Rat Naphthalene Cataract Studies: Mechanisms and Prevention
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Xu, Guo-Tong, Rat Naphthalene Cataract Studies: Mechanisms and Prevention. Doctor of Philosophy (Biomedical Sciences/Pharmacology), June, 1994, 134 pp., 16 tables, 34 figures, references, 153 titles. The mechanism of naphthalene-induced cataract in rats and the preventive action of AL01576 (an aldose reductase inhibitor, ARI) were studied in both in vivo and in vitro systems. In the in vivo studies, cataracts were induced in five strains of rats (2 pigmented, 3 albino) by naphthalene feeding (1g/kg/day). The cataractous changes occurred in 1 week as watercleft and spoke-like opacities which merged to form a shell-like opacity in the deep cortex by 3 weeks. Semi-quantitation of the opacities with an arbitrary six-score grading system showed little difference in the cataract development between the pigmented and albino strains. Major biochemical changes observed were a decrease of 20%-30% in GSH by one week of feeding, the appearance of disulfide cross-linking of lens proteins by 3 weeks, and a more than ten fold increase in the content of protein-GSH mixed disulfide. Neither damage to lens membrane functions as measured by 3H-choline or 86Rb uptake or loss of Na+/K+-ATPase activity was detected AL01576 (10 mg/kg/day) completely prevented the naphthalene-induced lens changes in both pigmented and albino rats. These results indicate that pigmentation is not required for induction of naphthalene cataract in rats and suggest that tyrosinase action on naphthalene metabolites (such as 1- or 2- naphthol) is not involved in this cataract formation. The in vitro “naphthalene cataract” was established by exposing rat lens to each of 5 potential naphthalene metabolites in organ culture system (in modified TC-199 medium) for 48 hrs. When naphthalene dihydrodiol was used, both the morphological and biochemical changes in the lens were very similar to those observed in lenses of naphthalene-fed rats, and AL01576 completely blocked these in vitro changes as it did in vivo. Other naphthalene metabolites (1,2-dihydroxynaphthalne, 1-naphthol, 2-naphthol and 1,2-naphthoquinone) caused changes which were different from those induced by naphthalene in vivo and one of them was prevented by AL01576. Therefore, naphthalene in vivo and none of them was prevented by AL01576. Therefore, naphthalene dihydrodiol is the key naphthalene metabolite which reaches the lens via blood and aqueous humor and causes cataract when it is metabolized to 1,2-naphthoquinone. This mechanism is further supported by the detection of naphthalene dihydrodiol in the lens and aqueous humor of naphthalene-fed rats. Examples of various classes of ARI (AL01576, AL04114, Sorbinil and Tolrestat) were compared for their effects on the formation of naphthalene cataract and a dual cataract induced with simultaneous feeding of galactose and naph-thalene. Both AL01576 and AL04114 (spirohydantoin derivatives) completely prevented the changes in the lenses of naphthalene-fed rats. However, Sorbinil (another spirohydantoin ARI) demonstrated a much weaker efficacy in this model and the carboxylic acid ARI, Tolrestat, showed no efficacy at all. In the dual cataract, Tolrestat prevented galactose cataract formation and reduced the lens dulcitol accumulation but showed no protection against the shell-like opacity caused by naphthalene. On the other hand, AL01576 protected the lens from the cataractogenic action of both compounds. These results rule out the involvement of aldose reductase in naphthalene cataract formation. Furthermore, AL04114 (not a cytochrome P-450 inhibitor) showed a similar efficacy as AL01576 (a inhibitor of cytochrome) in naphthalene cataract prevention. Therefore, the inhibition of cytochrome P-450 may not be involved in the prevention of this cataract. Based on these findings and the fact that AL01576 prevents the changes induced by naphthalene dihydrodiol (ND) but not 1,2-naphthoquinone (NQ), a new mechanism for rat naphthalene cataract formation is proposed: naphthalene is converted by cytochrome P-450 to ND, which reaches the eye via the blood and penetrates into the lens. By the action of dihydroxynaphthalene which autoxidizes to form NQ and H2O2 production and thus causes cataract. AL01576 and AL04114 inhibit DDD activity, block NQ and H2O2 production and thus prevent the cataract formation.