The effect of N-acetylcysteine on the microscopic fluid dynamics of rat tears

BACKGROUND: The classical view of the tear film is of a 10-micron film of aqueous tears, sandwiched between thin layers of lipid and mucus. This has been challenged recently by the revelation that the tear film may be considerably thicker than 10 microns and that dissolved mucus and protein may play a much more important role than simply promoting tear adherence. In particular, the primary role of mucus may be to form a structured aqueous gel that adheres closely to the corneal surface and evens out its irregularities, thus providing a high-quality optical surface. METHODS: We have used the robust tear film of the rat eye as an animal model to investigate the contribution of mucus and low-molecular-weight (LMW) proteins to tear film structure. The ocular surface was first exposed to saline, which washed away the tear film. Single drops of a tear/saline mixture, treated with various concentrations of the thiol-reducing agent N-acetylcysteine (NAC), were placed on the ocular surface and the resulting fluid behaviour was recorded with video-microscopy. RESULTS: At five per cent concentration, NAC appeared to degrade the gap-filling and anti-evaporative qualities of the tears, features that give the rat tear film its robust characteristics. Lower concentrations had no significant effect. DISCUSSION: In a previous publication, we showed that five per cent NAC alters the profile of LMW proteins in rat tears. The present observations suggest that the robust wetting properties of rat tears depend critically on their mucus and/or LMW protein content and possibly are related to the formation of an aqueous/mucous gel.