EFFECTS OF HIGH-CALCIUM CONCENTRATION ON THE FUNCTIONS AND INTERACTIONS OF OSTEOBLASTIC CELLS AND MONOCYTES AND ON THE FORMATION OF OSTEOCLAST-LIKE CELLS

The present study was performed to clarify the role of high calcium co ncentration and the appearance of mononuclear cells at the resorptive site in bone remodeling. Our recent study revealed that the high conce ntration of extracellular calcium ([Ca2+]e) stimulated DNA synthesis i n osteoblastic MC3T3-E1 cells not only directly but also indirectly vi a monocytes. Human monocyte-conditioned medium (CM) significantly stim ulated DNA synthesis and inhibited alkaline phosphatase (ALP) activity . In contrast, when monocytes were cultured at high [Ca2+]e concentrat ions (more than 3 mM), CM from these monocytes significantly stimulate d ALP activity in MC3T3-E1 cells. Such stimulatory effect of CM was no t observed at a high magnesium concentration (Mg2+, 5 mM). Treatment o f monocytes with the calcium ionophore A23187 did not affect the CM-in duced effect on DNA synthesis and ALP activity in these cells. To dete rmine the migration potency of MC3T3-E1 cells and monocytes toward the high [Ca2+]e, chemotaxis assay was performed. The increasing [Ca2+]e (more than 3 mM) induced a chemotactic response of MC3T3-E1 cells as w ell as monocytes, but the high concentration of Mg2+ (5 mM) did not in duce it. On the other hand, treatment with high [Ca2+]e (more than 3 m M) or CM significantly inhibited the 1,25-(OH)2D3-induced formation of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated ce lls (MNC) from their precursors derived from mouse spleen cells. The p resent study indicated that an increase in [Ca2+]e stimulated DNA synt hesis and ALP activity of osteoblasts via monocytes, induced chemotaxi s of osteoblasts as well as monocytes, and inhibited the formation of TRAP-positive MNC, suggesting the importance of the high Ca2+ Concentr ation and mononuclear cells at the resorptive site in bone remodeling.