Endocrine Abstracts

Parathyroid hormone (PTH) secretion maintains extracellular calcium (Ca²⁺_o) homeostasis under the control of the calcium-sensing receptor (CaR). Supra-physiological pH_o changes (±1 pH unit) significantly alter CaR activity in a heterologous expression system. Interestingly, metabolic acidosis and hyperparathyroidism are concomitant in both ageing and chronic kidney disease. Therefore, here we examined whether smaller, pathophysiological pH_o changes modulate CaR activity and in isolated (bovine) parathyroid cells as well as in CaR stably-transfected HEK-293 (CaR-HEK) cells.

Fura2-loaded CaR-HEK cells were stimulated with 2.5 mM Ca²⁺_o (pH 7.4) to elicit CaR-induced Ca²⁺_i mobilisation and then switched to the same buffer at either pH 7.2 (acidosis) or 7.6 (alkalosis) resulting in rapidly attenuated (−59±7%; P=0.012) or elevated (+31±9%; P=0.011) responses respectively. Furthermore, in isolated parathyroid cells, acidosis (pH 7.2) elicited a similar inhibitory effect on CaR-induced Ca²⁺_i mobilisation (−42±9%; P=0.017) while, again, alkalosis (pH 7.6) increased it (+35±10%; P=0.039). All responses were rapidly reversible upon return to pH 7.4 and are unlikely to result from altered intracellular pH as 0.4 unit pH_o changes (5 min) failed to alter pH_i in BCECF-loaded cells. The abundance of serum albumin in vivo might be expected to attenuate this effect, however, the pH_o effects on CaR activity were preserved in the presence of 5% albumin (equivalent to serum concentration). Furthermore, acidosis also attenuated the CaR-induced activation of extracellular signal-regulated kinase (3.5 mM Ca²⁺_o) by 56±11% (P<0.05) while alkalosis potentiated the response (+125±52; P<0.01). Finally, the most likely explanation for CaR’s pH_o-sensitivity is extracellular histidine residue ionisation. However, serial mutation of all 15 histidine residues (to valine) failed to significantly attenuate CaR pH_o sensitivity.

Therefore, pathophysiological changes in pH_o significantly modulate CaR sensitivity with extracellular alkalinisation potentiating and extracellular acidification inhibiting CaR activity. This could indicate a mechanistic link between metabolic acidosis and hyperparathyroidism in ageing and renal disease.

Declaration of interest: The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.

Funding: This work was supported, however funding details unavailable.