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 cytosol ic free Ca2+ ion concentration ([Ca2+](i)) in fura-2 loaded undifferen tiated HL-60 cells and activated the release of the lysosomal enzyme b eta-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-glucuronidas e release was two orders of magnitude higher (about 20 mu M). These da ta imply that the ATP-induced elevation in [Ca2+](i) is not sufficient to provoke beta-glucuronidase release. The role of [Ca2+](i) in ATP-i nduced beta-glucuronid ase release was further investigated using Ca2-free solutions and by loading HL-60 cells with the non-fluorescent Ca 2+-chelator BAPTA-AM. Ca2+ ions from both intracellular and extracellu lar sources were necessary for the activation of beta-glucuronidase se cretion by extracellular ATP. However, the internal store is the major source of Ca2+ ions for the response.