Calcium-sensing receptor activation induces intracellular calcium oscillations

Parathyroid hormone secretion is exquisitely sensitive to small changes in serum Ca²⁺ concentration, and these responses are transduced via the Ca²⁺-sensing receptor (CaR). We utilized heterologous expression in HEK-293 cells to determine the effects of small, physiologically relevant perturbations in extracellular Ca²⁺ on CaR signaling via phosphatidylinositol-phospholipase C, using changes in fura 2 fluorescence to quantify intracellular Ca²⁺. Chronic exposure of CaR-transfected cells to Ca²⁺ in the range from 0.5 to 3 mM modulated the resting intracellular Ca²⁺ concentration and the subsequent cellular responses to acute extracellular Ca²⁺ perturbations but had no effect on thapsigargin-sensitive Ca²⁺ stores. Modest, physiologically relevant increases in extracellular Ca²⁺ concentration (0.5 mM increments) caused sustained (30-40 min) low-frequency oscillations of intracellular Ca²⁺ (∼45 s peak to peak interval). Oscillations were eliminated by 1 μM thapsigargin but were insensitive to protein kinase inhibitors (staurosporine, KN-93, or bisindolylmaleimide I). Staurosporine did increase the fraction of cells oscillating at a given extracellular Ca²⁺ concentration. Serum Ca²⁺ concentrations thus chronically regulate cells expressing CaR, and small perturbations in extracellular Ca²⁺ alter both resting intracellular Ca²⁺ as well as Ca²⁺ dynamics.