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.