Corneal epithelial tight junctions and the localization of surface mucin.

Abstract

Studies have shown that the corneal epithelial surface can be protected with artificial tears that have an ionic composition similar to natural tears.1 More recent studies have reported that the corneal epithelial mucin layer is also maintained to protect the epithelial tight junctions with artificial tears that contain bicarbonate.2 Although the mechanism of this effect is not yet known, bicarbonate is the physiological buffer in tears.2–5 The maintenance of a slightly alkaline environment is more compatible with the corneal epithelium than neutral or acidic conditions. Bicarbonate also appears to be important in maintaining the normal electrophysiological parameters of the corneal epithelium. Preliminary data from our laboratory has suggested that an artificial tear solution with bicarbonate will protect and maintain the corneal epithelial potential and resistance of rabbit corneas mounted in modified Ussing chambers. The role of bicarbonate in corneal epithelial homeostasis is not clear; however, Conroy et al.6 have reported that carbonic anhydrase is present in the epithelium, and recently Candia7 has shown that there is a net flux of bicarbonate from the cornea to the tears. Rismondo et al.8 have shown in rabbit tears (HCO- 3 = 27.5 mM/L) that the concentration of bicarbonate is higher than that obtained from the lacrimal gland (HCO- 3 = 14.7 mM/L). Thus, tear bicarbonate comes from two sources: lacrimal gland fluid and the corneal epithelium.