We describe a novel method to monitor the endoplasmic reticulum (ER) free Ca2+ in intact cells. Continuous perfusion of HeLa cells, expressing ER-targeted apoaequorin, with coelenterazine allowed the apoprotein to act as a pseudo-luciferase capable of reporting free Ca2+ from 0.1–100 µM. In intact HeLa cells, addition of ionomycin increased apoaequorin-generated light by 91%, indicating that resting ER free Ca2+ was approx. 2 µM. Agonist stimulation decreased the ER apoaequorin signal and proportionally increased cytosolic free Ca2+ consistent with agonist-induced release of Ca2+ from the ER.

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