Calcium and insulin-like growth factor I stimulation of sodium-dependent phosphate transport and proliferation of cultured rat osteoblasts.

Calcium (Ca) stimulates proliferation of osteoblasts in vitro, an effect proposed to be mediated by IGF I. Addition of 1 mM Ca or of 1 nM IGF I to the medium (0.3 mM Ca) of a rat bone-derived cell line, PyMS, stimulated not only DNA synthesis but also sodium-dependent (Nad) phosphate (Pi) uptake, the latter, within 2 h. These cells barely express and produce IGF I. IGF binding protein-3 which inhibits IGF action decreased neither basal nor Ca-stimulated but IGF I-stimulated NadPi transport and DNA synthesis, indicating that Ca stimulated NadPi transport and DNA synthesis independently of IGF I. The effects of Ca and IGF I on DNA synthesis were additive. 1 microM nifedipine blocked IGF I- and Ca-stimulated DNA synthesis but not NadPi transport, suggesting that Ca influx is not mediating the NadPi transport-enhancing IGF I signal but is required for IGF I-induced osteoblast proliferation.