The effect of replacement of extracellular Na+ with N-methyl-D-glucamine (NMG) on P2 receptor signaling pathways was investigated in dibutyryl cyclic AMP-differentiated NG108-15, cells. Benzoylbenzoic ATP (BzATP) dose-dependently increased the cytosolic Ca2+ concentration ([Ca2+]i) with an EC50 value of 230 microM. Replacement of Na+ with NMG as well as removal of Mg2+ from the bathing buffer potentiated ethidium bromide uptake, [Ca2+]i increase, and 45Ca2+ uptake in response to ATP or BzATP. In contrast, in the presence of 5 mM Mg2+ to limit the amount of ATP4-, replacement of Na+ with NMG had no effect on the ATP-induced [Ca2+]i increase but caused a markedly larger [Ca2+]i increase when the calculated concentration of ATP4- was > 10 microM. The calculated EC50 value for ATP4- stimulation of the [Ca2+]i increase was 23 microM in NG108-15 cells. In vascular smooth muscle cells, intracellular Ca2+ release was the major pathway for the ATP-induced [Ca2+]i increase; both removal of Mg2+ and replacement of Na+ with NMG did not affect the action of ATP. These data suggest that ATP(4-)-promoted pores are antagonized by Na+ and Mg2+ in dibutyryl cyclic AMP-differentiated NG108-15 cells.