Regulation of Na-dependent Pi transport by parathyroid hormone in osteoblast-like cells.

In the present work we investigated the influence of parathyroid hormone (PTH) on the transport of inorganic phosphate (Pi) in the osteoblast-like cell line UMR-106. Pi was transferred from the extra- to the intracellular compartment by means of a Na-dependent transport system with an apparent binding affinity for both Pi and Na similar to that recently observed in the osteoblast-like cell line ROS 17/2.8 (Calcif. Tissue Int. 43: 83-87, 1988). Exposure of confluent UMR-106 cells to PTH (10(-9)-10(-7) M) induced a concentration-related stimulation of the Na-dependent Pi transport (NaPiT). An increase in NaPiT was observed after a 1-h exposure to 10(-7) M PTH, with the maximal response occurring at 4-6 h. (PTH, 35.6 +/- 0.3; vehicle, 27.4 +/- 0.2 pmol.microgram DNA-1.4 min-1, P less than 0.001). The stimulatory effect of PTH on NaPiT was not associated with a change in the Na-dependent alanine transport. A positive correlation was observed between the increase of NaPiT and that of cAMP in response to various concentrations of PTH. Stimulation of cAMP by forskolin (10(-4) M) mimicked the effect of PTH on NaPiT. Kinetic analysis of the PTH-induced stimulation of NaPiT indicated an increase in Vmax (PTH, 226.9 +/- 6.9; vehicle, 182.9 +/- 1.9 pmol Pi/microgram DNA, P less than 0.001), with no change in Km. The increase in NaPiT by either PTH or forskolin was followed by a transient inhibition from 6 to 24 h that was associated with a decrease in the Na-dependent alanine transport.(ABSTRACT TRUNCATED AT 250 WORDS)