Peroxynitrite exposure of cells cocultured with macrophages.

Peroxynitrite is a potent oxidant that is formed endogenously from NO and O2-. One approach for studying its toxicity involves the coculture of activated macrophages (producers of NO and O2-) with "target cells" (which make O2- but not NO). Because peroxynitrite concentrations in such experiments are too small and localized to measure, reaction-diffusion models were developed for situations ranging from isolated cells to many cells randomly distributed on a plate. In these models the macrophage population created a constant NO concentration in and near all cells, whereas fluxes of O2- into the media from individual macrophages led to localized extracellular formation of peroxynitrite. Target cells were found to have no effect on the peroxynitrite concentrations in other cells, whereas the macrophage effects were additive. The average concentration inside randomly distributed target cells increased with increasing macrophage number density, as entry of extracellular peroxynitrite grew in importance relative to intracellular formation. For high cell densities, large peroxynitrite membrane permeabilities, and low rates of intracellular synthesis, the surrounding macrophages were calculated to double or triple the peroxynitrite concentration in an average target cell. It was estimated that a macrophage enveloping a target cell could cause as much as a 10-fold increase in the concentration inside that target cell.