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+ioscillations in HEK-293 cells expressing human CaR. NPS R-568 was more potent than Ca2+eat eliciting Ca2+ioscillations, 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+eand NPS R-568. Finally, NPS R-568 induced sustained (>15 min after drug removal) Ca2+ioscillations, suggesting slow release of the drug from its binding site. We exploited the potency of NPS R-568 for eliciting Ca2+ioscillations 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+ioscillations, but further truncation (to 868 amino acids) eliminated both highly cooperative Ca2+-dependent activation and regular Ca2+ioscillations. Alanine scanning within the amino acid sequence from Arg873to His879reveals 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.
Extracellular calcium sensing receptor stimulation in human colonic epithelial cells induces intracellular calcium oscillations and proliferation inhibition
