Opacification of the lens nucleus is a major cause of blindness and is thought to result from oxidation of key cellular components. Thus, long-term preservation of lens clarity may depend on the maintenance of hypoxia in the lens nucleus. We mapped the distribution of dissolved oxygen within isolated bovine lenses and also measured the rate of oxygen consumption (Q O 2 ) by lenses, or parts thereof. To assess the contribution of mitochondrial metabolism to the lens oxygen budget, we tested the effect of mitochondrial inhibitors onQ O 2 and partial pressure of oxygen (P O 2 ). The distribution of mitochondria was mapped in living lenses by 2-photon microscopy. We found that a steep gradient of P O 2 was maintained within the tissue, leading to P O 2 < 2 mmHg in the core. Mitochondrial respiration accounted for approximately 90% of the oxygen consumed by the lens; however, P O 2 gradients extended beyond the boundaries of the mitochondria-containing cell layer, indicating the presence of non-mitochondrial oxygen consumers. Time constants for oxygen consumption in various regions of the lens and an effective oxygen diffusion coefficient were calculated from a diffusion-consumption model. Typical values were 3 × 10 −5 cm 2 s −1 for the effective diffusion coefficient and a 5 min time constant for oxygen consumption. Surprisingly, the calculated time constants did not differ between differentiating fibres (DF) that contained mitochondria and mature fibres (MF) that did not. Based on these parameters, DF cells were responsible for approximately 88% of lens oxygen consumption. A modest reduction in tissue temperature resulted in a marked decrease inQ O 2 and the subsequent flooding of the lens core with oxygen. This phenomenon may be of clinical relevance because cold, oxygen-rich solutions are often infused into the eye during intraocular surgery. Such procedures are associated with a strikingly high incidence of postsurgical nuclear cataract.

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SUMMARY In the vertebrate lens, nuclei and other cytoplasmic organelles are degraded in fiber cells situated in the center of the tissue. This is believed to ensure the transparency of the tissue. The mechanism that triggers this process is unknown. We hypothesized that standing gradients of oxygen generated within the tissue may serve as a spatial cue for organelle degradation. To examine this possibility, we incubated fertilized chicken eggs under hyperoxic (50% O2) or normoxic (21% O2)conditions. Hyperoxic treatment was initiated on the seventh day of embryonic development (E7), five days before organelle degradation normally commences in the lens core. Hyperoxia was maintained until E17. Under normoxic conditions,the partial pressure of oxygen (PO) within the vitreous compartment was low. Direct measurement of PO using an optode oxygen sensor indicated values of 1.3 kPa and 0.4 kPa for the mid- and anterior vitreous, respectively. Similarly, treatment with pimonidazole, a bio-reductive hypoxia marker, led to the formation of immuno-positive protein adducts within the lens, suggesting that the embryonic lens is chronically hypoxic in situ. Following hyperoxic treatment, vitreous PO significantly increased, although pimonidazole staining in the lens was not markedly affected. Confocal microscopy of slices prepared from hyperoxic lenses revealed a significant increase in the size of the lens relative to age-matched normoxic controls. By E13, an organelle-free zone(OFZ) was present in the center of normoxic and hyperoxic lenses. However, in hyperoxic lenses, the OFZ was consistently smaller, and the distance from the lens surface to the border of the OFZ significantly larger, than in normoxic controls. These observations suggest that hyperoxia delays organelle breakdown and are consistent with a model in which hypoxia in the deep cortical layers of the normal lens serves as a trigger for the organelle loss process.

Adverse reactions to NAC were less common with the two-bag regimen. These results add to the accumulating evidence that reducing the initial NAC infusion rate reduces the risk of adverse reactions.

Discontinuing acetylcysteine based on laboratory testing after 12 hours of treatment is feasible and likely safe in selected patients at very low risk of liver injury from acetaminophen overdose. This article is protected by copyright. All rights reserved.