1,25-Dihydroxyvitamin D₃ inhibits Na⁺-dependent phosphate transport in osteoblastic cells

In the present work we investigated the influence of vitamin D₃ metabolites on Na⁺-dependent phosphate (P(i)) transport in the clonal osteoblastic cell line UMR-106. The vitamin D₃ metabolite 1,25- dihydroxyvitamin D₃ [1,25(OH)₂D₃] dose-dependently inhibited P(i) transport with a half-maximal concentration of ~5 x 10^-11 M. The effect of 1,25(OH)₂D₃ was first observed after 8 h of preincubation period. Inhibition of phosphate uptake was relatively specific for the 1,25(OH)₂D₃ analogue of vitamin D₃. The potency order was 1,25(OH)₂D₃ >> 24,25- dihydroxyvitamin D₃ > 25-[³H]hydroxyvitamin D₃. Kinetically, 1,25(OH)₂D₃ decreased the maximal velocity of the phosphate uptake system, whereas the affinity for phosphate was unaffected. Activation of protein kinase C (PKC) in UMR-106 cells stimulated Na⁺-dependent P(i) transport. Nonetheless, the inhibitory effect of 1,25(OH)₂D₃ on P(i) transport was not related to downregulation of PKC. Chemical determination of intracellular P(i) showed a 50% reduction after 24-h preincubation with 10^-8 M 1,25(OH)₂D₃. We conclude that 1,25(OH)₂D₃ inhibits Na⁺-dependent phosphate transport in osteoblastic cells. This in turn leads to intracellular P(i) depletion. The physiological implication of this phenomenon on the effects of vitamin D on osteoblasts in situ is discussed.