ATP-induced beta-glucuronidase release from undifferentiated HL-60 cells is dependent on Ca2+ ions

Extracellular ATP (0.1-100 mu M) evoked a biphasic increase in cytosolic free Ca2+ ion concentration ([Ca2+](i)) in fura-2 loaded undifferentiated HL-60 cells and activated the release of the lysosomal enzyme beta-glucuronidase. The EC(50) for the elevation of [Ca2+](i) was about 0.2 mu M. However, the EC(50) for the activation of beta-glucuronidase release was two orders of magnitude higher (about 20 mu M). These data imply that the ATP-induced elevation in [Ca2+](i) is not sufficient to provoke beta-glucuronidase release. The role of [Ca2+](i) in ATP-induced beta-glucuronid ase release was further investigated using Ca2+-free solutions and by loading HL-60 cells with the non-fluorescent Ca2+-chelator BAPTA-AM. Ca2+ ions from both intracellular and extracellular sources were necessary for the activation of beta-glucuronidase secretion by extracellular ATP. However, the internal store is the major source of Ca2+ ions for the response.