FLUORIDE INCREASES NET CA-45 UPTAKE BY SAOS-2 CELLS - THE EFFECT IS PHOSPHATE DEPENDENT

Previous in vitro studies have shown that the effect of fluoride to in crease avian osteoblast-like cell proliferation was dependent on the p hosphate concentration. In vitro studies have further revealed that fl uoride could also have direct effects on osteoblast-like cells to incr ease phosphate uptake and transiently increase cytosolic calcium. The current studies were intended to determine whether fluoride could incr ease net Ca-45 uptake by human osteosarcoma (SaOS-2) cells and, if so, whether those effects would also be phosphate dependent. The results of these studies indicate that fluoride increased net Ca-45 uptake by SaOS-2 cells, with biphasic dose and time dependencies. After 30 minut es of exposure, net Ca-45 uptake was increased to a greater extent by 50 muM fluoride (217 +/- 16% of control, P < 0.001) than by 200 muM fl uoride; and the stimulatory effect of 100 muM fluoride on net Ca-45 up take was greater after 20 minutes (187 +/22% of control, P < 0.001) th an after 60 minutes (122 +/- 7% of control, P < 0.05). These effects o f fluoride to increase net Ca-45 uptake were dependent on the phosphat e concentration in the medium. Fluoride had no effect on net Ca-45 upt ake in medium containing 0.4 mM phosphate, but increased net Ca-45 upt ake in medium containing 1.2 or 2.0 mM phosphate (P < 0.005). As the p hosphate concentration was increased, the biphasic fluoride dose-respo nse curve was shifted to a lower range of fluoride concentrations. The se effects of fluoride were not unique to SaOS-2 cells with very high steady-state levels of skeletal alkaline phosphatase; similar effects were seen in a subpopulation of SaOS-2 cells with much lower alkaline phosphatase levels. Further studies indicated that the effects of fluo ride to increase SaOS-2 cell proliferation and skeletal alkaline phosp hatase activity showed a similar pattern of phosphate dependency. As t he fluoride-dependent increases in 3[H]-thymidine incorporation and ne t Ca-45 uptake were blocked by verapamil, these data are consistent wi th the general hypothesis that the osteogenic effects of fluoride are associated with acute effects to increase net Ca uptake.