ release during incubation of cells in monolayer culture for periods up to 24 h. Macrophages appeared to release H₂O₂ with or without PMA, although PMA greatly increased the amount of H₂O₂ released in short (1 to 4 h) incubations. Tumor cells did not replace PMA as a triggering agent for H₂O₂ release. Instead, tumor cells inhibited H₂O₂ release. The probable basis for inhibition was competition between macrophages and tumor cells for the supply of oxygen (O₂). Tumor cells did not inhibit H₂O₂ release when the O₂ concentration was held constant. The rates at which macrophages took up O₂ and released H₂O₂ were proportional to the O₂ concentration, as measured with the O₂ electrode. Rates of H₂O₂ release could be calculated from the difference in the rate constants for O₂ uptake measured in the presence of two different extracellular H₂O₂-consuming systems (HRP-scopoletin vs catalase). PMA-stimulated uptake of O₂ and release of H₂O₂ were highest in a small subpopulation of macrophages, obtained at the lowest-density position on gradients of bovine serum albumin. These cells also released H₂O₂ in the absence of PMA. Tumor cells had no effect on the rate constants for O₂ uptake and H₂O₂ release by the unfractionated macrophages or the macrophage subpopulations.