Effect of experimental dry eye on tear sodium concentration in the mouse.

PURPOSE: To examine the changes in tear volume, sodium concentration, and osmolarity in BALB/c and C57BL/6J mice in response to experimental dry eye (EDE).
METHODS: Tear volumes were determined at baseline and after 2 days of EDE using phenol red-impregnated cotton thread. Tear fluid was collected by instilling 1 microl of distilled water into the conjunctival sac and collecting the diluted tear fluid in a 0.5-microL glass capillary tube. CoroNa Red in dimethyl sulfoxide DMSO was added, and the fluorescent intensity was measured, which was compared to a standard curve of sodium concentrations. The resulting concentration was then corrected by the proper dilution factor. The tear osmolarity owing to sodium was calculated by comparing the calculated tear sodium concentration to a standard curve established by a vapor pressure osmometer. Serum osmolarity was also determined using a vapor pressure osmometer.
RESULTS: After 2 days of EDE, tear volume significantly decreased from 0.093 microL to 0.028 microL in BALB/c mice and from 0.066 microL to 0.026 microL in C57BL/6J mice. There was a concomitant significant increase in tear osmolarity from 177 mOsm/L to 300 mOsm/L in C57BL/6J mice. Tear osmolarity nearly doubled from 285 mOsm/L to 559 mOsm/L in BALB/C mice with EDE, approaching statistical significance (P=0.12). No change in the serum osmolarity was observed in either mouse strain.
CONCLUSIONS: These experiments show the ability of these new techniques for determining tear volume and estimating tear osmolarity in mice. By mimicking the findings of human dry eye disease, these findings validate the relevance of the mouse model for studying the pathogenesis of human keratoconjunctivitis sicca.