Characterization of intracellular calcium oscillations induced by extracellular nucleotides in HEp-2 cells

The effect of extracellular nucleotides on the cytosolic calcium concentrat ion of fluo-3-loaded HEp-2 cells was examined using confocal microscopy. Ex tracellular ATP and UTP at micromolar concentration induced cytosolic calci um oscillations in 42-66% of the cells. Oscillations were usually sinusoid and their frequency depended only slightly on agonist concentration. Oscill ations developed in calcium-free medium but were diminished by depletion of intracellular calcium stores with thapsigargin, indicating periodic calciu m release from internal stores. Inhibition of phospholipase C with U73122 p revented the development of oscillations, while ryanodine did not abolish t he response to extracellular nucleotides. Activation of protein kinase C wi th 4 beta -phorbol 12-myristate 13-acetate also prevented the development o f oscillations. These results indicate that extracellular nucleotides induc e periodic calcium release from inositol 1,4,5-trisphosphate-sensitive pool s in HEp-2 cells and that the inhibitory effect of protein kinase C on the phosphatidylinositol signaling pathway can contribute to the development of intracellular calcium oscillations. (C) 2000 Elsevier Science B.V. All rig hts reserved.