Effects of inositol trisphosphate on calcium mobilization in bone cells.

The effect of inositol 1,4,5 trisphosphate (IP3) on calcium mobilization was studied in human osteosarcoma lines, Saos-2 and G292, as well as isolated rat osteoblastic and osteoclastic cells. Cells were permeabilized with saponin and calcium mobilization was studied with the fluorescent dye, fura-2 in a recording spectrofluorometer. IP3 (10 microM) increased calcium release in all cell types studied. The effect was dependent on ATP and occurred in the presence of mitochondrial inhibitors. The effect was not seen with inositol 1-phosphate (IP) or inositol 1,4-diphosphate (IP2). Inositol 1,3,4,5 tetrakisphosphate (IP4) appeared to elicit a decrease in the calcium released. Depletion of the intracellular pool with the calcium ionophore, ionomycin, as well as incubation with the inhibitor of intracellular calcium mobilization, TMB-8, obliterated the IP3 effect. The results are consistent with the hypothesis that increases in IP3 can cause a rapid elevation of bone cell cytosolic calcium.