Two distinct ATP signaling mechanisms in differentiated neuroblastoma × glioma hybrid NG108-15 cells

The ATP signaling mechanism in neuroblastoma × glioma hybrid NG108-15 cells differentiated by exposure to dibutyryl-cAMP was characterized. In cells loaded with fura-2, ATP rapidly raised the cytosolic Ca2+ concentration ([Ca2+]i); the magnitude of the rise was inversely proportional to the extracellular Na+ concentration. Large increases in cytosolic Na+ concentration, measured with the fluorescent Na+ indicator sodium-binding benzofuran isophthalate, were dose-dependently elicited by ATP. ATP also evoked the entry of ethidium bromide into cells, and this process was inhibited by Mg2+. Inositol-1,4,5-trisphosphate (IP3) generation induced by ATP was totally blocked by removal of extracellular Ca2+, but residual IP3 generation still remained in nondifferentiated cells. In addition, ATP produced a concentration-, time-, and Mg2+-dependent biphasic uptake of 45Ca2+. A range of nucleotides and ATP analogues, including CTP, UTP, and GTP, induced only 9-29% of the ATP response. However, adenosine 5′-thiotriphosphate evoked 79% of ATP-induced 45Ca2+ uptake. 45Ca2+ uptake elicited by ATP could be potently blocked by purinoceptor antagonists, but other tested reagents less effectively blocked the action of ATP. When bradykinin was used as an agonist, the [Ca2+]i rise was transient and was insensitive to the extracellular Na+ concentration. Na+ influx, entry of ethidium bromide, and 45Ca2+ uptake were unaffected by bradykinin. Furthermore, bradykinin-evoked IP3 generation was insensitive to extracellular Ca2+. Neither ATP nor bradykinin had any effect on cAMP levels within cells. These data suggest that ATP induces a [Ca2+]i rise in differentiated NG108-15 cells via two distinct Ca2+ influx mechanisms, i.e., a receptor-operated cation channel and pores formed by ATP4-. These mechanisms are distinct from those elicited by bradykinin.