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F. Bai, P. Lei, Y.-B. Shui, R. Gupta, N. Holekamp, C. Siegfried, D. Beebe; Differences in Ascorbate Distribution and Oxygen Consumption Between the Eyes of Humans and Other Animals. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1180.
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Exposure of the lens to elevated oxygen can cause nuclear cataracts. Our previous study found that the ascorbic acid (AsA) in human vitreous humor reacts with oxygen, presumably lowering oxygen around the lens. We compared the AsA concentration in aqueous and vitreous humor and the rate of oxygen consumption in vitreous from human, cow, pig and rabbit eyes.
Samples of aqueous and vitreous were obtained from rabbit, pig, cow eyes immediately postmortem and from human eyes at the time of vitrectomy, cataract or glaucoma surgery. AsA was measured colorimetrically in triplicate samples. Oxygen consumption was measured using a microrespirometer fitted with an Oxylab pO2TM optical oxygen sensor.
AsA levels in aqueous and vitreous have been measured previously, but usually not from the same eyes and in different species at the same time. Humans had the highest AsA concentration in the vitreous and aqueous (Table). In humans, AsA levels were consistently higher in the vitreous than in the aqueous, while in the three animal species this ratio was reversed. The average rate of oxygen consumption (µl/ml/hr) was also highest in human vitreous (rabbit 0.99 ± 0.17; n=7, pig 0.49 ± 0.18; n=9, cow 0.90 ± 0.15; n=8, human, 1.32 ± 0.13; n=27).
The high AsA concentration in the human eye suggests that the ocular environment protects the human lens from exposure to oxygen. AsA is believed to enter the eye in the aqueous humor by transport across the ciliary epithelium. Our measurements raise the possibility that an additional transporter pumps AsA into human vitreous, accounting for the higher AsA concentration in the vitreous than in the aqueous. Experiments are in progress to test this possibility in human patients. Differences between humans and animal models must be accounted for in studies of ocular physiology.
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