Artificial tear composition and promotion of recovery of the damaged corneal epithelium.

In severe dry eye syndromes the corneal epithelium is compromised with development of punctate erosions and increased permeability. In the present study the ability of artificial tear solutions to promote recovery of the corneal epithelial barrier was determined by measurement of corneal uptake of 5,6 carboxyfluorescein (CF). Corneas of anesthetized rabbits were exposed to 0.01% benzalkonium for 5 min to increase epithelial permeability. The cornea was then exposed to an artificial tear solution for 1.5 h followed by measurement of CF uptake. During exposure to three commercial isotonic, nonpreserved solutions and a solution preserved with polyquaternium-1, CF uptake decreased significantly but did not return to control. No recovery of the epithelial barrier occurred during exposure of corneas to nonpreserved hypotonic solutions. During exposure to an experimental tear solution with an electrolyte composition similar to human tears, buffered with bicarbonate, CF uptake returned to control levels. Bicarbonate is an essential component of this solution because the same formula buffered with borate or without buffer was ineffective in promoting recovery of the damaged corneal epithelium.

Bile salt-independent retinyl ester hydrolase in the rat lacrimal gland.

The lacrimal gland secretes retinol and synthesizes and stores retinyl esters. In this study, presence of retinyl ester hydrolase in rat lacrimal gland was investigated. A bile salt-independent retinyl ester hydrolase was identified in lacrimal gland homogenates and microsomes which were incubated in a tris-maleate buffer with retinyl palmitate as substrate. Retinol produced was measured by HPLC and activity expressed as pmoles retinol produced/mg protein/hr. In homogenates, the enzyme activity had a pH optimum at pH 6 and maximum specific activity of 57.7 pmoles/mg/hr. In microsomes, the activity was optimal at pH 7 and maximum specific activity was 1073 pmoles/mg/hr. In contrast, rat liver retinyl ester hydrolase had a pH optimum at pH 8 and a specific activity of 24.7 pmoles/mg/hr. These observations demonstrate that a complete cycle of retinyl ester metabolism is present in the lacrimal gland.

Quantitative evaluation of the corneal epithelial barrier: effect of artificial tears and preservatives.

The effect of artificial tear solutions on the corneal epithelial barrier was evaluated by measuring corneal uptake of 5, 6 carboxyfluorescein (CF) after exposure of rabbit corneas to various formulations in a conjunctival cup. Four tear solutions containing 0.01% benzalkonium chloride (BAC), a formulation containing 0.001% Polyquad, a contact lens re-wetting solution containing 0.004% thimerosal, and each of the above preservatives in balanced salt solution (BSS) were evaluated. Four non-preserved solutions were also tested. After treatment with the solutions corneas were exposed to the CF, removed from the eyes and dialyzed in balanced salt solution. The CF concentration in the dialysate was measured by fluorometry. Solutions containing 0.01% BAC caused a 9.24 to 99.28 fold increase in CF uptake as compared to control. Solutions preserved with Polyquad or thimerosal caused only a 0 to 4 fold increase in CF uptake while non-preserved solutions caused no change compared to control. Corneas prepared for transmission electron microscopy using fixative containing ruthenium red exhibited damage which correlated well with CF uptake; the ruthenium red penetrated the epithelium to the basal cell layer after corneal exposure to solutions containing BAC while only superficial cell layers were stained after exposure to the other test solutions. The method used in this study allows statistical comparison of artificial tear formulations. The data show that patients with severe dry-eye who use artificial tears frequently should avoid tear solutions containing BAC and that non-preserved solutions are preferable for treatment of these patients.