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W. Lu, H. Hu, A. M. Laties, S. Jean, C. H. Mitchell; Upregulation of Retinal NTPDase 1 and Vitreal ATP Levels in an Experimental Rat Glaucoma Model. Invest. Ophthalmol. Vis. Sci. 2008;49(13):869.
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ATP is released in numerous tissues to signal a change in pressure or other mechanical perturbation. The increased intraocular pressure that can accompany glaucoma may also trigger the release of ATP. Retinal ganglion cells die in glaucoma, and stimulation of the P2X7 receptors for ATP can kill ganglion cells. We have demonstrated that the enzyme ecto-nucleoside triphosphate diphosphohydrolase 1 (NTPDase1; aka CD39) is upregulated in response to increased levels of elevated ATP in extracellular space. We have also found that NTPDase1 is increased in retinas from models of primate glaucoma. This study asked whether expression of the NTPDase1 or levels of extracellular ATP were increased in the retinas of rat with elevated intraocular pressure (IOP).
Elevation of IOP was induced in one eye of adult Brown Norway rats by injection of hypertonic saline into an episcleral vein. The IOP was monitored with TonoLab tonometer 4-6 x weekly. After 14 days, animals were sacrificed and eyes were fast frozen. The vitreous was dissected on dry ice and samples assayed for ATP content using the luciferase assay. Retinas were dissected, with proteins purified using standard techniques and run on a SDS-PAGE. Gels were blotted with a polyclonal antibody to rat NTPDase1. Staining was quantified and the mean ratio from experimental to control retinas determined from 3 independent immunoblots for each retina.
The injection of hypertonic saline, while challenging to learn, did lead to elevation of IOP in the treated eye of 6 rats. During the 14 days after injection, treated eyes had a mean IOP of 30.1 ± 1.4 mm Hg, while pressure in the contralateral control was 18.8 ±0.3 mm Hg. Pressure generally was higher in the first 5 days after injection, but remained elevated throughout the two weeks, with peak IOP of 44.4 ± 4.5 mm Hg while that on the final day 26.2 ± 2 mm Hg. Levels of NTPDase1 protein were higher in 5/6 of the experimental eyes as compared to control, and increased most closely with the peak pressure. ATP levels in the vitreous of treated eyes were 20 fold higher than controls, with levels related to the pressure on the final day.
While preliminary, these observations are consistent with a growing body of evidence suggesting that extracellular ATP levels are increased for an extended period in chronic glaucoma.
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