The phrase "intracellular stores" refers to the compartments within a cell where it can store different molecules or nutrients that it needs for various functions. Full definition
PKA phosphorylates IP3 receptors located on cell membranes, inducing them to release intracellular stores of calcium ions. (scientificamerican.com)
Patients may have normal plasma concentration of magnesium yet have depleted intracellular stores [31]. (journals.plos.org)
DAG is a lipid that remains in the membrane, whereas the soluble headgroup Ins (1,4,5) P3 diffuses into the cytosol and releases calcium from intracellular stores. (molecularcytology.nl)