HGF/SF (0.5-100 ng/ml) induced rapid membrane ruffling, formation of microspikes, and increased motility of HT115 cells within 5 min of addition. These effects were not accompanied by any change in cytosolic free Ca2+ concentration. However, ATP (0.5-10 mM) induced a transient rise in cytosolic free Ca2+ concentration in HT115 cells from a resting concentration of 100 nM to a peak of 400 nM before returning to baseline within 3 min. The addition of ATP to cells treated with HGF/SF inhibited both membrane ruffling and cell movement. The effect of ATP was attributed to the transient rise in cytosolic free Ca2+ concentration, because cytosolic BAPTA, which prevented the rise in cytosolic free Ca2+ concentration, also abolished the inhibitory effect of ATP. Raising cytosolic free Ca2+ concentration with ionomycin and ADP also inhibited membrane ruffling. It was thus concluded that transiently raised cytosolic free Ca2+ concentration inhibited HGF/SF-induced membrane ruffling of HT115 cells.