Parathyroid hormone inhibits phosphate transport in OK cells but not in LLC-PK1 and JTC-12.P3 cells

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Abstract
Na+-dependent phosphate transport and its response to parathyroid hormone (PTH) has been investigated in three continuous cell lines of renal epithelial origin (LLC-PK1, JTC-12.P3, and OK). The apparent Km for phosphate was similar, but the maximal transport rate (Vmax) was markedly different in the three cell lines. PTH and forskolin produced an increase of cellular adenosine 3',5'-cyclic monophosphate (cAMP) in all cell lines, but Na+-dependent phosphate transport was inhibited exclusively in the OK cells (a threefold reduction of influx after 4 h of exposure to 10(-10) M PTH). The change in phosphate transport is accounted for by a lowered Vmax (30.8 +/- 5.3 vs. 10.2 +/- 1.1 pmol X mg-1 X 3 min-1). The reduction in phosphate transport was reversible, such that 5 h after removal of PTH the Vmax had increased threefold over the inhibited state. Addition of PTH did not alter Na+-dependent L-alanine influx in the OK cells. Experiments with apical membrane vesicles showed that the change in Vmax occurred at the membrane level. It is concluded that the regulatory event responsible for PTH-reduced phosphate transport is beyond cAMP. Of the cell lines studied, only OK cells have a complete regulatory cascade.