Effects of oxygen pressure and medium volume on the toxicity of paraquat in rat and human type II pneumocytes

The herbicide, paraquat is highly toxic for mammals, with the lungs being the main target organ, because of the active accumulation of the compound in this organ. The cellular toxicity of paraquat has been shown to be an O₂-driven process and hyperoxia is known to increase the lethality of paraquat. In this study we have examined the effect of various O₂ concentrations on the toxicity of paraquat in rat and human type II pneumocytes in culture, and we have tested whether the thickness of the liquid layer above the cells would influence the toxicity of paraquat. Type II pneumocytes were isolated from rat or human lung tissue using trypsin digestion, Percoll density gradient centrifu gation and differential attachment. Adherent cells (day 2) were incubated for 20 h in different volumes of culture medium (thickness of liquid layer), whether or not in the presence of paraquat, in the presence of different O₂ tensions. The viability of the cells was assessed by the release of LDH in the culture medium. In both rat and human type II pneumocytes the toxicity of paraquat was independent of the thickness of the liquid layer (2.5 to 10 mm height). The toxicity of paraquat in rat type II pneumocytes decreased from a TC₅₀ value of 28 μM paraquat at 21% O₂ to 107 μM at 10% O₂ and increased to 12 μM and 8 μM at 60% and 85% O₂, respectively. For human type II pneumocytes the TC₅₀ values were 7 μM; 25 μM and > 1000 μM paraquat at 60%, 21% and 10% O₂, respectively. In this study we have shown that the diffusion of O₂ through a liquid layer does not limit the toxicity of paraquat and that, as in vivo, increasing O₂ partial pressure enhances the toxicity of paraquat.