Oxygen Distribution in the Rabbit Eye and Oxygen Consumption by the Lens

Abstract

Purpose: : Excessive exposure to oxygen has been proposed as a risk factor for nuclear cataracts. To better understand the metabolism of oxygen in the eye, we mapped oxygen distribution in the intraocular fluids/cavities and calculated the rate of oxygen consumption by the lens in rabbits breathing different levels of oxygen.

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 using 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/cavities 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 to 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/hr under normoxic conditions. Oxygen consumption by the posterior of the lens decreased or increased when animals inspired lower or higher concentrations of oxygen, respectively, reaching over 0.6 µl/hr at 40% oxygen.

Conclusions: : Intraocular oxygen is mostly derived from the retinal, choroidal and iris vasculature and by diffusion across the cornea. Freshly secreted aqueous humor and the aqueous humor in the anterior chamber angle is 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. This observation may provide a link between exposure of the lens to increased oxygen and the accompanying increase in the risk of nuclear cataracts.

Keywords: crystalline lens • oxidation/oxidative or free radical damage • vitreous