Retinoid permeability and uptake in corneas of normal and vitamin A-deficient rabbits

The influence of vitamins and trace element supplements on the stability of the pre-corneal tear film

The tear stability af 60 normal healthy subjects was measured and repeated 10 days later. Using a double-blind protocol, during the interim period 2/3 of the subjects (the treatment groups) took a commercially available daily dietary supplement and the remaining 1/3 acted as controls. The recommended daily dose was applied to the treatment groups. 20 of the treatment group took a daily dietary supplement of a mixture consisting of vitamins (e.g. A, B1, B2, B6, E) and trace elements (e.g. calcium iron, manganese), the other 20 took purely vitamin C tablets. The tear stability of both treated groups increased, however, the multi-vitamin and trace element group demonstrated the more consistent and individually predictable improvement. Within this group, the characteristics of the regression line correlating the initial mean (x) and final mean (y) tear stabilities were, y = 5.94 + 1.02x. The tear stability of the control group individuals did not significantly change.

Characterization of corneal proteoglycans under vitamin A deficiency

Comparison of in vivo radiolabeled corneal proteoglycans from vitamin A deficient, pair-fed control and normal rabbits by chromatographic and enzymatic methods, reveals subtle but reproducible changes in the proteoglycans present in the vitamin A deficient corneas relative to those of pair-fed and normal control corneas. Although the total amounts of [3H]leucine and [35S]sulfate incorporated in vivo into the proteoglycans per cornea are similar in the three types of cornea, the proteoglycan affinity for DEAE-Sepharose is different, with more of the vitamin A deficient proteoglycans requiring a higher NaCl concentration for elution. Analysis of in vivo labeled rabbit corneal proteoglycans reveals that in vitamin A deficient animals, relative to pair-fed controls, there is (1) an increase in the proteoglycans digested by chondroitinase AC indicative of decreased epimerization of glucuronic acid to iduronic acid; (2) an increase in the amount of keratan sulfate proteoglycan with high affinity for an anion exchange resin, consistent with a greater negative charge; (3) differences in proteoglycan affinities for octyl-Sepharose, reflecting differences in hydrophobicity; (4) increased susceptibility to proteolysis by trypsin; (5) no significant difference in sulfation.

In vivo metabolism of topically applied retinol and all-trans retinoic acid by the rabbit cornea

Corneas of normal and vitamin A-deficient rabbits were treated topically with [11, 12-3H] retinol or [11, 12-3H] all-trans retinoic acid. Methanol extracts of these corneas were analyzed by high pressure liquid chromatography. Radiolabeled compounds were extracted from the corneas which co-migrated chromatographically with known retinoid standards. In agreement with studies on other tissues and organs, retinol was metabolized to retinoic acid and more polar compounds by corneas of normal and vitamin A-deficient rabbits. All-trans retinoic acid was isomerized to 13-cis retinoic acid in normal rabbit corneas; however, this trans-cis isomerization did not occur in vitamin A-deficient, xerophthalmic corneas.

Possible mechanisms for the retention of topically applied vitamin A (retinol) in the albino rabbit eye

The concentration of vitamin A in various anterior segment tissues of the albino rabbit eye following topical instillation of a 1 mg/ml drug solution in peanut oil was determined using radiotracer techniques. It was found that 50-80% of the vitamin A absorbed into the albino rabbit eye was in the corneal epithelium and the conjunctiva and that over 50% of the vitamin A in the cornea was in its epithelial layers. While trace amounts of topically applied peanut oil remained in the precorneal area as late as 2 hours post-dosing, the vitamin A dissolved in it did not appear to play a major role in sustaining vitamin A concentration within the eye. Rather, solubilization of vitamin A by the proteins in tears as well as by the cellular lipids and retinol-binding proteins in the corneal epithelium appear to play a more important role. It is suggested that further work is necessary to delineate the specific involvement of retinol-binding proteins in ocular vitamin A pharmacokinetics.

Vitamin A is present as retinol in the tears of humans and rabbits

Vitamin A is required for the normal growth maintenance and maturation of the corneal epithelium and is effective in the treatment of xerophthalmia and experimental corneal epithelial wounds when applied topically as retinoic acid. The normal route of delivery of vitamin A to the cornea has remained undefined. We collected tears from normal and vitamin A deficient rabbits and from humans and analyzed them by high pressure liquid chromatography. A peak corresponding to a retinol standard was eluted from normal rabbit and human tears but was absent from the vitamin A deficient rabbit tears. The retinol concentration in rabbit tears was 69 ng/ml (0.2 X 10(-6)M) and in human tears was 16 ng/ml (0.5 X 10(-7)M). This demonstration that vitamin A is present in the tears as retinol establishes the rationale for treatment of corneal disease with topical vitamin A.

Healing of experimental corneal wounds treated with topically applied retinoids

We treated experimental corneal epithelial wounds in rabbits with topical retinoids. Treatment with 0.1% all-trans-retinoic acid three times per day resulted in a 21% increase in the healing rate compared to the control eyes. Treatment five times a day resulted in a 35% increase in healing rate. Treatment with topical retinoic acid also promoted corneal deturgescence. Retinyl palmitate, retinyl acetate, retinol, and 13-cis-retinoic acid had no effect on corneal wound healing. These data suggested that topically applied all-trans-retinoic acid may be effective in promoting corneal healing after surgery and in the treatment of persistent and recurring corneal epithelial defects.