Model of Ca2+ concentration controlled by sarcoplasmic reticulum of skeletal muscle, using the state transition

[Ca²⁺] in a muscle cell is controlled by the sarcoplasmic reticulum (SR) that releases Ca²⁺ through the channels, takes up Ca²⁺ by the pumps on the SR membrane, and stores up Ca²⁺ with Ca²⁺ binding protein called calsequestrin (CS). This report proposed a model that represents [Ca²⁺] in a muscle cell controlled by the SR using a state transition probability model in which one state means that protein in the SR is binding ligands, and the other is releasing them. The proposed model consists of 4 modules: calsequestrin, voltage dependent Ca²⁺ release channels, Ca²⁺ induced Ca²⁺ release channels, and Ca²⁺ pumps. Estimating the amount of Ca²⁺ both released and pumped up with the model, it was indicated that [Ca²⁺] rapidly increases from the static state as soon as nerve impulses arrive at a muscle. We further reveal that the fact that Ca²⁺ pumps are located apart from Ca²⁺ release channels has an important influence on generating a Ca²⁺ spike signal.