Adaptation to phosphate deprivation in osteoblast-like cells

The rat osteosarcoma cell line UMR-106–01 has an osteoblast-like phenotype. When grown in monolyer culture these cells transport inroganic 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 V_max and was blocked completely by pretreatment with cycloheximide (70 μM). 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.