Effect of Dexamethasone on Parathyroid Hormone (PTH) and PTH-Related Protein Regulated Phosphate Uptake in Opossum Kidney Cells*

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Abstract
Effects of dexamethasone (DEX) on receptor binding of bovine PTH (1-34) [bPTH (1-34)] and human PTH-related protein (1-34) [hPTHrP(1-34)] and cAMP accumulation, and on the inhibition of Na(+)-dependent phosphate uptake were studied in opossum kidney (OK) cells. Maximal specific binding of [125I]hPTHrP(1-34) was reduced by 40% in cells treated with 100 nM DEX, but half-maximal inhibition of binding of [125I]hPTHrP(1-34) by bPTH(1-34) or hPTHrP(1-34) was unaffected by DEX (0.5 nM and 1.4 nM, in the absence and presence of DEX, respectively). Moreover, the EC50 of bPTH(1-34) and hPTHrP(1-34)-evoked cAMP accumulation ranged from 3-7 nM and was the same with and without DEX. The EC50 of the inhibition of phosphate uptake by bPTH(1-34) and hPTHrP(1-34) ranged from 0.2-0.5 nM. Treatment with 100 nM DEX increased phosphate uptake 1.5-fold (EC50 3 nM), and the inhibition of phosphate uptake by bPTH(1-34) and hPTHrP(1-34) was suppressed. The inhibition of phosphate uptake by forskolin was also suppressed in cells treated with 100 nM DEX. DEX, on the other hand, enhanced forskolin-stimulated cAMP accumulation 1.6- to 1.8-fold. 12-O-Tetradecanoylphorbol-13-acetate did not affect cAMP production, but phosphate uptake was inhibited both in the absence and the presence of 100 nM DEX (EC50 3 nM). In conclusion, treatment of OK cells with DEX only minimally affected PTH receptor binding, and cAMP accumulation evoked by bPTH(1-34) and hPTHrP(1-34) remained unaltered. The inhibition of phosphate uptake by PTH, however, was suppressed.

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