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The human osteosarcoma cell line MG-63 displays a number of phenotypic properties typical of mature osteoblasts (OB) (1 ), the cells responsible for bone formation in the skeleton. The MG-63 cell line should, therefore, provide a good model for studying the control of Pi handling by osteoblasts. We have previously shown that insulin-like growlh factor-1 (IGF-1) increases Pi influx in . Although this change in Pi handling could be linked to changes in cell growth, the regulation of the flux of phosphate across the OB and hence into bone fluid may also affect the supply of Pi for the mineralisation process (3) Abnormal Pi transport in OBs has been implicated in the mineralisation defect associated with vitamin D-resistant rickets (4). In addition, acute Pi depletion has been shown to modulate the response of MG-63 cells to parathyroid hormone and agonist-induced CAMP accumulation (5) indicating again the value of these CellS in studying the connection of Pi handling to bone turnover We report here the partial characterisation 01 Pi influx in MG-63 Cells. We also studied the influx of 3-o-methylglucose (MeG). a process unlikely to be directly involved in bone turnover.Monolayer culures of MG-63 cells at passage number 25-40 were used in these studies. The cells were seeded at an initial density of 1.5 x lo5 celkJ2.5 cm well and incubated lor 24h in CUltUre medium. All cultures were then exposed to serum free medium with 0.1% BSA for 24h before measurement of the uptake Of either 32Pi or 3H-MeG. In time course studies, the Pi or MeG was added at time zero, with 6 wells for each time point. The uptake was stopped by rapid washing with Hepes-Ringer (HR) solution (6) at 37OC, followed by protein denaturation with ice-cold 0.67M perchloric acid. The HR wash solution contained phloretin to inhibit the efflux of MeG during washing. The wells were scraped and the radioactivity accumulated determined by liquid scintillation counting. When Na+ dependency of the transpod processes was studied, the plates were washed with a Tris-Ringer (TR) solution at 37% either with Na+, or wlth Na+ replaced with equimolar choline; and with glucose replaced by mannitol. The cells were then incubated for 4 minutes in TR of the same composition containing either 32Pi or 3H-MeG. Uptake was again stopped by rapid washing with HR and further processed as described above.In five separate experiments. the influx of 32Pi was linear for the first 10-25 minutes and started to plateau after 1.5-3.5 h. The uptake of 3H-MeG was rapid and came to an apparent plateau after a mean incubation time of 110 seconds (range 30-180 seconds n=5). The Na+dependancy studies revealed that 32Pi uptake was decreased 90+3% on Na+ deprivation (n=15) whereas the uptake of 3H-MeG decreased only 14+2% (n=18).These results show that the uptake of both Pi and MeG are different from those previously observed for the rat osteoblast-like line, UMR 106 subclone-06 [7]. Here Pi uptake showed significant deviation from linearity in under 60 minutes and a period of at least 60 minutes...