1 alpha,25-dihydroxyvitamin D3 stimulates sodium-dependent phosphate transport in osteoblast-like cells

1 alpha,25-Dihydroxyvitamin D3 [1,25(OH)2D3] stimulates calcium and phosphate absorption in the duodenum. Because osteoblastic cells appear to be responsible for Pi deposition in bone, we analyzed the effect of 1,25(OH)2D3 on sodium-dependent phosphate (NadPi) transport in newborn rat calvaria-derived osteoblasts and in a rat bone-derived cell line, PyMS. 1,25(OH)2D3 at 10(-9) mol/L stimulated NadPi uptake 1.6-fold in PyMS and 1.8-fold in calvaria cells after 24 h, but not after 2 h and 6 h. Insulin-like growth factor (IGF)-I stimulated NadPi transport 1.8-fold in both cell types within 2 h. There was no change in Na-dependent alanine transport or in Na-independent uptake of Pi and alanine, and the effects of 1,25(OH)2D3 on NadPi transport were not associated with corresponding changes in cell number, protein content, or alkaline phosphatase activity. Actinomycin D, an inhibitor of transcription, prevented the stimulatory effect of 1,25(OH)2D3 (but not that of IGF-I) on NadPi transport; stimulation of NadPi transport by 1,25(OH)2D3 may depend on an increased de novo synthesis of transporters. Our studies confirm that IGF-I stimulates NadPi uptake and show, for the first time, a specific stimulatory effect of 1,25(OH)2D3 on NadPi transport in osteoblasts in vitro.