Calcium-sensing receptors (CaSRs) are class C G protein-coupled receptors that respond to physiological activators, including extracellular Ca2+ (Cao2+) and L-amino acids as well as the pharmaceutical calcimimetic, cinacalcet. Unlike Cao2+, which is an orthosteric agonist, L-amino acids and cinacalcet are positive allosteric modulators. CaSR expression levels vary considerably between tissues, but the physiological significance of these differences in expression for the effects of its activators is unknown. To investigate the impact of receptor expression on CaSR-mediated signaling we used a tetracycline-inducible expression system and focused on intracellular Ca2+ (Cai2+) responses in single cells and considered both population and single-cell behavior. Increased receptor expression positively modulated CaSR-mediated Cai2+ mobilization in response to elevated Cao2+, the amino acid L-phenylalanine, or the calcimimetic cinacalcet. It lowered threshold concentrations for the initiation of Cai2+ oscillations and for their transformation to sustained Cai2+elevations, and it increased the proportions of responding cells. It also positively modulated the frequency of Cai2+ oscillations with the order of effectiveness: cinacalcet equal to or greater than Cao2+ greater than L-phenylalanine. The results indicate that receptor expression modulates key characteristics of the Cai2+ response at the single-cell level as well as the amplitude of whole-tissue CaSR-mediated responses by recruiting quiescent cells into the active pool of responding cells. By lowering the threshold concentrations for Cao2+- and L-amino acid-induced responses below the physiological levels of these nutrients in plasma, mechanisms that up-regulate receptor expression can control tissue function in the absence of dynamic changes in ligand concentration.
Brennan, S. C., Mun, H., Leach, K., Kuchel, P. W., Christopoulos, A., & Conigrave, A. (2015). Receptor expression modulates calcium-sensing receptor mediated intracellular Ca2+ mobilization. Endocrinology, 156(4), 1330 - 1342. https://doi.org/10.1210/en.2014-1771