Adaptation to phosphate deprivation in osteoblast-like cells.

The rat osteosarcoma cell line UMR-106-01 has an osteoblast-like phenotype. When grown in monolayer culture these cells transport inorganic phosphate and L-alanine via Na(+)-dependent transport systems. Exposure of these cells to a low phosphate medium for 4 h produced a 60-70 per cent increase in Na(+)-dependent phosphate uptake compared to control cells maintained in medium with a normal phosphate concentration. In contrast, Na(+)-dependent alanine uptake and Na(+)-independent phosphate uptake were not changed during phosphate deprivation. The increased phosphate uptake was due, in part, to an increased Vmax and was blocked completely by pretreatment with cycloheximide (70 microM). In these cells recovery of intracellular pH after acidification with NH4Cl is due primarily to the Na+/H+ exchange system. The rate of this recovery process, monitored with a pH sensitive indicator (BCECF), was decreased by more than 50 per cent in phosphate-deprived cells compared to controls indicating that Na+/H+ exchange was inhibited during phosphate deprivation.