Activation of P2Y-purinoceptors crucial to the reduction of astrocytic intracellular ATP during Ca²⁺ paradox-like insult

Abstract

"Ca²⁺ paradox" is the phenomenon whereby the intracellular concentration of Ca²⁺ paradoxically increases during reperfusion with normal Ca²⁺-containing media after brief exposure to a Ca²⁺-free solution. We found that prior exposure of the cultured astrocytes to a low Ca²⁺ solution for 60 min significantly decreased their intracellular ATP level ([ATP]_i) and injured them after reperfusion with a normal Ca²⁺ medium for 72 h. The present study aims at elucidating what mechanisms are involved in this ATP reduction leading to cell injury. In our previous study, we revealed that the intracellular concentration of Na⁺ ([Na⁺]_i) in astrocytes increased significantly during the Ca²⁺-free treatment. Thus, we first thought that this elevation of [Na⁺]_i increased the function of Na⁺/K⁺ ATPase, leading to the increase of ATP consumption. However, ouabain treatment didn't significantly change their [ATP]_i. Previous studies have revealed that Ca²⁺-free or low Ca²⁺ treatment induces the release of ATP to the extracellular space via either connexin hemi-channels or anion channels. When cultures were treated with either FFA or heptanol, inhibitors of these channels, the rise in extracellular ATP ([ATP]_o) was reduced, without maintaining intracellular amount of ATP. Both the ATP release and the [ATP]_i reduction were significantly attenuated by treatment with suramin, an inhibitor of P2Y receptors. These results suggested that astrocytic ATP release during Ca²⁺-free treatment induced their fall in [ATP]_i via the activation of P2Y1 receptor. [J Physiol Sci. 2007;57 Suppl:S117]