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Abstract: TH-PO172

Phosphate Inhibits Calcium-Sensing Receptor Expressed Endogenously in TT Cells

Session Information

  • CKD-MBD: Targets and Outcomes
    November 03, 2022 | Location: Exhibit Hall, Orange County Convention Center‚ West Building
    Abstract Time: 10:00 AM - 12:00 PM

Category: Bone and Mineral Metabolism

  • 401 Bone and Mineral Metabolism: Basic

Authors

  • Alghamdi, Khaleda Ali, The University of Manchester Faculty of Biology Medicine and Health, Manchester, United Kingdom
  • Salman, Sheherzad, The University of Manchester Faculty of Biology Medicine and Health, Manchester, United Kingdom
  • Ward, Donald T,, The University of Manchester Faculty of Biology Medicine and Health, Manchester, United Kingdom
Background

Calcium-sensing receptor (CaR) is the key controller of parathyroid hormone (PTH) secretion and extracellular calcium homeostasis. Hyperphosphataemia increases PTH secretion and we reported recently that pathophysiologic phosphate (Pi) concentrations can attenuate CaR activity directly in transfected HEK-293 cells and can increase PTH secretion from human and murine parathyroid cells (Centeno et al., 2019, Nature Communications).

Methods

This was investigated further using thyroidal C cell-derived TT cells (ATCC), which express CaR endogenously. Intracellular Ca2+ (Ca2+i) mobilisation was assayed by epifluorescence microscopy, protein expression by immunoblotting and calcitonin secretion by ELISA.

Results

Co-stimulation of TT cells with the (CaR-activating) calcimimetic R568 (1µM) and spermine (1mM) elicited robust Ca2+i mobilisation, that was inhibited 33±4% (P<0.001) by 2mM (pathophysiologic) Pi-containing buffer vs 0.8mM Pi control. In contrast, raising Pi concentration was without effect on carbachol-induced Ca2+i mobilisation (acting via muscarinic receptors). Also, 1.2mM (high) sulphate elicited a similar CaR inhibition as for Pi (-28±16%; P<0.05 vs 0.3mM control). Next it was found that CaR-mediated stimulation of the TT cells with 1µM R568 & 1mM spermine (in 2mM Ca2+ and 0.8mM Pi) increased calcitonin secretion, as expected, whereas the same stimulation in 2mM Pi (in the presence of 2.2mM Ca2+ to correct for any reduction in free Ca2+) significantly blunted this response (P<0.05).

CaR protein expression was confirmed in the TT cells by immunoblotting. However, CaR abundance was unaffected by overnight culture in media containing 2mM Pi (vs 0.8mM control) whereas CaR stimulation with 1µM R568 & 1mM spermine (in 0.8mM Pi) decreased CaR abundance by 20±4% (P<0.05). Therefore, sustained exposure to high Pi did not produce CaR downregulation at least in the thyroid-derived TT cells.

Conclusion

Therefore, pathophysiologic Pi treatment inhibits endogenous CaR-induced signalling and calcitonin secretion from thyroidal TT cells but without affecting CaR abundance. These results further support the idea that the CaR represents a mineral sensor, at which Pi acts directly as a non-competitive antagonist to limit CaR-induced suppression of PTH secretion.

Funding

  • Government Support – Non-U.S.