Regulation of phosphate (Pi) transport and NaPi-III transporter (Pit-1) mRNA, in rat osteoblasts

In osteoblasts only the type III Na+-dependent phosphate (NaPi) transporter isoforms Pit-1 and Pit-2 have been identified. We tested the effects of extracellular Pi, Ca2+ and IGF-1 on Na(d)Pi transport and Pit-1 or Pit-2 mRNA expression in rat osteoblastic (PyMS) cells. The v(max) of Na(d)Pi transport was higher in cells kept in Pi-free, serum-free medium for 24 h than in controls at 1 mM Pi (2(.)47 +/- 0(.)20 vs 1(.)83 +/- 0(.)17 nmol/mg protein x 10 min). The apparent affinity constant (K-M) for Pi remained unchanged. Pi withdrawal for 24 h did not impair cell viability whereas increasing the extracellular Pi to 5 mM resulted in cell death. Pit-1 (but not Pit-2) mRNA was upregulated following Pi deprivation, Ca2+ treatment or after treatment with 1 nM IGF-1, known to stimulate Na(d)Pi transport and cell proliferation. IGF-I also stiniuhited Na(d)Pi transport and Pit-1 mRNA in primary rat calvarial ostcoblasts. Expression of Pit-1 mRNA in vivo and the coordinate regulation of Pit-1 mRNA and Pi transport in osteoblastic cells suggest that Pit-1 is a candidate transporter of physiological relevance in bone.