Figure 1.

Increase in the cytoplasmic Ca²⁺-concentration can be due to Ca²⁺-influx from the extracellular space or due to Ca²⁺-release from the endoplasmic reticulum (ER). The equilibrium of the ER Ca²⁺-content is maintained by sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPases (SERCA) pumping calcium into the ER and inositol-1,4,5-phosphate- (IP₃R) and ryanodine-receptors (RYR) releasing calcium out of the ER. Within the ER, calcium is mainly buffered by calreticulin.