Human PTH‐(3–34) inhibited the effects of human parathyroid hormone‐related protein on phosphate uptake in a cultured renal cell line (OK cells)

The action mechanism of hPTH and hPTHrP-(1–34) on phosphate uptake in opossum kidney (OK) cells was studied using [Nle8,18Tyr34]hPTH-(3–34)-NH2, a potent competitive inhibitor of adenylate cyclase-coupled PTH receptor. We examined the effects of hPTH-(1–34), hPTHrP-(1–34), and hPTH-(3–34) separately or in combination on the change in renal cyclic AMP production and phosphate uptake in OK cells. Both hPTH-(1–34) and hPTHrP-(1–34) stimulated intracellular cyclic AMP production to the same degree at concentrations between 10−10 and 10−7 M and inhibited phosphate uptake equipotently on a molar basis (27.5 ± 2.0 and 33.2 ± 1.2% inhibition at 10−7 M, respectively). Both exogenous addition of (Bu)2cAMP and endogenous stimulation of cAMP by forskolin inhibited phosphate uptake in a dose-dependent manner. Cyclic AMP production induced by either hPTH-(1–34) or hPTHrP-(1–34) was inhibited by both [Nle8,18Tyr34]-hPTH-(3–34)-NH2 and [Tyr34]-hPTH-(7–34)-NH2. However, [Nle8,18Tyr34]hPTH-(3–34)-NH2 and [Tyr34]-hPTH-(7–34)-NH2 inhibited hPTH-induced cAMP production more strongly. The inhibitory action of phosphate uptake by hPTH-(1–34) and hPTHrP-(1–34) was prevented in the presence of a 100-fold greater concentration of [Nle8,18Tyr34]hPTH-(3–34)-NH2. The antagonistic action of [Nle8,18Tyr34]hPTH-(3–34)-NH2 on the inhibition of phosphate uptake induced by hPTH-(1–34) and hPTHrP-(1–34) became weaker with time (0–120 minutes), and [Nle8,18Tyr34]hPTH-(3–34)-NH2 did not antagonize the inhibition of phosphate uptake induced by hPTHrP-(1–34) at 120 minutes of incubation. Our results indicated that PTHrP inhibits renal phosphate transport, at least in part through an adenylate cyclase-cAMP-coupled PTH receptor, but the response to the competitive inhibition of this peptide-induced cAMP production and inhibition of phosphate uptake with PTH receptor antagonists was different. PTH receptor heterogeneity in OK cells may exist and contribute to this phenomenon.