NaF-induced Ca2+ mobilization is dependent upon the culture density in a parathyroid hormone-responsive osteoblast-like cell line

Abstract The effect of NaF on cytosolic free Ca 2+ concentrations ([Ca 2+ ]i) was examined in a clonal osteoblast-like cell line (MOB 3–4) loaded with Fura 2. MOB 3–4 cells in a sparse culture, which exhibited neither alkaline phosphatase (ALP) activity nor the response to parathyroid hormone (PTH), responded to NaF (0.1–10 mM) to increase [Ca 2+ ]i transiently. In contrast, the cells in a dense culture, which exhibited both ALP activity and the response to PTH, responded to NaF (above 4 mM) to increase [Ca 2+ ]i slowly. [Ca 2+ ]i in osteoblasts in primary culture slowly increased in response to both NaF (above 4 mM) and PTH (3 U/ml). Thus, the sensitivity and the response of MOB 3–4 cells to NaF and PTH varied with the culture density, and high culture density matured the cells like osteoblasts in primary culture. These NaF-induced Ca 2+ mobilizations were not dependent upon external Ca 2+ and were enhanced by Al 3+ (1 μM), whereas the PTH-induced Ca 2+ mobilizations were due to Ca 2+ influx. These results suggest that the maturation of MOB 3–4 cells, dependent upon the culture density, modulates intracellular signal transduction pathways and thereby alters the NaF-induced Ca 2+ mobilization, and that the culture density must be taken into consideration in studying Ca 2+ mobilization in such an osteoblast-like cells line as MOB 3–4 cell line.