ATP (1 mM) induced a biphasic increase in intracellular Ca2+ concentration ([Ca2+]i), i.e., an initial transient increase decayed to a level of sustained increase, in NG108-15 cells. The transient increase was inhibited by a phospholipase C inhibitor, 1-[6[[17beta-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H- pyrrole-2,5-dione (U73122), whereas the sustained increase was abolished by removal of external Ca2+. We examined the mechanism of the ATP-elicited sustained [Ca2+]i increase using the fura-2 fluorescent method and the whole-cell patch clamp technique. ATP (1 mM) induced a membrane current with the reversal potential of 12.5 +/- 0.8 mV (n = 10) in Tyrode external solution. The EC50 of ATP was approximately 0.75 mM. The permeability ratio of various cations carrying this current was Na+ (defined as 1) > Li+ (0.92 +/- 0.01; n = 5) > K+ (0.89 +/- 0.03; n = 6) > Rb+ (0.55 +/- 0.02; n = 6) > Cs+ (0.51 +/- 0.01; n = 5) > Ca2+ (0.22 +/- 0.03; n = 3) > N-methyl-D-glucamine (0.13 +/- 0.01; n = 5), suggesting that ATP activated a nonselective cation current. The ATP-induced current was larger at lower concentrations of external Mg2+. ATP analogues that induced the current were 2-methylthio-ATP (2MeSATP), benzoylbenzoic-ATP, adenosine 5'-thiotriphosphate (ATPgammaS), and adenosine 5'-O-(2-thiodiphosphate), but not adenosine, ADP, alpha,beta-methylene-ATP (AMPCPP), beta,gamma-methylene-ATP (AMPPCP), or UTP. Concomitant with the current data, 2MeSATP and ATPgammaS, but not AMPCPP or AMPPCP, increased the sustained [Ca2+]i increase. We conclude that ATP activates a class of Ca2+-permeable nonselective cation channels via the P2z receptor in NG108-15 cells.