Balance of purines may determine life or death of retinal ganglion cells as A3 adenosine receptors prevent loss following P2X7 receptor stimulation

The purines ATP and adenosine can act as a coordinated team of transmitters. As extracellular adenosine is frequently derived from the enzymatic dephosphorylation of released ATP, the distinct actions of the two purines can be synchronized. In retinal ganglion cells (RGCs), stimulation of the P2X(7) receptor for ATP leads to increased intracellular Ca2+ and death. Here we define the contrasting effects of adenosine and identify protective actions mediated by the A(3) receptor. Adenosine attenuated the rise in Ca2+ produced by the P2X(7) agonist 3'-O-(4-benzoylbenzoyl)ATP (BzATP). Adenosine was also neuroprotective, increasing the survival of ganglion cells exposed to BzATP. The A(3) adenosine receptor agonist 2-chloro-N-6-(3-iodobenzyl)-adenosine-5'-N-methyluronimide (Cl-IB-MECA) mimicked the inhibition of the Ca2+ rise, whereas the A(3) antagonist 3-Ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1,4-(+/-)-dihydropyridine-3,5-dicarboxylate (MRS-1191) reduced the protective effects of adenosine. Both Cl-IB-MECA and a second A(3) receptor agonist IB-MECA reduced the cell loss triggered by BzATP. The actions of BzATP were mimicked by ATP gamma S, but not by ATP. In summary, adenosine can stop the rise in Ca2+ and cell death resulting from stimulation of the P2X(7) receptor on RGCs, with the A(3) adenosine receptor contributing to this protection. Hydrolysis of ATP into adenosine and perhaps inosine shifts the balance of purinergic action from that of death to the preservation of life.