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J. Xia, W. Lu, A. M. Laties, C. H. Mitchell; Retinal Ganglion Cells Release ATP and Autostimulate P2X7 Receptors. Invest. Ophthalmol. Vis. Sci. 2009;50(13):146.
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While elevated intraocular pressure can injure retinal ganglion cells (RGCs), the processes linking this elevation to neuronal damage remain unclear. Stimulation of the ionotropic P2X7 receptor for ATP can raise calcium and kill ganglion cells, and increased ocular pressure can release ATP from the whole retina, suggesting excess extracellular ATP could convert mechanical strain into a neurotoxic signal. The requirement for high levels of ATP by the P2X7 receptor makes a local source of released ATP likely, leading to the hypothesis that RGCs themselves release ATP and autostimulate their own P2X7 receptors to activate inward currents. As the patch clamp technique complicates the application of pressure, cell swelling was used to apply mechanical stress. We asked whether swelling triggered ATP release from ganglion cells and whether this ATP was capable of autostimulating P2X7 receptors.
ATP release from RGCs isolated using the immunopanning technique was determined using the luciferase assay. RGCs were whole cell patch clamped from -100 to -60 mV in 5 mV steps in the presence of changing solutions using standard techniques.
Exposure of RGCs to hypotonic solution increased whole cell conductance five-fold. Hypotonicity shifted the reversal potential positive by 24 mV, consistent with the opening of a Na+ permeable channel. The specific P2X7 receptor antagonist A438079 (1 microM) blocked the activated current by >60% and hyperpolarized the reversal potential. Removal of extracellular ATP with apyrase reduced the hypotonically-activated current by >80%, confirming the activated current was largely due to ATP. Direct measurement of extracellular ATP indicated that swelling triggered a release of ATP from isolated RGCs.
These observations suggest that swelling causes retinal ganglion cells to release ATP into the extracellular space where it autostimulates P2X7 receptors. Whether this autostimulation occurs with other forms of mechanical perturbation remains to be determined.
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