Before revision: Intracellular calcium is regulated  by the release of Ca^{2+} ions from the endoplasmic reticulum via inisitol-4 ,5-triphosphate receptor channels .
After revision: Intracellular calcium is regulated by the release of Ca^{2+} ions from the endoplasmic reticulum via inisitol-4 ,5-triphosphate receptor (IP_3R) channels (among other possibilities such as RyR and L-type calcium channels) .