The mechanisms by which Pi depletion rapidly regulates gene expression and cellular function have not been clarified. Here, we found a rapid increase in intracellular ionized calcium [Ca2+](i) by phosphate depletion in LLC-PK1 cells using confocal microscopy with the green-fluorescence protein based calcium indicator "yellow cameleon 2.1." The increase of [Ca2+], was observed in the presence or absence of extracellular Ca2+. At the same time, an approximately twofold increase in intracellular mositol 1,4,5-triphosphate (IP3) occurred in response to the acute Pi depletion in the medium. Furthermore, 2-aminoethoxydiphenyl borate completely blocked the [Ca2+], increase induced by Pi depletion. These results suggest that Pi depletion causes IP3-mediated release of Ca2+ from intracellular Ca2+ pools and rapidly increases [Ca2+], in LLC-PK1 cells. (C) 2003 Elsevier Science (USA). All rights reserved.