Diffusion and Reaction of Nitric Oxide in Suspension Cell Cultures

AbstractA reaction-diffusion model was developed to predict the fate of nitric oxide (NO) released by cells of the immune system. The model was used to analyze data obtained previously using macrophages attached to microcarrier beads suspended in a stirred vessel. Activated macrophages synthesize NO, which is oxidized in the culture medium by molecular oxygen and superoxide (O2−, also released by the cells), yielding mainly nitrite (NO2−) and nitrate (NO3−) as the respective end products. In the analysis the reactor was divided into a “stagnant film” with position-dependent concentrations adjacent to a representative carrier bead and a well-mixed bulk solution. It was found that the concentration of NO was relatively uniform in the film. In contrast, essentially all of the O2− was calculated to be consumed within ∼2μm of the cell surfaces, due to its reaction with NO to yield peroxynitrite. The decomposition of peroxynitrite caused its concentration to fall to nearly zero over a distance of ∼30μm from the cells. Although the film regions (which had an effective thickness of 63μm) comprised just 2% of the reactor volume and were predicted to account for only 6% of the NO2− formation under control conditions, they were calculated to be responsible for 99% of the NO3− formation. Superoxide dismutase in the medium (at 3.2μM) was predicted to lower the ratio of NO3− to NO2− formation rates from near unity to