The conjunctiva is a thin, mucus-secreting, vascular tissue that covers most of the inner surface of the eyelids, and is part of the anterior sclera where the cornea begins. The conjunctiva is thought to function as a passive physical protective barrier and to participate in the maintenance of tear-film stability due to the mucus secreted by the resident goblet cells (64). Our laboratory as well as others have provided ample evidence for additional functional features of the conjunctiva, viz. acting as a conduit for drug delivery to the posterior segment of the eye following ocular drug instillation and contributing to the regulation of electrolyte and fluid balance in the microenvironment of its mucosal surface (65). The dynamic nature of conjunctiva was demonstrated from the identification of several transport mechanisms for Na+ absorption: Na+-glucose (66, 67), Na+-amino acid (68) and Na+-nucleoside cotransporters (69), in addition to active Cl- secretion (70). The permeability of the conjunctiva to a wide variety of hydrophilic and lipophilic molecules was also reported (71-73).
The ocular epithelium is very different in structure and function from the rest of the dead epidermis covering the mammalian body, which serves as a primary barrier from the environment. This small area (compared to the rest of the body surface) is a combination of two unique tissues, the conjunctiva, covering more than 85% of the surface, and the cornea. These epithelia are relatively unprotected and constantly exposed to light radiation, atmospheric oxygen, environmental chemicals, and physical abrasion. Each one of these forms of stress has the potential to generate reactive oxygen species (ROS) that contribute to ocular damage and disease if left unchecked. While the eye as an organ contains natural protective components, viz. water-soluble antioxidants such as vitamin C, cysteine, GSH, uric acid, pyruvate, and tyrosine; lipid-soluble antioxi-dants such as tocopherols and retinols; and highly specialized enzymes such as superoxide dismutase, catalase, and glutathione peroxidase, the ocular epithelium serves as the front line for protection equipped rather moderately with proper ammunition. Studies employing advanced analytical methods have demonstrated the presence of several unambiguous water-soluble antioxidants in human tear fluid collected at normal and stimulated flow rates. The mechanisms of action or the glandular sources of these anti-oxidants have not been addressed in any detail so far.
Hydrogen peroxide has been detected in aqueous humor and other ocular fluids; however reports of basal H2O2 levels within mammalian tear fluid are scarce. Concentrations of H2O2 present in aqueous humor range from 25 to 70 |M. Furthermore, the levels of H2O2 have been shown to increase up to 20-fold in certain age-related diseases of the eye with marked oxidative stress characteristics (74). Since H2O2 is an ultimate byprod uct of cellular respiration, it may be present in tear-fluid secretions under normal conditions. In vitro it can also arise spontaneously through a reaction between ascorbic acid, riboflavin, and light (or trace amounts of unbound metals in the absence of light). Given that ascorbate exists in the tear fluid at concentrations of 0.8-0.9 mM (75), and maximal levels of riboflavin have been detected in ocular surface tissues of the rabbit (76), under physiological conditions the former may also assume a pro-oxidant role by generating H2O2 as it is consumed. A dynamic equilibrium is essential for H2O2 through its continuous production in the ocular surface and tear film, and subsequent elimination (74). It is of interest that we found glutathione secretion by conjunctival epithelial cells may be a critical input for the maintenance of this equilibrium (77). Tear specimens from different human hosts showed a marked inhibition of hydroxy-radical formation, but did not affect superoxide or H2O2 levels (78). Minimally stimulated tear samples collected from human subjects do not display a detectable catalase or GSH-peroxidase activity suggesting that the tear film may lack conventional cellular mechanisms that provide significant protection from oxidative properties of H2O2 (79). The contribution of vitamin C and its transporters may thus be important in this regard.
Vitamin C given as a dietary supplement has been found to improve the tear-film stability, tear secretion, and health of the ocular surface in general. In normal subjects, improvement was found in tear-film stability with daily administration of vitamin C tablet although the magnitude of improvement was somewhat smaller than a multi-vitamin/trace element administered group (80). Peponis et al. found that oral vitamin C supplementation along with vitamin E, to non-insulin-dependent diabetic patients, attenuated free radical production, NO levels and improved the tear-function parameters significantly (81, 82). Vitamin C was also found to improve cholesterol-induced micro-circulatory changes in rabbits (83).
Animal studies have shown that deficiencies in water- and lipid-soluble vitamins (e.g., C, A, and E) result in the loss of goblet cells in the conjunctiva and abnormal chromatin distribution in the nucleus of epithelial cells (84). Inflammation and loss of vascularity were considered as potential mechanisms for goblet cell loss in ocular surface disorders (85). Given the high vitamin C levels in tears and the antiinflamma-tory role of vitamin C in the eye (41, 75, 86), further studies will be needed to delineate mechanisms involved in beneficial effects of nutritional supplementation of vitamin C in ocular disorders.
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