Figure 3.

Regulation of ER Ca²⁺ concentrations. (A) Under normal conditions, Ca²⁺ concentrations are higher in the ER lumen than in the cytosol. This balance is maintained by SERCA pumps that bring Ca²⁺ into the ER lumen, and Ca²⁺ channels (RyR and IP₃R) that release Ca²⁺ into the cytoplasm as needed for normal cellular signaling. (B) During ER stress, the Ca²⁺ gradient across the ER membrane is disturbed, leading to Ca²⁺ release from the ER and increased Ca²⁺ concentrations in the cytoplasm.