Solubility of nickel oxide particles in various solutions and rat alveolar macrophages

The solubility of five types of commercial nickel oxide particles was determined in different types of solutions, including distilled water, physiological saline, buffered saline, and tissue-culture medium, in order to estimate their solubility in the human respiratory tract. In addition, we examined the solubility of the two types of particles that were the most and least soluble particles of the above five types of nickel oxide, in rat alveolar macrophages cultured in vitro. The solubility of the nickel oxide particles in these solutions varied remarkably with their types, suggesting that, even though they are called as “nickel oxide,” their solubilities are different among the manufacturer and the product lot. Their solubilities were also influenced by the types of solution and the existence of carbon dioxide in the atmosphere. The solubility of nickel oxide particles in the alveolar macrophages was significantly larger than that observed in the culture medium without macrophages, but smaller than that observed in the distilled water. These results suggest that the actual solubility of nickel oxide particles in the respiratory tract may be difficult to estimate by the conventional solubility analysis method using distilled water, and that the enhancement of particle dissolution by the alveolar macrophages and the depression of particle solubility by the coexisting salt and carbon dioxide should be taken into consideration for the accurate estimation.