Carbon dioxide differentially affects the cytokine release of macrophage subpopulations exclusively via alteration of extracellular pH

The improved outcome after endoscopic surgery has been attributed to less surgical trauma. However, the underlying mechanisms are not fully understood, and direct effects of CO2 used for pneumoperitoneum, cellular acidification, and/or the lack of air contamination have been postulated to additionally modulate immune functions during endoscopic surgery. We investigated the effects of CO2 incubation, extracellular acidification, and air contamination on the inflammatory response of two distinct macrophage populations. R2 and NR 8383 rat macrophage cell lines were used. Interleukin-6 (IL-6) and nitric oxide after lipopolysaccharide (LPS) stimulation were determined in these sets of experiments: incubation in 100% CO2, 5% CO2, and room air for 2h; incubation at pH 7.4, 6.5, and 5.5 for 2 h in 5% CO2; and incubation in 100% CO2, 5% CO2 and room air in fixed pH 6.3. The extracellular pH was monitored during incubation. We determined the alteration of intracellular pH in cells subjected to extracellular acidification by fluorescence microscopy. Extracellular pH decreased to 6.3 during 100% CO2 incubation. IL-6 release was reduced after CO2 incubation in NR 8383 cells and increased in R2 cells (p < 0.05). It was not altered by air incubation. Decreasing the extracellular pH to 6.5 mimicked the effects of CO2 and a decrease to 5.5 suppressed IL-6 release in both cell lines. In fixed pH at 6.3, CO2 and air incubation had no effect. CO2 and pH had no impact on nitric oxide release and vitality. Intracellular pH decreased with extracellular acidification without significant difference between the two cell lines. A decrease in extracellular pH during incubation in CO2 differentially affects IL-6 release in macrophage subpopulations. This may explain contradictory results in the literature. Moreover, we demonstrated that air contamination does not affect macrophage cytokine release. The decrease in extracellular pH is the primary underlying mechanism of the alteration of macrophage cytokine release after CO2 incubation, and it appears that the ability to maintain intracellular pH is not determined by the effects of CO2 or extracellular acidification.