Functional characterization of P2Y 1 versus P2X receptors in RBA‐2 astrocytes: Elucidate the roles of ATP release and protein kinase C

A physiological concentration of extracellular ATP stimulated biphasic Ca 2+ signal, and the Ca 2+ transient was decreased and the Ca 2+ sustain was eliminated immediately after removal of ATP and Ca 2+ in RBA‐2 astrocytes. Reintroduction of Ca 2+ induced Ca 2+ sustain. Stimulation of P2Y 1 receptors with 2‐methylthioadenosine 5′‐diphosphate (2MeSADP) also induced a biphasic Ca 2+ signaling and the Ca 2+ sustains were eliminated using Ca 2+ ‐free buffer. The 2MeSADP‐mediated biphasic Ca 2+ signals were inhibited by phospholipase C (PLC) inhibitor U73122, and completely blocked by P2Y 1 selective antagonist MRS2179 and protein kinase C (PKC) activator phorbol 12‐myristate 13‐acetate (PMA) whereas enhanced by PKC inhibitors GF109203X and Go6979. Inhibition of capacitative Ca 2+ entry (CCE) decreased the Ca 2+ ‐induced Ca 2+ entry; nevertheless, ATP further enhanced the Ca 2+ ‐induced Ca 2+ entry in the intracellular Ca 2+ store‐emptied and CCE‐inhibited cells indicating that ATP stimulated Ca 2+ entry via CCE and ionotropic P2X receptors. Furthermore, the 2MeSADP‐induced Ca 2+ sustain was eliminated by apyrase but potentiated by P2X 4 allosteric effector ivermectin (IVM). The agonist ADPβS stimulated a lesser P2Y 1 ‐mediated Ca 2+ signal and caused a two‐fold increase in ATP release but that were not affected by IVM whereas inhibited by PMA, PLC inhibitor ET‐18‐OCH 3 and phospholipase D (PLD) inhibitor D609, and enhanced by removal of intra‐ or extracellular Ca 2+ . Taken together, the P2Y 1 ‐mediated Ca 2+ sustain was at least in part via P2X receptors activated by the P2Y 1 ‐induced ATP release, and PKC played a pivotal role in desensitization of P2Y 1 receptors in RBA‐2 astrocytes. J. Cell. Biochem. 104: 554–567, 2008. © 2007 Wiley‐Liss, Inc.