title: Divalent cations in tears, and their influence on tear film stability in humans and rabbits.
abstract: OBJECTIVE
  Reduced tear film stability is reported to contribute to dry eye. Rabbits are known to have a more stable tear film than humans. Thus, we sought to examine the tears of rabbits and humans for metal cations, and to test how they influence tear film stability.
  
  
  METHODS
  Tears were collected from 10 healthy humans and 6 rabbits. Tear osmolality was measured by vapor pressure osmometer, and metals analyzed using inductively coupled plasma (ICP) mass spectrometry or ICP atomic emission spectroscopy. The influence of divalent cations on tears was analyzed by measuring surface tension using the Langmuir trough in vitro, using different concentrations of cations in the subphase, and grading the tear break-up in rabbits in vivo after instillation of chelating agents.
  
  
  RESULTS
  Rabbit tears had a higher osmolality compared to humans. Major metals did not differ between species; however, rabbits had higher levels of Mg(2+) (1.13 vs. 0.39 mM) and Ca(2+) (0.75 vs. 0.36 mM). In rabbit tears in vitro, diminishing divalent cations resulted in a decrease in the maximum surface pressure from 37 to 30 mN/m. In vivo, an increase in the amount of tear film that was broken-up was found. In contrast, when changing divalent cation concentrations in human tears, the maximum surface pressure remained at 26 mN/m.
  
  
  CONCLUSIONS
  The normal osmolality of rabbit tears is significantly higher than that in humans. While divalent cations had little influence on human tears, they appear to have an important role in maintaining tear film stability in rabbits.
authors.affiliation: Brien Holden Vision Institute and School of Optometry & Vision Science, University of New South Wales, Sydney NSW 2052, Australia.
authors.forename: Xiaojia Eric
authors.initials: XE
authors.lastname: Wei
authors.forename: Maria
authors.initials: M
authors.lastname: Markoulli
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Millar
authors.forename: Mark D P
authors.initials: MD
authors.lastname: Willcox
authors.forename: Zhenjun
authors.initials: Z
authors.lastname: Zhao
mesh_terms: D000328:Adult; D000818:Animals; D002413:Cations, Divalent / Q000032:analysis*; D005260:Female; D006801:Humans; D007765:Lacrimal Apparatus / Q000378:metabolism*; D008297:Male; D013058:Mass Spectrometry; D009994:Osmolar Concentration; D011817:Rabbits; D012016:Reference Values; D013500:Surface Tension; D013666:Tears / Q000737:chemistry*
chemical_list: D002413:Cations, Divalent