The effect of -5-(isopropyl)-3-tert-butylthioindol-2-yl]-2, 2dimethylpropanoic acid), a compound widely used to inhibit leukotriene synthesis, on cytosolic free Ca 21 concentrations ([Ca 21 ] i ) in osteosarcoma cells has not been explored. This study examined whether MK-886 altered [Ca 21 ] i levels in suspended MG63 human osteosarcoma cells using fura-2. MK-886 at 0.1 mM and above increased [Ca 21 ] i in a concentration-dependent manner. The Ca 21 signal was reduced partly by removing extracellular Ca 21 . MK-886 induced Mn 21 quenching of fura-2 fluorescence, implicating Ca 21 entry. MK-886induced Ca 21 influx was inhibited by store-operated Ca 21 entry inhibitors, nifedipine, econazole, and SKF96365; and by the protein kinase C modulators, phorbol 12-myristate 13-acetate (PMA) and GF109203X. In Ca 21 -free medium, after pretreatment with 5 mM MK-886, 1 mM thapsigargin (an endoplasmic reticulum Ca 21 pump inhibitor)-induced [Ca 21 ] i rises were abolished; conversely, thapsigargin pretreatment nearly abolished MK-886-induced [Ca 21 ] i rises. Inhibition of phospholipase C with U73122 did not change MK-886-induced [Ca 21 ] i rises. Collectively, in MG63 osteosarcoma cells, MK-886 induced [Ca 21 ] i rises by causing phospholipase C-independent Ca 21 release from the endoplasmic reticulum and Ca 21 influx via protein kinase C-regulated store-operated Ca 21 entry.