Multiple pathways for ATP-induced intracellular calcium elevation in pheochromocytoma (PC12) cells

Extracellular ATP excites neurons in both the peripheral and central nervous system. To elucidate the mechanisms involved, we used spectrofluorometric analysis to study the pathways by which extracellular ATP elevates the intracellular Ca2+ concentration ([Ca2+]i) in individual, fura-2-loaded, rat pheochromocytoma PC12 cells. ATP (> 1 microM) increased [Ca2+]i. The ATP effect on [Ca2+]i was completely abolished by a nominally Ca(2+)-free extracellular medium, which indicates that the ATP-induced increase in [Ca2+]i was due to an influx of extracellular Ca2+. We next applied specific blockers of voltage-dependent Ca2+ channels and used experimental protocols with depolarizing external K(+)-rich solutions. Our results show that ATP induces influx of extracellular Ca2+ through (a) dihydropyridine-sensitive (Ln-type) Ca2+ channels, (b) Ca(2+)-permeable, voltage-independent, Cd(2+)-insensitive cation channels, and (c) an as yet unidentified, voltage-dependent, Cd(2+)-sensitive Ca2+ influx system.