Macrophages as a major source of oxygen radicals in the hyperoxic newborn rat lung

The lungs of newborn rats exposed to 60% O2 for 14 d were found to have a greatly increased cyanide-insensitive O2 consumption, reflecting increased reactive oxygen species (ROS) formation. Exposure of the lung to hyperoxia is known to increase the production of ROS by mitochondria. We hypothesized that macrophages may also be a major contributor to this increase. Newborn rat pups were exposed to either air or 60% O2 for 14 d and received either intraperitoneal gadolinium chloride (GdCl3) to abrogate macrophage influx, or inert vehicle. Lung homogenates were equilibrated in either 21% or 100% O2 and total and cyanide-insensitive O2 consumption, as well as nitric oxide accumulation were measured polarographically. Citrate synthase, a marker of mitochondrial mass, and nitrotyrosine, a marker of peroxynitrite formation, were quantified by Western blot. In addition to increased macrophage numbers, the lungs of 60% O2-exposed animals had greatly increased cyanide-insensitive O2 consumption (p < .05 compared to air controls) and immunoreactive nitrotyrosine (p < .05), which were all completely abrogated by treatment with GdCl3. Exposure to 60% O2 for 14 d had no effect on peroxynitrite-independent nitric oxide release or mitochondrial mass. We conclude that increased ROS in the lungs of newborn rats exposed to 60% O2 for 14 d was likely to be caused, in significant part, by the presence of increased numbers of macrophages.