Calciotropic hormones raise the chemically detectable [Pi] in UMR 106–06 osteoblast-like cells

Uptake of orthophosphate (Pi) by osteoblast-like cells is known to be stimulated by parathyroid hormone (PTH), but effects on intracellular [Pi] have not been investigated. Here we show in rat osteoblast-like cells (UMR 106-06) that PTH (10^?11 to 10^?7 M ) increases both ³²Pi uptake and cellular [Pi] by up to 50 per cent. 1,25 Dihydroxyvitamin D₃ (1,25D) (10^?12 to 10^?6 M ) and salmon calcitonin (CT) (10^?12 to 10^?6 g ml^?1) also increased cellular [Pi] (by up to 60 per cent), but the percentage increases in total cellular ³²Pi uptake were smaller. The effects of 1,25D were transient (observable at 80 min and 6 h but not 24 h), and were also observed with 24,25 dihydroxy- and 25 hydroxyvitamin D₃. Transient degradation of organic phosphorus pools to Pi might contribute to this increased [Pi]. These pools remain to be identified but were not shown to be phospholipids. Foetal bovine serum also affected cellular [Pi]. Care is therefore needed in distinguishing direct hormonal effects on cellular [Pi] from indirect effects arising from changes in the rate of cell growth.