Abstract
Purpose: :
Adenosine protects the brain, heart and retina from damage. Of the four adenosine receptors (A1, A2a, A2b or A3), protective actions have most frequently been attributed to the A1 receptor, including protection of retinal ganglion cells (RGCs) from excitotoxic and ischemic challenge. Although A3 receptors protect the heart and brain, a protective role for the A3 receptor in the retina is unknown. This study investigates whether the adenosine A3 receptor can prevent the calcium influx following stimulation by either glutamate or the P2X7 receptor agonist BzATP.
Methods: :
RGCs from juvenile Long–Evans rats were purified by immunopanning. Cells were loaded with Fura–2 and the intracellular calcium concentrations were measured in the presence of glutamate (10µM glutamate+10µM glycine) or BzATP (50µM).
Results: :
BzATP and glutamate both caused a significant rise in cellular Ca2+. BzATP is known to activate P2X7 receptors to trigger an influx of Ca2+ in RGCs, and NMDA acted like glutamate to raise Ca2+. Adenosine (10 µM) blocked the response to BzATP by 29% and to glutamate by 40%. The A3 receptor agonist Cl–IB–MECA (2 µM) mimicked the actions of adenosine and reduced the Ca2+ response to BzATP by 34%, and to glutamate by 42%. The response to BzATP was enhanced by exposure to adenosine in the presence of 1 µM of the A3 receptor antagonist MRS–1191 (273 nM vs. 147 nM for adenosine alone). The response to glutamate was also enhanced by MRS–1191 (532 nM vs. 341 nM for adenosine alone). The effect of MRS–1191 indicates at lease some adenosine actions are mediated by the A3 receptor.
Conclusions: :
Stimulation of the A3 adenosine receptor prevents the calcium rise triggered by BzATP or glutamate in rat RGCs, suggesting a role in neuroprotection. The block of responses to both glutamate and BzATP argues against a direct effect on receptors, and suggests adenosine alters a downstream Ca2+ influx pathway recruited by both agonists.
Keywords: adenosine • excitatory amino acid receptors • second messengers