Calcium and phosphate supplementation promotes bone cell mineralization: Implications for hydroxyapatite (HA)-enhanced bone formation

Organic phosphate, in particular beta-glycerophosphate (beta-GP), has been used to induce mineralization in cell culture systems. It serves as a sourc e of inorganic phosphate when hydrolyzed by alkaline phosphatase. This stud y examined the effect of supplemental calcium and phosphate as well as the influence of various metabolic inhibitors on mineralization in a rat osteob last-like cell-culture system. Mineralization was induced by supplementatio n of 1.8 mM of Ca+2 and 5 mM of beta-GP or Pi. Mineral deposits associated with in vitro mineralization were revealed under SEM and TEM. Levamisole (1 0-100 mu M) inhibited alkaline phosphatase activity and effectively reduced Mineral formation. Actinomycin (500 ng/mL) and cycloheximide (50 mu g/mL) also reduced mineral depositions by blocking RNA synthesis and protein synt hesis, respectively Levamisole and beta-GP did not appear to influence DNA synthesis. Spontaneous precipitation of calcium phosphate mineral was not d etected in the culture medium with calcium and phosphate supplements in the absence of cell culture. The findings suggest that an elevated concentrati on of calcium and phosphate is crucial for in vitro mineralization. Further more, the mineralization process is associated with biologic events rather than with a spontaneous precipitation of calcium phosphate mineral. In view of the degradation potential of hydroxyapatite (HA)-coated implants, these results may be a viable indication that HA enhances bone formation through a similar mechanism. (C) 2000 John Wiley & Sons, Inc.