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S.J. Mangers, B. Kramer, N.M. Holekamp, Y.–B. Shui, D.C. Beebe; Oxygen Consumption By Human Vitreous Humor . Invest. Ophthalmol. Vis. Sci. 2006;47(13):4114.
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Recent studies suggest that exposure of the human lens to molecular oxygen contributes to the formation of nuclear cataracts. Measurement of the distribution of oxygen in the living eye raised the possibility that human vitreous humor consumes oxygen (Holekamp, Shui and Beebe, AJO. 139;302–10 2005). The present study was performed to test this possibility.
Oxygen consumption by vitreous humor was measured using a micro–respirometer that employed an optical oxygen sensor (optode; Oxylab pO2, Oxford Optronics, Oxford, UK). Changes in oxygen levels were measured in fresh or frozen vitreous humor from postmortem human, rabbit and bovine eyes. Human vitreous humor was also boiled for 5 minutes or exposed to 5% oxygen overnight.
Typically, oxygen levels in human vitreous humor declined rapidly, reaching nearly undetectable levels. 12 of 13 human eyes yielded vitreous samples that consumed oxygen at a mean initial rate of 0.83 µl•hr –1•ml–1 (minimum, 0.08, maximum, 4.48). Vitreous humor from one human eye did not consume oxygen. Boiling human vitreous did not decrease its ability to consume oxygen. Exposing stirred human vitreous humor to 5% oxygen overnight abolished its ability to consume oxygen. Addition of 1.5 or 5 mM ascorbate restored the ability of this vitreous humor to consume oxygen. Vitreous humor obtained from rabbits consumed oxygen, but at a much slower mean rate than human vitreous (mean initial rate 0.06± 0.01 µl•hr –1•ml–1; n = 4). In preliminary tests, bovine vitreous humor did not consume oxygen at a detectable rate.
Human vitreous humor consumes oxygen in an ascorbate–dependent manner. This is probably due to the non–enzymatic oxidation of ascorbate. Human vitreous consumed oxygen at least ten times faster than the animals studied, suggesting that rapid oxygen consumption serves an important function in the human eye. In humans, oxygen consumption in the vitreous humor contributes to the low oxygen levels found at the posterior of the lens, which may help prevent nuclear cataracts.
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