Kidney Week

Abstract: FR-PO488

Hyperphosphatemia Is a Negative Modulator of the Calcium-Sensing Receptor

Session Information

• Bone and Mineral Metabolism: Basic
  October 26, 2018 | Location: Exhibit Hall, San Diego Convention Center
  Abstract Time: 10:00 AM - 12:00 PM

Category: Bone and Mineral Metabolism

• 401 Bone and Mineral Metabolism: Basic

Authors

• Pacios centeno, Patricia, The University of Manchester, Manchester, United Kingdom

Patricia Pacios centeno,

• Tu, Chia-Ling, San Francisco VA Medical Center, San Francisco, California, United States

Chia-Ling Tu,

• Herberger, Amanda L., University of California, San Francisco/ San Francisco Veterans Affairs Medical Center, San Francsicso, California, United States

Amanda L. Herberger,

• Chang, Wenhan, Endocrine Unit, SF-VAMC, University of California San Francisco, San Francisco, California, United States

Wenhan Chang,

• Conigrave, Arthur D., University of Sydney, Sydney, New South Wales, Australia

Arthur D. Conigrave,

• Ward, Donald T,, University of Manchester, Manchester, United Kingdom

Donald T, Ward,

Group or Team Name

• Ward

Background

Chronic kidney disease (CKD) is associated with hyperphosphataemia and secondary hyperparathyroidism (SHPT). The key controller of parathyroid hormone (PTH) secretion is the calcium-sensing receptor (CaSR), whose crystallised extracellular domain has revealed four putative phosphate-binding sites in its inactive conformation. Our aim was to determine whether extracellular phosphate, at concentrations found in CKD, can inhibit CaSR and increase PTH secretion, and thus potentially contribute to the aetiology of SHPT.

Methods

CaSR activity was measured as Ca²⁺_i mobilisation (Fura-2) and extracellular signal-regulated kinase (ERK) phosphorylation in HEK-293 cells stably transfected with human CaSR. The HEPES buffer contained 0.5mM CaCl₂ with phosphate added as Na₂HPO₄ and KH₂PO₄ in a 4:1 ratio (pH 7.4). PTH secretion was measured in dispersed human parathyroid cells obtained ethically following neck surgery and in cultures of intact mouse parathyroid glands.

Results

Raising phosphate concentration from 0.8 mM (physiological) to 2 mM (CKD-like) inhibited CaSR-induced Ca²⁺_i mobilisation (-52 ± 4%; P<0.001) and ERK activation (-18 ± 3%; P<0.01). Raised phosphate concentration (2 mM) inhibited the maximal (E_max) Ca²⁺_o response suggesting non-competitive antagonism (-32 ± 3%; P<0.001). Further, phosphate attenuated CaSR activity with an IC₅₀ of 1.2 mM (95%CI 1.0 to 1.4). Mutation of CaSR^R62 (a putative phosphate-binding site) to alanine ablated this inhibitory effect. Finally, pathophysiologic phosphate concentrations elicited rapid (within minutes) and reversible increases in PTH secretion in freshly-isolated human parathyroid cells (+39 ± 10%, 0.8 vs 2mM, P<0.05; +58 ± 15%, 0.8 vs 3mM; P<0.001) consistent with a receptor-mediated action (with thanks to Ryan Mun). Similarly, in cultures of mouse parathyroid glands phosphate again elicited rapid, concentration-dependent increases in PTH secretion (by up to 180%, 3 vs 0.8mM).

Conclusion

Therefore CaSR is able to sense pathophysiologic deviations in phosphate concentration apparently via non-competitive antagonism at CaSR^R62 resulting in increased PTH secretion. This mechanism may help explain the contribution of hyperphosphatemia to SHPT.

Funding

• Other NIH Support