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1From the Departments of Ophthalmology and Visual Sciences and 4Cell Biology and Physiology, the 2Barnes Retina Institute, and the 3Division of Comparative Medicine, Washington University, St. Louis, Missouri.
PURPOSE. Excessive exposure to oxygen has been proposed to be a risk factor for nuclear cataracts. For a better understanding of the metabolism of oxygen in the eye, oxygen distribution was mapped in the intraocular fluids, and the rate of oxygen consumption by the lens in rabbits breathing different levels of oxygen was calculated.
METHODS. Young albino rabbits were anesthetized, intubated, and exposed to normoxic, hypoxic, or hyperoxic conditions. The hemoglobin saturation of the blood was monitored with a pulse oximeter, and arterial oxygen levels were measured with a blood gas analyzer. A fiberoptic optical oxygen sensor (optode) was used to determine oxygen levels in different regions of the eye. Oxygen flux across the posterior of the lens was calculated from the measured oxygen gradients in the vitreous chamber.
RESULTS. Oxygen levels in the ocular fluids changed markedly when rabbits breathed air made hypoxic or hyperoxic. Oxygen levels were highest near the retinal vasculature, the iris vasculature, and the inner surface of the central cornea. Compared with nearby regions, oxygen levels were decreased in the aqueous humor closest to the pars plicata of the ciliary body and near the anterior chamber angle. Oxygen levels were generally lower closer to the lens. From the oxygen gradients in the vitreous body, oxygen consumption by the posterior half of the lens was calculated to be 0.2 to 0.4 µL/h under normoxic conditions. Oxygen consumption by the posterior of the lens increased in proportion to the amount of oxygen supplied.
CONCLUSIONS. Intraocular oxygen is mostly derived from the retinal and iris vasculature and by diffusion across the cornea. Freshly secreted aqueous humor and the aqueous humor in the anterior chamber angle are relatively depleted of oxygen. The marked increase in oxygen consumption that occurs when the lens is exposed to increased oxygen is likely to result in the production of higher levels of reactive oxygen species and may provide a link between elevated oxygen levels and the risk of nuclear cataracts.
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