Osteoblast response to bioactive glasses in vitro correlates with inorganic phosphate content

Inorganic phosphate (Pi) is a physiological regulator of osteoblasts and chondrocytes, suggesting that phosphate may contribute to the biological response of these cells to bioactive glasses like Bioglass® 45S5, which is composed of 45% SiO₂, 24.5% CaO, 24.5% Na₂O, and 6% P ₂O₅. We investigated the effect of varying the Pi content of bioactive glass disks (0%, 3%, 6% and 12% P₂O₅) using human osteoblast-like MG63 cells as the model. Cell number on 6% Pi disks was comparable to cultures on tissue culture plastic, but was reduced at higher and lower Pi concentrations. Alkaline phosphatase specific activity of isolated cells and cell layer lysates, as well as PGE₂, TGF-β ₁ and NO levels in conditioned media, were elevated in cultures grown on bioactive glass and varied with the Pi content. The greatest effects were observed in cultures grown on disks with the lowest Pi concentrations. Thus, growth on the bioactive glasses enhances cell function in comparison with tissue culture plastic and lower Pi content favors osteoblast differentiation.