Abstract
Abstract: :
Purpose: Acute elevations of intraocular pressure can lead to the death of retinal ganglion cells (RGCs), but the link between pressure and cell death remains unclear. RGCs express the P2X7 receptor for ATP; in other systems P2X7 activation can lead to cell death. As mechanical perturbation can lead to release of ATP from many cell types, including retinal glial cells, we asked whether elevations in ocular pressure can lead to an increase in vitreal ATP, and whether stimulation of P2X7 receptors for ATP can kill RGCs. Methods: Bovine retinal eyecups were placed in a pressurized chamber and the ATP content of the vitreal solution was determined with the luciferin/luciferase assay. Cell viability was assessed by determining the percentage of labeled rat neonatal RGCs cells present after 24 hrs culture with drugs. Intracellular calcium measurements were made from unlabeled rat neonatal RGCs cultured for 24 hrs and loaded with fura–2. Results: : Elevation of pressure led to a 3–fold increase in vitreal ATP levels. The ATP levels in vitreous of eyes exposed to 100 mm Hg for 10 min rose to 26.3 ± 5.2 nM; the levels in eyes maintained at atmospheric pressure were only 8.4 ± 2.0 nM. Concentrations are expected to be substantially higher in the retina. Pressure did not lead to any signs of gross damage indicative of cell lysis. Stimulation of P2X7 ATP receptors with agonist BzATP in culture raised intracellular calcium and killed RGCs: P2X7receptor involvement was confirmed by the ability of antagonist OxATP to prevent cell death and by an enhanced Ca2+ response in the absence of extracellular Mg2+. Conclusions:The ability of increased pressure to elevate vitreal ATP levels, combined with the ability of P2X7 receptor stimulation to kill RGCs, suggests ATP may connect acute elevations in IOP to RGC death. The ability of OxATP to prevent cell death suggests a neuroprotective potential of P2X7 antagonists in glaucoma.
Keywords: excitatory neurotransmitters • retina: neurochemistry • neuroprotection