What is the concentration of calcium ions in the endoplasmic reticulum?

@article{Bygrave1996WhatIT,
  title={What is the concentration of calcium ions in the endoplasmic reticulum?},
  author={Fyfe L. Bygrave and Angelo Benedetti},
  journal={Cell calcium},
  year={1996},
  volume={19 6},
  pages={547-51}
}
Consideration of the data from a number of sources indicates that the concentration of Ca2+ in the endoplasmic reticulum is very high and perhaps in the mM range. A number of implications flow from this-an important one being that the magnitude of Ca2+ gradients across the endoplasmic and plasma membranes are very similar.