Abstract
The endoplasmic reticulum (ER) is an important regulator of Ca2+Ca2+<math><mrow><msup><mtext>Ca</mtext><mrow><mn>2</mn><mo>+</mo></mrow></msup></mrow></math> in cells and dysregulation of ER calcium homeostasis can lead to numerous pathologies. Understanding how various pharmacological and genetic perturbations of ER Ca2+Ca2+<math><mrow><msup><mtext>Ca</mtext><mrow><mn>2</mn><mo>+</mo></mrow></msup></mrow></math> homeostasis impacts cellular physiology would likely be facilitated by more quantitative measurements of ER Ca2+Ca2+<math><mrow><msup><mtext>Ca</mtext><mrow><mn>2</mn><mo>+</mo></mrow></msup></mrow></math> levels that allow easier comparisons across conditions. Here, we developed a ratiometric version of our original ER-GCaMP probe that allows for more quantitative comparisons of the concentration of Ca2+Ca2+<math><mrow><msup><mtext>Ca</mtext><mrow><mn>2</mn><mo>+</mo></mrow></msup></mrow></math> in the ER across cell types and sub-cellular compartments. Using this approach we show that the resting concentration of ER Ca2+ in primary dissociated neurons is substantially lower than that in measured in embryonic fibroblasts.