Voltage-dependent phosphate transport in osteoblast-like cells

Phosphate ion (Pi) in sufficient concentrations is crucial for bone mineralization. The osteoblast (OB) may be responsible for the transport of Pi into the bone interstitium, where mineralization occurs. We previously characterized a Na+-dependent Pi transporter (NaPi) in the osteoblastic UMR-106–01 cell line. In the present study, the alteration of Na+-dependent Pi transport by changes in membrane potential was investigated. Depolarizing the cells with increasing concentrations of ambient K+ and valinomycin resulted in a progressive decline in Na+-dependent Pi uptake to a maximum of 28% at a membrane potential of −18 mV compared to control Na+-dependent Pi uptake at a membrane potential of approximately −60 mV. Hyperpolarizing the cells with SCN− increased Na+-dependent Pi uptake over control by 50% at an SCN− concentration of 70 mM. Determination of membrane potential by using the fluorescent probe, DiSC3(5), showed that the addition of Pi to cells in Na+-containing medium resulted in a small depolarization. These data show that NaPi activity can be altered by membrane potential changes and that the initiation of Na+-dependent Pi uptake is associated with depolarization of the plasma membrane of UMR-106–01 cells. Taken together, the cotransport of Na+ and Pi results in the movement of a net positive charge into the cell.