Stimulation of P2X7 receptors elevates Ca2+ and kills retinal ganglion cells.

Purpose: Retinal ganglion cells are known to express ionotropic P2X(7) receptors for ATP. Stimulation of these receptors in other cells can elevate Ca(2+) and sometimes lead to cell death. This study asked whether P2X(7) receptor stimulation alters the Ca(2+) levels and viability of retinal ganglion cells.
Methods: P2X(7) agonists were applied to retinal ganglion cells from neonatal rats loaded with fura-2 to examine their effect on intracellular Ca(2+) levels. The effect of P2X(7) receptor stimulation on cell viability was examined in rat retinal ganglion cells back-labeled with aminostilbamidine.
Results: The P2X(7) agonist benzoylbenzoyl adenosine triphosphate (BzATP) led to a large, sustained increase in Ca(2+). BzATP was >100-fold more effective than ATP at raising intracellular Ca(2+), when both agonists were applied at 10 microM. The response to BzATP was enhanced threefold by removal of extracellular Mg(2+), was dependent on extracellular Ca(2+), and was prevented by brilliant blue G (BBG). BzATP led to a concentration-dependent reduction in the number of cells with a median lethal dose (LD(50)) of 35 muM. Cell death was prevented by the P2X(7) antagonists BBG and oxidized ATP, but not by 30 microM suramin, consistent with the actions of the P2X(7) receptor. BzATP activated caspases in ganglion cells, but did not lead to membrane blebbing or increased permeability to Yo-Pro-1. The L-type Ca(2+) channel blocker nifedipine attenuated cell death, suggesting excessive Ca(2+) influx contributes to the lethal effects of BzATP.
Conclusion: Short-term stimulation of the P2X(7) receptor can raise Ca(2+) in rat retinal ganglion cells, whereas sustained stimulation of the receptor can kill them.