Calcium sensing receptor activation by a calcimimetic suggests a link between cooperativity and intracellular calcium oscillations

Activation of the calcium sensing receptor (CaR) by small increments in extracellular calcium (Ca²⁺_e) induces intracellular calcium (Ca²⁺_i) oscillations that are dependent on thapsigargin-sensitive intracellular calcium stores. Phenylalkylamines such as NPS R-568 are allosteric modulators (calcimimetics) that activate CaR by increasing the apparent affinity of the receptor for calcium. We determined, by fluorescence imaging with fura-2, whether the calcimimetic NPS R-568 could activate Ca²⁺_i oscillations in HEK-293 cells expressing human CaR. NPS R-568 was more potent than Ca²⁺_e at eliciting Ca²⁺_i oscillations, particularly at low [Ca²⁺]_e (as low as 0.1 mM). The oscillation frequencies elicited by NPS R-568 varied over a 2-fold range from peak to peak intervals of 60-70 to 30-45 s, depending upon the concentrations of both Ca²⁺_e and NPS R-568. Finally, NPS R-568 induced sustained (>15 min after drug removal) Ca²⁺_i oscillations, suggesting slow release of the drug from its binding site. We exploited the potency of NPS R-568 for eliciting Ca²⁺_i oscillations for structural studies. Truncation of the CaR carboxyl terminus from 1077 to 886 amino acids had no effect on the ability of Ca²⁺ or NPS R-568 to induce Ca²⁺_i oscillations, but further truncation (to 868 amino acids) eliminated both highly cooperative Ca²⁺-dependent activation and regular Ca²⁺_i oscillations. Alanine scanning within the amino acid sequence from Arg⁸⁷³ to His⁸⁷⁹ reveals a linkage between the cooperativity for Ca²⁺-dependent activation and establishment and maintenance of intracellular Ca²⁺ oscillations. The amino acid residues critical to both functions of CaR may contribute to interactions with either G proteins or between CaR monomers within the functional dimer.