Calcium in osteoblast-enriched bone cells

The exchangeability, location, and amount of the total calcium in bone cells were studied in relation to their osteoblastic activity. Cells were isolated from neonatal rat calvariae by sequential collagenase digestion and incubated with 45Ca2+ before or after various treatments. 45Ca2+, 40Ca2+, and DNA were determined on each cell sample. Long-term experiments were performed on cultured cells. The cells closest to the forming bone had the highest alkaline phosphatase activity and the highest Ca2+ content (i.e., 17 mm Ca2+/l cell water, as compared to soft tissue cells, which have 2-3 mm Ca2+/l (Borle 1981). About 50% of this Ca2+ exchanges readily with 45Ca2+ and appears to be located almost entirely in the cell membrane. Thirty-five percent exchanges only slowly with a t1/2 of 27 hours, and is located within the cell, principally in the mitochondria as seen in pyroantimonate fixed cells (Neuman et al. 1985). In spite of its slow exchange-ability, this Ca2+ fraction can be mobilized or augmented rapidly when Ca2+ supply is reduced or increased in vitro or in vivo. 1,25(OH)2D3 given in vivo increased this intracellular Ca2+ when the Ca2+ supply was low and released it when the Ca2+ supply was high. About 15% of the Ca2+ in these cells was nonexchangeable. These results suggest that osteoblasts do process more Ca2+ when closer to the mineralization site. Whether this Ca2+ is en route from blood to bone or from bone to blood will require further study.