Is ascorbate in human tears from corneal leakage or from lacrimal secretion?

This study investigated whether fresh main lacrimal gland secretion contains ascorbate, with a view to providing indirect evidence of an immediate source of this antioxidant in tears. Our hypothesis was that, if the source is corneal leakage, continuous tearing or rinsing of the eye will result in a marked decrease, by dilution, in ascorbate concentration in the reflex tears collected. Alternatively, the ascorbate concentration will be relatively constant if the main lacrimal gland secretion is the main immediate source.

Five successive samples of yawn-induced reflex tears were collected from the same eye of each of 42 subjects. In 36 of these volunteers, the testing eye was then flushed with 10 ml of saline and a sixth tear sample ('post-flush') was collected immediately. Tear ascorbate concentrations were measured using a validated high performance liquid chromatography method.

The ascorbate concentration of the first sample (baseline) was slightly but significantly (P < 0.0001) lower (17.3 +/- 8.9 micro M) than the four subsequent samples in all subjects (average 21.4 microM). Ascorbate concentrations of post-flush samples were very similar to pre-flush values. Mean +/- SD ascorbate concentrations of pre- and post-flush samples were, respectively, 22.5 +/- 10.9 and 17.3 +/- 5.8 microM.

Results show that ascorbate is present in fresh reflex tears. Data do not support the view that the cornea is the source of tear ascorbate in healthy eyes. Rather, results indicate that ascorbate is present in main lacrimal gland secretion and that this antioxidant is depleted in basal tears. Measurement of tear ascorbate may offer useful information regarding antioxidant supply to and protection of the cornea.