Fluoride rapidly and transiently raises intracellular calcium in human osteoblasts

We examined the effect of fluoride (F) on intracellular ionic calcium [Ca²⁺]_i in normal human osteoblasts maintained in culture. Cells were grown on glass coverslips to near‐confluency and loaded with the Ca‐sensitive dye, fura‐2AM. Fluorescence changes were monitored in single cells using an inverted microscope coupled by fiberoptic to a microspectrofluoremeter. The addition of F (100 ng/mL) to the medium promoted a rapid and significant increase in free [Ca²⁺]_i from a resting level of 245 ± 36 SE nM to a peak concentration of 440 ± 51 nM (p < 0.04). This increase in [Ca²⁺]_i began at 10–20 s after addition of F and was maximal by 30 s. Intracellular [Ca²⁺]_i levels then returned to near resting values by 60–80 s after F addition. This response was evident with as little as 25 ng/ml of fluoride and was dose dependent up to 500 ng/ml. At concentrations greater than 500 ng/ml, there appeared to be an attenuation of the rise in [Ca²⁺]_i. The observed rise in [Ca²⁺]_i was dependent on extracellular calcium since lowering extracellular calcium concentration or incubation with calcium channel blockers abolished the response. This observation supports a role of increased [Ca²⁺]_i as one of the initial events of fluoride on action osteoblasts.