Protective role of oral antioxidant supplementation in ocular surface of diabetic patients. (Clinical Science).

Aim: To investigate the effect of vitamin C and E supplementation in the levels of nitrite, nitric oxide (NO) related metabolite, and ocular surface parameters in diabetic patients.

Methods: 50 patients with non-insulin dependent diabetes mellitus were given vitamin C (1000 mg/day) and vitamin E (400 IU/day) supplementation for 10 days. Nitrite levels in tears were measured by photometric determination before and after vitamin supplementation. Tear function parameters (Schirmer test I, BUT, ocular ferning test) and brush cytology analysis of the conjunctival epithelium were also evaluated.

Results: Nitrite levels were found to be significantly reduced (p<0.05) after 10 days of vitamin C and E supplementation. Improved values for Schirmer test, BUT test, and ocular ferning test were also found. Goblet cell density and grading of squamous metaplasia showed a significant improvement.

Conclusions: Oxidative stress and free radical production are elevated in diabetes mellitus. Antioxidants, such as vitamin C and vitamin E, probably have an important role in reducing the oxidative damage produced by nitric oxide and other free radicals and improving the ocular surface milieu.

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Diabetes mellitus is associated with a number of ocular complications which can lead even to blindness. The study of ocular surface manifestations during the course of diabetes mellitus has increased in recent years. For example, 47%-67% of diabetic patients have primary corneal lesions during their lifetime. (1) In addition many diabetic patients complain of dry eye symptoms, indicating a clear role for tear film abnormalities. (2)

The ocular surface is relatively unprotected and constantly exposed to radiation, atmospheric oxygen, environmental chemicals and physical insults, resulting in the generation of reactive oxygen species (ROS) which are thought to contribute to ocular damage? Vitamin C (ascorbic acid) is found in high concentration in the eye and is thought to be a primary substrate in ocular protection. (4) Moreover, most studies have found that people with diabetes mellitus have circulating ascorbic acid concentrations at least 30% lower than people without diabetes mellitus. (5)

Nitric oxide (NO) serves a wide variety of functions in various biological systems, both in intracellular compartments as a second messenger that responds to activation of plasma membrane receptors and in extracellular compartments as a paracrine factor that carries information between cells. (6)

Nitric oxide is synthesised from L-arginine by three distinct isoforms of nitric oxide synthase (NOS) which are products of three different genes. (7) The three isoforms of NOS are classified as follows: neuronal NOS (NOS1), inducible NOS (NOS2), and endothelial NOS (NOS3). (8) Nitric oxide is constantly produced by NOS1 and NOS3, which are activated via the calcium/calmodulin complex. By contrast, the activity of NOS2 is independent of calcium and capable of generating large amounts of NO in the presence of immunological and inflammatory stimuli. (8)

Nitric oxide has been implicated in various physiological and pathological processes in the eye. It contributes to the regulation of aqueous humour dynamics, retinal neurotransmission, phototransduction, regulation of retinal vascular tone, ocular inflammatory diseases (uveitis, retinitis), degenerative diseases (glaucoma, retinal degeneration), allergic eye disease, and diabetic retinopathy. (6-8)

Induction of iNOS (inducible NOS) by cytokines leads to the production of large amounts of NO which mediate the destructive responses in ocular inflammation. (6) Peroxynitrire ([ONOO.sup.-]) formed by the reaction between nitric oxide and superoxide ([O.sub.2.sup.-]) is a powerful oxidant capable of causing tissue injury. (9)

There are observations that high glucose levels lead to increased NOS expression and increased NOS activity. (10) This could probably be attributed to increased oxidative stress that is evident in diabetes mellitus. (9)

The above observations raise the question of whether antioxidant supplementation could modify NO levels as well as tear film characteristics in diabetes mellitus.

In this study the potential effect of oral vitamins C and E on NO levels and various clinical and cytological parameters of tear film and ocular surfaces in diabetic patients was investigated.

PATIENTS AND METHODS

Ninety seven eyes of 50 patients diagnosed with non-insulin dependent diabetes mellitus (NIDDM) were enrolled in this study from the department of ophthalmology and the diabetes centre of the general hospital of Piraeus. Patients characteristics are summarised in Table 1.

Exclusion criteria from the study were (a) any systemic disease other than NIDDM, (b) medications other than hypoglycaemic drugs or insulin that could interfere with tears and ocular surface parameters, (c) topical eye medications within the past 6 months, (d) history of ophthalmic surgical or laser procedures, (e) eye diseases …