Regulation of the phosphate (Pi) concentration in UMR 106 osteoblast-like cells: Effect of Pi,Na+ and K+

Osteoblast-like cells possess Na-dependent transporters which accumulate orthophosphate (Pi) from the extracellular medium. This may be important in bone formation. Here we describe parallel measurements of Pi uptake and cellular Pi] in such cells from the rat (UMR 106–01 and UMR 106–06) and human (OB), and in non-osteoblastic human fibroblasts (Detroit 532 (DET)). In UMR 106–01, cellular Pi] was weakly dependent on extracellular Pi] and higher than expected from passive transport alone. ³²Pi]-uptake was inhibited by Na deprivation, but paradoxically increased on K deprivation. With Na, 87 per cent of cellular ³²P was found in organic phosphorus pools after only 5 min. Na deprivation also decreased cellular Pi], in both UMR 106–01 and DET, but the decrease was smaller than that in ³²Pi]-uptake. Ouabain decreased ³²Pi]-uptake and cellular Pi] in DET, but not in UMR 106–01. Regulation of cellular Pi] is therefore at least partly dependent on Na/Pi co-transport, but this does not seem to be an exclusive property of osteoblasts.