Characteristics of phosphate transport in osteoblastlike cells

The characteristics of the transport of inorganic phosphate (Pi) in osteoblastic cells have been determined using the osteosarcoma cell line ROS 17/2.8. The initial rate of the Pi transfer from the extracellular into the intracellular osteoblastic compartment is mediated by a sodium-dependent process. The stoichiometric analysis of the cotransport system suggests that two sodium ions would be transferred with each Pi molecule. In the presence of sodium, the Pi transfer was saturable with increasing extracellular Pi concentration. In the absence of extracellular sodium, only a negligible amount of Pi enters the osteoblastic cells, with a kinetic compatible with a simple diffusion process. The kinetic parameters of the saturable component of the Pi transport measured at an external sodium concentration of 143 mmol/liter were Km = 448 +/- 12 mumol/liter; Vmax = 37.1 +/- 0.7 nmol/mg prot. 4 min. In the presence of 0.1 mmol/liter Pi, the half-maximal activation by sodium was obtained at 43 +/- 1.3 mmol/liter. The Pi transport rate was reduced by arsenate, by metabolic inhibitors such as FCCP and by ouabain, an inhibitor of Na-K ATPase. These results strongly suggest that the Pi transfer into osteoblastic cells is a carrier-mediated process which is driven by the transmembrane electrochemical gradient of sodium.